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PSYC 367 > L16-L20 (Vision cont.) > Flashcards

L16-L20 (Vision cont.) Flashcards

1
Q

4 components of photoreceptors

A
  1. outer segment: stores photopigments
  2. inner segment: makes photopigments
  3. nucleus (outer nuclear layer)
  4. axon (outer plexiform layer)

each synaptic terminal admits 2 horizontal cells and at least 2 bipolar cells

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2
Q

Bleaching in frog retina

photoactivation

A
  • light turned on but no bleaching yet (red due to rod photopigment called rhodopsin)
  • photoactivation: bleaching of photopigment
  • color fades as photopigment bleaches with more time in the light
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3
Q

2 components of rhodopsin photopigment molecules

in rods

A
  • opsin: determines which wavelengths the photopigment absorbs
  • retinal: captures light photons

  • protein (opsin) connected to a light-sensitive chromophore (retinal)
  • no rods in fovea!
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4
Q

2 components of cone photopigment molecules

A
  • chromophore (retinal)
  • protein: photopsin I (L cones), photopsin II (M cones), photopsin III (S cones)

no S cones in the center of the fovea

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5
Q

Which cells do graded potentials occur in?

A

photoreceptors, horizontal cells, bipolar cells

a slow change in membrane potential that varies in size (not all-or-nothing)

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6
Q

Duplex vision

adapting to light and dark conditions

A
  • scotopic: rod-mediated vision in dim light
  • photopic: cone-mediated vision in bright light

  • cones are less sensitive than rods in dim light and rod response is saturated in bright light (rhodopsin bleached)
  • similar to indoor and outdoor film in a camera
  • pupil diameter ranges from 2-8 mm
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7
Q

Adaption and sensitivity of rods vs. cones

A
  • rods: slow adaptation, good sensitivity
  • cones: fast adaptation, poor sensitivty
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8
Q

Dark adaptation

A

increase in sensitivity with time in the dark as we switch from photopic to scotopic vision

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9
Q

Diffuse bipolar cells

A

depolarize in response to an increase in photon catch by photoreceptors due to opening of (+) ion channels

connect to rods or cones in the peripheral retina with 50 photoreceptors converging onto each bipolar cell

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10
Q

ON midget bipolar cells

A

depolarize in response to an increase in photon catch by photoreceptors

connect to cones in the fovea with 1 cone per bipolar cell

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11
Q

OFF midget bipolar cells

A

depolarize in response to a decrease in photon catch by photoreceptors

connect to cones in the fovea with 1 cone per bipolar cell

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12
Q

What happens when bipolar cells depolarize?

A
  • increased glutamate release into synaptic cleft
  • binds to receptors on retinal ganglion cell
  • ion channels open and retinal ganglion cell depolarizes

depolarization in retinal ganglion cells happen with more light in ON bipolar cells and less light in OFF bipolar cells

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13
Q

Which cells do action potentials occur in?

A

amacrine cells and retinal ganglion cells

spike or rapid depolarization

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14
Q

What happens to ON bipolar cells when light is on?

midget or diffuse

A
  • depolarize and increase glutamate release
  • ON-center retinal ganglion cells fire more action potentials
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15
Q

What happens to OFF bipolar cells when light is on?

midget

A
  • hyperpolarize and decrease glutamate release
  • OFF-center retinal ganglion cells fire fewer action potentials
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16
Q

What happens to ON bipolar cells when light is off?

midget or diffuse

A
  • hyperpolarize and decrease glutamate release
  • ON-center retinal ganglion cells fire fewer action potentials
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17
Q

What happens to OFF bipolar cells when light is off?

midget

A
  • depolarize and increase glutamate release
  • OFF-center retinal ganglion cells fire more action potentials
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18
Q

Midget retinal ganglion cells

aka P cells

A
  • project to parvocellular LGN layers
  • comprise 70% of RG cells
  • have small cell bodies, short dendrites, and thin axons
  • synapse with midget bipolar cells
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19
Q

