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Chapter 6

PSYA01 FINAL EXAM NOTES Chapter 6.docx

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University of Toronto Scarborough
Steve Joordens

CHAPTER 6: PERCEPTION Perception: rapid, automatic unconscious process by which we recognize what is represented by the information provided by our sense organs BRAIN MECHANISMS OF VISUAL PERCEPTION The Primary Visual Cortex Hubel and Wiesel experiment – unconscious animals with open eyes, still brain activity with pictures being shown to them. Concluded geography of visual field is retained in the primary visual cortex. Map on brain is distorted giving most space to the centre of visual field. Map is like mosaic made up of tiles/modules Module: block of cortical tissue that receive info from same group of receptor cells 0.5mm*0.7mm 150000 neurons All neurons within module receive info from the same small region of the retina, Primary Cortex contains 2500 modules Each module receives info from small region of one retina, it receives info from a small region of the visual field Hubel and Wiesel found neural circuits within each module analyzed various characteristics of their own particular part of the visual field, that is of their receptive field: portion of visual field in which the presentation of visual stimuli will produce an alternation in the firing rate of a particular neuron some detect orientation of lines, width, movement, direction of movement, color etc. The Visual Association Cortex Individual modules are combined here Two Streams of Visual Analysis Visual info analyzed by primary is further analyzed by the association Investigators discovered more than two dozen distinct regions and subregions of visual cortex of a rhesus monkey Circuits of neurons analyze particular aspects of visual info and send results to other circuits which perform further analysis, as it goes on more complex features are analyzed, in milliseconds, the process leads to perception of the scene and objects in it; higher level also interact with memories Neurons in primary visual cortex send axons to region of visual association cortex that surrounds the striate cortex and divides them into two pathways: ventral stream(flow of info from primary visual to visual association area in the lower temporal lobe; used to form perception of object’s shape, color, orientation) (continues forward and ends in the inferior temporal cortex) and dorsal stream (flow of info from primary visual to visual association area in the parietal lobe; used to form the perception of an object’s location in 3d space). (ascends into posterior parietal cortex) The Ventral Stream: Perception of Form Recognition of visual patterns and identification takes place in the inferior temporal lobe, here analysis of form and color are put together and 3D objects emerge Brain damage can cause category of defects known as visual agnosia Visual agnosia: inability of a person to recognize the identity of an object visually; caused by damage to the visual association cortex Person can still determine objects by other means and no loss of intellect Prosopagnosis: form of visual agnosia characterized by difficulty in the recognition of people’s faces, can recognize a face but not whose face it is, but can still recognize by voice Is common symptom of visual agnosia Studies with brain damaged people and functional-imaging studies show face recognizing circuits are found in the fusiform face area (FFA): a region of the ventral stream located at base of brain FFA also same effect for other objects such as a farmer’s cows, driver’s car, bird expert and birds, etc. Autistic people have deficit in ability to recognize faces and looking at faces fails to active 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, stick figures, body parts When deactivated temporarily by transcranial magnetic stimulation, lost ability to recognize photo of body parts but not of faces or motorcycles Parahippocampal Place Area (PPA): region of the ventral stream below the hippocampus that is activated by visual scenes and backgrounds Woman with bilateral damage unable to recognize specific objects but PPA unharmed was still able to recognize the overall scene The Ventral Stream: Perception of Colour Individual neurons in a region of ventral stream involved with combining in combining info from red/green and blue/yellow ganglion cells (color), damage to the area impaired ability to distinguish colors Cerebral achromatopsia: inability to discriminate among different hues due to damage to the visual association cortex, if only one side, lose color on one side, if both, lose all color an unable to even remember colors they saw before The Dorsal Stream: Perception of Spatial Location Parietal lobe receives visual, auditory, somatosensory, and vestibular info and is involved in spatial and somatosensory perception Damage disrupts performance on tasks that require 1)perceiving and remembering the location of objects and 2) controlling the movement of the eyes and limbs End of Dorsal stream located in posterior parietal cortex Neurons in dorsal stream involved in visual attention and control of eye movements, the visual control of reaching and pointing and visual control of grasping and other hand movements and depth perception Goodale and colleagues show that dorsal is not just where but how as well, woman with damage to her posterior parietal unable to adjust her hand size to grab an object though she knew what it was and it’s size The Dorsal Stream: Perception of Movement Without being able to perceive the velocity (distance and speed) of an object we can’t predict where they will be and unable to catch them or avoid the catching us Region located in extrastriate cortex that surrounds primary visual contains neurons that respond differently to movement Bilateral damages causes akinetopsia: inability to perceive motion Walsh, Ellison, Battelli and Cowey did this with electromagnets, subjects could still tell what objects were not which was moving Form from Motion Perception of movement can even help us perceive 3D forms Johansson demonstrated how much info we can derive from movement: dressed actors up in black with lights on certain parts of body and filmed them in dark doing things. People who watched could only see the lights but could tell the movements were by humans and what the movements were Subsequent studies showed the gender could even be told Grossman- functional study show when people viewed form from motion video, bilateral region , specifically ventral bank of posterior end of superior temporal sulcus became active. More activity observed on right hemisphere no matter which visual field video presented to , even if the video was just imagined Visual Agnosia can still perceive actions and recognize the actions VISUAL PERCEPTION OF OBJECTS Figure and Ground Classify most of what we see as object or background Objects have particular shape and location in space, background are formless and mainly help us judge location of objects we see in front of them Figure: visual stimulus perceived as self contained object Ground: visual stimulus perceived as formless background against which objects are seen Figure/Ground not intrinsic property but depends on what you are looking at Boundary one of most important aspects of form perception Sharp distinct change in brightness/color/texture we perceive an edge, if continuous, assumed boundary Gestalt Laws of Perceptual Organization Presence of boundary not necessary for form Illusory contours: lines that don’t exist Gestalt psychology: asserts perception of objects is produ
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