L16-L20 (Vision cont.) Flashcards
4 components of photoreceptors
- outer segment: stores photopigments
- inner segment: makes photopigments
- nucleus (outer nuclear layer)
- axon (outer plexiform layer)
each synaptic terminal admits 2 horizontal cells and at least 2 bipolar cells
Bleaching in frog retina
photoactivation
- 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
2 components of rhodopsin photopigment molecules
in rods
- opsin: determines which wavelengths the photopigment absorbs
- retinal: captures light photons
- protein (opsin) connected to a light-sensitive chromophore (retinal)
- no rods in fovea!
2 components of cone photopigment molecules
- chromophore (retinal)
- protein: photopsin I (L cones), photopsin II (M cones), photopsin III (S cones)
no S cones in the center of the fovea
Which cells do graded potentials occur in?
photoreceptors, horizontal cells, bipolar cells
a slow change in membrane potential that varies in size (not all-or-nothing)
Duplex vision
adapting to light and dark conditions
- 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
Adaption and sensitivity of rods vs. cones
- rods: slow adaptation, good sensitivity
- cones: fast adaptation, poor sensitivty
Dark adaptation
increase in sensitivity with time in the dark as we switch from photopic to scotopic vision
Diffuse bipolar cells
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
ON midget bipolar cells
depolarize in response to an increase in photon catch by photoreceptors
connect to cones in the fovea with 1 cone per bipolar cell
OFF midget bipolar cells
depolarize in response to a decrease in photon catch by photoreceptors
connect to cones in the fovea with 1 cone per bipolar cell
What happens when bipolar cells depolarize?
- 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
Which cells do action potentials occur in?
amacrine cells and retinal ganglion cells
spike or rapid depolarization
What happens to ON bipolar cells when light is on?
midget or diffuse
- depolarize and increase glutamate release
- ON-center retinal ganglion cells fire more action potentials
What happens to OFF bipolar cells when light is on?
midget
- hyperpolarize and decrease glutamate release
- OFF-center retinal ganglion cells fire fewer action potentials
What happens to ON bipolar cells when light is off?
midget or diffuse
- hyperpolarize and decrease glutamate release
- ON-center retinal ganglion cells fire fewer action potentials
What happens to OFF bipolar cells when light is off?
midget
- depolarize and increase glutamate release
- OFF-center retinal ganglion cells fire more action potentials
Midget retinal ganglion cells
aka P cells
- project to parvocellular LGN layers
- comprise 70% of RG cells
- have small cell bodies, short dendrites, and thin axons
- synapse with midget bipolar cells
Parasol retinal ganglion cells
aka M cells
- project to magnocellular LGN layers
- comprise 10% of RG cells
- synapse with diffuse bipolar cells
- large cell bodies, long dendrites, thick axons
Bistratified retinal ganglion cells
aka K cells
- 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
What photopigment do retinal ganglion cells contain?
intrinsically photosensitive
in < 5% of RG cells
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
Receptive field of a retinal ganglion cell
and what determines its size
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)
Acuity vs sensitivity
- ability to resolve fine detail
- ability to detect low levels of light
What yields high sensitivty vs. high acuity in the receptive fields of RG cells?
size depends on # of photorepectors connected (through bipolar cells!
- 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
Positive vs. negative regions of the RG cell receptive field
- positive regions are where light leads to excitation (increase in APs)
- negative regions are where light leads to inhibition (decrease in APs)
Spatial opponency
retinal ganglion cells
light elicits opposite responses in the center and surround of the receptive field
What determines the size of the center and surround of the RG cell receptive field?
- center size is determined by photoreceptor connections through bipolar cells
- surround size is determined by photoreceptor connections through horizontal cells
Lateral inhibition
retinal ganglion cells
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
Spatial responses of the 3 kinds of RG cells
- 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
Temporal responses of the 3 kinds of RG cells
- 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
Visual angle
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
Snellen ratio
smallest resolvable visual angle
* numerator is always 6m or 20ft (optical infinity)
* denominator = size of the smallest letter each person can correctly identify
What snellen ratio entails normal acuity?
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
How is recognition acuity measured in the snellen chart?
