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PSYC 101 CHPT. 6

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PSYC 101
Barbara Cox

PSYC 101, 007 CHPT. 6 PERCEPTION I. Brain Mechanisms of Visual Perception Perception- a rapid, automatic, unconscious process by which we recognize what is represented by the info provided by our sense organs The Primary Visual Cortex  Visual info proceeds from retina to the thalamus then to the primary visual cortex  Primary visual cortex is organized into modules  Module- block of cortical tissue that receives info from the same group of receptor cells  All of the neurons within a module receive info from the same small region of the retina  Primary visual cortex contains approx. 2500 modules  Neural circuits within each module analyzes specific info from their own particular part of the visual field- the receptive field  Receptive field- portion of the visual field in which the presentation of the visual stimuli will produce an alternation in the firing rate of a particular neuron  Different circuits detect orientation & width of lines, colour & movement of those lines  However, perception of objects & the totality of the visual scene does not happen here The Visual Association Cortex  To perceive objects & entire visual streams, info from individual modules must be combined  That combination takes place at different levels in the visual association cortex  Visual info analyzed by primary visual cortex is further analyzed in visual association cortex  Neurons in primary visual cortex send axons to region of the visual association cortex that surrounds the striate cortex  Visual association cortex then divides into pathways: ventral stream & dorsal stream  Ventral stream- flow of info from the primary visual cortex to visual association area in the lower temporal lobe  Used to form the perception of an object’s shape, colour & orientation (recognition of what an object is)  Dorsal stream- flow of info from primary visual cortex to the visual association area in the parietal lobe  Used to form the perception of an object’s location in 3D space (identifies where an object is located & whether it is moving) The Ventral Stream: Perception of Form  Damage to ventral stream can cause various forms of visual agnosia, such as prosopagnosia  Visual agnosia- inability of a person who is not blind to recognize the identity of an object visually  Prosopagnosia- form of visual agnosia characterized by difficulty in recognition of people’s faces  Know that you are looking at a face but cannot say whose face it is, even if it belongs to someone you know  Studies with brain-damaged people & functional imaging studies indicate that face-recognizing circuits are found in the fusiform face area (FFA)  Some evidence suggests that face-recognition circuits develop as a result of experience with seeing people’s faces  Specific regions of the ventral stream are devoted to particular categories of visual stimuli:  Extrastriate body area (EBA)- region of the occipital cortex, next to primary visual cortex that responds to forms resembling human body  Parahippocampal place area (PPA)- region of the ventral stream, below the hippocampus, that is activated by visual streams & backgrounds The Ventral Stream: Perception of Colour  Individual neurons in a region of the ventral stream respond to particular colours  Damage to this region can disrupt one’s ability to distinguish between different colours  Cerebral achromatopsia- inability to discriminate among different hues (vision without colour)  If brain damage occurs on only one side of the brain, people lose their colour vision in only half of their visual field  If brain damage is bilateral, they lose all colour vision & cannot even imagine colours of objects they saw before the damage occurred The Dorsal Stream: Perception of Spatial Location  Dorsal stream is located in parietal lobe- damage to parietal lobe disrupts perceiving & remembering location of objects as well as controlling the movement of eyes & the limbs  Neurons in dorsal stream are involved in: o Visual attention & control of eye movements o Visual control of reaching, pointing, grasping & other hand movements o Perception of depth  Suggested that primary function of dorsal stream is to guide actions rather than simply perceiving spatial locations The Dorsal Stream: Perception of Movement  Akinetopisa- inability to see motion  Form from motion- phenomenon that perception of movement can help us perceive 3D forms  Involves different brain mechanisms than those of perception of objects II. Visual Perception of Objects Figure and Ground  Perception of form requires recognition of figure & ground  We classify most of what we see either as object or background  Objects are things that have particular shapes & locations in space  Backgrounds are formless & serve to help us judge the location of objects we see in front of them  Figure- visual stimulus that is perceived as self- contained object  Ground- bisual stimulus that is perceived as formless background against which objects are seen  Boundary is an edge perceived in the visual field  if the edge forms a continuous boundary, we perceive the space enclosed by the boundary as a figure  most figures are defined by boundary, but a boundary is not necessary for perception of form Gestalt Laws of Perceptual Organization  Gestalt psychology- branch of psychology that asserts that the task of perception is to recognize objects in the environment according to the organizations of their elements  Perception of an organized whole, tendency to integrate pieces of info into meaningful wholes  concepts: thereness (location in space, not consciously aware of) & thatness (consciously aware of)  Components of a visual scene can combine in various ways to produce different forms  Several principals of grouping can predict the combination of these elements: o Law of proximity- elements located closest to each other are perceived as belong to the same figure o Law of similarity- similar elements perceived as belonging to the same figure o Good continuation- given 2 or more interpretations of elements that form the outline of the figure, the simplest interpretation will be preferred o Law of closure- elements missing from the outline of a figure are “filled in” by the visual system o Law of common fate- elements that move together give rise to the perception of a particular figure Models of Pattern Perception  Our ability to recognize shapes of objects can be explained by our use of templates  Template- hypothetical pattern that resides in the nervous system & is used to perceive objects or shapes by a process of comparison  Visual system searches through its set of templates & compares each of them with the pattern provided by a stimulus  If it finds a match, it knows the pattern is a familiar one  Connection between appropriate template & memories on other parts of the brain could provide the name of the object & info about it  Most psychologies do not believe that th
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