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Chapter 4


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University of Toronto Scarborough
Steve Joordens

Chapter 4 Signal Detection  Measuring absolute and difference threshold depends on the assumption that a threshold exists  Humans don’t suddenly and rapidly switch between perceiving and not perceiving o Going from not sensing to sensing is gradual  Accurate perception of a sensory stimulus is haphazard o Sensory signals face a lot of competition (noise: all the other stimuli coming from the internal and external environment o Internal noise -> memories, moods, and motives  Competes with your ability to detect a stimulus with perfect, focused attention o Rarely able to just focus on one stimulus at a time. Noise takes away the ability to perceive everything that you sense and may also cause you to perceive things that you haven’t sensed (hearing beeps that aren’t really there)  Signal detection theory: the response to a stimulus depends on a person’s sensitivity to the stimulus in the presence of noise and on a person’s decision criterion o Observers consider the sensory evidence and then compare it to an internal decision criterion o Exceed criterion -> Admit detection o Falls short of criterion -> Admit non-detection o Quantify an observer’s response in the presence of noise  Hit: Present, yes  Miss: Present, no  False Alarm: Not present, yes  Correct rejection: Not present, no  SDT -> a way to measure perceptual sensitivity: how accurate the perceptual system represents sensory events  Real-life situations must take into account the consequences of hits, misses, false alarms, and correct rejection Sensory Adaptation  Smells fading, getting used to cold water  Sensory adaptation: sensitivity to prolonged stimulation tends to decline over time as an organism adapts to current conditions  Perceptual systems emphasize change in responding to sensory events  Useful process for most organisms  Sensory systems respond more strongly to changes in stimulation that to constant stimulation VISION I: HOW THE EYES AND THE BRAIN CONVERT LIGHT WAVES TO NEURAL SIGNALS  Visual acuity: the ability to see fine detail  Humans have sensory receptors in their eyes that respond to wavelengths of light energy Sensing Light  Visible light is the portion of the electromagnetic spectrum that we can see -> extremely small slice o Length: Hue and colour 1 | P a g e Chapter 4 o Intensity/Amplitude: Brightness o Purity: number of distinct wavelengths -> saturation/richness of colours  Light doesn’t need a human to have the property it does. Length, amplitude, and purity are properties of the light waves themselves The Human Eye  Eyes have evolved as specialized organs to detect light 1. Light reaches eye 2. Passes through the cornea: clear, smooth, outer tissue -> bends light wave 3. Cornea send light wave through the pupil: a hole in the coloured part of the eye (iris) 4. Iris: translucent donut-shaped muscle controlling the size of the pupil -> amount of light that can enter the eye 5. Muscles behind the iris control the shape of the lens to bend the light again and focus it onto the retina: light sensitive tissue lining the back of the eyeball 6. Flatter lens: far away objects 7. Rounder lens: nearby objects 8. 6&7 are called accommodation: the process by which the eye maintains a clear image on the retina Photo transduction into the Retina  Wavelength of light -> meaningful image  Retina is what turns the lights outside into vision on the inside of our body  Two types of photoreceptor cells in the retina  Cones: detect colour, operate under normal daylight conditions, and allow us to focus on fine detail o Only 6 million in each retina o Densely packed in the fovea o Sparsely distributed in the fovea o Reason why objects in peripheral vision isn’t as clear  Rods: active under low-light conditions for night vision o Much more sensitive than cones o All rods contain the same photo pigment -> no information about colour and sense on shades of gray o 120 million rods are distributed relatively evenly around each retina except in the very center (fovea: an area of the retina where vision is the clearest and there are no rods at all) o Absence of rods in the fovea decreases the sharpness of vision in reduced light -> look a little off to the side to compensate  Retina is thick with cells o Innermost layer: photoreceptor cells o Middle layer: bipolar cells: collect neural signals from the rods and cones and transmit them to neurons calls retinal ganglion cells o Outermost layer: RGC: organize the signals and send them to the brain  RGC axons form the optic nerve: leaves the eye through a hole in the retina and contains neither rod not cones, so it has no mechanism to sense light. This hole creates a blind spot: a location in the