Retinal Processing of Photoreceptor Signal

Responses of
photoreceptors to stimuli

Define stimuli

Michelson contrast (MC)
MC=EmaxEminEmax+Emin

Weber contrast (WC)
\( WC=\frac{E_{max}-E_{min}}{E_{min}} \)

useful when using stimuli modulating around mean excitation

useful when using pulsed stimuli

Selective desensitization

Silent substitution method

disadvantanges

Complete isolation is difficult.

Disadvantages of pulsed stimuli also count here.

Mean excitation changes between pulse duration and resting.

Different backgrounds and adaptation states need affect the outcome.

Desensitize photoreceptors that were not in interest with background and isolate the photoreceptor under study with a flash stimuli.

use periodic stimulus around mean luminance and chromaticity

Replace one stimulus by another of different spectral composition

The intensities of two stimuli are chosen such that the number of photo-isomerizations of one photoreceptor is not altered, but that of another in interest is strongly altered.

Advantages

image

Can deliver stimuli with well-defined spectral, temporal, and spatial properties.

State of adaptation can be kept constant (in continuous stimuli).

Isolation of the responses of different photoreceptor types can be complete.

Post-receptoral processing

horizontal cells

H1

H2

Large cell bodies

radiate stout primary dendrites

85% of H1 cells make synapes with L and M cones

medium size cell bodies

thin, curved, interwined, branched dendrites

contact to all cone pedicles

S cone pedicles make more synapses with H2 dendrites tahn M or L cone pedicles

horizontal cells in primates exhibit cone specificity without cone opponency

Bipolar cells

rod bipolar (RB)

midget bipolar (MB)

diffuse bipolar (DB)

S-cone bipolar (BB)

giant cone bipolar (GB)

form the M/L con opponent parvocellular pathway

a single dendrite contacts with one L or M cone in central region

FMB

make flat synapses with cone pedicles

IMB

make invaginating synapses with cone pedicles

send axons to upper half of the IPL

send axons to lower half of the IPL

every M or L cone pedicle contacts a FMB and an IMB cell

convey info. from S cones to specific ganglion cells

long, smoothly-curved dendrites

make invaginating synapses with one or more S cones

DB1, DB2, DB3

DB4, DB5, DB6

make flat synapses with cone pedicles

send axons to the upper half of the IPL

make invaginating or flat synapses with cone pedicles

send axons to the lower half of the IPL

sequential naming due to depths of their axons to IPL

contact all cones in their dendritic trees

some of them provide M or L cone inputs to S/ML pathways

some are biased against S cone

Ganglion cells

Magnocellular (MC-) pathway

Parvocellular (PC-) pathway

Kinocellular (KC-) pathway

consist of DB cells and parasol ganglion cells

project to two ventral layers of the LGN

MC-ganglion cells receive L+M signal

respond to high temporal freq. and have relatively large RFs

motion processing

consist of MB cells and midget ganglion cells

process exclusively L-M signals

cone strengths are more balanced compared to MC-path

have smaller RFs

form perception

consist of DB, BB cells, and small-field bi-stratified ganglion cells

DB cells provide L+M inhibitory signals, while BB cells provide S excitatory signals (S-[L+M])

the antagonistic inputs are spatially co-extensive (type 2 cells)

Electroretinogram (ERG) studies

Electrical signal of retinal origin that is elicited by the same excitation of photopigments that leads to a visual response.

ERG components

a-wave

b-wave

d-wave

photopin negative response (PhNR)

early negative wave

originates in activity of the photoreceptors and OFF-bipolar cells

positive wave following a-wave

by activity of ON-bipolar cells

addition of a positivity as a response to stimulus onset and a slightly delayed positive d-wave response to stimulus offset

identical implicit times to stimulus offset as a-wave, implying the same cellular origins

reflect activity of retinal ganglion cells

Early studies

Recent developments

use short flashed stimuli

ERG data were hard to interpret as being related to cone opponency

use repetitive stimuli

Heterochromatic flicker photometry
a ref. light and a test light are modulated in counter-phase at high temporal freq.

spectral sensitivities are identical in psychophysical and ERG exp., resulting in luminosity function

inter-individual variability also present in ERG data and is related to L\M ratio in luminance channel

ERG response to intermediate freq. stimuli containing red-green contrast reflect activity of L-M opponent PC-path