The relationship of vibrissal motor cortex unit activity to whisking in the awake rat

Rats actively sweep their whiskers back and forth to locate and palpate objects within their immediate environment. Microstimulation studies in anesthetized rats have demonstrated the presence of a large vibrissal motor representation in agranular cortex. However, the manner in which motor cortex neurons contribute to whisking behavior in the awake animal is unknown. This study represents an initial investigation of the relationship between the activity of task-related neurons in vibrissal motor cortex and the electromyographic (EMG) activity of the deep vibrissal pad muscles in the awake, freely whisking rat. Each animal was gently held in an experimenter's hands while the animal whisked the air. A spring-loaded, metal microelectrode mounted in a removable, miniature microdrive provided stabile recordings of motor cortex unit activity. Fine-wire electrodes implanted in the mystacial pad simultaneously recorded facial muscle activity. Results showed that the discharge of task-related neurons was correlated with changing levels of muscle output. Unit discharge was related in a tonic or phasic-tonic fashion to EMG activity. No units were found to discharge rhythmically in a 1:1 fashion with the periodicity of the whisking pattern. These findings support a role for vibrissal motor cortex in the initiation and modulation of the overall level of mystacial pad muscular output, but not in the generation of bursts of EMG activity responsible for individual whisking sweeps.