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Flashcards in vision I-III Deck (62)
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1

components of light

Wavelength: corresponds to color. Amplitude: corresponds to intensity or brightness

2

Components of eye that contribute to focusing/refractive power

cornea: 2/3. Lens: 1/3

3

size of pupil is controlled by

ciliary muscles

4

what is the blind spot

the optic disc contains no photoreceptors

5

Retinal neurons and flow of information

Photoreceptors (rods and cones) capture photons of light and convert (transduction) them to an electrical signal (change in membrane potential), which is passed synaptically to bipolar cells and horizontal cells, and then to the output cells of the retina, the ganglion cells.

6

Location of retinal neurons

From front (where light enters) to back: ganglion cells > bipolar and horizontal cells > cones and rods. This set up means that light must pass through all of the other cells to get to the photoreceptors, then the signal is passed "backwards" to the optic nerve. The other retinal cells are nearly transparent, making this possible

7

what is the fovea

region where acuity is greatest- cones are concentrated here and only work well in bright light

8

compare the anatomy of rods and cones

rods: Have intracellular membranous sacks/disks containing membrane proteins in the outer segment, and mitochondria/nucleus in inner segment, then a synaptic terminal. Longer than cones. Cones: Outer segment contains surface membrane infoldings, and the rest is the same as rods

9

What are the major steps in phototransduction

Photons are absorbed by rhodopsin which is membrane bound in disc > G protein, transducin, activation > cGMP phosphodiesterase activation > cGMP is degraded > nonselective cation channels in surface membrane close > cell is hyperpolarized

10

What is unique about ganglion cells

Only the ganglion cells make action potentials; all of the other retinal cells communicate by graded changes in membrane potential, which alter the rate of exocytosis of neurotransmitters in a graded fashion. Ganglion cells fire action potentials spontaneously in dark (cell is depolarized)

11

What is a receptive field

The best stimulus to get a sensory neuron to change action potential firing rate

12

What are the receptive field properties of retinal ganglion cells?

Donut shaped receptive fields. Two types: "On" center ganglion cells are excited by shining light in center (and inhibited by light in periphery). "Off" center ganglion cells are turned off by shining light in center (and turned on by light in periphery)

13

Compare receptive field in peripheral vs central vision

receptive field is 100 times larger in peripheral vision than in central vision

14

When is central vs peripheral vision used

Central vision (fovea, high conc of cones) is used in daylight wth high resolution. Peripheral vision (rods) is used in dark, with poor resolution.

15

compare numbers of rods and cones

100 million rods, 8 million cones

16

What neurotransmitter do photoreceptors release

glutamate

17

What action does glutamate have on bipolar cells

Excitatory (Off center) or inhibitory (on center), due to different receptor types

18

will a bi polar cell with inhibitory glutamate receptors be inhibited or excited in the dark? In the light?

dark: tonically inhibited. Light: turned on

19

function of horizontal cells

Mediate receptive field: They behave as though they have excitatory receptors for glutamate from photoreceptors, and make inhibitory synapses on neighboring photoreceptors in receptive field.

20

center vs sourround in receptive field

the center is represented by rods which are connected to ganglion cells directly via bipolar cells The surround represents rods which are connected to bipolar cells, then horizontal cells, then the ganglion cell

21

In receptive field, which synapses are always excitatory

The surround photoreceptor to horizontal cell synapses, and bipolar cell to ganglion cell synapses

22

In receptive field, which synapses are always inhibitory

horizontal cell to photoreceptor synapses.

23

In receptive field, which synapses may be either excitatory or inhibitory

field center photoreceptor to bipolar cell

24

What is the rebound response in inhibitory area

When light is shone on an inhibitory area (center or surround), there will be a rebound response when the light is turned off. This is from the abrupt removal of inhibition

25

compare the responses in on-center vs off-center ganglion cells when light shines in the middle of receptive field, outside of receptive field only, small portion of outer receptive field, entire field

Middle only: on center fires Aps, off center has no Aps. Outside only: on center doesn’t fire AP's, off center fires Aps. Small portion of outer field: on center fires one AP, off center first few AP's. Entire field: on center fires a few AP's, off center fires a few AP's

26

Do ganlgion cells repond to absolute or relative light

relative only

27

Describe the decussation of optic nerve fibers

At chiasm, axons from nasal half of each eye crosses midline then continues as optic tract to lateral geniculate nucleus of thalamus.

