Demonstration of three-dimensional contact point determination and contour reconstruction during active whisking behavior of an awake rat

Abstract

The rodent vibrissal (whisker) system has been studied for decades as a model of active touch sensing. There are no sensors along the length of a whisker; all sensing occurs at the whisker base. Therefore, a large open question in many neuroscience studies is how an animal could estimate the three-dimensional (3D) location at which a whisker makes contact with an object. In the present work we simulated the shape of a real rat whisker to demonstrate the existence of several unique mappings from triplets of mechanical signals at the whisker base to the three-dimensional whisker-object contact point. We then used high speed video to record whisker deflections as an awake rat whisked against a peg, and used the mechanics resulting from those deflections to extract the contact points along the peg surface. These results demonstrate that measurement of specific mechanical triplets at the base of a biological whisker can enable 3D contact point determination during natural whisking behavior. The approach is viable even though the biological whisker has non-ideal, non-planar curvature, and even given the rat’s real-world choices of whisking parameters. Visual intuition for the quality of the approach is provided in a video that shows the contour of the peg gradually emerging during active whisking behavior. Author summary When a rodent explores its environment, it often rhythmically taps its whiskers against objects to obtain tactile (touch) information. This behavior is called “whisking.” In the field of neuroscience, whisking is used to investigate how animals combine tactile information with movement to explore objects. Like a hair, a whisker has no sensors along its length–all sensors are at the whisker base. So how do a rat’s whiskers convey information about the three-dimensional (3D) location of an object to the rat’s brain? When a whisker touches an object, six mechanical signals are generated at the whisker base. Previous work has shown that only three of these six mechanical signals are required to determine 3D contact location, but this previous work was performed only in simulation, using idealized deflections of idealized whiskers. Here we identify mappings between mechanical signals and 3D object location during active whisking of a real rat. We then use one mapping to show a sequence of whisker-object contact points that gradually reveals the contour of the object. This work shows that at least one triplet of mechanical signals is sufficient to extract 3D contours through touch, even though real-world whisking includes friction, dynamics, and non-idealized whisker geometries.