The Morphology of the Rat Vibrissal Array: A Model for Quantifying Spatiotemporal Patterns of Whisker-Object Contact

Abstract

In all sensory modalities, the data acquired by the nervous system is shaped by the biomechanics, material properties, and the morphology of the peripheral sensory organs. The rat vibrissal (whisker) system is one of the premier models in neuroscience to study the relationship between physical embodiment of the sensor array and the neural circuits underlying perception. To date, however, the three-dimensional morphology of the vibrissal array has not been characterized. Quantifying array morphology is important because it directly constrains the mechanosensory inputs that will be generated during behavior. These inputs in turn shape all subsequent neural processing in the vibrissal-trigeminal system, from the trigeminal ganglion to primary somatosensory (“barrel”) cortex. Here we develop a set of equations for the morphology of the vibrissal array that accurately describes the location of every point on every whisker to within ±5% of the whisker length. Given only a whisker's identity (row and column location within the array), the equations establish the whisker's two-dimensional (2D) shape as well as three-dimensional (3D) position and orientation. The equations were developed via parameterization of 2D and 3D scans of six rat vibrissal arrays, and the parameters were specifically chosen to be consistent with those commonly measured in behavioral studies. The final morphological model was used to simulate the contact patterns that would be generated as a rat uses its whiskers to tactually explore objects with varying curvatures. The simulations demonstrate that altering the morphology of the array changes the relationship between the sensory signals acquired and the curvature of the object. The morphology of the vibrissal array thus directly constrains the nature of the neural computations that can be associated with extraction of a particular object feature. These results illustrate the key role that the physical embodiment of the sensor array plays in the sensing process.

Biomimetic vibrissal sensing for robots

“…Still frames taken from a simulated rendering of (a) SCRATCHbot and (b) Shrewbot emphasizing the form of their whisker fields and inferred sensory surface, interpolated between the whisker tips (shown in brown), at different phases of whisking. Shrewbot is able to form a radially symmetric sensory surface with a concave or convex profile similar to that observed in rats [27,22]. The less biomimetic whisker array morphology of SCRATCHbot is incapable of forming a convex sensory surface.…”

“…From this graphical rendering of such a simulation, it is clear that the sensory surface of Shrewbot has potential advantages over SCRATCHbot for spatial exploration. Specifically, the surface surrounds the head in a continuous fashion and can be formed into both concave and convex shapes as has been described for rats [22]. The ability to form a smooth convex surface (with respect to the robot), when the whiskers are fully protracted, is largely owing to the exponential increase in length of the whiskers along the row.…”

“…the more rostral macrovibrissae are exponentially shorter than the more caudal whiskers in the same row. In constrast, measures of the distribution in lengths of macrovibrissae across the array have found no strong dorsal-ventral trend [21,22].…”

“…The morphological properties of the rat vibrissal system have been analysed in detail by Towal et al [22], Hartmann and co-workers [23] and Birdwell et al [24] who have shown that they make a significant contribution to the types of signals obtained through whisker-environment contacts. Here, we briefly discuss morphological features of biological vibrissal systems and their artificial counterparts together with sensor transduction and aspects of active sensing control.…”

Radial distance determination in the rat vibrissal system and the effects of Weber's law

“…Obviously, if the rat ensured that the tip of a particular vibrissa of known linear base-to-tip length made contact with an object, then it could infer that the object was located at a distance equal to that length. This strategy may work particularly well for the most rostral vibrissae, which tend to point concave forward during the course of a whisk, increasing the chances for tip contact [17,22]. Two problems arise with this method for distance determination.…”