PSYC 367: Quiz 3

photoreceptors

photoreceptors

light sensitive receptors in the retina that initiate the act of seeing through producing chemical signals - two types: rods and cones

rods

a photoreceptor specialized for night vision

• have more than cones (95% of photoreceptors), absent from centre of fovea

• nocturnal animals have all-rod retinas

• all have the same photopigment, so can’t signal differences in colour

rod structure

• outer segment: stores photopigments

• inner segment: makes photopigments

• nucleus

• axon (in one synaptic terminal)

synaptic terminals in photoreceptors

each synaptic terminal admits 2 horizontal cells and at least 1 bipolar cell

cones

photoreceptor specialized for daylight vision, fine visual acuity, and colour (central vision - trusted more than peripheral)

• less of them (4-5 million)

• most concentrated at centre of fovea (smaller and more tightly packed)

• require brighter illumination (central vision is almost blind in dark environments)

• have 1/3 different photopigments that signal colour

cone structure

outer segment: stores photopigments

inner segment: makes photopigments

nucleus

axon - in the several synaptic terminals

photoreceptor synaptic terminals

specialized structures for contacting other retinal neurons - horizontal and bipolar cells

photoreceptor outer segments

contain photopigment molecules

photoreceptor inner segments

makes visual pigments, each one a specific structure that absorbs a certain wavelength and also captures light photons with chromophore

3 different cone types

• short wavelength-sensitive cones (S-cones)

• medium wavelength-sensitive cones (M-cones)

• long wavelength-sensitive cones (L-cones)

S-cones

most absorbent wavelength at 420 nm - 5-10% of of cone population

• essentially missing from fovea centre

M-cones

most absorbent wavelength at 535 nm - in fovea, less than L-cones

L-cones

most absorbent wavelength at 565 nms - in fovea, more than M-cones

rhodopsin

visual pigment (1/4 in the retina) found in rods, mainly in membranous discs in outer segment

• consist of a protein (opsin) connected to a light-sensitive chromophore (retinal)

what happens when a photo meets the photoreceptors?

Photoactivation: absorbed by a molecule of rhodopsin in outer segment of rod → transfers energy to chromophore portion of visual pigment molecule

• biochemical cascade of events resulting in closing of cell membrane channels that normally allow ions to flow into rod’s outer segment

opsin

determines which wavelengths photopigment absorbs

chromophore (retinal)

captures light photons

• also in in cone photopigments, but protein part is different

photoactivation process

activated by light, leads to transduction (process by which light is transformed into electrical activity)

light transduction

hyperpolarization: imbalance of electrical current between inside and outside of rod outer segment causes closing of cell membrane channels, inside is more negatively charged

• closes voltage-gated calcium channels at synaptic terminals

• reduces concentration of glutamate released in synapse

• signals to bipolar cells that rod has captured a photon

• potentials are graded (instead of the all-or-none in nervous system)

photoreceptor activity in the dark

• cyclic GMP keeps sodium flowing (-40mV membrane potential)

• glutamate is released into synaptic cleft and binds to bipolar cell membrane receptors

photoreceptor activity with light absorption

• deactivation of cGMP closes sodium channels

• photoreceptor hyperpolarizes (-70mV)

• calcium channels close; glutamate concentration reduced

graded potential

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

• occurs in photoreceptors, bipolar cells, horizontal cells

pupil diameter

ranges from 2 (bright light) to 8 (darkness) mm

duplex vision

scotopic (rod-mediated, more sensitive in dim light) and photopic (cone-mediated, less sensitive vision in bright light)

• similar to indoor and outdoor film in a camera

• cones much less sensitive than rods in dim light

• rod response saturates in bright light (rhodopsin bleached)

adapting to light and dark conditions (Hecht, Haig & Chase, 1937)

participant adapts to bright light → lights then turned off - frequently measure absolute threshold for circles for 30 mins (large violet circle, small red circle, small green peripheral circle)

