CENTRAL VERSUS PERIPHERAL DETERMINANTS OF PATTERNED SPIKE ACTIVITY INRAT VIBRISSA CORTEX DURING WHISKING

We report on the relationship between single-unit activity in primary somatosensory vibrissa cortex of rat and the rhythmic movement of vibr issae. Animals were trained to whisk freely in air in search of food. Electromyographic (EMG) recordings from the mystatial pads served as a reference for the position of the vibrissae. A fast, oscillatory comp onent in single-unit spike trains is correlated with vibrissa position within the whisk cycle. The phase of the correlation for different un its is broadly distributed. A second, slowly varying component of spik e activity correlates with the amplitude of the whisk cycle. For some units, the phase and amplitude correlations were of sufficient strengt h to allow the position of the whiskers to be accurately predicted fro m a single spike train. To determine whether the observed patterned sp ike activity was driven by motion of the vibrissae, as opposed to cent ral pathways, we reversibly blocked the contralateral facial motor ner ve during the behavioral task so that the rat whisked only on the ipsi lateral side. The ipsilateral EMG served as a reliable reference signa l. The fast, oscillatory component of the spike-EMG correlation disapp ears when the facial motor nerve is blocked. This implies that the pos ition of vibrissae within a cycle is encoded through direct sensory ac tivation. The slowly varying component of the spike-EMG correlation is unaffected by the block. This implies that the amplitude of whisking is likely to be mediated by corollary discharge. Our results suggest t hat motor cortex does not relay a reference signal to sensory cortex f or positional information of the vibrissae during whisking.