Spike-triggered covariance and synthetic image replay reveal nonlinearities in V1 color processing

Abstract

Event Abstract Back to Event Spike-triggered covariance and synthetic image replay reveal nonlinearities in V1 color processing Gregory Horwitz1, 2* 1 University of Washington, United States 2 Washington National Primate Research Center, United States The processing of color information in cortical area V1 remains poorly understood. The vastness of the stimulus space is a fundamental problem: not every stimulus pattern can be displayed during a standard neurophysiology experiment. In a previous study, we addressed this problem by stimulating V1 neurons in awake, fixating monkeys with random, colorful patterns and used dimensionality-reduction techniques (averaging and principal components analysis) to isolate features of the stimulus patterns to which the neurons responded1. The stimulus tuning of some neurons was well characterized by a single dimension in the stimulus space. For these neurons, we observed structure in the spike-triggered average stimulus and little or no structure in the principal components of the spike-triggered stimuli. Other neurons responded to multiple spike-triggered stimulus features. These features were manifest in the spike-triggered average and in one or more of the principal components. A subpopulation of neurons had a spike-triggered average that indicated sensitivity to a preferred color throughout the receptive field and a first principal component that indicated sensitivity to luminance edges. These results are inconsistent with a linear model of cone signal integration. We investigated the relationship between chromatic and luminance signals in these neurons by fitting a 2-dimensional linear-nonlinear cascade model to the data. The fits suggested that these neurons carry color-opponent signals, the gain of which is enhanced by luminance contrast. In the current study, we tested this prediction directly by performing the spike-triggered averaging and principal components analyses on the data stream as it was being collected. Spike-triggering stimulus features were extracted from these analyses and linearly combined to create a battery of synthetic images. Synthetic images were displayed at the neuron’s receptive field for 200 ms each, and spikes following each presentation were counted. Results of this experiment confirmed the model predictions: a subset of V1 neurons responded modestly to their preferred color in isolation but strongly to their preferred color superimposed on a luminance edge. We conclude that color processing across a subpopulation of V1 neurons is enhanced at luminance edges. This physiological result may be related to interactions between color and luminance observed psychophysically. Acknowledgments We thank J. Gold and C. Hass for assistance with the computer communication code. This work was supported by the McKnight Foundation and NIH grant RR000166.