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Sensation and Perception (PSYC 2390) Chapter Summaries

98 Pages

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PSYC 2390
Naseem Al- Aidroos

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Sensation and Perception Chapter  Summaries Chapter 1: Introduction to Perception The Perceptual Process • Perceptual process: a sequence of processes that work together to determine our experience of and reaction to stimuli in the environment, which include the following categories: 1) Stimuli  Refers to what is out there is the environment, what we actually pay attention to, and what stimulates our receptors 2) Electricity  Refers to the electrical signals that are created by the receptors and transmitted to the brain 3) Experience and Action  Refers to our goal—to perceive, recognize and react to the stimuli 4) Knowledge  Refers to knowledge we bring to the perceptual situation  This category is located above the other 3 boxes because it can have its effect at many different points in the process The Stimuli • Steps for the stimulus category: 1) Environmental stimulus  Example: everything in our wood scene is the environmental process 2) Attended stimulus  Example: when the individual focuses on the moth, which becomes the attended stimulus 3) Stimulus on the receptors  Example: an image of the moth is formed on the individual’s retina 1­2. Environmental Stimuli and Attended Stimuli • Environmental stimuli: is all the things in our environment that we can potentially perceive • Attended stimulus: when the individual focuses on a particular object, making it their center of attention o It changes from moment to moment, as their attention shifts from place to place 3. The Stimulus on the Receptors • This step is important because the stimulus is transformed into another form: an image on the individual’s retina • Since the stimulus is transformed into an image, this image is actually a representation o It isn't the actual stimulus but it stands for the stimulus Electricity • Everything we perceive is based on electrical signals in our nervous system o They are created in the receptors, which transform energy from the environment into electrical signals in the nervous system • Steps for the electricity category 1) Transduction  Occurs when the receptors create electrical energy in response to light 2) Transmission  Occurs as one neuron activates the next one 3) Processing  This electrical energy is processed through networks of neurons 4. Transduction • Transduction: is the transformation of one form of energy into another form of energy • It occurs in the nervous system when energy in the environment—such as light energy, mechanical pressure or chemical energy—is transformed into electrical energy 5. Transmission • After transduction, these signals activate other neurons, which in turn activate more neurons • Eventually these signals travel out of the eye and are transmitted to the brain • The transmission step is crucial because if signals don’t reach the brain, there is no perception 6. Processing • Neural processing: involves interactions between neurons • The original electrical representation of the stimulus that is created by the receptors is transformed by processing a new representation of the stimulus in the brain Experience and Action • Steps for experience and action 1) Perception  Example: Ellen has conscious perception of the moth 2) Recognition  Example: Ellen recognizes the moth 3) Action  Example: Ellen takes action by walking toward the tree to get a better view 7. Conscious Perception • Perception: is conscious sensory experience • It occurs when the electrical signals that represent the stimulus are transformed by the brain into the individual’s experience of seeing the stimulus 8. Recognition • Recognition: is our ability to place an object in a category that gives it meaning o Although perception and recognition seem similar, they are actually separate processes • Dr. P’s problems was diagnosed as visual form agnosias: an inability to recognize objects, caused by a brain tumor o He could perceive the object and recognize parts of it, but he couldn’t perceptually assemble the parts in a way that would enable them to recognize the objects as a whole 9. Action • Action: includes motor activities such as moving the head or eyes and moving through the environment o Action is important for survival • Goodale propose that early in the evolution of animals the major goal of visual processing was to help the animal control navigation, catch prey, avoid obstacles and detect predators o These are all crucial functions for the animal’s survival • Perception is a continuously changing process o For example, the scene that Ellen is observing changes every time she shifts her attention to something else or moves to a new location, or something in the scene moves o The steps of the perceptual process are arranged in a circle because the overall process is so dynamic and continually changing Knowledge • Knowledge: is any information that the perceiver brings to the situation o It affects the number of the steps in the perceptual process o Information of things learned years ago or knowledge obtained from events that have just happen affect the perceptual process • The rat-man demonstration: shows how recently acquired knowledge can influence perception • Bottom-up processing or data-based processing o Is processing that is based on incoming data o Incoming data always provides the starting point for perception because without it, there is no perception o Example: for Ellen, the incoming data are the patterns of light and dark on her retina created by light reflected from the moth and the tree • Top-down processing or knowledge-based processing o Refers to processing that is based on knowledge o Example: for Ellen, this knowledge includes what she knows about moths • Bottom-up and top-down processing often work together to create perception How to Approach the Study of Perception • Perception has been studied using 2 approaches: o Psychophysical approach o Physiological approach • The 3 relationships that are usually measured to study the perceptual process are the: o Psychophysical (PP) relationship between stimuli and perception o Physiological (PH1) relationship between stimuli and physiological processes o Physiological (PH2) relationship between physiological processes and perception • Psychophysical approach (PP) o Introduced by Gustav Fechner, who in his book “Elements of Psychophysics” coined the term psychophysics o Psychophysics: refers to the use of quantitative methods to measure relationships between stimuli (physics) and perception (psycho)  Also refers to any measurement of the relationship between stimuli and perception o Example: asking an observer to decide whether 2 very similar patches of colour are the same or different • Physiological approach (PH1 or PH2) o Involves measuring the relationship between  Stimuli and physiological processes (PH1)  Physiological processes and perception (PH2) o These physiological processes are most often studies by measuring electrical responses in the nervous system, but can also involve studying anatomy or chemical processes o Example of PH1  Measuring how different coloured light result in electrical activity generated in neurons in a cat’s cortex o Example of PH2  Astudy in which a person’s brain activity is measured as the person describes the colour of an object they are seeing • Our goal is to understand how neurons and the brain creates perception • Cognitive influences on perception: the starting place for top-down processing o This is studied by measuring how knowledge and other factors, such as memories and expectations, affect each of the 3 relationships o Knowledge can also affect physiological responding, as shown in the rat-man demonstration Measuring Perception Description • Phenomenological method: the method used when a researcher asks a person to describe what they are perceiving or to indicate when a particular perception occurs o This method is the first step in studying perception because it describes what we perceive o This description can be very basic or complex Recognition • Recognition: is measured when we categorize a stimulus by naming it Detection • Fechner’s book described a number of quantitative methods for measuring the relationship between stimuli and perception • These methods are called classical psychophysical methods: they were the original methods used to measure the stimulus-perception relationship and include: o Limits o Adjustment o Constant stimuli The Absolute Threshold • Absolute threshold: is the smallest amount of stimulus energy necessary to detect a stimulus • There are 3 basic methods for determine the absolute threshold: o The method of limits o The method of adjustment o The method of constant stimuli • The method of limits o The experimenter presents a stimuli (such as a tone) in either ascending order (increasing intensity) or descending order (decreasing intensity) o The observer’s job is to indicate a ‘yes’response if they notice the stimulus, or ‘no’if they don’t  The change from ‘yes’to ‘no’indicated by the dashed line on the graph is the crossover point o The absolute threshold is calculated by averaging all the crossover points • The method of adjustment o The observer or the experimenter adjusts the stimulus intensity continuously until the observer can just barely detect the stimulus o This method differs from the method of limits because the observer does not say ‘yes’or ‘no’as each stimulus intensity is presented o The absolute threshold is calculated by taking the average setting • The method of constant stimuli o The experimenter presents 5-9 stimuli with different intensities in random order o The experimenter determines the percentage of times that the observer perceives each intensity o The absolute threshold is defined as the intensity that results in detection on 50% of the trial • The choice among the different methods is usually determined by the accuracy needed and the amount of time available o Constant stimuli:  Advantages: • Most accurate method • It involves many observations • Stimuli are presented in random order, which minimizes how presentation on one trail can affect the observer’s judgement on the next trial  Disadvantage: Time-consuming o Adjustment:  Advantage: faster because observers can determine their threshold