Parasol retinal ganglion cells

aka M cells

A
  • project to magnocellular LGN layers
  • comprise 10% of RG cells
  • synapse with diffuse bipolar cells
  • large cell bodies, long dendrites, thick axons
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20
Q

Bistratified retinal ganglion cells

aka K cells

A
  • project to koniocellular LGN layers
  • comprise 10% of RG cells
  • synapse with diffuse or midget bipolar cells
  • small or large cell bodies and dendritic fields, intermediate axons
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21
Q

What photopigment do retinal ganglion cells contain?

intrinsically photosensitive

in < 5% of RG cells

A

melanopsin on dendrites
* peak absorption at 480 nm (blue wavelengths)
* role in vision may be brightess discrimination and contrast detection
* involved in pupil reflexes and circadian rhythms

e.g. stimulated by blue light that leads to reduced melatonin in the evening, which reduces sleepiness

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22
Q

Receptive field of a retinal ganglion cell

and what determines its size

A

the region on the retina and the corresponding region in visual space in which visual stimuli influence the neuron’s firing rate

size is determined by # of photoreceptors connected to that neuron (through bipolar cells)

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23
Q

Acuity vs sensitivity

A
  • ability to resolve fine detail
  • ability to detect low levels of light
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24
Q

What yields high sensitivty vs. high acuity in the receptive fields of RG cells?

size depends on # of photorepectors connected (through bipolar cells!

A
  • convergence of rods (outside fovea) onto retinal ganglion cells yields high sensitivity
  • lack of convergence in cones (in fovea) onto retinal ganglion cells yields high acuity

Examples
* 1 parasol RG cell has 1 large receptive field
* 3 midget RG cells have 3 small receptive fields

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25
Q

Positive vs. negative regions of the RG cell receptive field

A
  • positive regions are where light leads to excitation (increase in APs)
  • negative regions are where light leads to inhibition (decrease in APs)
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26
Q

Spatial opponency

retinal ganglion cells

A

light elicits opposite responses in the center and surround of the receptive field

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27
Q

What determines the size of the center and surround of the RG cell receptive field?

A
  • center size is determined by photoreceptor connections through bipolar cells
  • surround size is determined by photoreceptor connections through horizontal cells
28
Q

Lateral inhibition

retinal ganglion cells

A

antagonistic neural interaction between adjacent regions of the retina

In ON-center cells:
* biggest response when the light is as big as the center of the receptive field
* light bigger than the center stimulates the inhibitory surround

29
Q

Spatial responses of the 3 kinds of RG cells

A
  • spatial opponency in midget and parasol RG cells
  • bistratified cells only have centers

midget RG cells have smaller receptive fields than parasol and most bistratified RG cells

30
Q

Temporal responses of the 3 kinds of RG cells

A
  • midget RG cells have a sustained response: lasts entire time light is on or off in receptive field
  • parasol RG cells have a transient response: brief response at onset and/or offset of light in receptive field
  • bistratified RG cells have either a sustained or transient response
31
Q

Visual angle

A

measure of retinal image size (α) that depends on the physical size of the object and its distance from the eye

  • larger and closer objects have larger retinal images
  • measured in degrees of arc
32
Q

Snellen ratio

A

smallest resolvable visual angle
* numerator is always 6m or 20ft (optical infinity)
* denominator = size of the smallest letter each person can correctly identify

33
Q

What snellen ratio entails normal acuity?

A

can read line on which bars subtend 1 minute of arc from 6m viewing distance (6/6)

  • 6/3 is better than normal: can read at 6m what a normal person can read at 3m
  • 6/12 is worse than normal: can read at 12m what a normal person can read at 6m
34
Q

How is recognition acuity measured in the snellen chart?

A
  • letters
  • numbers
  • shapes
  • requires identification

  • ability to correctly identify a target
  • depends on photoreceptor properties (density and convergence) and higher cortical factors
35
Q

How is resolution acuity measured in the snellen chart?

