| It is widely held that visual input is encoded by firing rate changes in retinal cells that are locked to stimulus events. However, retinal firing patterns are not only influenced by external stimuli but also by dynamics of intrinsic neuronal networks such as oscillations. To study the function of oscillatory activity in early sensory systems, we developed methods to quantify how much more information a neuron in a sensory stream can transmit by phase-locking its spikes to an intrinsic oscillation. We used the new method to analyze data from the Hirsch lab at USC who used whole-cell recordings in vivo to measure retinal inputs and the spikes they evoke from single thalamic relay cells. Our analysis showed that relay cells transmit information using two separate channels. The first channel, as has been described before, transmits information localized to the neural receptive field by using changes in spike rate evoked by the stimulus; this channel relays visual signals slower than 30 Hz. The second, novel, channel encodes information by using spike timing relative to intrinsic retinal oscillations and occupies a separate frequency band, in the gamma range (50-70 Hz). Remarkably, the amount of information in the second channel could match or even exceed that conveyed by the first, a result that we were able to reproduce with a simple model. Because retinal oscillations involve large-scale networks, the novel channel could convey "gist", or contextual aspects, of the visual scene. |
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