Such a signal could originate from a hypothesized brainstem pattern generator (CPG; Figure 3), perhaps relayed via vM1 cortex. In this case fast modulation of neuronal signals in vS1 cortex by whisking

could be altered, but not eliminated, if whisking is blocked. Concepts from control theory suggest that both signals could be present in cortex as INCB024360 a means to compare actual versus intended vibrissa position (Ahissar et al., 1997 and Kleinfeld et al., 2002). Recordings from primary sensory neurons during muscular activation of the follicle could distinguish between peripheral reafference and efference copy. Such recordings in the trigeminal ganglion are facilitated by the technique of fictive whisking, in which electrical stimulation of the facial nerve is used to rhythmically drive vibrissa motion in anesthetized animals (Brown and Waite, 1974 and Zucker and Welker, 1969). Measurements of single-unit activity revealed a population of neurons in the trigeminal ganglion that spiked in response to a change in vibrissae position but not contact (Szwed et al., 2003). This established that muscular movement of the follicle alone is sufficient to drive spiking in primary sensory

neurons. Further, different neurons spiked at different positions into the fictive whisk (Figure 6A). The histogram of spiking by different units covered the full range of protraction and part of retraction (Figure 6A). These data support a reafferent pathway that carries NSC 683864 cost only reafferent signals of vibrissa position, as opposed to both position and touch signals. Yet details of the angle or phase response for different units are unlikely to reflect their response in the awake animal. The motor drive in fictive whisking consists only of protraction, as opposed to both retraction Methisazone and protraction in awake animals (Berg and Kleinfeld, 2003). Further, the mechanics of the follicles are different for fictive whisking than when

the follicle sinuses are gourged with blood in awake animals (Rice, 1993), so that the sensitivity of the receptors in the follicle to both self-motion and touch may be diminished in the anesthetized state. Measurements from neurons in the trigeminal ganglion in awake animals are difficult as the ganglion lies in a cranial fossa. Reports from two laboratories provide evidence that different units will spike in different phases of the whisking cycle (Khatri et al., 2009 and Leiser and Moxon, 2007). However, these same units invariably respond to touch as well. While this speaks against the possibility of a solely reafferent pathway, technical considerations suggest that the unit data contained contributions from more than one neuron (Hill et al., 2011b).