• cones: quick adaptation (then saturation), poor sensitivity

• rods: slow adaptation, good sensitivity

• purple curve = change in threshold intensity to detect a spot (represents the more sensitive of the rods or cones at a given time)

ways the visual system adjusts to changes in illumination

• pupil size

• photopigment regeneration

• duplex retina

• neural circuitry (ganglion cells measuring difference in light intensity between central and peripheral vision

horizontal cells

run perpendicular to photoreceptors and make contact with nearby photoreceptors - enable lateral inhibition (signals that reach retinal ganglion cells are based on differences in activation)

amacrine cells

run perpendicular to photoreceptors in retina - receive inputs from bipolar cells and other amacrine cells, send signals to bipolar, other amacrine, and retinal ganglion cells

bipolar cells

intermediaries between photoreceptors → ganglion cells

diffuse bipolar cells

depolarize with increase in photon catch by photoreceptors due to opening of positive ion channels

• increase rate of neurotransmitter release

• connect to rods or to cones in peripheral retina

• synapse with up to 50 receptors per bipolar cell (convergence)

ON midget bipolar cells

depolarizes to an increase in photon catch by photoreceptors in central retina

• increase rate of neurotransmitter release

• connect to cones in fovea

• receive input from 1 cone per bipolar cell → pass to single ganglion cells (why images in this part of retina are the clearest)

OFF midget bipolar cells

hyperpolarizes in response to an increase in light captured by cones

• connect to cones in fovea

• 1 cone per bipolar cell → pass to single ganglion cells

neural transmission between a bipolar cell and a retinal ganglion cell

1. Bipolar cell depolarizes

2. Glutamate released into synaptic cleft

3. Ion channels open

4. Retinal ganglion cell depolarized

depolarization happens with more light in ON cells and less light in OFF cells

action potential

rapid depolarization - occurs in amacrine cells and retinal ganglion cells

cells when lights are on

• ON bipolars (midget or diffuse): depolarize/increase nt release, ON-centre retinal ganglion cells fire more action potentials

• OFF bipolars (midget): hyperpolarize/decrease nt release, OFF-centre retinal ganglion cells fire fewer action potentials

cells when lights are off (luminance)

• ON bipolars: hyperpolarize/decrease nt release, ON-centre retinal ganglion cells fire fewer action potentials

• OFF bipolars: depolarize/increase nt release, OFF-centre retinal ganglion cells fire more action potentials

mighet retinal ganglion cells/P ganglion cells

project to parvocellular LGN layers

• small cell bodies, short dendrites, thin axons

• 70% of ganglion cells in retina

• synapse with midget bipolar cells

parasol/M retinal ganglion cells

project to magnocellular layers of LGN

• larger cell bodies, long dendrites, thick axons

• 10% of ganglion cells in retina

• synapse with diffuse bipolar cells

bistratified/K retinal ganglion cells

project to koniocellular layers of LGN

• small or large cell bodies and dendritic fields, intermediate axons

• 10% of ganglion cells in retina

• synapse with diffuse or midget bipolar cells

intrinsically photosensitive/melanopsin-containing) retinal ganglion cells (4th type of retinal ganglion cells)

involved in pupil reflexes, circadian rhythms - stimulated by blue light = reduced melatonin production in evening which reduces sleepiness (brightness discrimination/contrast detection)

• <5% of retinal ganglion cells

• photopigment melanopsin on dendrites

• peak absorption at 480 nm

receptive field

region on retina and corresponding region in visual space in which visual stimuli influence neuron’s firing rate - neuron firing rate increases when light shines on the specific spot

• ganglion cell rf size determined by number of photoreceptors connected to that neuron (through bipolar cell)

receptive field convergence

• convergence of rods onto ganglion cells yields high sensitivity

• lack of convergence in cones yields high acuity

ON-centre cell

a cell that increases firing in response to an increase in light intensity in its receptive-field centre - neuron fires more when light is shone directly in the centre, fires less when light is shone in peripheral (spatial opponency)

