Active sensing: Head and vibrissal velocity during exploratory behaviors of the rat

The vibrissal-trigeminal pathway of the rat has become an increasingly important model in neuroscience to study how sensory and motor signals are encoded, processed, and integrated in the nervous system, ultimately yielding perception of an object. In this chapter, we focus specifically on the role of head and vibrissa (whisker) velocity during exploratory movements. The chapter begins by describing basic vibrissal anatomy and mechanics, and shows that different studies measure vibrissa velocity under very different mechanical conditions, which will give rise to very different types of mechanoreceptor activation. It is thus critical to consider forces and bending moments at the whisker base in addition to vibrissa velocity when quantifying vibrissa-object contact during natural behavior. To illustrate this point, we summarize recent results demonstrating that whisking velocity at the time of collision with an object may influence the rat's ability to determine the radial distance to the object as well as the horizontal angle of contact. Further, we present evidence suggesting that the rat may actively select velocities at different points in the whisking trajectory, perhaps to aid localization behavior in these two dimensions. Finally, because the whiskers are always acting in concert with the head, we describe correlations between whisking behavior and head velocity. Preliminary data suggest that the position, orientation, and velocity of the head - which moves at a very different spatial and temporal scale than the vibrissae - will have a large effect on the tactile information acquired by the vibrissal system.