A
  • landolt Cs
  • tumbling Es
  • grating acuity
  • doesn’t require identification

  • smallest spatial detail that can be resolved within a target
  • relies on photoreceptor properties (density and convergence)
36
Q

Grating size

resolution acuity

A
  • smallest stripe width that can be resolved in a pattern of stripes
  • measured in cycles/degree, where 1 cycle = 1 black and 1 white stripe

minimum resolvable grating cycle is 1 minute of arc (0.17 degree)

37
Q

When is recognition acuity normal?

A

if you can identify letters with a 1 minute of arc stroke width

resolution acuity is often better than recognition acuity (e.g. only half as good as grating acuity)

38
Q

Foveal viewing

best visual acuity

A

mediated by cones in photopic light conditions
* 0.5 minutes of arc between cone centers in fovea vs. 0.75 minutes of arc between rod centers outside fovea
* no convergence of cones onto RG cells

39
Q

Sinewave grating

contrast sensitivty

A
  • used to assess spatial vision from very course to very fine detail
  • sinusoidal change in intensity across pattern
40
Q

3 parameters of contrast sensitivity in a sinewave grating

A
  1. spatial frequency: # of cycles in 1 degree of visual angle
  2. contrast: luminance difference between light (Lmax) and dark (Lmin) bars
  3. phase: position of grating relative to some landbark (e.g. begins in the middle of a black bar)
41
Q

2 reasons why contrast sensitivity is low at high spatial frequencies

A
  1. optics: optical appartus of eye can’t transmit higher spatial frequencies
  2. anatomy: spacing of cones in fovea limits the highest spatial frequency that can be resolved

if the bars in a sinewave grating are closer together than the distance between 2 cones, you can’t resolve them

42
Q

2 reasons why contrast sensitivity is low at low spatial frequencies

A
  1. physiology: fewer neurons tuned to low spatial frequencies
  2. experimental design: grating contains only a few bars
43
Q

Which frequency range do retinal ganglion cells give the strongest response?

A

medium frequency when the receptive field center matches the width of the grating bars

different neurons are tuned to different spatial frequencies

44
Q

How does contrast sensitivity (and grating acuity) change with age?

A
  • before age 20, improves at all sfs due to longer, thinner, denser cones
  • after age 20, decreases at high spatial frequencies due to changes in the optical properties of the eye
45
Q

Geniculostriate pathway

A
  1. axons from the nasal half of each eye crossover at the optic chiasm; axons from the temporal half of each eye stay on the same side
  2. optic tracts synapse at the LGN
  3. optic radiations project to V1

primary visual cortex (V1) is also called area 17 and striate cortex

46
Q

Which LGN layers do axons from the ipsilateral vs. contralateral eye synapse?

A
  • axons from the ipsilateral eye (temporal half) synapse on layers 2, 3, and 5
  • axons from the contralateral eye (nasal half) synapse on layers 1, 4, and 6
47
Q

Which LGN layers do each kind of retinal ganglion cell synapse?

A
  • midget (P) cells synapse on the parvocellular layers (upper 4 layers of LGN)
  • parasol (M) cells synapse on the magnocellular layers (bottom 2 layers of LGN)
  • bistratified cells synapse on the koniocellular layers (K3 and K4)

there are 6 koniocellular layers, one below every magnocellular and parvocellular layer

48
Q

P-pathway

retinal ganglion cells to LGN to V1

A
  • midget (P) cells synapse on parvocellular layers of LGN
  • neurons from parvocellular layers project to layers 2/3 and 4Cβ of V1
49
Q

M-pathway

retinal ganglion cells to LGN to V1

A
  • parasol (M) cells synapse on magnocellular layers of LGN
  • neurons from magnocellular layers project to layers 2/3, 4B, 4Cα
50
Q

K-pathway

retinal ganglion cells to LGN to V1

A
  • bistratified cells synapse on koniocellular layers of LGN (K3 and K4)
  • neurons from koniocellular layers project to layers 2/3 and 4A
51
Q

Visual field

A

extent of visual space over which vision is possible when the eyes are held in a fixed position

~190 degrees in humans

52
Q

Visual field representations of each eye

nasal, temporal, upper, lower regions

A
  • right visual field on the nasal half of the right eye and the temporal half of the left eye (i.e. the left halves of each eye)
  • left visual field on the nasal half of the left eye and the temporal half of the right eye
  • upper visual field on the lower half of the retina
  • lower visual field on the upper half of the retina

