The Visual System I-III

Question 1

Characteristics of visual field deficits

Answer 1

• due to an occipital lesion
• = absence of vision

Question 2

Characteristics of visual agnosia

Answer 2

• 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.

Question 3

Characteristics of Retina

Answer 3

• retina = sensory organ of the eye
• retinal ganglion cells = outsput neurons ==> group @ optic disk ==> optic nerve

Question 4

Neurons @ retina

Answer 4

• 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

Question 5

Characteristics of phototransduction

Answer 5

• 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

Question 6

Receptive field properites of retinal ganglion cells

Answer 6

• 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

Question 7

Rules of terinal processing ==> ganglion receptive field types

Answer 7

• 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

Question 8

Mediation of the ganglion cell receptive field surround

Answer 8

• mediated by horizontal cells
• horizontal cells behave as though excited by glutamate released from photoreceptors + make inhibitory synapses on neighboring photoreceptors in field center

Question 9

Info representate @ Lateral geniculate nucleus

Answer 9

• optic tracts (post-optic chiasm) ==> LGN
• LGN represents the contralateral visual field

Question 10

Characteristics of parvocellular system

Answer 10

• made up of parvocellular ganglion cells
• responsible for object vision: color, form, detail

1. high acuity
2. small receptive fields
3. not responsive to motion
4. color vision (inputs from cones)

Question 11

Characteristics of magnocellular system

Answer 11

• made up of magnocellular gangilion cells
• responsible for spatial vision: motion and depth

1. low acuity
2. large receptive fields
3. responsive to motion
4. no color vision (input from rods)

Question 12

Characteristics of hypercolumns

Answer 12

• 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

Question 13

Receptive field characteristics of simple cells

Answer 13

• 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

Question 14

Hierarchical processing definition

Answer 14

• 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)

Question 15

Characteristics of binocular cells

Answer 15

• 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

Question 16

Receptive field characteristics of complex cells

Answer 16

• 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

Question 17

Receptive field shapes: photoreceptor, ganglion cell, simple cell, complex cell

Answer 17

• photoreceptor = tiny spot
• ganglion = donut
• simple = bar
• complex = edge

Question 18

Location: photoreceptor, ganglion cell, simple cell, complex cell

Answer 18

• photoreceptor = retina
• ganglion = retina
• simple cell = cortex (layer 4 of hypercolumns)
• complex cell = cortex

Question 19

Response to diffuse light: photoreceptor, ganglion cell, simple cell, complex cell

Answer 19

• photoreceptor = ok
• ganglion = so-so
• simple cell = no
• complex cell = no

Question 20

Orientation selective?: photoreceptor, ganglion cell, simple cell, complex cell

Answer 20

• photoreceptor = no
• ganglion = no
• simple cell = yes
• complex cell = yes

Question 21

Binocularly driven?: photoreceptor, ganglion cell, simple cell, complex cell

Answer 21

• photoreceptor = no
• ganglion = no
• simple cell = yes
• complex cell = yes

Question 22

Position sensitive?: photoreceptor, ganglion cell, simple cell, complex cell

Answer 22

• photoreceptor = yes
• ganglion = yes
• simple cell = yes
• complex cell = no

Question 23

Perception of color

Answer 23

• 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

Question 24

Characteristics of color-opponent ganglion cells

Answer 24

• 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

Question 25

Parallel processing of visual info

Answer 25

• 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

Question 26

Primary parallel pathways ==> ascending visual systems

Answer 26

• 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

Question 27

Central color processing

Answer 27

• 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

Question 28

Ocular dominance definition

Answer 28

• 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

Question 29

Characteristics of senstivie/critical period

Answer 29

• = 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)

Question 30

Result of monocular deprivation experiment

Answer 30

• 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

Question 31

Results of binocular deprivation experiment

Answer 31

• 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

Question 32

Results of strabismus experiment

Answer 32

• 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

Question 33

Result of monocular deprivation experiment

Answer 33

• 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

Question 34

Results of binocular deprivation experiment

Answer 34

• 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

Question 35

Results of strabismus experiment

Answer 35

• 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

Question 36

Characteristics of senstivie/critical period

Answer 36

• = 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)

Question 37

Characteristics of visual field deficits

Answer 37

• due to an occipital lesion
• = absence of vision

Question 38

Characteristics of visual agnosia

Answer 38

• 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.

Question 39

Experiments that reverse the impact of abnormal development/deprivation

Answer 39

• blocking optic nerves w/tetrodotoxin ==> blindness
• electrical stimulation bilaterally and synchronously beyond the TTX block ==> normal binocular development
• electrical stimulation unilaterally/asynchronously ==> monocular development/