The Timing of Spikes in Primary and Secondary Somatosensory Cortex Contributes to Perception
The activity of sensory neurons can carry more information when the timing of spikes is registered with millisecond precision than when the spike count is registered over windows of tens or hundreds of milliseconds. In generating a percept and selecting an action based on that percept, does the brain use the simpler code of spike counts or the more information-rich, but more complex, code of temporal patterns? To investigate whether precise timing contributes to behavior, we analysed spike trains recorded from neurons in rat primary (S1) and secondary (S2) somatosensory cortex. On each trial the rat was presented one of three textures and used its whiskers to palpate the stimulus. It was required to withdraw and turn to the reward spout specified by the identity of the contacted texture. We computed the information about texture carried by spike counts and spike times during each whisker contact with the texture. In trials where the rat made a correct choice, S1 and S2 neurons carried approximately twice as much information in spike times as in spike counts. We found that a stimulus contact signal can provide a “clock” by which to decode spike patterns. Next, we reasoned that if a neuronal coding mechanism contributes to behavior, then neurons would be found to transmit less stimulus information in trials in which the rat made the wrong choice. When we included error trials in the analysis, the texture information carried both by spike counts and spike patterns decreased significantly, but the effect was much more pronounced for patterns. Taken together, these results suggest that in S1 and S2 the time at which spikes occur, not just the number of spikes, makes a crucial contribution to tactile perception. This talk reports joint work with Zou Yanfang and Mathew Diamond (SISSA) and ALberto Mazzoni and Giuseppe Notaro (IIT).