  • portions of each eye that look at the same regions of the visual field send axons to the same regions of the brain
  • right visual field is connected to the left visual cortex and left visual field is connected to the right visual cortex
53
Q

Visual field map in V1

A
  • upper visual field is represented below the calcarine fissure (pink and yellow)
  • lower visual field is represented above the calcarine fissure (orange and blue)
54
Q

Cortical magnification

A
  • magnified fovea in the visual field map on V1
  • results in the decline of visual acuity with distance from the fovea/fixation
55
Q

Scotoma

A

a small region of blindness in the visual field due to damage to a corresponding small region of the retina or V1

e.g. left hemisphere V1 damage below the calcarine fissure produces blindness in the upper right visual field

56
Q

Macular degeneration

A
  • disease of aging that damages cones in the macula
  • simulated scotoma in central vision
57
Q

Retinitis pigmentosa

A

hereditary diseases that progressively destroy photoreceptors

58
Q

Quadrantanopia

A

damage to half of V1 (above or below calcarine fissure) in one hemisphere produces blindness in the corresponding quadrant of the visual field

e.g. damage to the right V1 below the calcarine fissure produces blindness in the upper left visual field

59
Q

Hemianopia

A

damage to the entire V1 (above and below calcarine fissure) in one hemisphere produces blindness in the contralateral visual field

  • e.g. damage to the right V1 produces blindness in the left visual field
  • all of V1 in both hemispheres have to be damaged to produce complete cortical blindness
60
Q

Orientation tuning of cortical (V1) neurons

binocular neurons in layers 2/3, 4B, 5, and 6

A
  • cortical neurons respond optimally to certain orientations (e.g. strongest to vertical)
  • arrangement of receptive fields of LGN neurons feeding into a V1 neuron may establish its orientation tuning
61
Q

Simple vs. complex cells

in layers 2/3, 4B, 5, and 6 of V1

A
  • simple cells have separate on and off regions; stimulus orientation and position (phase) are important
  • complex cells do not have separate on and off regions; only stimulus orientation is important

complex cells given ON and OFF, only ON, or only OFF response throughout their receptive field

62
Q

End-stopped cells

in layers 2/3, 4B, 5, and 6

A

simple and complex cells that are tuned to the same length

63
Q

Ocular dominance

A

relative strength of a cortical neuron’s connection with the 2 eyes

left and right eye ocular dominance columns in monkey V1

  • white stripes: left eye injected with radioactive tracer
  • black stripes: right eye not injected
64
Q

2 columns in layer 2/3 of V1

A
  1. blobs (dark stain): contain color-selective neurons
  2. interblobs (light stain): contain orientation-selective neurons (simple and complex cells)
cytochrome oxidase staining (metabolic activity marker)

  • blobs are most prominent in layers 2/3
  • layer 4B has motion-selective neurons
65
Q

Hypercolumn

A

chunk of cortex containing a set of ocular dominance columns, each with a set of orientation columns (interblobs) and color columns (blobs)

2 ocular dominance columns, each with a pinwheel of orientation columns around a blob

each hypercolumns processes a small piece of the world, size of which depends on cortical magnification factor

66
Q

What are the corresponding columns of V1 in V2?

extrastriate visual

A
  • interblobs = interstripes (orientation-selective), connect to V3 and V4
  • blobs = thin stripes (color-selective), connect to V4
  • layer 4B = thick stripes (motion-selective), connect to V3 and V5

  • layer 4B doesn’t stain for cytochrome oxidase (can’t see blobs and interblobs)
  • all V2 neurons are binocular
67
Q

2 cortical visual streams

extrastriate visual cortex

A
  1. dorsal: parasol (M) cells to magno LGN to V1 to parietal lobe
  2. ventral: midget (P) cells to parvo LGN to V1 to temporal lobe

ventral stream controls conscious perception while dorsal stream controls unconscious action

PSYC 367 (9 decks)

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