Anticipatory activity of motor cortex in relation to rhythmic whisking

Abstract

Rats characteristically generate stereotyped exploratory (5-12 Hz) whisker movements, which can also be adaptively modulated. Here we tested the hypothesis that the vibrissal representation in motor cortex (vMCx) initiates and modulates whisking by acting on a subcortical whisking central pattern generator (CPG). We recorded local field potentials (LFPs) in vMCx of behaving Sprague-Dawley rats while monitoring whisking behavior through mystacial electromyograms (EMGs). Recordings were made during free exploration, under body restraint, or in a head-fixed animal. LFP activity increased significantly prior to the onset of a whisking epoch and ended prior to the epoch's termination. In addition, shifts in whisking kinematics within a whisk epoch were often reflected in changes in LFP activity. These data support the hypothesis that vMCx may initiate and modulate whisking behavior through its action on a subcortical CPG.

Figure 1

A: Whisker trajectory (up=protraction) evoked by microstimulation of vMCx and recorded with a CCD. B: Nissl-stained coronal sections depicting electrode tracks through the agranular-medial (AGm) region of the motor cortex.

Figure 2

Raw trace of EMG activity (A & B, top row) with corresponding LFP traces below. Onset (green arrowhead) and offset (red arrowhead) in EMG and LFP activity, determined using statistical algorithms (see Materials and Methods) are depicted on the corresponding traces. Increases in LFP just before whisking onset and again just prior to whisking offset are evidenced as an increase in power in the LFP spectrogram (third row). Changes in power that are statistically significant above the 95% confidence level are plotted in the bottom panels. Instances in which a change in whisking dynamics during an epoch is reflected in LFP activity are indicated with blue arrowheads.

Figure 3

Time-course of increases in LFP activity (hatched bars) and of EMG activity (filled bars) recorded from 49 epochs in 5 animals. Data are aligned to whisking onset. Changes in LFPs preceded whisking onset in 39 of the 49 epochs, and ended prior to whisking offset in all samples. Overlaps in activity are illustrated by bar overlaps. Bars marked A & B correspond to data depicted in Figs. 2A and 2B.