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Flashcards in The Visual System I-III Deck (39)
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1

Characteristics of visual field deficits

  • due to an occipital lesion
  • = absence of vision

2

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.

3

Characteristics of Retina

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

4

Neurons @ retina

  • 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

Characteristics of phototransduction

  • 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

Receptive field properites of retinal ganglion cells

  • 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

Rules of terinal processing ==> ganglion receptive field types

  • 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

Mediation of the ganglion cell receptive field surround

  • 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

Info representate @ Lateral geniculate nucleus

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

10

Characteristics of parvocellular system

  • 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)

11

Characteristics of magnocellular system

  • 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)

12

Characteristics of hypercolumns

  • 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

Receptive field characteristics of simple cells

  • 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

Hierarchical processing definition

  • 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

Characteristics of binocular cells

  • 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

Receptive field characteristics of complex cells

  • 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

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

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

18

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

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

19

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

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

20

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

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

21

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

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

22

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

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

23

Perception of color

  • 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

Characteristics of color-opponent ganglion cells

  • 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