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Midterm 1 Sensory and Perception.rtf

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Todd Ferretti

Chapter 1: Introduction to Perception The Perceptual Process Perception does not just happen, but it is the end result of complex “behind the scenes” processes, many of which are not available to your awareness The perceptual process, is a sequence of processes that work together to determine our experience of an reaction to stimuli in the environment The perceptual process is divided into four categories: stimulus, electricity, experience, and action, and knowledge The Stimulus The stimulus exists both “out there,” in the environment, and within the person’s body Environmental stimulus is all of the things in our environment that we can potentially perceive The attended stimulus changes from moment to moment Electricity One of the central principles of perception is that everything we perceive is based on electrical signals in our nervous system. These electrical signals are created in the receptors, which transform energy from the environment into electrical signals in the nervous system—a process called transduction. Transduction Is the transformation of one form of energy into another form of energy Occurs in the nervous system when energy is in the environment—such as light energy, mechanical pressure, or chemical energy—is transformed into electrical energy. Transmission After an image has been transformed into electrical signals, these signals activate other neurons, which in turn activate other 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 Processing Neural processing which involves interactions between neurons Interactions between neurons is like how signals are transmitted by your cell phone In the nervous system the signal that reaches the brain is transformed so that, although it represents the original stimulus, it is usually very different from the original signal Neural processing happens as the signals that originate in the receptors travel through a maze of interconnected pathways between the receptors and the brain within the brain In the nervous system, the original electrical representation of the stimulus that is created by the receptors is transformed by processing into a new representation of the stimulus in the brain Experience and Action “Backstage activity” of transduction, transmission, and processing is transformed into things we are aware of—perceiving, recognizing, and acting on objects in the environment Perception Perception is a conscious sensory experience. It occurs when your brain into your experiences of seeing that object transforms the electrical signals that represent an object. Just because you see and perceive and object doesn’t mean it stops there. Other things have happened as well, you have recognized the object, or differentiated the object from something similar, and you take action on what to do next after you have perceived the object. These are all still continuing steps of perception. Recognition Recognition is our ability to place an object in a category that gives meaning. Although we might be tempted to group perception and recognition together, it should be known that these are separate processes. You can very well perceive something without recognizing exactly what it is. Visual form agnosia—an inability to recognize objects that was caused by a brain tumor. E.g., Dr. P who could perceive the parts of objects but couldn’t identify the whole object. Action Action includes motor activities such as moving the head or eyes and locomotion through the environment Some researchers see action as an important outcome of the perceptual process because of its importance for survival. Knowledge Is any information that the perceiver brings to a situation Information that a person brings to a situation can be things learned years ago and can influence the perceptual process is the ability to categorize objects Bottom-up processing is processing that is based on income data. Incoming data always provide the starting point for perception because without incoming data, there is no perception. Top-down processing refers to processing that is based on knowledge. Knowledge isn’t always involved in perception but it often is—sometimes without our even being aware of it. Bottom-up is essential for perception because the perceptual process usually beings with stimulation of the receptors. Bottom-up and top-down processing often work together to create perception How to Approach the Study of Perception The psychophysical approach to perception was introduced by Gustav Fechner, who coined the term psychophysics to refer to the use of quantitative methods to measure relationships between stimuli (physics) and perception (psycho) The physiological approach to perception involves measuring the relationship between stimuli and physiological processes and between physiological processes and perception The knowledge, memories, and expectations that people bring to the situation influence their perceptions. These factors that have been described as the starting point for top-down processing are called cognitive influences on perception. Measuring Perception Description: The phenomenological method is a first step in studying perception because it describes what we perceive. The description can be at a very basic level, such as when we notice that we can perceive some objects as being further away than others, or that there is a many other perceptual qualities Recognition: when we categorize a stimulus by naming it, we are measuring recognition Detection: methods, limits, adjustments, and contrast stimuli are called the classical physcophysical methods because they were the original methods used to measure the stimulus perception relationship Thresholds The Absolute Threshold is the smallest amount of stimulus energy