The Visual System I-III Flashcards
1
Q
Characteristics of visual field deficits
A
- due to an occipital lesion
- = absence of vision
2
Q
Characteristics of visual agnosia
A
- due to a variety of occipital lesions leading to damage @ the occipitotemporal (“what” stream) or occipitoparietal (“where” strem)
- = visual image is seen normally, but is not adequately reconized
- e.g. object agnosia, face agnosia, etc.
3
Q
Characteristics of Retina
A
- retina = sensory organ of the eye
- retinal ganglion cells = outsput neurons ==> group @ optic disk ==> optic nerve
4
Q
Neurons @ retina
A
-
Photoreceptors = rods and cones ==> capture light photons and transduce them to electrical signals
- cones = color vision
- rods = color insensitive, work best in dim light
- Photoreceptors ==> bipolar cells and horizontal cells ==> ganglion cells
5
Q
Characteristics of phototransduction
A
- phototransduction = process of conversion of light to changes in membrane potential (by photoreceptors)
- light absorption ==> hyperpolarization of photoreceptors
- photon absorption:
- photorecptors pack membrane proteins very densely
- Vitamin A (pigment) attached to memphrane proteins absorb photons
- membrane protein ==> G-protein activiation ==> decreased cGMP ==> closing of NSC channels
6
Q
Receptive field properites of retinal ganglion cells
A
- ganglion cells = only cells that make APs
- receptive field = best stimulus to get cell to change AP firing rate
- “on” center ganglion cells =
- excited by light shining in centers
- inhibited by light in periphery
- “off” center ganglion cells =
- excited by light shining in periphery
- inhibited by light shining in center
7
Q
Rules of terinal processing ==> ganglion receptive field types
A
- Photoreceptors are hyperpolarized by light ==> decreased NT release
- Photoreceptors release glutamate ==>
- OFF-center bipolar cells = excited
- ON-center bipolar cells = inhibited
- Bipolar receptors ==> excitation @ ganglionic cells
8
Q
Mediation of the ganglion cell receptive field surround
A
- mediated by horizontal cells
- horizontal cells behave as though excited by glutamate released from photoreceptors + make inhibitory synapses on neighboring photoreceptors in field center
9
Q
Info representate @ Lateral geniculate nucleus
A
- optic tracts (post-optic chiasm) ==> LGN
- LGN represents the contralateral visual field
10
Q
Characteristics of parvocellular system
A
- made up of parvocellular ganglion cells
- responsible for object vision: color, form, detail
- high acuity
- small receptive fields
- not responsive to motion
- color vision (inputs from cones)
11
Q
Characteristics of magnocellular system
A
- made up of magnocellular gangilion cells
- responsible for spatial vision: motion and depth
- low acuity
- large receptive fields
- responsive to motion
- no color vision (input from rods)
12
Q
Characteristics of hypercolumns
A
- hypercolumns = microregions of V1 (primary visual cortex)
- 6 layers (layer 1 = surface; layer 6 = bottom, borders white matter)
- LGN axons terminate @ layer 4
- each colum divided in half ==> one half for each eye = “ocular dominance columns”
- cells near the border of the ocular dominance colums = binocular
- blobs = @ central regions of hypercolumns = handle color
13
Q
Receptive field characteristics of simple cells
A
- receptive fields = narrow line of light covering all of center ON areas without intruding on the flanking OFF areas
- some cells are opposite ==> OFF-center w/ ON-surround
- spatial positition and oreination of line is crucial
- various simple cells attuned to different orientations ==> orientation columns organized as pinwheels
14
Q
Hierarchical processing definition
A
- several cells w/similar but spatially offest receptive fields converge on a higher order cell to create an altogether new type of receptive field
- e.g. receptive fields of lower order cells (ganglion cells/LGN neurons) are slightly offset ==> all synapse on a single simple cell ==> sum to form the receptive field of higher order cell (simple cell)
15
Q
Characteristics of binocular cells
A
- found near borders of ocular dominance columns
- receive inputs from LGN from both eyes
- receive virtually identical visual fields
- sensitive to and mediate depth perception
16
Q
Receptive field characteristics of complex cells
A
- receptive field = ~ simple cells + abstract for position (i.e. position does not have to be specifically located in visual field)
- especially good at detecting lines/edges moving across visual field
- created by convergence of several simple cells whose positions are slightly offset onto complex cell
17
Q
Receptive field shapes: photoreceptor, ganglion cell, simple cell, complex cell
A
- photoreceptor = tiny spot
- ganglion = donut
- simple = bar
- complex = edge
18
Q
Location: photoreceptor, ganglion cell, simple cell, complex cell
A
- photoreceptor = retina
- ganglion = retina
- simple cell = cortex (layer 4 of hypercolumns)
- complex cell = cortex
19
Q
Response to diffuse light: photoreceptor, ganglion cell, simple cell, complex cell
A
- photoreceptor = ok
- ganglion = so-so
- simple cell = no
- complex cell = no
20
Q
Orientation selective?: photoreceptor, ganglion cell, simple cell, complex cell
A
