Band-Pass Response Properties of Rat SI Neurons

Rats typically employ 4- to 12-Hz “whisking” movements of their vibrissae during tactile exploration. The intentional sampling of signals in this frequency range suggests that neural processing of tactile information may be differentially engaged in this bandwidth. We examined action potential responses in rat primary somatosensory cortex (SI) to a range of frequencies of vibrissa motion. Single vibrissae were mechanically deflected with 5-s pulse trains at rates ≤40 Hz. As previously reported, vibrissa deflection evoked robust neural responses that consistently adapted to stimulus rates ≥3 Hz. In contrast with this low-pass feature of the response, several other characteristics of the response revealed bandpass response properties. While average evoked response amplitudes measured 0–35 ms after stimulus onset typically decreased with increasing frequency, the later components of the response (>15 ms post stimulus) were augmented at frequencies between 3 and 10 Hz. Further, during the steady state, both rate and temporal measures of neural activity, measured as total spike rate or vector strength (a measure of temporal fidelity of spike timing across cycles), showed peak signal values at 5–10 Hz. A minimal biophysical network model of SI layer IV, consisting of an excitatory and inhibitory neuron and thalamocortical input, captured the spike rate and vector strength band-pass characteristics. Further analyses in which specific elements were selectively removed from the model suggest that slow inhibitory influences give rise to the band-pass peak in temporal precision, while thalamocortical adaptation can account for the band-pass peak in spike rate. The presence of these band-pass characteristics may be a general property of thalamocortical circuits that lead rodents to target this frequency range with their whisking behavior.