• + regions = light increases action potentials (excitation)

• - regions = light decreases action potentials (inhibition)

• centre size determined by photoreceptor connections through bipolar cells

• surround size determined by photoreceptor connections through horizontal cells

OFF-centre cell

a cell that increases firing in response to a decrease in light intensity in its receptive-field centre - neuron fires more when light hits peropheral of cell, less when it hits the centre

size of light and neuron firing

cell fires fastest when size of light matches size of excitatory centre, reduces rate when spot of light begins to encroach on its inhibitory surround

• lateral inhibition

• cells are more sensitive to differences in intensity of light in centre and in surround, less affected by average intensity of light

receptive fields in M, P, and K-ganglion cells

• M-cells have smaller receptive fields than P and K-cells

• M and P-cells have spatial opponency

• K-cells have centres only (ON and OFF responses)

• M-cells are the most senitive

retinal ganglion cells temporal responses

• M-cells = sustained response: lasts entire time light is on or off in ganglion cell receptive field

• P-cells = transient response: brief response at onset and/or offset of light in ganglion cell receptive field

• K-cells = sustained OR transient response at onset and offset of light in receptive field

Describe 3 new technological developments that may lead to
treatments for age-related macular degeneration or retinitis
pigmentosa.

1. Substitute an electronic prosthesis (artificial device that replaces or augments a missing or impaired body part) into the retina - uses camera to convert light into energy and generates a stimulation pattern for neurons

2. Gene therapy to express light-activated channels in surviving photoreceptors using adeno-associated viral vectors

3. Chemically modify endogenous channels in retinal ganglion cells to make them light sensitive

age-related macular degeneration

disease associated with aging that affects the macula - gradually destroys sharp central vision, making it difficult to read, drive, and recognize faces

retinitis pigmentosa

progressive degeneration of the retina that affects night vision and peripheral vision - commonly runs in families and can be caused by defects in a number of different genes

Discuss how lateral inhibition allows retinal ganglion cells to highlight contrast and discount ambient light intensity.

• enhances contrast by shaping ganglion cell receptive field to emphasize differences in light intensity between adjacent areas of the retina - make’s ganglion cell’s response to a boundary between light and dark stronger

• responds primarily to relative changes in light intensity rather than absolute levels - allows visual system to focus on changes or patterns in visual scene, rather than overall brightness

visual angle

measure of retinal image size that depends on physical size of object and object’s distance from eye (retinal image size also changes with distance)

• formed by lines going from ex. top and bottom of a stripe cycle, passing through the centre of the lens and ending on the retina

measuring visual angle

divide the size of cycle by viewing distance at which you ca barely see a difference between two objects → take arctangent of this ratio

cycle

for grating, a pair consisting on one dark bar and one bright bar

size of visual angle

• measured in degrees of arc - 60 minutes of arc in 1 degree

• 1 degree is ~0.3 mm on retina

• Rule of thumb: 1 cm object at 57 cm away subtends 1 degree (index finger at arm’s length)

visual acuity

ability of visual system to resolve fine spatial detail - smallest resolvable visual angle

• depends on separation distance of photoreceptors from each other/convergence of receptors on each ganglion cell

• worse in peripheral vision because more cones converge on each other

• central vision is slower at responding to light - foveal cones have longer axons = denser packing

minimum visible acuity

smallest objects one can detect - ex. can see a dark wire against a bright background when they subtend an angle of 0.5 arc second (0.00014)

why is minimum visible acuity so small?

