Rhythmic whisking area (RW) in rat primary motor cortex: an internal monitor of movement-related signals?

Vibrissae-related sensorimotor cortex controls whisking movements indirectly via modulation of lower-level sensorimotor loops and a brainstem central pattern generator (CPG). Two different whisker representations in primary motor cortex (vM1) affect whisker movements in different ways. Prolonged microstimulation in RF, a larger anterior subregion of vM1, gives rise to complex face movements and whisker retraction while the same stimulation evokes large-amplitude rhythmic whisker movement in a small caudo-medial area (RW). To characterize the motor cortex representation of explorative whisking movements, here we recorded RW units in head-fixed rats trained to contact a moving object with one whisker. RW single units were found to encode two aspects of whisker movement independently, albeit on slow time scales (hundreds of milliseconds). The first is whisker position. The second consists of speed (absolute velocity), intensity (instantaneous power), and frequency (spectral centroid). The coding for the latter three parameters was tightly correlated and realized by a continuum of RW responses-ranging from a preference of movement to a preference of rest. Information theory analysis indicated that RW spikes carry most information about position and frequency, while intensity and speed are less well represented. Further, investigating multiple and single RW units, we found a lack of phase locking, movement anticipation, and contact-related tactile responses. These findings suggest that RW neither programs detailed whisker trajectories nor initiates them. Nor does it play a role in processing object touch. Its relationship to whisking is thus indirect and may be related to movement monitoring, perhaps using feedback from the CPG.