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PSYA01H3 Chapter Notes -Visual Cortex, Torsten Wiesel, Visual Agnosia

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Steve Joordens

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Perception is the (rapid, automatic, and unconscious) process by which we recognize what
is represented by the information provided by our sense organs.
Brain Mechanisms of Visual Perception
Perception takes place in the brain. Optic nerves send info to the thalamus primary visual
cortex (occipital lobe) in turn neurons in the primary visual cortex send info to 2 levels of
the visual association cortex. The higher levels of the perceptual process interact with
memories: The viewer recognizes familiar objects and learns the appearance of new,
unfamiliar ones.
David Hubel and Torsten Wiesel conducted an experiment using microelectrodes. They
connected these microelectrodes to regions of the visual system of certain animals (to
detect action potentials produced by individual neurons). The animals were anaesthetized
and this causes them to be unconscious but their nerve impulses still work, as they should.
When they moved the stimuli around they located the largest effect on the electrical activity.
They concluded that the geography of the visual field is retained in the primary visual
Receptive field is the portion of the visual field in which the presentation of visual stimuli
will produce an alteration in the firing rate of a particular neuron.
The visual association cortex
The first level of the visual association cortex contains subdivisions and each contains a map
of the visual scene. One of these subdivisions is responsible for the width and orientation
and gives the perception of shapes. Another receives info about movements and another
about colour. Three-dimensional perception takes place on the second layer. Partial lobe is
responsible for the location of objects.
Effects of Brain Damage on Visual Perception
When the primary visual cortex is damaged a person becomes
blind in some portion of the visual field.
Although a person loses sight they are
still able to perceive objects and backgrounds
Perception takes place in the visual association cortex.
Damage to the visual association cortex doesn’t prevent
one from seeing fine details.
Achromatopsia is the inability to discriminate among different hues; caused by damage to
the visual association cortex.
Damage to a subregion of the visual association cortex makes it difficult for a person to
perceive movement.
If the visual association cortex in the parietal lobe is damaged they will have difficulties with
their abilities to keep track of the location of objects, this is known as Balint’s Syndrome.
Visual agnosia is 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 is a form of visual agnosia characterized by difficulty in the recognition of
people’s faces; caused by damage to the visual association cortex.
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