1. optics of the eye spread the image of the line, making it wider on the retina

2. fuzzy retinal image of the line casts a shadow that reduces the light on a row of cones to levels that is just detectably less than rows of cones on either side

Snellen ration (20/20) - visual acuity

• numerator is viewing distance of 6m (20 ft) - optical infinity (lens relaxed in emmetropic eye)

• denominator calculated for each person based on size of smallest letter they can correctly identify (distance at which a person with normal vision can identify letters_

• normal = read line of bard of letters that subtend 1 min of arc from 6m viewing distance (6/6)

• 6/3 = better than normal (can read at 6 meters what normally read at 3)

• 6/12 = worse than normal (can read at 6 meters what normally read at 12)

testing visual acuity on people not familiar with English language

• Landolt Cs: where is the gap in the C

• Tumbling Es: what direction are the E prongs facing

recognition acuity

requires identification - letters, numbers, shapes

• angular size of smallest feature that one can recognize or identify

• normal if you can identify letters with a 1 min of arc stroke width (0.017 degree)

• many people can see smaller letters

• depends on photoreceptor properties + higher cortical factors

resolution acuity

does not require identification, finest high-contrast detail that can be resolved - Landolt Cs, tumbling Es, grating acuity (stripe width)

• often better than recognition

• depends on photoreceptor properties (density and convergence)

• ex. can resolve black and white stripes when one cycle subtends angle of approximately 1 min of arc

visual acuity is best when

mediated by the cones in the fovea

• foveal viewing: 0.5 mins of arc between cone centres vs. 0.75+ mins of arc between rod centres outside fovea - no convergence of photoreceptors onto retinal ganglion cells and fit nicely into acuity limit of 1 minute of arc (2 cones per cucle)

• photopic light conditions

vertical meridian asymmetry

we have better acuity a fixed distance below the midline of the visual field than above - acuity falls off more rapidly along vertical midline (horizontal and vertical asymmetry

contrast sensitivity is measured by

sinewave grating - used to assess spatial vision over a range from very coarse to very fine detail, sinusoidal change in intensity across pattern

sinewave grating varies in

• spatial frequency: # of cycles in 1 degree of visual angle

• contrast: luminance difference between light (Lmax) and dark (Lmin) bars

different spatial frequencies

low, medium, high

phase

position of grating relative to some fixed position (measured in degrees)

which parameters differ in the 2 left gratings and in the 2 right gratings?

• 2 left: contrast and phase

• 2 right: orientation and spatial frequency

contrast detection threshold for sinwave gratings is a function of

spatial frequency - most sensitive to intermediate spatial frequencies (~6 cycles/degree

• steep falloff due to neural factors

as contrast goes down and spatial frequency goes up

sensitivity gets lower

contrast sensitivity

reciprocal of (Lmax - Lmin)/(Lmax + Lmin) → divide that answer by 1

• contrast of 100% = sensitivity of 1

why does our sensitivity drop off as frequency reaches 60 cycles/degree?

high spatial frequencies transmitted poorly due to

1. optics: eye can’t transmit higher spatial frequencies

2. anatomy: spacing of cones

factors that influence exact form of CSF

1. adaptation level of eye

2. Temporality modulation of targets (how it varies over time)

3. age and refractive state of individual

when do retinal ganglion cells give the best response?

when width of receptive field centre matches the specific width of the grating bars and the phase fills the receptive field perfectly

• different neurons respond to different spatial frequencies; fewer tuned to low

• respond weakly when spatial frequency is too low or too high

why do respond to sine wave gratings the most?

although pure sine waves are rare, all objects have some edge that may or may not be fuzzy - visual system breaks down images into components that essentially are sine wave gratings (same as the Fourier analysis in the auditory system)

• track how often changes from light to dark occur over a region of space (spatial frequencies)

spatial frequency

number of light/dark changes across 1 degree of a person’s visual field

why is contrast sensitivity low at low spatial frequencies?

