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Chapter 6


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University of Toronto Scarborough
Steve Joordens

CHAPTER 6: PERCEPTION - The primary function of the sense organs is to provide info. to guide behavior. - Perception: a rapid, automatic, unconscious process by which we recognize what id represented by the info. Provided by our sense organs. Brain Mechanisms of Visual Perception. - Visual perception is often described as a hierarchy of info processing- circuits of neu- rons analyze particular aspects of visual info and send the results of their analysis to an- other circuit. etc. The Primary Visual Cortex - David and Wiesel: inserted micro-electrodes into various regions of the visual systems of cats and monkeys to detect the action potentials produced by individual neurons. - The signals picked up by the micro-electrodes are electronically amplified and sent to a recording device for further analysis. - Concluded: the geography of the visual field is retained in the primary visual cortex. - Module: a block of cortical tissue that receives info from the same group of receptor cells (in the retina). - Hubel + Wisel: found that neural circuits within each module analyzed various charac- teristics of their own particular part of the visual field. - Receptive field: that portion of the visual field in which the presentation of visual stimuli will produce an alternation of the firing rate of a particular neuron. The Visual Association Cortex - > 2 Streams of Visual Analysis - Visual info analyzed by the PVC is further analyzed in the VAC - Neurons in the PVC send axons to the region of the VAC that surrounds the striate cor- tex. - At this point, VAC divides into 2 pathways: the ventral and dorsal stream. - Ventral stream: the flow of info from the PVC to the VA area in the lower temporal lobe; used to form the perception of an object’s shape, color, orientation (the “what” system). - Dorsal stream: the flow of info. from the primary visual cortex to the visual association area in the parietal lobe; used to form the perception of an object’s location in 3-dimen- sional space (the “where” system). - The ventral stream continues forward and ends in the inferior temporal cortex. - The dorsal stream ascends into the posterior parietal cortex. - > The Ventral Stream: Perception of Form - Recognition of visual patterns and identification of particular objects takes place in the inferior temporal cortex - Here that analyses of form and color are put together and perceptions of 3-dimension- al objects emerge. - Visual agnosia: the inability of a person who is not blind to recognize the identity of an object visually; caused by damage to the visual association cortex. - Prosopagnosia: a form of visual agnosia characterized by difficulty in the recognition of people’s faces; caused by damage to the visual association cortex. - Fusiform face area (FFA): a region of the ventral stream of the visual system that con- tains face-recognizing circuits. (located at the base of the brain). - H/e the failure to recognize is not confined to faces. - Greld, Gauther and Schultz (2002): found that people with autism showed a deficit in the ability to recognize faces and that looking at faces failed to make active the FFA. - Extrastriate body area (EBA): a region of the occipital cortex, next to the primary visual cortex, that responds to forms resembling the human body. - Specifically activated by photographs, silhouettes, or stick figures of human bodies or body parts and not by controlled stimuli such as photographs, or drawing of tools, scrambled silhouettes, or scrambled stick drawings of bodies. - Berlucchi, Agility and Urgesi (2004): found that when the EBA was temporarily inacti- vated by transcranial magnetic stimulation, people lost the ability to recognize photos of body parts but not parts of faces or motorcycles. - Parahippo campal place area (PPA): a region of the ventral steam, below the hip- pocampus, that is made active by visual scenes. (collections of visual objects) and backgrounds. - > The Ventral Stream: Perception of Color. - Individual neurons in a region of the V.S respond to particular colors. Suggests -region involved in combining the info. From red/green and yellow/blue signals that originate in retinal ganglion cells. - Lesions to this area can also cause loss of color vision. - Cerebral achromatopsia: the inability to discriminate among different hues; caused by damage to the visual association cortex. - > The Dorsal Stream: Perception of Spatial Location - Neurons in the dorsal stream are involved in visual attention and control of eye move- ments, the visual control of reaching and pointing, and grasping + perception of depth. - Guide actions (primary function). - > The Dorsal Stream/ Perception of Movement - Akinetopsia: an inability to see motion. - Region of the visual association cortex. - > Form from Motion - Perception of movement can help us perceive 3-dimensional forms (form from motion). - The ventral bank of the posterior end of the superior temporal sulcus active. PART II: VISUAL PERCEPTION OF OBJECTS Figure and Ground - Figure: a visual stimulus that is perceived as a self-contained object. - Ground: a visual stimulus that is perceived as a formless background against which objects are seen. Gestalt Laws of Perceptual Organization - Gestalt psychology: a branch of psychology that asserts that the perception of objects is produced by particular configurations of the elements of stimuli. - Task of perception was to recognize objects in the environment according to the orga- nization of their elements. - Argue that in perception the whole is more than the sum of its parts. - Observed that several principles of groupings can predict the combination. - Law of proximity: a gestalt la w of organization; elements located closet to each other are perceived as belonging to the same figure. - Law of similarity: a gestalt law of organization; similar elements are perceived as be- longing to the same figure. - Good continuation: a gestalt law of organization; given 2 or more interpretations of ele- ments that form the outline of the figure; the simplest interpretation will be preferred. - Law of closure: a gestalt law of organization; elements missing from the outline of a figure are “filled in” by the visual system. - Law of common fate: a gestalt law of organization; elements that move together give rise to the perception of a particular figure. - Alias, Blake, Lee (1998): found that the principle of common fate applies to changes other than movement. Models of Pattern Perception - > Templates and Prototypes - Template: a hypothetical pattern that lives in the nervous system and is used to per- ceive objets or shapes by a proses of comparison. - Prototype: a hypothetical idealized pattern that resides in the Nervous system and is used to perceive objects or shapes by a process of comparison; r
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