PSYC 101 Chapter Notes -Two-Streams Hypothesis, Visual Agnosia, Parahippocampal Gyrus

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Published on 22 Apr 2013
School
UBC
Department
Psychology
Course
PSYC 101
Professor
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)
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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
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Document Summary

Perception- a rapid, automatic, unconscious process by which we recognize what is represented by the info provided by our sense organs. 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. 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.

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