necessary to detect a stimulus. For example, the smallest amount of light energy that enables a person to just barley detect a flash of light would be the absolute threshold for seeing that light. The Difference Threshold is the smallest difference between two stimuli that a person can detect. Weber’s Research Found that when the difference between the standard and comparison weights was small, his observers found it difficult to detect the difference in weights, but they easily detected larger differences His research was stated mathematically by Fechner as DL/S=K and was called Weber’s Law. K is a constant called the Weber Fraction. Magnitude Estimation If we double the intensity of a tone, does it sound twice as loud? If we double the intensity of a light, does it look twice as bright? Although a number of researchers, proposed equations that related perceived magnitude and stimulus intensity, it wasn’t until 1957 that S.S Stevens developed a technique called scaling, or magnitude estimation that accurately measure this relationship. As intensity is increased, perceptual magnitude increases more than intensity is called response expansion. The beauty of the relationships derived from magnitude estimation is that the relationship between the intensity of a stimulus and our perception of its magnitude follows the same general equation for each sense. These functions are called power functions. This relationship is called Steven’s power law. Threshold Measurement can be influenced by how a person chooses to respond A way to describe the difference between two people is that each have a different response criterion which means that everybody responses differently. Response criterion is not very important if we are testing many people and averaging their responses. However, if we wish to compare two people’s responses, their differing response criteria could influence the results. Luckily there is a way to take differing response criteria into account. This procedure is described as signal detection theory. Chapter 2 Review – Intro. To Physiology of Perception History - seat of the mind was seen as being/coming from the heart - this then changed to be the brain as the seat of the mind - two ideas were held regarding structure of nervous system 1- reticular theory: held that nervous system consisted of a large network of fused nerve cells 2- neuron theory: stated that nervous system consisted of distinct elements or cells - acceptance of neuron theory was based on discovery of staining—chemical technique that caused cells to become coloured as they stood out from surrounding tissue Basic Structure of the Brain - cerebral cortex: layer that covers the surface of the brain and contains the machinery for creating perception, as well as other functions, such as language, memory, and thinking - basic principle of cortical function is modular organization—specific functions are served by specific areas of the cortex - primary receiving areas: first areas in cerebral cortex to receive signals initiated by each senses’ receptors - occipital lobe: receiving area for vision - temporal lobe: area for hearing - parietal lobe: area for skin senses—touch, pain, temperature - frontal lobe: receives signals from all senses and plays role in perceptions that involve coordination of information received through two or more senses Neurons: - purpose...1) to respond to stimuli from the environment, transducer these stimuli into electrical signals, and 2) to communicate with other neurons, so that signals can travel long distances— transmission - structure: - cell body contains mechanisms that keep the cell alive - dentrites branch out from cell body to receive electrical signals from other neurons - axon, or nerve fibre, is filled with fluid that conducts electrical signals - receptors: type of neuron important for perceptions—specialized to respond to environmental stimuli such as pressure for touch – part of each receptor reacts to environmental stimuli and triggers the generation of electrical signals, which are eventually transmitted to neurons with axons - resting potential: -70mV; the value when nerve fibre is at rest—no signals in the neuron - action potential: +40mV; happens when neuron’s receptor is stimulated so that signal is transmitted down the axon Properties of Action Potential - propagated response: once the response is triggered, it travels down the axon without decreasing in size - refractory period: interval between the time one nerve impulse occurs and the next one can be generated in the axon - upper limited of neuron’s firing rate is about 500-800 impulses per second - spontaneous activity: action potentials that occur in absence of stimuli from the environment - synapses: small space between neurons—neurotransmitter Events at the Synapse - when action potentials reach the end of a neuron, they trigger release of neurotransmitters - neurotransmitter molecules flow into the synapse to small aerates on the receiving neuron called receptor sites that are sensitive to specific neurotransmitters - excitatory transmitters: cause the inside of the neuron to become more positive, a process called depolarization - inhibitory transmitters: cause the inside of the neuron to become more negative, a process called hyperpolarization - inhibitory response can prevent the neuron form reaching the level of depolarization needed to generate action potentials Neural Processing - neural circuits: groups of interconnected neurons - convergence: the synapsing of more than one neuron into a single neuron - increases firing rate of a neuron because stimulating more receptors increases the amount of excitatory transmitter released onto another neuron Receptive Fields: area on the receptors that influences the firing rate of the neuron - center-surround receptive field: areas of receptive field are arranged in a center region that responds