in just a few trials by adjusting the intensity themselves The Difference Threshold (DL) • Ernst Weber created another type of threshold: the difference threshold • Difference threshold (DL): is the smallest difference between 2 stimuli that a person can detect • How is the difference threshold measured? o Participants are asked to indicate whether they detect a difference between 2 stimuli, without actually looking at them o In this case, 2 weight sizes:  The standard weighing 100 g  The comparison weighing 100 g + 2 g • Weber found when the difference between the standard and comparison weights were small, the observer found it difficult to detect the differences in the weights, but they easily detected larger differences o As the magnitude of the stimulus increased, so does the DL o The ratio of the DL to the standard stimulus is constant DL K= • Weber’s law: S o Where K is a constant called the Weber fraction  Each sensory judgement has its own K o Where S is the value of the standard stimulus o Where DL is the difference threshold • Fechner’s proposal of the 3 psychophysical methods demonstrated that mental activity could be measured quantitatively and is still used today Magnitude Estimation • Magnitude estimation: above-threshold perceptions developed by S. S. Stevens, and the method includes: o The experimenter presents a standard stimulus to the observer and assigns it a value, such as 10 o He then presents lights at different intensities and the observer is asked to assign a number to each of these lights that is proportional to the brightness of the standard • Response compression: as intensity increases, the magnitude increases, but not as rapidly as the intensity o To double the brightness, it is necessary to multiply the intensity by 9 o Example: using light intensity as the stimulus o Represented by the red line • Response expansion: as intensity increases, the perceptual magnitude increases more than the intensity o Example: using electric shock as the stimulus o Represented by the orange line • The relationship between the intensity of a stimulus and our perception of its magnitude follows the same general equation for each sense n o Steven’s power law: P = KS  The power functions are described in the equation  Where P is the perceived magnitude  Where K and n are constants  Where S is the stimulus intensity • One of the properties of power functions is that taking the logarithm of the terms on the left and right sides of the equation changes the function into a straight line • Steven’s power law illustrates how the operation of each sense is adapted to how organisms function in their environment o For instance, when you are looking outside you are not blinded by the sudden brightness o The opposite situation occurs for electric shock  Small increase in shock intensity causes large increases in pain  This rapid increase in pain even to small increases in shock intensity serves to warn us of impending danger, and we therefore tend to withdraw even from weak shocks Search • Visual search: the observer’s task is to find one stimulus among many, as quickly as possible o An everyday example of this would be searching for a friend’s face in a crowd • Reaction time: the time between presentations of the stimulus and the observer’s response to the stimulus o This provides important information about mechanisms responsible for perception Something to Consider: Threshold Measurement can be influenced by how a Person  Chooses to Respond • In a hypothetical experiment 2 people are asked to respond with a yes or no, if they are able to detect a light o Away of describing this difference between these 2 people is that each has a different response criterion o Response criterion: in a signal detection experiment, the subjective magnitude of a stimulus above which the participants will indicate that the stimulus is present  Julie • Response criterion is low • She says yes if there is the slightest change a light is present  Regina • Response criterion is high • She says yes only when she is sure that she sees the light • Response criterion is not every important if we are testing many people and averaging their responses o However, if we wish to compare 2 people’s responses, their differing response criteria could influence the results o Luckily, there is a way to take differing response criteria into account, which is known as signal detection theory Chapter 2: Introduction to the Physiology of Perception The Brain: The Mind’s Computer Brief History of the Physiological Approach Early Hypotheses about the Seat of the Mind • 4 century B.C.: PhilosopherAristotle stated that the heart was the seat of the mind and tst soul • 1 century A.D.: Greek physician Galen saw human health, thoughts and emotions as being determined by 4 different ‘spirits’flowing through the ventricles or the cavities of the brain • Early 1630: Philosopher Rene Descartes specified the pineal gland, which was thought to be located over the ventricles, as the seat of the soul The Brain as the Seat of the Mind • In 1664, Thomas Willie wrote the book, “The Anatomy of the Brain”, where he concluded that o The brain was responsible for mental functioning o Different functions were located in different regions in the brain o Disorders of the brain were disorders of chemistry Signals Travelling in Neurons • In the 1800s, there were two opposing ideas about the nervous system 1) Reticular theory: held that the nervous system consisted of a large network of fused nerve cells 2) Neuron theory: stated that the nervous system consisted of distinct elements or cells • Camillo Gogli (1873) discovered staining: a chemical technique that caused nerve cells to become coloured so they stood out from surrounding tissues o This made it possible to see the structure of the entire neuron o This lead to the acceptance of neuron theory • Late 1800s, researchers had shown that a wave of electricity is transmitted in groups of neurons o Johannes Mueller in 1842 proposed the doctrine of specific nerve energies, which stated that our perceptions depend on ‘nerve energies’reaching the brain and that the specific quality we experience depends on which nerves are stimulated Recording from Neurons • In the 1920s, Edgar Adrian was able to record electrical signals from single sensory neurons o This provided important information about what is happening in the nervous system o This ushered in the modern era of brain research Basic Structure of the Brain • Much of the research on the connection between the brain and perception has focused on the activity in the cerebral cortex, which is responsible for perception, language, memory and thinking • Modular organization: specific functions are served by specific areas of the cortex o An example of this is how the senses are organized into primary receiving areas, the first areas in the cerebral cortex to receive the signals initiated by each sense’s receptors, which include:  Occipital lobe: area for vision  Temporal lobe: area for hearing  Parietal lobe: area for skin senses, such as touch, temperature and pain  Frontal lobe: receives signals from all the senses and plays an important role in perception that involve the coordination of information received through 2 or more senses Neurons: Cells that Create and Transmit Electrical Signals Structure of Neurons • Structure of neurons o Cell body: contains mechanisms to keep the cell alive o Dendrites: branch out from the cell body to receive electrical signals from other neurons o Axon/nerve fiber: is filled with fluid that conducts electrical signals • Receptors: a type of neuron, which is specialized to respond to environmental stimuli, examples of receptors that are specialized for responding to: o Light (vision) o Pressure changes in the air (hearing) o Pressure on the skin (touch) o Chemicals in the air (smell) o Chemicals in liquid form (taste) Recording Electrical Signals in Neurons • Nerve: consists of the axons/nerve fibers of many neurons • Micro-electrodes: small shafts of glass/metal with very fine tips, that are used to record signals from single neurons • The key principle for understanding electrical signals in neurons is that we are always measuring the difference in charge between 2 electrodes o Recording electrode is the electrode located where the electrical signals will occur o Reference electrode is located some distance away so it is not affected by the electrical signals • Resting potential: when the nerve is at rest, the difference in potential of –70 millivolts; this value stays the same as long as there are no signals in the neuron • When the neuron’s receptor is stimulated o The signal is transmitted down the axon o As the signal passes the recording electrode, the charge inside the axon rises to +40 millivolts o As the signal continues past the electrode, the charge inside the fiber reverses course and start becoming negative again, until it returns to the resting level o Action potential: is the signal which lasts about 1 millisecond Action Potentials Chemical Basis • The electrical signal in neurons are created by and conducted through liquid • Neurons are surrounded by a solution rich in ions, molecules that carry an electrical charge o Ions are created when molecules gain or lose electrons, as happens with compounds are dissolved in water • The solution ____ the axon of a neuron is rich in: o Outside: positively charged sodium (Na ) ions + o Inside: positively charged potassium (K ) ions • Action potential charges are caused by the flow of sodium and potassium ions across the cell membrane o The changes in sodium and potassium flow that create the action potential are caused by changes in the fiber’s permeability to sodium and potassium • Permeability: is a property of the cell membrane that refers to the ease with which a molecule can pass through the membrane o Selective permeability: occurs when a membrane is highly permeable to one specific type of molecule, but not others • Chemical steps of the action potential: 1) Resting potential at –70 mV 2) The receptor is stimulated and causes an action potential to increase to +40 mV  This triggers a process that causes the membrane to become selectively permeable to sodium  Sodium flows into the axon  This causes a rapid increase in positive charge inside of the neuron 3) The action potential reaches +40 mV, followed by a rapid return to the baseline of –70 mV  The membrane suddenly becomes