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Abstract:
Natural visual environments contain a variety of continuously changing visual features . In order to represent the richness of such environments, neurons in visual areas need to encode information both about “what” aspects of the visual world (for example, the level of contrast) and how these aspects change over time. So far, little is known about how the responses of visual cortical neurons can encode both types of information concurrently.
Here we explored the hypothesis that aspects of visual features defined on different time scales are to some extent encoded by different aspects of the neural responses. We recorded single unit activity and LFPs in primary visual cortex of anaesthetized macaques during the binocular presentation of naturalistic color movies. By means of computational analysis, we extracted two visual features from the area of the movie inside the receptive fields (RFs) of each single neuron. The first was the Michelson contrast in the RF. The second was a form of temporal contrast, quantifying the average frame to frame variations of pixel luminance in the RF. We then used information theoretic analysis to investigate systematically which types of neural codes carried information either about the current value of these features in each frame or about the time course of these features on slower time scales.
We found that spike rates encoded both the current value and the frame by frame change of the Michelson contrast, but did not encode information about the time course of these features on slower time scales. We then considered the information carried by the “phase of firing”, defined as the timing of spikes measured with respect to the phase of low frequency [1-4 H] Local Field Potential fluctuations. The phase of firing carried information about the temporal changes of contrast over a time scale of several hundred milliseconds (which are the scales carrying the most power in natural movies), but did not carry information about the current value of contrast in each frame or about its frame to frame variations.
These results demonstrate that different aspects of visual features, such as their current value and their dynamics on slow time scales, are represented in complementary neural codes operating on different time scales. They hence suggest that the nervous system uses multiplexing to keep a simultaneous representation of several important aspects of the external world.
