Abstract
Rodent whisker input consists of dense microvibration sequences that are often temporally integrated for perceptual discrimination. Whether primary somatosensory cortex (S1) participates in temporal integration is unknown. We trained rats to discriminate whisker impulse sequences that varied in single-impulse kinematics (5–20-ms time scale) and mean speed (150-ms time scale). Rats appeared to use the integrated feature, mean speed, to guide discrimination in this task, consistent with similar prior studies. Despite this, 52% of S1 units, including 73% of units in L4 and L2/3, encoded sequences at fast time scales (≤20 ms, mostly 5–10 ms), accurately reflecting single impulse kinematics. 17% of units, mostly in L5, showed weaker impulse responses and a slow firing rate increase during sequences. However, these units did not effectively integrate whisker impulses, but instead combined weak impulse responses with a distinct, slow signal correlated to behavioral choice. A neural decoder could identify sequences from fast unit spike trains and behavioral choice from slow units. Thus, S1 encoded fast time scale whisker input without substantial temporal integration across whisker impulses.
Highlights
Natural sensory input comprises dense temporal series of discrete events, which animals often temporally integrate to guide perceptual decisions
Rats can discriminate vibrotactile sequences applied to the whiskers, and prior studies show that this often involves behavioral integration over time to calculate mean whisker-speed
We found that neurons in the primary somatosensory cortex encoded whisker sensory information at very fast time scales (
Summary
Natural sensory input comprises dense temporal series of discrete events, which animals often temporally integrate to guide perceptual decisions. In the whisker tactile system, active whisking generates dense streams of stick-slip events on surfaces (5–10 ms duration, ~60 ms interval) [3,4] and contact events on object edges [5,6] These temporal series constitute the whisker vibrotactile signal. While animals can perceive individual brief whisker impulses alone or within trains [7,8,9,10,11], behavioral discrimination of vibrotactile sequences is often based on a time-averaged composite feature, mean whisker speed, rather than the kinematics or precise pattern of individual deflections [12,13] This suggests that the brain generates both short time-scale (individual impulse) and temporally integrated, long time-scale (mean speed or intensity) representations of whisker input. How these time scales are represented in the cortex is unknown
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