selectively permeable to potassium  Potassium flows out of the axon  This causes a rapid increase in negative charge inside of the neuron 4) Once the action potential has passed, the neuron returns to its resting potential of –70 mV Basic Properties • Basic properties of action potentials include: o Propagated response o It remains the same size o Refractory period o Spontaneous activity • An important property of the action potential is that it is a propagated response o Propagated response: once the response is triggered, it travels all the way down the axon without decreasing in size o This enables neurons to transmit signals over long distances • Another property is that the action potential remains the same size no matter how intense the stimulus is o Changing the stimulus intensity does not affect the size of the action potential but does affect the rate of firing • There is an upper limit to the number of nerve impulses that can occur per second, which occurs because of the: o Refractory period: the interval between the time one nerve impulse occurs and the next one can be generated in the axon o The refractory period for most neurons is about 1 ms, the upper limit of a neurons’ firing rate is about 500-800 impulses per second • The action potentials that occurs in the absence of stimuli from the environment is called spontaneous activity o This establishes a baseline level of firing for the neuron o The presence of stimulation usually causes an increase in activity above this spontaneous level • What do these properties of the action potential mean in terms of their function for perceiving? o The action potential’s function is to communicate information and provide information about the intensity of a stimulus o This information must be transmitted to other neurons and eventually to the brain or other organs that can react to the information Events at the Synapse • Synapse: is the small space between the neurons o Its discovery raised the question of how the electrical signals generated by one neuron are transmitted across the space separating the neurons • Action potentials trigger the release of chemicals called neurotransmitters, that are stored in structures called the synaptic vesicles in the sending neuron o The neurotransmitter molecules flow into the synapse to small areas on the receiving neuron o This area is called the receptor sites that are sensitive to specific neurotransmitters  The receptor sites exist in a variety of shapes that match the shapes of particular neurotransmitter molecules o When a neurotransmitters makes contact with a receptor site matching its shape, it activates the receptor site and triggers a voltage in the receiving neuron  The direction of this voltage change depends on the type of neurotransmitters released and the nature of the cell body of the receiving neuron • There are 2 types of neurotransmitters: o Excitatory transmitters: cause the inside of the neuron to become more positive, a process called depolarization  This response is much smaller than an action potential  To generate an action potential, enough excitatory transmitters must be released to increase depolarization  Excitatory response: depolarization can trigger an action potential o Inhibitory transmitters: cause the inside of the neuron to become more negative, a process called hyperpolarization  Hyperpolarization is considered an inhibitory response because it can prevent the neuron from reaching the level of depolarization needed to generate action potentials • In summary: o The release of excitatory transmitters  Increase the chances that a neuron will generate action potentials  Is associated with high rates of nerve firing o The release of inhibitory transmitters  Decreases the chances that a neuron will generate action potentials  Is associated with lower rates of nerve firing • The function of neurons is to process information from both excitatory and inhibitory sources Neural Processing: Excitation, Inhibition, and Interactions between Neurons • Neural circuits: are groups of interconnected neurons that consist of just a few neurons or many thousands of them Excitation, Inhibition and Neural Responding • Convergence: the synapsing of more than one neuron into a single neuron • The diagram is of a complex neural circuit, which includes: o 2 inhibitory synapses—Aand C inhibit B • What happens when we increase the number of receptors stimulated? o Stimulating receptor 4  Through its excitatory connection, it increases the firing rate of neuron B o Stimulating receptor 3, 4, 5  This adds the output of two more excitatory synapses to B  It increases the firing rate o Stimulating receptors 2, 3, 4, 5, 6  Receptors 2 and 6 stimulate neuronsA and C, which releases inhibitory transmitters  This decreases firing rate • In this circuit, neuron B fires weakly to small stimuli or longer stimuli, but fires best to a stimulus of medium length o The combination of convergence and inhibition caused neuron B to respond best to a light stimulus of a specific size o The neuron that synapse with neuron B are therefore doing much more than simply transmitting electrical signals  They are acting as part of the neural circuit that enables the firing of neuron B to provide information about the stimulus falling on the receptors o The firing of a neuron like B, for example, might help signal the presence of a small spot of light, or detail of a larger pattern Introduction to Receptive Fields • Receptive field: is the area on the receptors that influence the firing rate of the neuron o Excitatory area: stimulating this area causes an increase in the neuron’s firing rate o Inhibitory area: stimulating this area causes a decrease in firing rate • Center-surround receptive field: the areas of the receptive field are arranged in a center region that responds one way and a surround that responds in the opposite way o Excitatory-center-inhibitory-surround receptive field: the center is excitatory, while the surround is inhibitory o Inhibitory-center-excitatory-surround receptive field: the center is inhibitory, while the surround is excitatory o Center-surround antagonism: the fact that the center and the surround of the receptor field respond in opposite ways • Center surround antagonism comes into play when the spot of light becomes large enough so that it begins to cover the inhibitory area o Stimulation of the inhibitory surround counteracts the center’s excitatory response, causing a decrease in the neuron’s firing rate o Thus, this neuron responds best to a spot of light that is the size of the excitatory center of the receptive field o This is similar to what happens when we increase the number of receptors stimulated in the circuit The Sensory Code: How the Environment is represented by the Firing Neurons • We have seen that respective fields enable us to specify a neuron’s response o It indicates the location on the receptor surface that causes a neuron to respond and the size/shape of the stimulus that causes the best response Specificity Coding • Specificity coding: is the representation of particular objects in the environment by the firing of neurons that are tuned to respond specifically to that object • The idea that there are single neurons that each respond only to a specific stimulus was proposed in the 1960s by Jerzy Konorski and Jerry Lettvin o Lettvin coined the term “grandmother cell” to describe this highly specific type of cell o Grandmother cell: is a neuron that responds only to a specific stimulus  This stimulus could be a specific image, such as a picture of your grandmother, or a concept, such as the idea of grandmothers in general • Is there evidence for grandmother cells? o Quiroga and coworkers (2005) recorded from 8 patients with epilepsy who had electrodes implanted in their hippocampus to help localize precisely where their seizures originated, in preparation for surgery o The function of the hippocampus, plus the fact that the people were tested had a history of past experiences with these stimuli, may mean that familiar, well- remembered objects may be represented by the firing of just a few very specialized neurons o But is this grandmother cells?  No, because it is unlikely that neurons respond to a single object/concept  If there were only one neuron that responded to a particular person/concept, it would be extremely difficult to find it Distributed Coding • Distributed Coding: is the representation of a particular object by the pattern of firing of groups of neurons • Advantages of this: o It doesn’t require a specialized neuron for every object in the environment o Instead, it allows for the representation of a large number of stimuli by the firing of just a few neurons Sparse Coding • Sparse coding: the idea that a particular object is represented by the firing of a relatively small number of neurons • This type of coding is considered in between distributed and specificity coding Something to Consider: the Body­Mind Problem • Mind-body problem: how do physical processes such as nerve impulses or sodium and potassium molecules flowing across membranes (body part) become transformed into the richness of perceptual experience (mind part) • Neural correlate of consciousness (NCC): Research focuses on determining the connections between stimuli in the environment and the firing of neurons o This is where consciousness can be roughly defined as our experiences • Does determining the NCC qualify as a solution to the mind-body problem? o Researchers call finding the NCC the easy problem of consciousness because it has been possible to discover many connections between neural firing and experience o Hard problem of consciousness  How do physiological responses become transformed into experience?  