1. physiology: fewer neurons turned to low

2. experimental design: harder to see grating since it only contains a few bars

contrast sensitivity and age

• during development: contrast sensitivity improves at all spatial frequencies, peak shifts to the right with age (mainly due to longer, thinner, denser cones

• old age: contrast sensitivity and grating acuity decreases at high spatial frequencies, no peak shift (probably due to changes in optical properties of eye)

geniculostriate pathway

1. axons from nasal half of each eye cross over to opposite side of brain, axons from temporal half of each eye stat on same side

2. optic chiasm: where nerves from each eye meet

3. optic tract: where nerves from both eyes travel to the LGN

4. lateral geniculate nucleus: 1st synapse, carried by optic radiations

5. primary visual cortex/V1/striate cortex/area 17

lateral geniculate nucleus (LGN) strucutre

2 magnocellular layers at bottom and 4 parvocellular layers at top - koniocellular layers in spaces between magno and parvo layers

koniocellular layers

each involved in a different aspect of processing - like relaying signals from S-cones

magnocellular layers

receive input from M ganglion/parasol cells in retina

• respond to large, fast, moving objects

parvocellular layers

receive input from P ganglion/midget cells in retina

• process stationary targets

lateral geniculate nucleus

structure in the thalamus that receives input from retinal ganglion cells and has input and output connections to the visual cortex - one in each cerebral hemisphere

visual input to LGN

each LGN keeps left and right eye inputs separate - left side of retina in each eye go to left LGN and right side of retina in each eye go to right LGN, but only right visual field goes to left LGN and only left visual field goes to right LGN

• gets input from both eyes, it is the actual visual field that is separated again

LGN layers

• layers 2, 3, 5 are from ipsilateral eye (same side of body)

• layers 1, 4, 6 are from contralateral eye (opposite side of body)

adjacent neurons in LGN are stimulated by adjacent points on retina or in visual field - each layer contains a highly organized map of a complete half of the visual field (topographic map)

cells synapsing in layers of the LGN

• midget cells project to parvocellular (P) layers (P3, P4, P5, P6)

• parasol cells project to magnocellular (M) layers (M1, M2)

• bistratified cells project to koniocellular (K) layers (K3, K4 - axons from superior colliculus synapse in K1 and K2)

LGN cell receptive fields

region of visual field or part of retina on which light produces a response

primary visual cortex (V1)/area 17/striate cortex

receiving area for LGN inputs in cerebral cortex

V1 layers

6 layers: layer 1 is closest to the outer surface of the brain, layer 6 is the deepest layer

• layer 4 has 3 sublayers - 4A, 4B, 4C (4C further subdivided in alpha and beta layers)

• fibres from the LGN project mainly to layer 4C - magnocellular axons mostly go to 4C alpha and parvocellular axons mostly project to 4C beta

where do LGN neurons mostly project?

layer 4C in V1

• magno LGN projects to V1 layer 4C alpha, further projects to V1 layer 4B and V1 layer 2/3

• parvo LGN projects to V1 layer 4C beta, further projects to V1 layer 2/3

• konio LGN projects to V1 layer 2/3 or V1 layer 4A

• layer 2/3 further projects to layers 5 and 6, or extrastriate cortex

P-pathway

midget retinal ganglion cells (sustained, small rfs) → parvocellular LGN layers (sustained, small rfs) → layer 4C beta in V1 → layer 2/3 interblobs in V1 (orientation) → interstripes (orientation) in V2 → V3 → V4

K-pathway

bistratified retinal ganglion cells (large rfs) → koniocellular LGN layers (large rfs) → layer 4A and layer 2/3 (colour) in V1 → thin stripes (colour) in V2 → V4

M-pathway

parasol retinal ganglion cells (transient, large rfs) → magnocellular LGN layers (transient, large rfs) → layer 4C alpha in V1 → layer 4B in V1 (motion) → thick stripes (motion) in V2 → V3, V5

visual field

extent of visual space over which vision is possible with eyes held in a fixed position

• about 190 degrees in humans

visual field representation in the eye

• right visual field on nasal half of right eye and temporal half of left eye - left field on temporal half of right eye, nasal half of left eye

• upper visual field on lower half of retina; lower visual field on upper half of retina

scotoma

small region of blindness in visual field due to damage in corresponding small region of retina

macular degeneration

disease of aging that damages cones in macula

retinitis pigmentosa

hereditary disease that progressively destroy photoreceptors