visual field that produces no sensation on the retina 2 | P a g e Chapter 4 Receptive Fields  Each axon in the optic nerve originates in an individual retinal ganglion cell  Most RGCs respond to input from an entire patch of adjacent photoreceptors  Receptive field: the region of the sensory surface that, when stimulated, causes a change in the firing rate of that neuron  GENERAL CONCEPT OF RECEPTIVE FIELDS APPLIES TO ALL SENSORY SYSTEMS  When the shining spot exactly fills the excitatory zone, it elicits the strongest response, whereas light falling on the surrounding inhibitory zone elicits the weakest response or none at all o The retina is organized in this way to detect edges – abrupt transitions from light to dark or vice versa o Edges: define the shapes of objects, and anything that highlights such boundaries improves our ability to see an object’s shape Perceiving Colour Seeing Colour  Sir Isaac newton o Colour is not something “in” light o Colour is nothing but our perception of wavelengths  Shortest visible wavelength -> deep purple  As wavelengths increase, the colour perceived changes gradually (ROY. G. BIV) -> visible spectrum  Cones contain one of three types of pigment o Red: long wavelength o Green: medium wavelength o Blue: short wavelength o The above are the primary colours of light  Additive Colour mixing: increasing the reflected wavelengths -> adding like to stimulate the cones -> all = white  Subtractive Colour mixing: removes wavelengths, thus absorbing light waves we see as red, blue or yellow -> all = black Trichromatic Colour Representation in the Cones  S-Cones = short wavelengths = blue  M-Cones = medium wavelength = green  L-Cones = long wavelength = red  Trichromatic colour representation: the pattern of responding across the three types of cones provides a unique code for each colour  Colour deficiency is caused by a genetic disorder which one of the cone types is missing, in rare cases, 2 or all 3 can be missing. This affects men more often than women Colour-Opponent Representation into the Brain  Cones need an occasional break 3 | P a g e Chapter 4  Staring too long at one colour fatigues the cones that respond to that colour, producing a form of sensory adaptation those results in a colour afterimage.  Colour-opponent system: pairs of visual neurons work in opposition o Red-sensitive against green-sensitive o Blue-sensitive against yellow sensitive The Visual Brain  A great deal of visual processing takes place within the retina  More complex aspects of vision require more powerful processing and that enlists the brain  Action potentials containing information encoded by the retina travel to the brain along the optic nerve  Thalamus receives inputs from all the senses except smell  Area V1: The part of the occipital lobe that contains the primary visual cortex o Information is systematically mapped into a presentation of the visual scene Neural Systems for Perceiving Shape  One of the most important functions of vision involves perceiving the shapes of object  Perceiving shape depends on the location and orientation of an object’s edges Pathways for What, Where, and How  One brain system identifies people and things and another tracks their movements or guides our movements in relation to them  Visual streams are projected from the occipital cortex to visual areas in other parts of the brain o Ventral (below) stream travels across the occipital lobe into the lower levels of the temporal lobes  What a shape is  Represents an objects shape and identity  Visual-form agnosia: the inability to recognize objects by sight  Not visual representation  Can recognize an object by touch  Can still accurately guide actions by sight  Inability to recognize familiar faces o Dorsal (above) stream travels up from the occipital lobe to the parietal lobes  Areas that identify the location and motion of an object  Where it is  Perceive spatial relations -> original called the “where pathway”  Also crucial for guiding movements -> “how pathway”  Optic Ataxia: Difficulty using vision to guide their reaching and grasping movements  Recognize what objects are o The two streams must integrate VISION II: RECOGNIZING WHAT WE PERCEIVE  How the system links together individual visual features into whole objects, allows us to recognize what those objects are, organizes objects into visual scenes and detects motion and change in those scenes 4 | P a g e Chapter 4 Attention: The “Glue” That Binds Individual Features into a Whole  Specialized feature detectors in different parts of the visual system analyze each of the multiple features of a visible object o Orientation o Colour o Size o Shape o How are the features mentioned above combined into a single object? o Binding problem in perception: how features are linked together so that we see unified objects in