How do sodium and potassium ions flowing across a membrane or the nerve impulses that result from this flow become the perception of a person’s face or the experiences of the colour red  This version of the problem is still unsolved  The first difficulty lies in figuring out how to go about studying the problem Chapter 3: Introduction to Vision - vision begins when visible light is reflected from objects into the eye Light: the stimulus for vision - vision is based on visible light, which is a band of energy within the electromagnetic spectrum * the electromagnetic spectrum is a continuum of electromagnetic energy that is produced by electric charges and is radiated as waves. * the energy in this spectrum can be described by its wavelength - visible light is the energy within the electromagnetic spectrum that humans can perceive. It has wavelengths ranging from about 400 to 700 nanometers (nm). - for humans the wavelength of visible light is associated with the different colors of the spectrum. - light can also be described as consisting of small packets of energy called photons, with one photon being the smallest possible packet of light energy The Eye - the eye is where vision begins - light reflected from objects in the environment enters the eye through the pupil and is focused by the cornea and lens to form sharp images of the objects on the retina, which contains the receptors for vision - there are two kinds of visual receptors, rods, and cones, which contain light-sensitive chemicals called visual pigments that react to light and trigger electrical signals - these signals flow through the network of neurons that make up the retina - the signals then emerge from the back of the eye in the optic nerve, which conducts signals toward the brain. - the cornea and lens at the front of the eye and the receptors and neurons in the retina lining the back of the eye shape what we see by creating the transformations that occur at the beginning of the perceptual process Light is Focused by the Eye - once light is reflected from an object into the eye, it needs to be focuses onto the retina - the cornea (transparent covering of the front of the eye), accounts for about 80% of the eye’s focusing power. Like the lenses in eyeglasses, it is fixed in place, so we cant adjust its focus - the lens, which supplies the remaining 20 percent of the eye’s focusing power, can change its shape to adjust the eye’s focus for stimuli located at different distances - accommodation makes sure that images on the retina don't become out of focus. * the ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker. * this increased curvature bends the light rays passing through the lens to pull the focus point back to create a sharp image on the retina - near point: the distance at which your lens can no longer adjust to bring close objects into focus - the distance of the near point increases as a person gets older, a condition called presbyopia (“old eye”) - the near point for most 20 year olds is at about 10 cm, but increases to 14cm by 30, 22cm at 40, and 100 cm at 60 years. - this loss of ability to accommodate occurs because the lens hardens with age, and the ciliary muscles become weaker. These changes make it more difficult for the lens to change its shape for vision at close range - at around the age of 45 your ability to accommodate begins to decrease rapidly, and the near point moves beyond a comfortable reading distance, therefore some people have to hold things farther away to read them. - myopia (near sightedness): an inability to see distant objects clearly. The reason for this difficulty is that the myopic eye brings parallel rays of light into focus at point in front of the retina so that the image reaching the retina is blurred. - this problem can be caused by either of two factors 1) refractive myopia, in which the cornea, and/or the lens bends the light too much, or 2) axial myopia, in which the eyeball is too long. Either way, images of faraway objects are not focused sharply, do objects look blurred - far point: the distance at which the spot of light becomes focused on the retina. When the spot of light is at the far point, a myope can see it clearly - Laser assisted in situ keratomileusis (LASIK) surgery: LASIK involves sculpting the cornea with a type of laser called an excimer laser, which does not heat tissue. - the cornea is sculpted by the laser so that it focuses light onto the retina. - hyperopia (farsightedness): can see distant objects clearly but has trouble seeing nearby objects. In the hyperopic eye, the focus point for parallel rays of light is located behind the retina, usually because the eyeball is too short. - by accommodating to bring the focus point back to the retina, people with hyperopia are able to see distant objects clearly - the constant need to accommodate when looking at nearby objects results in eyestrain and in older people, headaches. - focusing the image clearly onto the retina is the initial step in the process of vision. But it is important to realize that although a sharp image on the retina is essential for clear vision, we do not see the image on the retina. - vision occurs not in the retina, but in the brain, and before the brain can create vision, the light on the retina must be transformed into electricity transforming light into electricity the visual receptors and transduction - transduction is carried out by receptors, neurons specialized for receiving environmental energy and transforming this energy into electricity. - the receptors for vision are the rods and cones.As we will see shortly the rods and cones have different properties that affect our perception. - the key part of the rod for transduction is the outer segment, because it is here that the light acts to create electricity - rod outer segments contain stacks of discs, and each disc contains thousands of visual pigment molecules - the molecules are long strands of protein called opsin , which loops back and forth across the disc membrane seven times. - retinal is one membrane inside the visual pigment molecules. Each molecule has one of these tiny retinal molecules and it is crucial for transduction, because it is part of the visual pigment that is sensitive to light - transduction is triggered when the light-sensitive retinal absorbs one photo of light - before light is absorbed, the retinal is next to the opsin. When a photon of light hits the retinal, it changes shape, so it is sticking out from the opsin. - this change in shape is called isomerization and it is this step that triggers the transformation of the light entering the eye into electricity in the receptors how does transduction occur? - saying that isomerization of the visual pigment results in transduction is just the first step in explaining how light is transformed into electricity - Hecht’s Psychophysical Experiment: the starting point for Hecht’s experiment was his knowledge that transduction is triggered by the isomerization of visual pigment molecules and that it takes just one photon of light to isomerizes a visual pigment molecule * he did a psychophysical experiment that enabled him to determine how many visual pigment molecules need to be isomerized for a person to see * he accomplished this by using a method of constant stimuli to determine a person’s absolute threshold for seeing a brief flash of light. * he found: a person could detect a flash of light that contained 100 photons. To determine how many visual pigment molecules were isomerized by this flash, he considered what happened to those 100 photons before they reached the visual pigment - his conclusions can be summarized as follows: 1) a person can see a light if 7 rod receptors are activated simultaneously 2) a rod receptor can be activated by the isomerization of just 1 visual pigment molecule pigments and perception - vision can occur only if the rod and cone visual pigments transform the light entering the eye into electricity distribution of the rods and cones 1) there is one small area, the fovea, that contains only cones. When we look directly at an object its image falls on the fovea 2) the peripheral retina, which includes all of the retina outside of the fovea, contains both rods and cones. The fovea only contains about 1 percent of the 6 million cones in the retina 3) there are many more rods than cones in the peripheral retina because most of the retina’s receptors are located there and because there are about 120 million rods and 6 million cones - macular degeneration is a condition which is most common in older people. It destroys the cone-rich fovea and a small area that surrounds it. This creates a “blind spot” in central vision, so when a person looks at something he or she loses sight of it - retinis pigmentosa is a condition which is the degeneration of the retina that is passed from one generation to the next. It first attacks the peripheral rod receptors and results in poor vision in the peripheral visual field. In severe cases, the foveal cone receptors are also attacked, resulting in complete blindness - we don’t see blind spots because some mechanism in the brain “fills in” the place where the image disappears dark adaptation of the rods and cones - a process called dark adaptation causes the eye to increase its sensitivity in the dark - dark adaptation curve is a plot of how visual sensitivity changes in the dark, beginning with when the lights are extinguished - dark adapted sensitivity occurs at the end of dark adaptation, and is about 100,000 times greater than the light adapted sensitivity that occurred before dark adaption began. Measuring ConeAdaptation - to measure dark adaptation of the cones alone, we have to ensure that the image of the test light stimulates only cones - this is achieved by having the observer look directly at the test light so its image will fall on the all-cone fovea, and by making the test light small enough so that its entire image falls within the fovea Measuring Rod Adaptation - need to measure dark adaptation in a person that has no cones. These people have a rare defect and they are called rod monochromats visual pigment regeneration - when light hits the light sensitive retinal part of the visual pigment molecule, it is isomerized and triggers the transduction process - it then separates from the opsin part of the molecule. This separation causes the retina to become lighter in colour, a process called visual pigment bleaching - William rushton (1961) devised a procedure to measure the regeneration of visual pigment that occurs during dark adaptation. - his measurements showed that cone pigment takes 6 minutes to regenerate completely, whereas a rod pigment takes more than minutes - his result demonstrated two important connections between perception and physiology 1) our sensitivity to light depends on the concentration of a chemical- the visual pigment 2) the speed at which our sensitivity is adjusted in the dark depends on a chemical reaction- the regeneration of the visual pigment rod and cone absorption spectra - the difference between the rod and cone spectral sensitivity curves is caused by differences