Lecture 22: Vision 3 What, where and how

Question 1

The two streams of visual processing

Answer 1

both streams process object shape; but processing is not identical: visual illusion affects perception and not actions –> ventral system; ventral does better complex shape-analysis, dorsal is limited

dorsal system can’t align; parietal system (intact in DF) has limited ability to process shape, however dorsal system can analyze simple contours when salient

Question 2

rods: duplex retina

Answer 2

1 of 2 kinds of photo-sensitive receptors on the retina; can be activated by a little bit of light –> individual sensitivity high; extensive convergence onto bipolar and ganglion cells; outcome: low resolution, under conditions of low illumination; scatter on other regions of the retina –> peripheral vision ; part of duplex retina

pooled activity –> good sensitivity, bad resolution (can’t see details), under low light you can still see

Question 3

P cells: duplex ganglion cells

Answer 3

duplex ganglion cells; small, slower, sustained response; receive input from small # of photoreceptors; good spatial resolution; poor light sensitivity –> need a lot of light to fire

Question 4

Ventral visual stream

Answer 4

can be divided into face and other objects areas; occipito-temporal lobe; object shape recognition; “what”; (on brain photo, goes down)

perception/objects: faces, scenes, human bodies, and other objects

Question 5

Evidence that ventral and dorsal streams are seprarable

Answer 5

1. evidence from monkey studies:
   - monkeys with damage to occipito-temporal lobe were OK with spatial task, but impaired at object task (couldn’t tell what shapes were present, but could tell where they were) and monkeys with damage to posterior parietal cotex were OK with object task, but impaired at spatial task (couldn’t tell which object was closer, but could recognize new objects from old objects)
2. bilateral posterior parietal damage: evidence from human patients; can recognize objects; cannot point to it, or describe its location –> patient with Balint’s syndrome can identify one object at a time, but cannot reach for that object –> shape perception and reaching rely on different brain regions
3. bilateral inferior occipito-temporal damage: can localize objects, cannot recognize what they are

Question 6

patient DF: whats impaired, what’s preserved, what does this mean

Answer 6

carbon monoxide poisoning–> damage to occipito-temporal lobe; low level visual functions normal (occipital cortex OK –> see colors); adept at tactual recognition; cannot perceive shape

can identify objects by touch and people by voice; when given a model of an image, cannot copy image; can draw image from memory, but can’t identify after

has visual agnosia

may be evidence for different functions of ventral and dorsal streams –> both “what” (dorsal also “how” do you perform an action)

Question 7

Visual illusion: perception is illusory, action is not

Answer 7

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Question 8

Limits to shape processing by the dorsal stream

Answer 8

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Question 9

cones: Duplex retina

Answer 9

1 of 2 kinds of photo-sensitive receptors on the retina; cluster near the fovea –> best visual acuity; limited convergence –> need more light in order for them to fire; high resolution image, provided there is ample light

no pool –> good resolution, bad sensitivity

Question 10

M cells: Duplex ganglion cells

Answer 10

duplex ganglion cells; large, faster, transient response; recieve input from large # of photoreceptors; good light sensitivity –> don’t need much light to fire; poor spatial resolution

Question 11

dorsal visual stream

Answer 11

occipito-parietal; object location, spatial attention; “where”

action/location: reaching, grasping, eye movement