Variability and color opponency in marmoset retinal ganglion cells
Response variability of marmoset parvocellular neurons
Jonathan D. Victor, Esther M. Blessing, Jason. D. Forte, Peter Buzás and Paul R. Martin
J. Physiol. (Lond.) 579, 29-51 (2007)
This study concerns the properties of neurons carrying signals for colour vision in primates.
We investigated the variability of responses of individual parvocellular lateral geniculate
neurons of dichromatic and trichromatic marmosets to drifting sinusoidal luminance and
chromatic gratings. Response variability was quantified by the cycle-to-cycle variation in
Fourier components of the response. Averaged across the population, the variability at low
contrasts was greater than predicted by a Poisson process, and at high contrasts the responses
were approximately 40% more variable than responses at low contrasts. The contrast-dependent
increase in variability was nevertheless below that expected from the increase in firing rate.
Variability falls below the Poisson prediction at high contrast, and intrinsic variability of
the spike train decreases as contrast increases. Thus, while deeply modulated responses in
parvocellular cells have a larger absolute variability than weakly modulated ones, they have
a more favourable signal : noise ratio than predicted by a Poisson process. Similar results
were obtained from a small sample of magnocellular and koniocellular (‘blue-on’) neurons.
For parvocellular neurons with pronounced colour opponency, chromatic responses were,
on average, less variable (10–15%, p<0.01) than luminance responses of equal magnitude.
Conversely, non-opponent parvocellular neurons showed the opposite tendency. This is
consistent with a supra-additive noise source prior to combination of cone signals. In summary,
though variability of parvocellular neurons is largely independent of the way in which they
combine cone signals, the noise characteristics of retinal circuitry may augment specialization
of parvocellular neurons to signal luminance or chromatic contrast.
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