in the absorption spectra of the rod and cone visual pigments - an absorption spectrum is a plot of the amount of light absorbed by a substance versus the wavelength of the light - rod pigment absorbs best at 500 nm, the blue-green area of the spectrum - there are three absorption spectra for the cones because there are three different cone pigments, each contained in its own receptor * the short wavelength pigment (s) absorbs best at about 419 nm; the medium- wavelength pigment (M) absorbs light best at about 531 nm; and the long-wavelength pigment (L) absorbs light best at about 558 nm why we use our cones to see details - rod vision is more sensitive than cone vision because it has more convergence, however the cones have better visual acuity-detail vision- because they have less convergence Lateral Inhibition and Perception - lateral inhibition is inhibition that is transmitted across the retina Chapter 4: Visual Cortex and Beyond Lateral Geniculate Nucleus Most of the signals from the optic nerve travel to the LGN, in the thalamus · for every 10 signals the LGN gets from the retina it only sends 4 · is a regulator to the rest of the brain · Center-surround receptive fields, same as a retinal ganglion · Regulates visual information as it flows from the retina to the V1 · The LGN also receives signals from the cortex and the thalamus · Signals are organized based on the eye they came from, the receptors that generated them, and the type of environmental information that is represented in them Retinotopic Organization The LGN is organized in the same way as the retina and V1 is also organized in the same way as the retina Magnocellular cells Larger cells in the LGN, input from rods Parvocellular cells Smaller cells in the LGM input from red and green cones Superior Colliculus Area involved in controlling eye movement, receives about 10% of the fibers form the optic nerve Cortical Magnification Factor The areas the cortex that correspond to the fovea has proportionally larger area dedicated to processing Simple Cortical Cells Cells in the V1 (striate cortex) with side by side receptive fields, respond somewhat to spots of light but better to bars of light, respond best to bars of light in a particular orientation · There are simples cells that respond to bars of light in every possible orientation Orientation Tuning Curves Is measurement of a simple cortical cells response to bars of light with different orientation, is the relationship between light bar orientation and the firing rate Complex Cells Respond to moving bars of light in a particular orientation End-Stop Cells Respond to moving lines of a particular length, moving corners or edges, do not responds to stim that are too long Contrast Threshold Minimum contrast at which a subject can still perceive an orientation Selective adaptation The idea is that if neurons fire for long enough they become fatigued or adapt. This causes the neurons firing rate to decrease and the neuron to fire less when the stimulus is immediately present · Is selective because only the neurons that respond to that type of stim adapt Grating Stimuli Alternating bars of black and white Selective Rearing Is an animal is raised in an environment that contains only one type of stim, then the neurons that respond to that type of stimuli will become more prevalent Neural Plasticity The idea that the neuronal response can be shaped by the perceptual experience · Example with the cats being raised in a completely vertical lined tubes PET Positron Emission Tomography, uses low doses of a radioactive tracer which allows for the indication of blood flow to an area Subtraction Technique A way of examining the brain in which there is a test condition when the stimulus of interest is not present and another when the stimulus of interest is i.e., an interest in the brain regions active when a person if manipulating an object, the first part would be the brain activity while the person is holding the object and the second part is when the person is manipulating the object, the first is then substituted form the second and this highlights the part of the brain that is used for the activity fMRI Functional magnetic resonance energy, uses the magnetic hemoglobin to ascertain blood flow Columns in V1 Each column is specific to one type of stimulus, orientation, spatial location, etc., Location Columns Receptive fields at the same location on the retina as in the column, one might respond to vertical and one might respond to slightly off form vertical Orientation Columns In the cortex, columns that respond best to lines in a particular orientation, adjacent columns have, cells with a slightly different preference to line orientation, slightly diagonal, 1 mm across the cortex represents an entire range of orientation · A column of cells that responds to 90 degrees is right next to the column that responds to 85 degrees Ocular Dominance Columns Most neurons respond better to one eye then to the other, 80% of the columns respond to both eyes, but they have a preference · There is an alternating pattern of columns that respond to each eye as you move along the cortex Hyper-columns Is a large grouped column that contains a location column, a ocular dominance column, and a complete orientation column (these 2are not real but are a useful concept) Ventral Pathway Is the what pathway, Dorsal Pathway Is the where pathway, · These pathways are not completely separate, there is some mixture · Is thought that the dorsal stream may also provide information about how to do things as well, such as picking things up Ablation Refers to destruction or removal of part of the NS, goal is to determine the function of an area · Monkeys who had part of their parietal lobe removed, could not perform the object discrimination task object discrimination · Monkeys that had part of their temporal lobe removed could not perform the object spatial task landmark discrimination Module A brain structure specializes that processes particular types of stimuli Fusiform Face area Located on the fusiform gyrus on the underside of the IT cortex, is specialized to respond to faces Prosopagnosia Difficulty recognizing faces of familiar people, the might not even be able to recognize themselves. Although they can recognize the person as soon as they hear them speak, caused by damage to the temporal lobe, Parahippocampal Place area Is activated by pictures depicting outdoor or indoor scenes, important for spatial processing Extra Striate body area Activated by images of bodies or parts of bodies Chapter 5: Perceiving Objects and Scenes Why is it so Difficult to Design a Perceiving Machine? The Stimulus on the Receptors is Ambiguous • Inverse projection problem: a particular image on the retina can be created by many different objects o If we know an object’s shape, distance, and orientation we can determine the shape of the object’s image on the retina o However, a particular image on the retina can be created by an infinite number of objects • For instance a rectangular image on the retina can be create by a trapezoid and other non- rectangular objects • The information from a single view on an object can be ambiguous and humans solve this problem by o Moving to different viewpoints o Making use of knowledge they have gained from past experiences in perceiving objects Objects can be Hidden or Blurred • Can you find the pencil or glasses in this image? • This problem of hidden objects occurs any time on object obscures part of another object o This occurs frequently o People easily understand that the part of the object that is covered continues to exist though o They use their knowledge of the environment to determine what is likely to be present • People are also able to recognize objects that are not in sharp focus o Can you recognize the individuals in this image? Objects Look Different from Different Viewpoints • The images of objects are continually changing, depending on the angle from which they are viewed • Viewpoint invariance: the ability to recognize an object seen from different viewpoints The Gestalt Approach to Object Perception • Wilhelm Wundt established the first laboratory of scientific psychology in 1879 and a new approach to psychology called structuralism o One of the basic ideas behind structuralism was that perceptions are created by combining elements called sensations o The image on the right, are dots are sensations that added together create our perception of the face o Problems that could not be explained by structuralism includes:  Apparent movement  Illusionary contours  Bi-stable figures • Apparent movement o When 2 stimuli that are in slightly different positions are flashed one after another with the correct timing, movement is perceived between the 2 stimuli o There is no actual movement in the display, just no stationary stimuli o Amodern example are electronic signs which display moving advertisements • Illusionary contours o Contour that is perceived even though it is not present in the physical stimulus o The above image • Bi-stable figure shown above • In the early 1900s, a group of psychologists called themselves Gestalt psychologists o Gestalt: a whole configuration that cannot be described merely as the sum of its parts o Having rejected the idea that perception is built up of sensations, the Gestalt psychologists proposed a number of principles, which they called laws of perceptual organization The Gestalt Laws of Perceptual Organization • Perceptual organization: involves the grouping of elements in an image to create larger objects • There are 9 laws of perceptual organization: the law of 1) Pragnanz/simplicity 2) Similarity 3) Good continuation 4) Proximity/Nearness 5) Common Region 6) Uniform Connectedness 7) Synchrony 8) Common Fate 9) Meaningfulness/Familiarity 1. Pragnanz/Simplicity • Law of pragnanz/good figure/simplicity: every stimulus pattern is seen in such a way the resulting structure is as simple as possible • For instance in the above image, we see imageAas 5 circles, instead of as image B, which is interpreted as 9 objects 2. Similarity • Law of similarity: similar things appear to be grouped together • Grouping can also occur because of similarity of: o Shape o Size o Orientation o Colour • The image on the right is perceived as horizontal rows or vertical columns that can be perceived as one object 3. Good Continuation • Law of good continuation: points that, when