in the opposite way - there is excitatory-center-inhibitory-surround receptive field (on- center-off-surround) and inhibitory-center-excitatory-surround receptive field - the fact that the center and surround respond in opposite ways causes effect-center-surround antagonism The Sensory Code - specific coding: representation of particular objects in the environment by the firing of neurons that are tuned to respond specifically to that object; grandmother cell—neuron that responds only to a specific stimulus - distributed coding: representation of a particular object by the pattern of firing of groups of neurons –there are different patterns of firing for each face - sparse coding: idea that particular object is represented by the firing of a relatively small number of neurons Chapter 3 – Vision Vision is based on visible light, a band of energy in the electromagnetic spectrum Two visual receptors: Rods and Cones, which contain light sensitive chemicals called visual pigments which react to light and trigger electrical signals. The signals emerge from the back of the eye in the optic nerve, which sends signals to the brain.Cornea and Lens are responsible for focusing. If you have an object 20+ feet away the light reaches you eye at parallel, and the lens or cornea do not have to focus the light.If you have an object <20 feet away, the light reaches your eye at an angle and the rays must be corrected by the lens. The lens is “squeezed” by the Ciliary muscles of the eye in order to bend the incoming light. This process is known as Accomodation. Accomodation has its limits, the point where something is so close your lens cannot bring it into focus is called a near point. The distance of the near point increases with age. Presbyopia: Condition where the lens hardens with age, making it harder to focus as it will not bend light as well. Myopia/Nearsightedness: An inability to see distant objects clearly because light is brought into focus by the lens TOO EARLY. Refractive Myopia: Near sightedness occurs because cornea and lens bend light TOO MUCH Axial Myopia: Eyeball is too long. Because of this light is focused too early and near sightedness occurs. Far Point: The distance at which light becomes focused on the retina. Hyperopia/Farsightedness: Inability to see objects which are close to you resulting from the focus point being located behind the eye.We don’t not “see” from the retina. We see from the brain. Transduction is carried out by receptors (neurons specialized for receiving environmental energy and transforming it into electricity) Each rod contains stacks of discs inside of it which contain the visual pigment required to see.The main molecule in vision is Retinal. One retinal molecule is contained in each Visual Pigment (VP) molecule. Retinal is light sensitive. When light strikes the VP and retinal molecules, the retinal, being light sensitive changes shape. This change in shape triggers the transformation for light to electrical energy for the brain to interpret as vision. Isomerisation: Technical term describing retinal changing shape after coming into contact with light. Hecht’s psychophysical Experiment: Determined : - a rod receptor can be activated by the isomerisation of just one molecule Enzyme Cascade: See textbook picture. Fovea: Area of retina containing only cones. When we look directly at an object it falls on the fovea Peripheral Retina: Everywhere else in the retina excluding the fovea. Contains rods and cones. Blind spot: exists where the optic nerve connects to the back of the eyeball. Dark Adaptation: The eye increases its sensitivity to the dark after prolonged exposure. (View graphs in textbook) This adjustment occurs in two stages: 1) Initial, rapid stage (cones adapt) 2) Secondary, slower stage (rods adapt) Why do rods take longer to reach maximum sensitivity to dark lighting?When light hits retinal it changes shape. This causes the VP to turn yellow. In the absence of light retinal rejoins the VP molecule and the pigment changes to red. Cone pigment takes 6 minutes to regenerate to the red colour. Rod pigments take 30 minutes to regenerate to the red colour. This is consistent with the dark adaptation curve (see text).Important points: 1) Our sensitivity in the light depends on the dark adaptation curve. 2) The speed @ which our sensitivity in the dark is adjusted depends on a chemical reaction of the visual pigments. Spectral sensitivity: An observer’s sensitivity to light at each wavelength across the visible spectrum.Rods are most sensitive to low wavelength light. Cones are most sensitive to high wavelength light, because of this, grens blues and purples stand out more at night (low wavelength light). This is known as the Purkinje Shift. Look at Diagram of Cross section of Retina in Text:Receptors Horizontal cellBipolar cellsAmacrine cells Ganglion cells Signals travel across the retinal via horizontal and amacrine cells Convergence: Synapsing of more than one neuron onto a single neuron. Important: Signals from rods converge more than cones. This leads to rods being used for SENSITIVE vision. And cones being used for DETAIL vision. Chapter 4 – Midterm 1 Study Notes 4.1 After the Retina Retina 1.5axons from each LGN (lateral Geniculate Nucleus) occipital cortex Receptive Fields (RF) in the LGN have the same centre-surround configuration as retinal ganglion cells. V1 of the occipital lobe – Striate cortex (because there is a stripe of light- coloured nerve fibers) Organized into layers Like the LGN, neurons from the same area of the retina also project on the same area of the cortex Hubel and Weisel – Receptive Fields of cortical neurons Cells in the striate cortex with receptive fields have excitatory and inhibitory areas Arranged side by side rather than in center-surround configuration Three types of neurons in the stria
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