28

Does the LGN represent the ipsilateral or contralateral visual field

contralateral- Fibers which see the right field of view (but are located on the left side of each eye) synapse in the left lateral geniculate nucleus

29

List the pathway from the eyes to the brain

optic nerve > optic chiasm > optic tract > LGN > optic radiation > visual cortex

30

pattern of optic tract projections to cortex

retinotopic projection- lower half of each retina projects to lower half of each visual cortex, etc.

31

lateral geniculate nucleus layout

Layers 1,4,6 receive input from contralateral eye, while layers 2,3,5 receive input from ipsilateral eye. Additionally, layers 1 and 2 receive inputs from magnocellular ganglion cells and layers 3-6 receive input from parvocellular ganglion cells

32

What kinds of stimuli do parvocellular ganglion cells detect

Object visioin- color, form, and detail. High acuity, small receptive fields, not responsive to motion, color vision from cones

33

What kinds of stimuli do magnocellular ganglion cells detect

spatial vision- motion and depth. Low acuity, large receptive fields, responsive to motion and no color vision (input from rods)

34

Pathway of magnocellular and parvocellular systems

Segregated at LGN, travel in parallel but separate pathways through visual cortical areas

35

What are the receptive field characteristics of cortical simple and complex cells? How are these receptive field properties achieved by synaptic inputs from lower order cells?

Simple: stimulated by a narrow line of light/ on area that is flanked on each side by off areas. Diffuse light is entirely ineffective. The spatial position and orientation of the line is also crucial. Orientation columns, which are up and down like the hypercolumns, are organized as pinwheels horizontally, such that each orientation column has the same orientation, but adjacent orientation columns have a slightly different orientaion.

36

Draw the major features of a hypercolumn.

In primary visual cortex (V1), these are 1mm regions layered from layer 1 at surface of brain to layer 6 at border btw grey and white matter. Each column has two ocular dominance columns (one half for each eye), with about half of the cells being binocular and receiving inputs from both eyes (these are the cells at the interface btw left and right eye). Each column also has pinwheels radiating from central blobs which orient lines in the visual field. The blobs are central regions of the columns which handle color

37

Where do LGN axons terminate in the primary visual cortex

layer 4

38

What are the receptive field characteristics of cortical simple cells

simple cells have straight line fields . Can have on centers and off flanking lines, or opposite. Diffuse light is entirely ineffective. The spatial position and orientation of the line is also crucial. Orientation columns, which are up and down like the hypercolumns, are organized as pinwheels horizontally, such that each orientation column has the same orientation, but adjacent orientation columns have a slightly different orientaion.

39

How are simple cell receptive fields achieved by synaptic inputs from lower order cells?

Hierarchical processing: Several cells with similar but
spatially offset receptive fields converge on a higher order cell to create an altogether new type of receptive field. Several overlapping ON-center LGN and ganglion cell receptive fields line up along a line, and many of these cells will converge on one cortical cell in V1. The cortical cell will have a receptive field that is the sum of the LGN cells receptive fields. Hierarchical processing: Several cells with similar but
spatially offset receptive fields converge on a higher order cell to create an altogether new type of receptive field. Several overlapping ON-center LGN and ganglion cell receptive fields line up along a line, and many of these cells will converge on one cortical cell in V1. The cortical cell will have a receptive field that is the sum of the LGN cells receptive fields.

40

function of binocular cells

receive inputs from LGN from both eyes. The receptive fields of the two eyes are identical (same orientation, same region of retina, same width of line, same on off organization). Mediate depth perception

41

What are the receptive field characteristics of cortical complex cells?

Complex cells have receptive fields like simple cells except they abstract for position, meaning position of the line is not as important

42

How are complex cell receptive fields achieved by synaptic inputs from lower order cells?

Convergence of several simple cells whose positions are slightly offset. The converging simple cells on the complex cell make excitatory synapses, and any single simple cell can cuase the complex cell to fire.

43

Where are simple and complex cells located

In the same hypercolumns- LGN axons terminate in layer 4, creating simple cells, then simple cells send axons up and down the same hypercolumn to higher and lower cortical layers, creating complex cells.