- photoreceptor = no
- ganglion = no
- simple cell = yes
- complex cell = yes
21
Q
Binocularly driven?: photoreceptor, ganglion cell, simple cell, complex cell
A
- photoreceptor = no
- ganglion = no
- simple cell = yes
- complex cell = yes
22
Q
Position sensitive?: photoreceptor, ganglion cell, simple cell, complex cell
A
- photoreceptor = yes
- ganglion = yes
- simple cell = yes
- complex cell = no
23
Q
Perception of color
A
- 3 types of cones = blue, green, red
- considerable overlap in wavelength sensitivities
- relative activities of of 3 cone types ==> encode colors
- color-opponent ganglion cells help perceive color
24
Q
Characteristics of color-opponent ganglion cells
A
- cones of different color preferences converge @ retina to produce various ganglion cells
- bipolar cells @ fovea connected to one color cone (e.g. red) @ field center and different color cone (e.g. green) @ field surround ==> RED ON-center, GREEN OFF-surround ==> ganglion cells
- Color opponent ganglion cells =
- red-green opponents
- blue-yellow opponents
25
Parallel processing of visual info
* requirement that dissimilar info (e.g. color and form) must be analyzed by separate, but parallel, neural pathways
* eventually brain unites all information into a unified perception
26
Primary parallel pathways ==\> ascending visual systems
* Dorsal pathway
* travels from V1 dorsally to parietal lobe
* responsible for spatial vision:
* motion
* depth perception
* Ventral pathway
* travels ventrally from V1 to temporal lobe
* responsible for object vision:
* color
* form
* pattern vision
27
Central color processing
* primary: occurs @ blobs
* blobs = sense COLOR ONLY, not spatial information
* receive inputs from many color-opponent inputs from a location @ retina ==\>
* red hues=excitatory and green hues=inhibitory
* secondary: more advanced color processing continues in higher cortical areas
28
Ocular dominance definition
* measure of relative synaptic input to a cell from each eye
* seven categories:
* category 1 = contralateral eye (to cortical cell) only
* category 4 = equally from both eyes
* category 7 = ipsilateral eye only
29
Characteristics of senstivie/critical period
* = period of time when the connections (between sensory cells and cortical cells) can be altered by visual experience
* e.g. kitten closes one eye, cortical cells lose connections to deprived eye (same experiment has no effect on adults)
30
Result of monocular deprivation experiment
* kitten closes single eye (i.e. deprived of any form visual stimuli) for a few days
* all cortical connections to that eye are lost
* all cells are monocular or lack any innervation
31
Results of binocular deprivation experiment
* kitten closes both eyes ==\> primary visual cortex = ~normal (activity can be induced)
* cortex still contains ~50% binocular cells
* higher order visual cells disrupted ==\> cats become behaviorally blind
* indicates that development of ocular dominance occurs via "competition" between inputs from the two eyes
32
Results of strabismus experiment
* cut kitten's medial rectus of one eye==\> lateral deviation of eye
* ==\> very few binocular cells
* all cells were driven by one eye OR the other (approx equal numbers of each)
* probably due to dysynchronous inputs b/c at any given moment, each eye was looking at a different part of visual world
33
Result of monocular deprivation experiment
* kitten closes single eye (i.e. deprived of any form visual stimuli) for a few days
* all cortical connections to that eye are lost
* all cells are monocular or lack any innervation
34
Results of binocular deprivation experiment
* kitten closes both eyes ==\> primary visual cortex = ~normal (activity can be induced)
* cortex still contains ~50% binocular cells
* higher order visual cells disrupted ==\> cats become behaviorally blind
* indicates that development of ocular dominance occurs via "competition" between inputs from the two eyes
35
Results of strabismus experiment
* cut kitten's medial rectus of one eye==\> lateral deviation of eye
* ==\> very few binocular cells
* all cells were driven by one eye OR the other (approx equal numbers of each)
* probably due to dysynchronous inputs b/c at any given moment, each eye was looking at a different part of visual world
36
Characteristics of senstivie/critical period
* = period of time when the connections (between sensory cells and cortical cells) can be altered by visual experience
* e.g. kitten closes one eye, cortical cells lose connections to deprived eye (same experiment has no effect on adults)
37
Characteristics of visual field deficits
* due to an occipital lesion
* = absence of vision
38
Characteristics of visual agnosia
* due to a variety of occipital lesions leading to damage @ the occipitotemporal ("what" stream) or occipitoparietal ("where" strem)
* = visual image is seen normally, but is not adequately reconized
* e.g. object agnosia, face agnosia, etc.
39
Experiments that reverse the impact of abnormal development/deprivation
* blocking optic nerves w/tetrodotoxin ==\> blindness
* electrical stimulation bilaterally and synchronously beyond the TTX block ==\> normal binocular development
* electrical stimulation unilaterally/asynchronously ==\> monocular development/
