Simulations of a Vibrissa Slipping along a Straight Edge and an Analysis of Frictional Effects during Whisking

Abstract

During tactile exploration, rats sweep their whiskers against objects in a motion called whisking. Here we investigate how a whisker slips along an object’s edge and how friction affects the resulting tactile signals. First, a frictionless model is developed to simulate whisker slip along a straight edge and compared with a previous model that incorporates friction but cannot simulate slip. Results of both models are compared to behavioral data obtained as a rat whisked against a smooth, stainless steel peg. As expected, the frictionless model predicts larger magnitudes of vertical slip than observed experimentally. The frictionless model also predicts forces and moments at the whisker base that are smaller and have a different direction than those predicted by the model with friction. Estimates for the friction coefficient yielded values near 0.48 (whisker/stainless steel). The present work provides the first assessments of the effects of friction on the mechanical signals received by the follicle during active whisking. It also demonstrates a proof-of-principle approach for reducing whisker tracking requirements during experiments and demonstrates the feasibility of simulating a full array of vibrissae whisking against a peg.

Mechanical responses of rat vibrissae to airflow

“…Shorter whiskers have higher intrinsic curvature Towal et al, 2011;Quist and Hartmann, 2012), explaining why the effect of orientation angle appears to be related to whisker length. From a mechanical point of view, the intrinsic curvature of the whisker allows it to 'twist' about its own axis, and will have a nonlinear effect on both axial force and bending (Huet et al, 2015;Huet and Hartmann, 2016). The results for the longer and shorter whiskers are averaged in Fig.…”

“…In contrast, when the whiskers are protracted against an object, they will tend to slip in different directions depending on their kinematic trajectory, their intrinsic curvature, object geometry and friction Hartmann, 2008, 2010;Hobbs et al, 2016;Huet et al, 2015;Huet and Hartmann, 2016).…”

Whisking mechanics and active sensing

“…Quantifying these contact variables requires either mechanical modeling [10,15,28,33,37–39,42,43,44••] or the use of geometric proxies. For example, change in curvature near the whisker base during contact is sometimes an appropriate proxy for change in bending moment at the whisker base [45•,46].…”

“…Because the whisker rolls as it protracts, the orientation of the whisker varies systematically through the whisking cycle [12,29]. In turn, the whisker’s orientation at the time of object contact will determine the direction in which it is deflected by the object [10,11••,13•,41,42]. The direction of whisker bending could thus provide a mechanism for the rat to determine the horizontal angle at which the whisker has made contact with the object [12,13•].…”

“…Recent advances in three-dimensional (3D) whisker mechanics [10,11••,12,13•] offer the opportunity to compute the complete set of tactile inputs transmitted by the vibrissae during active tactile exploration. The goal of the present paper is to review recent literature so as to establish a unified framework for describing the geometric and mechanical variables relevant to whisking behavior.…”