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M2 - vision (the retina
neurons
multi-layered
cover rear, inner…
M2 - vision
the retina
- neurons
- multi-layered
- cover rear, inner wall of eyeball
1. visual receptors
- cells that respond to light
- spontaneous, graded release of NT
rods
- larger, in periphery
- convergent
- sensitive to motion and low light
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high sensitivity in dim light - insufficient release of ex. NT from bipolar, but it converges on 1 ganglion w/ sum of NT
so AP happens
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cones
- smaller, dispersed throughout retina
- divergent
- sensitive to color and detail
fovea
- small central area of high concentration of cones only
- high detail resolution
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1.5 horizontals (interneurons)
- graded potentials
- inhibtory NTs
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2. bipolar cells
- spontaneous firing
- graded potentials
- release excitatory NT
2.5 amacrines (interneurons)
- graded potentials
- inhibitory NT
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in bright light ->
- receptors shut down
- bipolars spont. release excit. NT for ganglions to pass firing threshold
- ganglions send message: bRIGHT LIGHT!
in dim light ->
- receptors spon. release is decreased (graded reaction
- bipolars somewhat inhibited, release some ex NT
- ganglions sometimes triggered
in dark light ->
- receptors release enough inhibitory NT
- prevents bipolars from triggering ganglions
- ganglions =/= fire -> @ brain: NO LIGHT!
3. ganglions
- action potentials
- axons of ganglions form
optic nerve
blind spot/optic disk
- optic nerve leaves eye and blood vessels enter/leave
- no receptors here
information pathways
dorsal stream
magnocellular pathway, where/how, parietal
magnocellular ganglions
- large cells w/ large RFs & transient (shorter) response
- motion, localization = RODS
- navigate/manipulate environment
- integrated w/ somatosensory info & premotor/motor cortexes
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PATH:
- rods/cones in retina periphery
- bottom 2 layers of LGN in thalamus
- V1 -> V2
- MT (medial temporal cortex)
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mst (medial superior temporal cortex
- responds best to optic flow
- environment move toward/from observer
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sts (superior temporal sulcus)
- in lateral temporal lobes
- responds to biological motion
ventral stream
parvocellular pathway, who/what, temporal
- color, detail = CONES
- shape, texture
- integrated with language
centers for naming identified objects
to determine detail, shape, texture
(V3 combines both)
simple cells
- in V1
- responds best to particular lines of orientation
- responds well to sine wave gradients
complex cells
- in V2
- responds best to moving lines of orientation
high spatial frequency
- sensitive to high # of dark-light changes
- high SFs for detail
parvocellular ganglions
- small cells w/ small RFs & sustained response
PATH:
- cones/fovea
- top 4 layers of LGN in thalamus
- V1 -> V2 -> V3-> V4 in occiptial
- IT
IT (inferior temporal cortex)
- active in recognition of complex stimuli
(hand, geometric shape, gaze)
- cells in fusiform gyrus (temporal) respond to faces
prospagnosia
- inability to recognize familiar faces
color
- spectra of visible light
- wavelengths ranging from 350nm - 700nm
trichromatic color vision
- 3 cone types (S, M, L / B, G, R)
- respond to its corresponding opsin
color opponency
- trichromatic system recoded for R/G, B/Y, B/W
- opponent cells (ganglion & LGN)
color constancy
- recognize colors under changing light conditions
retinex theory
- retina + cortex = visual cortex (V4)
- "subtracts" shared tint
visual cortex
columnal organization
column
- respond to the same preferred stimulus
- 6 layers of cells
hypercolumn
- set of cells w/ same RF
- include set of orientations + blobs (color processing)
retinotopic map
- topological map that preserves spatial relationships found on retina
magnification factor
- cortical cells w/ small RGS fill a dis-proportionally large are of the visual projection areas
- ex: fovea = .01% of retina, 8% of V1
center-surround RFs
For example, if a Ganglion Cell has an Excitatory Center-Inhibitory Surround RF…
- light the center of its RF = the ganglion is excited
(by activity of bipolars from that area)
- light the center more than the surround = ganglion more likely to reach threshold or to fire at higher rate
- light the surround more than center = ganglion less likely to reach threshold or to fire at lower rate
- light the surround of its RF = the ganglion is inhibited
(by activity of amacrines from that area)
lateral inhibtion
- helps create center-surround RFs
- exaggerate differences btwn. areas of dark/light
simultaneous contrast
- the illusions
- ex: gray on white vs gray on black
uni-directional lateral inhibition
- direction of inhibition in direction-sensitive motion circuit
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