connected, results in straight/smoothly curving lines are seen as belong together and the lines tend to be seen in such as a way as to follow the smoothest path • The above image can be perceived as a pattern of continuous interwoven strands 4. Proximity (Nearness) • Law of proximity/nearness: things that are near each other appear to be grouped together • The image on the right can be perceived as 2 pairs of circles 5. Common Region • Principle of common region: elements that are within the same region of space appear to be grouped together o Common region can overpower proximity o Since objects can be in different regions now and not considered close together anymore • The image on the right can be perceived as 3 pairs of circles, since they are confined by the outlines 6. Uniform Connectedness • Principle of uniform connectedness: a connected region of visual properties that are perceived as a single unit, such as: o Lightness o Colour o Texture o Motion • The image on the right can be perceived as 3 pairs of circles, since they are connected by the lines 7. Synchrony • Principle of synchrony: visual events that occur at the same time are perceived as belonging together • Synchrony occurs when the yellow lights blink on and off together 8. Common Fate • Law of common fate: things that are moving in the same direction appear to be grouped together o Common fate is similar to synchrony, because they are both dynamic, except synchrony can occur without movement • The above image shows a flock of birds that are moving in the same direction, and are thus seen as grouped together o When a portion of the flock changes direction, their movement creates a new group 9. Meaningfulness/Familiarity • Law of meaningfulness/familiarity: things that form patterns that are familiar/meaningful are likely to become grouped together • The above image on the left is first perceived as a forest with a rider and his horse, however once you begin to perceive the images of faces in the image, you give meaning to these perceptions o The image on the right shows where these faces are with red circles Perceptual Segregation: How Objects are separated from the Background • Perceptual segregation: perceptual organization in which one object is seen as separate from other objects, which is caused by o Figure-ground segregation: the perceptual separation of an object from its background  Figure: when an object is seen as separate from the background  Ground: the background 108 What are the Properties of Figure and Ground? • Reversible figure-ground: a figure-ground pattern that perceptually reverses as it is viewed, so that the figure becomes the ground and the ground becomes the figure o Rubin created this pattern, which can be seen as either 2 blue faces looking at each other in front of a white background, or as a white vase on a blue background • Border ownership: when two areas share a border, as occurs in figure-ground displays, the border is usually perceived as belonging to the figure What Factors determine which Area is the Figure? • Vecera and coworkers used a method to show that regions in the lower part of a display are more likely to be perceived as figure than regions in the upper part • Other factors that help determine which area will be seen as the figure include: o Symmetry o Smaller areas o Vertical/horizontal areas o Meaningfulness The Gestalt “Laws” as Heuristics • The reason for rejecting the term ‘laws’is that the rules of perceptual organization and segregation don’t make strong enough predictions to qualify as ‘laws’ • Instead, the Gestalt principles are more accurately described as heuristics o Rules of thumb that provide a best-guess solution to the problem o They may not result in a correct solution every time, but they will most of the time o They are also faster • Gestalt heuristics reflect properties of the environment Recognition­by­Components Theory • Recognition-by-components (RBC) theory: a theory proposed by Biederman, which proposes that we recognize objects by decomposing them into volumetric features called geons • The theory states that our recognition of objects is based on features called geons o Geons: a term that stands for ‘geometric ions,’and are the basic units of objects o Biederman proposed 36 different geons and suggested that this is enough to enable us to mentally represent a large proportion of the objects that we can easily recognize • Non-accidental properties (NAP): properties of edges in the retinal image (2D) that correspond to the properties of edges in the 3-dimensional environment o The above image are alternating perspectives of a coin seen through the retina o In this case, the property of curvature is called a non-accidental property because the it’s the view that is seen often o Accidental viewpoint: a viewpoint that occurs rarely and where the NAP is not visible  In this case, the accidental viewpoint on the coin is the flat straight edge • RBC proposes that a key property of geons is that each type of geons has a unique set of NAP o For example, consider a rectangular-solid geon, in which the NAP are the three parallel straight edges (seen in figureAand B)  In this case, the accidental viewpoint would be the 2 parallel lines you see when looking at the rectangular on a flat surface (seen in figure C) o For example, consider a cylinder geon, in which the NAP are the two parallel straight edges  In this case, the accidental viewpoint would be the circle you see looking at the object straight on • The fact that geon has a unique set of NAP results in a property of geons called discriminability o Discriminability: each geon can be discriminated from other geons o The fact that NAP are visible from most viewpoints results in another property of geons known as viewpoint invariance  Viewpoint invariance: the ability to recognize an object seen from different viewpoints • The main principle of RBC theory is the principle of componential recovery o Principle of componential recovery: the ability to identify an object if we can identify its geons o This principle is what is behind our ability to identify objects in the natural environment even when parts of the objects are hidden by other objects • RBC theory also states that we can recognize objects based on a relatively small number of geons Perceiving Scenes and Objects in Scenes • We usually see objects within a scene • Scene: is a view of a real-world environment that contains o Background elements o Multiple objects that are organized in a meaningful way relative to each other and the background • One way of distinguishing objects and scenes is that: o Objects are compact and are acted upon o Scenes are extended in space and are acted within Perceiving the Gist of a Scene • Gist of a scene: general description of a scene 1) People can identify most scenes after viewing them for only a fraction of a second, as when they flip rapidly from one TV channel to another • Research has shown that it is possible to perceive the gist of a scene within a fraction of a second • Mary Potter’s experiment 1) She showed observers a target picture and then asked them to indicate whether they saw that picture as they viewed a sequence of 16 rapidly presented pictures 2) Her observers could do this with almost 100% accuracy even when the pictures were flashed for only 250 ms • Li Fei-Fei’s experiment 1) He presented pictures of scenes for times ranging from 27 ms to 500 ms and asked observers to write a description of what they saw 2) They needed to take persistence of vision to consideration  Persistence of vision: a phenomenon in which perception of any stimulus persists for about 250 ms after the stimulus is physically terminated 3) To eliminate this it is necessary to flash a masking stimuli to make sure the observer saw the pictures for exactly the desired duration  Masking stimuli: a visual pattern that, when presented immediately after a visual stimulus, decreases a person’s ability to perceive the stimulus 4) The results from experiment showed that:  With brief presentations, observers only saw the light and dark areas of the pictures • This is known as the overall gist of the scene, which seems to be perceived first  By 67 ms, observers could identify some large objects in the picture  By 500 ms, observers were able to identify smaller objects and details Global Image Features • Olivia and Torralba propose that observers use information called global images features, which can be perceived rapidly and are associated with specific types of scenes • Five main global image features were proposed are: 1) Degree of naturalness  High: include scenes from nature with horizontal and vertical lines • Such as the beach and the forest  Low: include man-made structures that are dominated by straight lines and angles • Such as the street 2) Degree of openness  High: includes wide spaces with a visual horizon and few objects • Such as the beach and the street  Low: includes tight spaces with lots of objects • Such as the forest 3) Degree of roughness  High: scenes that contain many elements and are more complex • Such as the forest  Low: smooth scenes that have fewer elements • Such as the beach 4) Degree of expansion  High: scenes that appear to keep going • Such as the street  Low: scenes that appear to be small 5) Colour  Scenes with characteristic colours, like the blue beach scene and the green and brown forest scene • Global images properties o Holistic: the scene is seen as a whole and does not depend on small details, recognizing individual objects, or separating one object from another o Rapidly perceived o Contains information that results in perception of a scene’s structure and spatial layout • The general property of perception o Our past experiences in perceiving properties of the environment plays a role in determining our perceptions Regularities in the Environment: Information for Perceiving • Regularities in the environment: characteristics of the environment that occur regularly and in many different situations • We easily use our knowledge of regularities in the environment to help us perceive, even though we may not be able to identify the specific information we are using • There are 2 types of regularities: o Physical regularities o Semantic regularities Physical Regularities • Physical regularities: regularly occurring physical properties of the environment o For example, there are vertical and horizontal orientations in the environment