44

How does one hypercolumn relate to its neighbors

Because of the retinotopic projection, neighboring hypercolumns attend to neighboring retinal regions. The ocular dominance columns line up in stripes. The orientation column pinwheels spin out over the
cortical surface, interconnected with neighboring hypercolumns Because of the retinotopic projection, neighboring hypercolumns attend to neighboring retinal regions. The ocular dominance columns line up in stripes. The orientation column pinwheels spin out over the
cortical surface, interconnected with neighboring hypercolumns

45

For photoreceptors List: location, diffuse light, receptive field shape, orientation selective, binocular driven and position sensitive?

retina, diffuse light ok, tiny spot, not orientation selective, not binocularly drive, yes position sensitive

46

For ganglion cells List: location, diffuse light, receptive field shape, orientation selective, binocular driven and position sensitive?

retina, diffuse light so-so, donut, not orientation selective, not binocularly drive, yes position sensitive

47

For simple cells List: location, diffuse light, receptive field shape, orientation selective, binocular driven and position sensitive?

cortex, diffuse light no, bar, yes orientation selective, yes binocularly drive, yes position sensitive

48

For complex cells List: location, diffuse light, receptive field shape, orientation selective, binocular driven and position sensitive?

cortex, diffuse light no, edge, yes orientation selective, yes binocularly drive,no position sensitive

49

Types of cones

blue, green and red

50

What are color-opponent ganglion cells?

Ganglion cells with receptive fields that are partial to particular colors. In fovea, most biopolar cells are connected directly to one kind of cone (ie red) and indirectly, via horizontal cells, to cones with different color (ie green). This would create a red-ON center and Green-OFF sourround receptive field, for example, which is passed to the ganglion cells. All combos of red-green and blue-yellow on-off fields exist.

51

Where is color processed in the cortex?

ventral pathway - ending in temporal lobe. Blob cells play role

52

What is parallel processing

For different dimensions of the image (e.g. shape, color, motion, spatial information) we have
analogous systems that use hierarchical processing to construct higher levels of representation in
their dimensions. This means that dissimilar dimensions (color and form) are analyzed by separate, but parallal, neural systems

53

What is hierarchical processing

successive synaptic integration of highly specific synaptic inputs to construct higher and higher levels of representation of the retinal image

54

dorsal vs ventral parallel vision pathways and functions

The Dorsal Pathway travels from V1 dorsally through thick stripe of V2, then onto V5 (MT, middle temporal) and finally to the parietal lobe and is responsible for spatial vision, including motion and depth perception. The Ventral Pathway travels ventrally from V1 to stripe and interstripe of V2, then to V4 and finally to the temporal lobe and is responsible for object vision, including color, form, and pattern vision

55

Stripe regions of V2 receive input from where?

blobs in V1- only care abour color. They do not have center-surround anatomy but rather a uniform area within which light of one color excites cell and light of another color inhibits cell

56

Categories of ocular dominance

Category 1 cells: monocular, driven only by the eye contralateral to cortical cell. Category 4: binocular, driven equally by both eyes. Category 7: monocular, driven only by eye ipsilateral to cortical cell. All other categories contain some degree of binocular cells

57

How is cortical wiring of binocularly driven cells determined

its genetic- when cell receives inputs from both eyes, the receptive field positions and orientations in the two eyes are identical from birth

58

what happens with monocular deprivatioin

if one eye of a kittne is closed for even a few days, the cortical cells lose all connections to the deprived eye andalmost all cells become monocular. If one eye of a cat is closed for a few days, there is no change. This suggested disuse atrophy as an explanation for visual cortex wiring

59

Critical/sensitive period in cortex development

Period of time when connections can be altered by visual experience. Lasts about 3 months in kittens, with maximal sensitivity at 4-6 weeks. In humans, it is about 2-3 years Once the connections are lost, they can not be recovered.

60

What happens with binocular deprivation

When both eyes are closed in a kitten for a short amount of time, the primary visual cortex is mostly normal, about 50% binocular cells, but the cats were behaviorally blind (higher order visual cells were completely disrupted). Showed competition btw converging synaptic inputs from two eyes is mechanism of wiring in visual cortex

61

What happens with alternating monocular deprivation

The first eye that is deprived will lose cortical connection. When the first eye is then kept open and the second eye is then deprived, the first eye recovers and the newly deprived eye loses its connections with the cortex. This suggests that there is active suppression by the active eye.

62

What happens when a strabismus is produced in one eye

If one extraocular muscle (medial rectus) is cut, this produces a strabismus so that the two eyes look at different parts of the world. This results in very few binocular cells, with all cells being driven exclusively by one eye or the other. This suggests that cells that fire together wire together