- letters
- numbers
- shapes
- requires identification
- ability to correctly identify a target
- depends on photoreceptor properties (density and convergence) and higher cortical factors
How is resolution acuity measured in the snellen chart?
- 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)
Grating size
resolution acuity
- 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)
When is recognition acuity normal?
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)
Foveal viewing
best visual acuity
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
Sinewave grating
contrast sensitivty
- used to assess spatial vision from very course to very fine detail
- sinusoidal change in intensity across pattern
3 parameters of contrast sensitivity in a sinewave grating
- spatial frequency: # of cycles in 1 degree of visual angle
- contrast: luminance difference between light (Lmax) and dark (Lmin) bars
- phase: position of grating relative to some landbark (e.g. begins in the middle of a black bar)
2 reasons why contrast sensitivity is low at high spatial frequencies
- optics: optical appartus of eye can’t transmit higher spatial frequencies
- 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
2 reasons why contrast sensitivity is low at low spatial frequencies
- physiology: fewer neurons tuned to low spatial frequencies
- experimental design: grating contains only a few bars
Which frequency range do retinal ganglion cells give the strongest response?
medium frequency when the receptive field center matches the width of the grating bars
different neurons are tuned to different spatial frequencies
How does contrast sensitivity (and grating acuity) change with age?
- 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
Geniculostriate pathway
- 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
- optic tracts synapse at the LGN
- optic radiations project to V1
primary visual cortex (V1) is also called area 17 and striate cortex
Which LGN layers do axons from the ipsilateral vs. contralateral eye synapse?
- 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
Which LGN layers do each kind of retinal ganglion cell synapse?
- 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
P-pathway
retinal ganglion cells to LGN to V1
- midget (P) cells synapse on parvocellular layers of LGN
- neurons from parvocellular layers project to layers 2/3 and 4Cβ of V1
M-pathway
retinal ganglion cells to LGN to V1
- parasol (M) cells synapse on magnocellular layers of LGN
- neurons from magnocellular layers project to layers 2/3, 4B, 4Cα
K-pathway
retinal ganglion cells to LGN to V1
- bistratified cells synapse on koniocellular layers of LGN (K3 and K4)
- neurons from koniocellular layers project to layers 2/3 and 4A
Visual field
extent of visual space over which vision is possible when the eyes are held in a fixed position
~190 degrees in humans
Visual field representations of each eye
nasal, temporal, upper, lower regions
- 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
Visual field map in V1
- upper visual field is represented below the calcarine fissure (pink and yellow)
- lower visual field is represented above the calcarine fissure (orange and blue)
Cortical magnification
- magnified fovea in the visual field map on V1
- results in the decline of visual acuity with distance from the fovea/fixation
Scotoma
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
Macular degeneration
- disease of aging that damages cones in the macula
- simulated scotoma in central vision
Retinitis pigmentosa
hereditary diseases that progressively destroy photoreceptors
Quadrantanopia
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
Hemianopia
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
Orientation tuning of cortical (V1) neurons
binocular neurons in layers 2/3, 4B, 5, and 6
- 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
Simple vs. complex cells
in layers 2/3, 4B, 5, and 6 of V1
- 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
End-stopped cells
in layers 2/3, 4B, 5, and 6
simple and complex cells that are tuned to the same length
Ocular dominance
relative strength of a cortical neuron’s connection with the 2 eyes
- white stripes: left eye injected with radioactive tracer
- black stripes: right eye not injected
2 columns in layer 2/3 of V1
- blobs (dark stain): contain color-selective neurons
- interblobs (light stain): contain orientation-selective neurons (simple and complex cells)
- blobs are most prominent in layers 2/3
- layer 4B has motion-selective neurons
Hypercolumn
chunk of cortex containing a set of ocular dominance columns, each with a set of orientation columns (interblobs) and color columns (blobs)
each hypercolumns processes a small piece of the world, size of which depends on cortical magnification factor
What are the corresponding columns of V1 in V2?
extrastriate visual
- 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
2 cortical visual streams
extrastriate visual cortex
- dorsal: parasol (M) cells to magno LGN to V1 to parietal lobe
- ventral: midget (P) cells to parvo LGN to V1 to temporal lobe
ventral stream controls conscious perception while dorsal stream controls unconscious action
