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Lecture 5

PSYB65H3 Lecture Notes - Lecture 5: Visual Agnosia, Optic Chiasm, Prosopagnosia

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Ted Petit

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PSYB65- Lecture 5
Sensory systems
Retinas are divided anatomically right down the middle; a visual field is what a person can see;
visual fields are split into two half-fields of left and right; consists of nasal portion of retina is the
half that is closest to one’s nose, and the other portion of the retina that is to the outside of the
head are referred to as the temporal portion; the nerves are divided exactly at the middle of the
eye; the two separate nerves leaving your eyes are referred to as the optic nerves; the point at
which they come together is referred to as the optic chiasm; they separate apart again and head
towards the brain which is referred to as the optic tract; the nasal portion of the retina crosses,
while the temporal potions stay on the same side when heading back to the left and right
cortex; something in your right visual field crosses over to the left cortex, and everything in your
left visual field goes to your right cortex; macular sparing refers to the little bit in the center that
you can see when the damage is near the cortex
The more central the neuron (associational vs. primary), the more advanced the area of the
brain, the more complex is the image that excites it; the more complex images excite it; for
example in the retina, a neuron respond to a spot of light falling on its receptive field; by the
time you get to area 17,lines of light oriented in a specific direction; when you get to
associational cortex, these neurons integrate information from other neurons, and it responds
to complex images like a face, hand
Complete damage to the right occipital cortex (major stroke), you will get complete blindness in
the left visual field except for the tiny little bit of macular sparing; smaller amount of damage in
the right occipital cortex, you get partial blindness in the opposite visual field; a little tiny area of
damage produces something called a schitoma which is a small area of blindness in the visual
field; lesions to higher visual processing areas (20,21) cause problems of perception of complex
visual material such as faces, patterns
Visual agnosia is the inability to recognize objects for what they are; cannot figure out what to
do with it or what it is used for; typically caused by damage to the left occipital lobe
Prosopagnosia is when a person cannot recognize faces; also may not be able to recognize other
complex objects; typically from bilateral damage from areas 18 and 19
Colouragnosia is when the colours seem less intense to people, and in extreme cases, they can
lose colour vision completely after a stroke; typically caused by bilateral damage to the occipital-
temporal area
Somatosensory information comes into the post central gyrus; damage to this area results in
altered sensory thresholds; cannot distinguish as precisely as people without damage as to
where they are being touched exactly or how many points are touching them;
Stereognosis is where a person cannot tell you what a shape is; a person is impaired in their
tactile/feeling; cannot distinguish shapes will enough because their sensation is impaired; they
also have a hard time in placing or locating objects in space, particularly true of their own limbs
Motor systems
If there is a very severe stroke involving a large area in most of the motor strip, you will get loss
of all voluntary movement in the opposite side of the body; initially referred to as a flaccid
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