Seminar: The contribution of millisecond spike timing of cortical neurons to sensory coding and perceptual decisions
When a neuron responds to a sensory stimulus, two fundamental codes may transmit the information specifying stimulus identity—spike rate (the total number of spikes in the sequence, normalized by time) and spike timing (the detailed millisecond-scale temporal structure of the response). Previous studies reported that millisecond-precise spike times of cortical neurons carry sensory information that cannot be extracted from spike rates defined over tens of milliseconds. However, it has remained unclear whether the extra information available in spike timing is actually used by the brain. To address this issue, we developed a mathematical approach based on information theory to relate sensory information content of spike rates and spike times to the behavioral outcome in the same trial of a perceptual discrimination task. Using this formalism to analyze neuronal responses in primary (S1) and secondary (S2) somatosensory cortex performing a whisker-based somatosensory discrimination task, we found that spike timing makes crucial contributions to tactile perception, complementing and surpassing those made by rate. The language by which somatosensory cortical neurons transmit information, and the readout mechanism used to produce behavior, appears to rely on multiplexed signals from spike rate and timing.
Stefano Panzeri from the Italian Institute of Technology