than oblique (angled) orientations o Oblique effect: enhanced sensitivity to vertically and horizontally oriented visual stimuli • Why should being exposed to more verticals and horizontals make it easier to see them? o One answer to this question is that experience-dependent plasticity, causes the visual system to have more neurons that respond best to these orientations • Another physical characteristic of the environment o Is that when object partially covers another, the contour of the partially covered object “comes out the other side”  This is an example of the Gestalt law of good continuation o Is that uniformly connected regions are regularities in the environment o Light-from-above heuristic: the assumption that most light in our environment comes from above • The Gestalt heuristics are therefore based on the kinds of things that occur very often Semantic Regularities • Semantic regularities: are the characteristics associated with the functions carried out in different types of scenes o Semantics refers to the meaning and function of the scene o We are focusing on what happens within the scene • During visualization tasks, many people report seeing not just a single object, but an object within a setting • Hollingworth’s experiment o He had observers see scenes like the one above (without the circles) o In this scene, the target object is the barbell, although the observers did not know this o There were 2 conditions:  Present: observers were shown the picture where the target object (barbell) was present  Absent: observers were shown the picture where the target object (barbell) was absent o Observers were asked to indicate where the target object was (present condition), or where the target object would have been (absent condition) o The circles indicate the average error of observer’s judgments of the position of the target object in which condition  The smaller circle shows that observers who saw the target object accurately located their positions  The larger circle shows that observers who had not seen the target object were not quite as accurate but were still able to predict where the target objects would be • Palmer’s experiment o He first presented a context scene, such as the one on the left o He then briefly flashed one of the target pictures on the right o He asked observers to identify the object in the target picture o Results:  Observers were able to correctly identify the loaf of bread 80% of the time  But, they correctly identified the mailbox or the drum only 40% of the time  In summary, observers were able to quickly and accurately identify target objects that were relevant to the context scene, more than irrelevant objects The Role of Inference in Perception • Helmholtz proposed a principle of perception called the theory of unconscious inference o Theory of unconscious inference: states that some of our perceptions are the result of unconscious assumptions we make about the environment • The theory accounts for our ability to create perceptions from stimulus information that can be seen in more than one way o For example, in the above image (figureA), people can perceive it as a blue rectangle in front of a red rectangle (figure B) o However, this same image can be formed by a blue rectangle in front or behind a red 6-sided figure (figure C) o We are more likely to infer the first figure more often, because of experiences we have had with similar situations in the past • Likelihood principle: states that we perceive the object that is most likely to have caused the pattern of stimuli we have received o This principle was proposed to deal with the ambiguity of perception • Helmholtz viewed the process of perception as being similar to the process of solving a problem o For perception, the task is to determine which object caused a particular pattern of stimulation o This problem is solved by a process in which the observer brings their knowledge of the environment to bear in order to infer what the object might be o This process is unconscious • Helmholtz’s idea of perception as inference was quantified using a statistical technique called Bayesian inference, which takes probabilities into account The Physiology of Object and Scene Perception Neurons that Respond to Perceptual Grouping and Figure–Ground • The above image is how a neuron in the striate cortex (V1) responds to various stimuli o Figure A: an oriented bar inside the neuron’s receptive field (the small square)  The neuron fires to this stimulation o Figure B: an oriented bar inside the neuron’s receptive field surrounded by randomly oriented bars  The lines that fall outside of the neuron’s receptive field, act as inhibition stimuli and cause a decrease in neuron firing to the bar  Contextual modulation: the effect of stimuli that fall outside of the neuron’s receptive field o Figure C: an oriented bar inside the neuron’s receptive field surrounded by randomly oriented bars, where the ones near the receptive field line up with the line inside of it  The neuron actually increases its firing response, despite the fact that the area outside of the receptive field (inhibition area) is being stimulated  The neuron’s response increases when good continuation and similarity causes the receptive-field line to become perceptually grouped with the other outside lines • The above image is from another experiment, in which a neuron, in the visual cortex, responds well when leftward slanted lines are positioned over the neuron’s receptive field (the green bar) o Figure A: we perceive the leftward slanting bars as a square on a background of right-slanted lines  When the receptive field is within the figure of similar left-slanted lines, the neuron fires o Figure B: the right-slanted lines (background) seen in figureA, are replaced with left-slanted lines, which changes from being the figure to becoming the new background  When the bars of the receptive field became the background, the neuron firing decreases to nearly nothing How Does The Brain Respond To Objects? Review of Sensory Coding • Objects typically cause activity not only in a number of neurons within a module, but also in a number of different areas of the brain • Firing is, therefore, distributed in 2 ways: 1) Across groups of neurons within a specific area 2) Across different areas in the brain More Evidence for Distributed Activity across the Brain • Faces provide one of the best examples of distributed representation across the brain • Processing of faces occurs in the: o Occipital cortex:  Begins the initial processing of the face  Sends signals to the fusiform gyrus, where visual information concerned with identification is processed o Amygdala:  Emotional aspects of the face  Facial expression  Observer’s emotional reaction to the face o Superior temporal sulcus  Evaluation of where a person is looking or gaze direction  Perceiving movements of a person’s mouth as they speak o Frontal cortex  Evaluation of a face’s attractiveness • This multiple forms of processing the face led to the conclusion that there is a distributed system in the cortex for perceiving faces Connecting Neural Activity and Perception • Grill-Spector’s experiment o She studied the question of how activation of the brain is related to whether a person recognizes an object  She did this my measuring brain activation as the human observer identified pictures of a well-known face—Harrison Ford  They focused on the fusiform face area (FFA) o Trials consisted of the observer viewing a picture of  Harrison Ford  Apicture of another person’s face  Arandom texture o Each picture was presented briefly (50 ms) followed immediately by a random- pattern mask o The observer’s task was to indicate whether the picture was of  Harrison Ford  Another object  Nothing o The figure shows the course of brain activation for the trials in which Harrison Ford’s face was presented  The red curve (top) shows that activation was greatest when observers correctly identified the stimulus as Harrison Ford’s face  The orange curve (middle) shows that the activation was less when they responded to ‘other object’to Harrison Ford’s face • They correctly identified the image as a face, but did not recognize it  The blue curve (bottom) indicates that there was little activation when observers could not even tell that a face was presented • Tong’s experiment o Binocular rivalry: if one image is presented to the left eye and a different image is presented to the right eye, perception alternates back and forth between the 2 eyes  We are only able to perceive one thing then another, but not both together o Observers in this experiment viewed the overlapping red house and green face through red-green glasses  The house image was presented to the right eye  The face image was presented to the left eye o When the observers perceived the:  House, activity occurred in the para-hippocampal place area (PPA), in the left and right hemispheres (red dot)  Face, activity occurred in the fusiform face area (FFA), in the right hemispheres (green dot) o Even though the image on the retina remained the same throughout the experiment, activity in the brain changed, depending on what the person was experiencing Something to Consider: Models of Brain Activity that can Predict What a Person is  Looking at • Is it possible to tell what scene a person is looking at by monitoring their brain activity? Kamitani and Tong’s Orientation Decoder • Kamitani and Tong took a step toward being able to ‘decode’brain activity by measuring observers’fMRI response to grating stimuli—alternating black and white bars o They presented gratings with a number of different orientations • They determined the response to these gratings in a number of fMRI voxels o Avoxel is a small cube-shaped area of the brain o There is some variability in how different voxels respond • By using the information provided by the responses of many voxels, the researchers were able to create an ‘orientation decoder’ o This decoder was able to determine what orientation a person was looking at based on their brain activity • They created this decoder by: o Measuring the response of 400 voxels in the primary visual cortex (V1) and a neighbouring area called V2 to grating with 8 different orientations • By consulting the orientation decode, the researchers could find out which orientation the observer was focusing on o The orientation decoder essentially provides a window into a person’s