Embodied information processing: vibrissa mechanics and texture features shape micromotions in actively sensing rats

Texture coding in the rat whisker system: slip-stick versus differential resonance

Rats discriminate surface textures using their whiskers (vibrissae), but how whiskers extract texture information, and how this information is encoded by the brain, are not known. In the resonance model, whisker motion across different textures excites mechanical resonance in distinct subsets of whiskers, due to variation across whiskers in resonance frequency, which varies with whisker length. Texture information is therefore encoded by the spatial pattern of activated whiskers. In the competing kinetic signature model, different textures excite resonance equally across whiskers, and instead, texture is encoded by characteristic, nonuniform temporal patterns of whisker motion. We tested these models by measuring whisker motion in awake, behaving rats whisking in air and onto sandpaper surfaces. Resonant motion was prominent during whisking in air, with fundamental frequencies ranging from approximately 35 Hz for the long Delta whisker to approximately 110 Hz for the shorter D3 whisker. Resonant vibrations also occurred while whisking against textures, but the amplitude of resonance within single whiskers was independent of texture, contradicting the resonance model. Rather, whiskers resonated transiently during discrete, high-velocity, and high-acceleration slip-stick events, which occurred prominently during whisking on surfaces. The rate and magnitude of slip-stick events varied systematically with texture. These results suggest that texture is encoded not by differential resonant motion across whiskers, but by the magnitude and temporal pattern of slip-stick motion. These findings predict a temporal code for texture in neural spike trains.

Whisker-mediated texture discrimination

Rats use their whiskers to rapidly and accurately measure the texture of objects. The authors evaluate recent evidence about how whisker movement across a surface produces texture-specific motion signals, and how the signals are represented by the brain.

Responses of Rat Trigeminal Ganglion Neurons to Longitudinal Whisker Stimulation

Responses of rat trigeminal ganglion neurons to longitudinal whisker stimulation. Rats use their mobile set of whiskers to actively explore their environment. Parameters that play a role to generate movement dynamics of the whisker shaft within the follicle, thus activating primary afferents, are manifold: among them are mechanical properties of the whiskers (curvature, elasticity and taper), active movements (head, body, and whiskers), and finally, object characteristics (surface, geometry, position, and orientation). Hence the whisker system is confronted with forces along all three axes in space. Movements along the two latitudinal axes of the whisker (horizontal and vertical) have been well studied. Here we focus on movement along the whisker's longitudinal axis that has been neglected so far. We employed ramp-and-hold movements that pushed the whisker shaft toward the skin and quantified the resulting activity in trigeminal first-order afferents in anesthetized rats. Virtually all recorded neurons were highly sensitive to longitudinal movement. Neurons could be perfectly segregated into two groups according to their modulation by stimulus amplitude and velocity, respectively. This classification regimen correlated perfectly with the presence or absence of slowly adapting responses in longitudinal stimulation but agreed with classification derived from latitudinal stimulation only if the whisker was engaged in its optimal direction and set point. We conclude that longitudinal stimulation is an extremely effective means to activate the tactile pathway and thus is highly likely to play an important role in tactile coding on the ascending somatosensory pathway. In addition, compared with latitudinal stimulation, it provides a reliable and easy to use method to classify trigeminal first-order afferents.

Vibrissal kinematics in 3D: tight coupling of azimuth, elevation, and torsion across different whisking modes

Perception is usually an active process by which action selects and affects sensory information. During rodent active touch, whisker kinematics influences how objects activate sensory receptors. In order to fully characterize whisker motion, we reconstructed whisker position in 3D and decomposed whisker motion to all its degrees of freedom. We found that, across behavioral modes, in both head-fixed and freely moving rats, whisker motion is characterized by translational movements and three rotary components: azimuth, elevation, and torsion. Whisker torsion, which has not previously been described, was large (up to 100 degrees), and torsional angles were highly correlated with whisker azimuths. The coupling of azimuth and torsion was consistent across whisking epochs and rats and was similar along rows but systematically varied across rows such that rows A and E counterrotated. Torsional rotation of the whiskers enables contact information to be mapped onto the circumference of the whisker follicles in a predictable manner across protraction-retraction cycles.

Biomechanics of the vibrissa motor plant in rat: rhythmic whisking consists of triphasic neuromuscular activity

The biomechanics of a motor plant constrain the behavioral strategies that an animal has available to extract information from its environment. We used the rat vibrissa system as a model for active sensing and determined the pattern of muscle activity that drives rhythmic exploratory whisking. Our approach made use of electromyography to measure the activation of all relevant muscles in both head-fixed and unrestrained rats and two-dimensional imaging to monitor the position of the vibrissae in head-fixed rats. Our essential finding is that the periodic motion of the vibrissae and mystacial pad during whisking results from three phases of muscle activity. First, the vibrissae are thrust forward as the rostral extrinsic muscle, musculus (m.) nasalis, contracts to pull the pad and initiate protraction. Second, late in protraction, the intrinsic muscles pivot the vibrissae farther forward. Third, retraction involves the cessation of m. nasalis and intrinsic muscle activity and the contraction of the caudal extrinsic muscles m. nasolabialis and m. maxillolabialis to pull the pad and the vibrissae backward. We developed a biomechanical model of the whisking motor plant that incorporates the measured muscular mechanics along with movement vectors observed from direct muscle stimulation in anesthetized rats. The results of simulations of the model quantify how the combination of extrinsic and intrinsic muscle activity leads to an enhanced range of vibrissa motion than would be available from the intrinsic muscles alone.