Beyond cones: an improved model of whisker bending based on measured mechanics and tapering

Abstract

The sense of touch is represented by neural activity patterns evoked by mechanosensory input forces. The rodent whisker system is exceptional for studying the neurophysiology of touch in part because these forces can be precisely computed from video of whisker deformation. We evaluate the accuracy of a standard model of whisker bending, which assumes quasi-static dynamics and a linearly tapered conical profile, using controlled whisker deflections. We find significant discrepancies between model and experiment: real whiskers bend more than predicted upon contact at locations in the middle of the whisker and less at distal locations. Thus whiskers behave as if their stiffness near the base and near the tip is larger than expected for a homogeneous cone. We assess whether contact direction, friction, inhomogeneous elasticity, whisker orientation, or nonconical shape could explain these deviations. We show that a thin-middle taper of mouse whisker shape accounts for the majority of this behavior. This taper is conserved across rows and columns of the whisker array. The taper has a large effect on the touch-evoked forces and the ease with which whiskers slip past objects, which are key drivers of neural activity in tactile object localization and identification. This holds for orientations with intrinsic whisker curvature pointed toward, away from, or down from objects, validating two-dimensional models of simple whisker-object interactions. The precision of computational models relating sensory input forces to neural activity patterns can be quantitatively enhanced by taking thin-middle taper into account with a simple corrective function that we provide.

Active Touch and Self-Motion Encoding by Merkel Cell-Associated Afferents

“…We treated each whisker as a cone based on its radius at base and its length. Consistent with prior work (Hires et al, 2016), we found that the shape of individual whiskers deviated slightly from that of a pure cone (not shown), with uncertainty in I p of ~50%. Taken together, our estimates of uncertainty in E , I p and Δ k p imply that our reported values of absolute bending moment and forces must be considered approximations, accurate to no better than a factor of two (Taylor, 1997).…”

The Sensorimotor Basis of Whisker-Guided Anteroposterior Object Localization in Head-Fixed Mice

“…Many whiskers are often missing from cohabitating mice, due to trimming associated with social hierarchy in C57/BL6 mice [46]. The progressive length of whiskers across the pad [13] prevents short whiskers from reaching distant objects. Rodents use tactile feedback to adapt their whisking pattern, minimizing impingement during object contact [47].…”

“…Rodents can determine the location of objects by active exploration with whiskers [8], even when head fixed [9]. High-speed videography [10] and physical models [11][12][13][14] can quantify motion and forces that drive whisker input with submillisecond resolution during behavior [15,16]. This input is transformed and integrated in a topographic arrangement of columns in primary somatosensory cortex (S1) that have a one-to-one mapping to individual whiskers.…”

Dynamic cues for whisker-based object localization: An analytical solution to vibration during active whisker touch

“…This supports the possibility that rodents could use vibration as a cue for distance to object. However, rodent whiskers are approximately conical [ 14 , 15 ], with the center of mass one quarter length from the whisker base. This provides conical whiskers with distinct vibrational properties.…”