mind Kay and coworkers’ Photograph Decoder • What about stimuli that are more complex than oriented gratings? o Key and coworkers have created a new decoder that can determine which photograph of a natural scene has been presented to an observer • They presented 1750 black and white photographs of a variety of natural scenes to an observer and measured the activity of in 500 voxels in the primary visual cortex • The goal of this part of the experiment was to determine how which voxel responses to specific features of the scene, such as: o The position of the image o The image’s orientation o The degree of the detail in the image • These experiments do show that it is possible to identify information in the activity of the primary visual cortex that can predict which image out of a group of images a person is looking at o However, we are still not able to create, from a person’s brain activity, a picture that corresponds to what the person is seeing Chapter 6: Visual Attention Attention and Perceiving the Environment • In everyday life, we often have to pay attention to a number of things at once, a situation called divided attention • Selective attention: focusing on specific objects and ignoring others o This is the focus of the chapter Why Is Selective Attention Necessary? • You selectively focus on certain things in your environment because you visual system has been constructed to operate that way • The problem the visual system faces is that there is no much information being sent from the retina towards the brain that if the visual system had to deal with all of it, if would rapidly become overloaded o To deal with this problem, the visual system is designed to select only a small part of this information to process and analyze • One of the mechanisms that help achieve this selection is the structure of the retina, which contains the all-cone fovea o This area supports detail vision, so we must aim the fovea directly at objects we want to see clearly How Is Selective Attention Achieved? • Mechanisms of selective attention include: o Overt attention  Eye movements scan a scene by aiming the fovea at places we want to process more deeply o Covert attention  Attention not based on eye movements What Determines How We Scan A Scene? • The first task in the study of eye movements is to devise a way to measure them o Modern researchers use camera-based eye trackers to determine the position of the eye by taking pictures of the eye and noting the position of a reference point, such as a reflection that moves as the eye moves • The above image shows eye movements that occurred when an observer viewed a picture of a fountain o Fixations: places where the eye pauses to take in information about specific parts of the scene  Shown as dots in the image o Saccades: small, rapid eye movements  Shown as lines connecting the dots in the image Stimulus Salience • Stimulus salience: characteristics of the environment that stand out because of physical properties such as: o Colour o Brightness o Contrast o Orientation • Capturing attention by stimulus salience is a bottom-up process o It depends solely on the pattern of stimulation falling on the receptors • Saliency map: a ‘map’of a visual display that takes into account characteristics of the display such as colour, contrast, and orientation that are associated with capturing attention o Created by Parkhurt and coworkers o This map predicts which areas a person is most likely to attend to o They found that the initial fixations were closely associated with the saliency map Knowledge about Scenes • The knowledge we have about the things that we often found in certain types of scenes and what things are found together within a scene can help determine where we look • Knowledge about specific types of scenes might influence where you look Nature of the Observer’s Task • Head-mounted eye trackers have been developed that make it possible to track a person’s eye movements as they perform tasks in the environment o This device shows that when a person is carrying out the task, the demands of the task override factors such as stimulus saliency o For instance, the above image shows the fixations and eye movements that occurred as a person was making a peanut butter sandwich Learning from Past Experiences • It is clear that a number of factors determine how a person scans a scene o Salient characteristics may capture a person’s initial attention o Cognitive factors become more important as the observer’s knowledge of the meaning of the scene begins determining where they fixate • Even more important than what a scene is, is what the person is doing within the scene o Specific tasks, such as making a peanut butter sandwich or driving, exerts strong control over where we look How does Attention affect our Ability to Perceive? Perception can occur without Focused Attention or • Reddy and coworkers’experiment showed that we can take in information from a rapidly presented photograph of a face that is located off to the side form where we are attending • Observers looked at the cross (+) on the fixation screen o They then saw the central stimulus, which is an array of 5 letters • Observers were instructed to keep looking at the center of the array of letters o These letters are change or stay the same on some trials • The letters were followed immediately by the peripheral stimulus o This stimulus is either a picture of a face or a disc that’s half green and half red o The stimulus flashes at a random position on the edge of the screen for 150 ms o The stimulus it is followed by a mask to limit the time it was visible • There are 3 main conditions: 1) Central task condition  The letters are flashed in the center of the screen, where the observer is looking  The observer’s task is to indicate whether all of the letters are the same  The non-relevant peripheral stimuli are still flashing on the side 2) Peripheral task condition  The observer’s task is to indicate whether the peripheral face is male or female or if the peripheral disc is red-green or green-red 3) Dual task condition  The observer’s task is to complete task 1 and 2 at the same time • The results of the experiment o When the observers only had one task to do at a time, they performed well o Central and peripheral task condition results  Performance was 80-90% o Dual task condition results  Performance on the face was near 90%  Performance on the red-green disc dropped to 54% (where chance is 50%) • What do these results mean? o These results show that it is possible to take in information about faces even when attention is not focused on the faces o But why do the results differ between faces and objects? o Let’s consider the differences between faces and discs, focusing on faces  Are meaningful  We have a large amount of experience perceiving them  We process them as a whole Perception can be affected by a Lack of Focused Attention • Inattentional blindness: failure to perceive a stimulus that isn’t attended, even if it is in full view Inattentional Blindness • Mack and Rock’s experiment o They demonstrated Inattentional blindness using the procedure above o The observer’s task is to indicate which arm of a briefly flashed cross is longer  The horizontal arm  The vertical arm o The inattention trial  Asmall test object is flashed close to where the observer is looking, along with the cross  When observers were then given a recognition test in which they were asked to pick out the object from 4 alternatives, they were unable to indicate which shape had been presented • Simons and Chabris’experiment o They created a 75-second film that showed 2 teams of 3 players each o One teams was passing a basketball around and the others were guarding o Observers were told to count the number of passes, a task that focused their attention on one of the teams o After about 45 seconds, one of the 2 events occurred that took 5 seconds  Either a woman carrying an umbrella  Or a person in a gorilla suit walked through the ‘game’ o After seeing the video, observers were asked whether they saw anything unusual happen or whether they saw anything other than the 6 players  46% of the observers failed to report • These experiments demonstrate that when observers are attending to on sequence of events, they fail to notice another even, even when it is right in front Change Detection • Rensink and coworkers’experiment o They presented one picture followed by a blank field, followed by the same picture but with an item missing, following by the blank field, and so on o The pictures were alternated in this way until observers were able to determine what was different about the two pictures o They found that the pictures had to alternated back and forth a number of times before the difference was detected • Change blindness: the difficulty in detecting changes in scenes o It is counter-intuitive result o Even though people believe they would detect such obvious changes, they fail to do so when actually tested • There is an important different between changes that occur in real life and those that in change detection experiments o Changes that occur in real life are often accompanied by motion o The appearance of new object in a change detection experiment is not signaled by motion, so your attention is not attracted to the place where the object appears Does Attention Enhance Perception? Effects of Attention on Information Processing Posner and coworkers’ experiment • They were interested in answering this question: 141 o Does attention to specific location improve our ability to respond rapidly to a stimulus presented at that location? • Pre-cueing: a procedure in which a cue stimulus is presented to direct an observer’s attention to a specific location where a test stimulus is likely to be presented • The observers kept their eyes stationary throughout the experiment, always looking at the cross (+) • They first saw an arrow cue indicating on which side of the target a stimulus was likely to appear • The observer’s task is to press a key as rapidly as possible when a target square is presented off to the side • There were 2 types of trials: o Valid trial: the square appears on the side indicated by the cue arrow 80% of the time o Invalid trial: the cue arrow indicate that the observer should attend to the left, but the target is presented on the right • The results o The observers react more ra
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