Whisker deafferentation and rodent whisking patterns: Behavioral evidence for a central pattern generator

Even in the absence of explicit stimulation, rats emit patterns of rhythmic whisking movements. Because of their stereotyped nature and their persistence after sensory denervation and cortical ablation, whisking movements have been assumed to reflect the output of a central pattern generator (CPG). However, identification of a movement pattern as the product of a CPG requires evidence that its generation, patterning, and coordination are independent of sensory input. To provide such evidence, we used optoelectronic instrumentation to obtain high-resolution records of the movement trajectories of individual whiskers in rats whose heads were fixed to isolate their exploratory whisking from exafferent inputs. Unconditioned whisking patterns were quantitatively characterized by a biometric analysis of the kinematics, rhythmicity, and coordination of bilaterally homologous vibrissa movements. Unilateral and bilateral sectioning of the infraorbital nerve, which innervates the whiskers, was then performed to block reafferent inputs generated by the animal's own whisking movements. Unilateral sectioning of the nerve has no effect on whisking kinematics but is followed by a significant but relatively transient bilateral increase in whisking frequency. However, bilateral deafferentation, when performed in a single-stage procedure, does not disrupt the generation, patterning, or bilateral coordination of whisking patterns in the rat. These findings provide strong behavioral evidence for a whisking CPG and are discussed in relation to its possible location and properties.