our visual world rather than free-floating or miscombined features Illusory Conjunctions: Perceptual Mistakes  Illusory conjunction: perceptual mistake where features from multiple objects are incorrectly combined o Reason why they occur: feature integration theory: focused attention is not required to detect the individual features that comprise a stimulus , such as the colour, shape, size, and location of letters, but is required to bind those individual features together o Attention provides the glue necessary to bind features together The Role of the Parietal Lobe  The binding process makes use of feature information processed by the ventral stream, but since binding also involves linking together features at a specific spatial location, it also depends on the dorsal stream  Parietal lobe damage -> dorsal -> how -> binding problems, because that area also affects attention focus Binding and Attention in Synesthesia  Synesthesia -> instance of atypical feature binding  Normal binding of colours and letters is a response to actual features of the external stimulus, but in synesthesia, the colour feature is not present in the external stimulus  However, the processes involved in normal feature binding also occur in synesthesia o Parietal lobe, which is active in the normal binding of colour and shape also becomes active during the experience of letter-colour synesthesia Recognizing Objects by Sight  Ability to recognize letters despite the varying fonts  Ability to recognize people despite change in hairstyle or accessories  Inefficient to not recognize an object due to a minor variation  How? o Modular View: Specialized brain areas, or modules, detect and represent faces or houses or even body parts.  fMRI showed that the brain has feature detectors to aid in visual perception and also “face detectors”, “building detectors” o Distributed representation: 5 | P a g e Chapter 4  Critique of Modular View: Although a sub region in the temporal lobes does respond more to faces than to any other category, parts of the brain outside this area may also be involved in face recognition. It is the pattern of activity across multiple brain regions that identifies any viewed object  The temporal lobe’s neurons respond to specific objects viewed in various ways  Perceptual constancy: Even as aspects of sensory signals change, perception remains consistent o Perception is sensitive to changes in stimuli, but perceptual constancies allow us to notice the differences in the first place Principles of Perceptual Organization  Before object recognition -> visual system must perform an important task o Group the image regions that belong together into a representation of an object  We tend to perceive a unified, whole object instead of the object in separate parts -> Gestalt psychology o Perceptual Grouping Rules  Simplicity(Pragnanz -> good form)  Basic rule: simplest explanation is usually the best  Visual system selects the best or most likely interpretation  Closure  Fill in missing elements of a visual scene  Filling in gaps  Continuity  Edges or contours that have the same orientation  Similarity  Similar in colour, lightness, shape, or texture  Proximity  Close together -> grouped together  Common fate  Elemenmts which move together -> parts of a single moving object Separating Figure from Ground  Perceptual Grouping o Ability to recognize objects by sight  Grouping: visually separating an object from its surroundings o Identifying a figure apart from the (back) ground o Can perceive elements differently  Focus attention on some objects as distant from their environment o Size  Smaller regions -> usually the figure o Movement  Moving around in a static environment o Edge assignment  Edge or boundary between figure and ground  If edge belongs to figure -> defines the object’s shape 6 | P a g e Chapter 4  Background continues behind the edge Theories of Object Recognition  Two broad explanations of object recognition: one based on the object as a whole and the other one its parts o Image-based object recognition  Object you’ve seen before is stored in the memory as a template: mental representation that can be directly compared to a viewed shape in the retinal image  Shape templates are stored with name, category, and other associations to that object  Compares templates to the current retinal image and selects the template that most closely matches the current image  Critique: Orientation? Inefficient system o Parts-based object recognition  Brain deconstructs viewed objects into a collection of parts  Objects are stored in memory as structural descriptions  Geons which are combined to make objects  Does not require a template  Limitations  Only allows for object recognition at the level of categories and not at the level of
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