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Retinal Processing of Photoreceptor Signal (Responses of photoreceptors…
Retinal Processing of Photoreceptor Signal
Responses of
photoreceptors to stimuli
Define stimuli
Michelson contrast (MC)
\( MC=\frac{E_{max}-E_{min}}{E_{max}+E_{min}} \)
useful when using stimuli modulating around mean excitation
Weber contrast (WC)
\( WC=\frac{E_{max}-E_{min}}{E_{min}} \)
useful when using pulsed stimuli
Mean excitation changes between pulse duration and resting.
Selective desensitization
disadvantanges
Complete isolation is difficult.
Disadvantages of pulsed stimuli also count here.
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.
Silent substitution method
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
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
Large cell bodies
radiate stout primary dendrites
85% of H1 cells make synapes with L and M cones
H2
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)
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
send axons to upper half of the IPL
IMB
make invaginating synapses with cone pedicles
send axons to lower half of the IPL
every M or L cone pedicle contacts a FMB and an IMB cell
diffuse bipolar (DB)
DB1, DB2, DB3
make flat synapses with cone pedicles
send axons to the upper half of the IPL
DB4, DB5, DB6
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
S-cone bipolar (BB)
convey info. from S cones to specific ganglion cells
long, smoothly-curved dendrites
make invaginating synapses with one or more S cones
giant cone bipolar (GB)
Ganglion cells
Magnocellular (MC-) 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
Parvocellular (PC-) pathway
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
Kinocellular (KC-) pathway
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
early
negative
wave
originates in activity of the
photoreceptors
and
OFF-bipolar cells
b-wave
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
d-wave
identical implicit times to stimulus offset as a-wave, implying the same cellular origins
photopin negative response (PhNR)
reflect activity of
retinal ganglion cells
Early studies
Recent developments
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
use short flashed stimuli
ERG data were hard to interpret as being related to cone opponency
