Mitra J. Z. Hartmann

Right–Left Asymmetries in the Whisking Behavior of Rats Anticipate Head Movements

Rats use rhythmic movements of their vibrissae (whiskers) to tactually explore their environment. This “whisking” behavior has generally been reported to be strictly synchronous and symmetric about the snout, and it is thought to be controlled by a brainstem central pattern generator. Because the vibrissae can move independently of the head, however, maintaining a stable perception of the world would seem to require that rats adjust the bilateral symmetry of whisker movements in response to head movements. The present study used high-speed videography to reveal dramatic bilateral asymmetries and asynchronies in free-air whisking during head rotations. Kinematic analysis suggested that these asymmetric movements did not serve to maintain any fixed temporal relationship between right and left arrays, but rather to redirect the whiskers to a different region of space. More specifically, spatial asymmetry was found to be strongly correlated with rotational head velocity, ensuring a “look-ahead” distance of almost exactly one whisk. In contrast, bilateral asynchrony and velocity asymmetry were only weakly dependent on head velocity. Bilateral phase difference was found to be independent of the whisking frequency, suggesting the presence of two distinct left and right central pattern generators, connected as coupled oscillators. We suggest that the spatial asymmetries are analogous to the saccade that occurs during the initial portion of a combined head–eye gaze shift, and we begin to develop the rat vibrissal system as a new model for studying vestibular and proprioceptive contributions to the acquisition of sensory data.

Biomechanics: Robotic whiskers used to sense features

Whiskers mimicking those of seals or rats might be useful for underwater tracking or tactile exploration.Several species of terrestrial and marine mammals with whiskers (vibrissae) use them to sense and navigate in their environment — for example, rats use their whiskers to discern the features of objects, and seals rely on theirs to track the hydrodynamic trails of their prey. Here we show that the bending moment — sometimes referred to as torque — at the whisker base can be used to generate three-dimensional spatial representations of the environment, and we use this principle to construct robotic whisker arrays that extract precise information about object shape and fluid flow. Our results will contribute to the development of versatile tactile-sensing systems for robotic applications, and demonstrate the value of hardware models in understanding how sensing mechanisms and movement control strategies are interlocked.

The Brain in Its Body: Motor Control and Sensing in a Biomechanical Context

Although it is widely recognized that adaptive behavior emerges from the ongoing interactions among the nervous system, the body, and the environment, it has only become possible in recent years to experimentally study and to simulate these interacting systems. We briefly review work on molluscan feeding, maintenance of postural control in cats and humans, simulations of locomotion in lamprey, insect, cat and salamander, and active vibrissal sensing in rats to illustrate the insights that can be derived from studies of neural control and sensing within a biomechanical context. These studies illustrate that control may be shared between the nervous system and the periphery, that neural activity organizes degrees of freedom into biomechanically meaningful subsets, that mechanics alone may play crucial roles in enforcing gait patterns, and that mechanics of sensors is crucial for their function.

An in silico central pattern generator: silicon oscillator, coupling, entrainment, and physical computation

Abstract. In biological systems, the task of computing a gait trajectory is shared between the biomechanical and nervous systems. We take the perspective that both of these seemingly different computations are examples of physical computation. Here we describe the progress that has been made toward building a minimal biped system that illustrates this idea. We embed a significant portion of the computation in physical devices, such as capacitors and transistors, to underline the potential power of emphasizing the understanding of physical computation. We describe results in the exploitation of physical computation by (1) using a passive knee to assist in dynamics computation, (2) using an oscillator to drive a monoped mechanism based on the passive knee, (3) using sensory entrainment to coordinate the mechanics with the neural oscillator, (4) coupling two such systems together mechanically at the hip and computationally via the resulting two oscillators to create a biped mechanism, and (5) demonstrating the resulting gait generation in the biped mechanism.

Haptically Linked Dyads: Are Two Motor-Control Systems Better Than One?

Are Two Motor-Control Systems Better Than One? Kyle Reed, Michael Peshkin, Mitra J. Hartmann, Marcia Grabowecky, James Patton, and Peter M. Vishton Department of Mechanical Engineering, Northwestern University; Department of Biomedical Engineering, Northwestern University; Department of Psychology, Northwestern University; Rehabilitation Institute of Chicago, Chicago, Illinois; and Department of Psychology, College of William and Mary

The morphology of the rat vibrissal array: a model for quantifying spatiotemporal patterns of whisker-object contact.

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 whiskers identity (row and column location within the array), the equations establish the whiskers 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.

Variability in Velocity Profiles During Free-Air Whisking Behavior of Unrestrained Rats

During exploratory behaviors, the velocity of an organisms sensory surfaces can have a pronounced effect on the incoming flow of sensory information. In this study, we quantified variability in the velocity profiles of rat whisking during natural exploratory behavior that included head rotations. A wide continuum of profiles was observed, including monotonic, delayed, and reversing velocities during protractions and retractions. Three alternative hypotheses for the function of the variable velocity profiles were tested: 1) that they produce bilateral asymmetry specifically correlated with rotational head velocity, 2) that they serve to generate bilaterally asymmetric and/or asynchronous whisker movements independent of head velocity, and 3) that the different profiles--despite increasing variability in instantaneous velocity--reduce variability in the average whisking velocity. Our results favor the third hypothesis and do not support the first two. Specifically, the velocity variability within a whisk can be observed as a shift in the phase of the maximum velocity. We discuss the implications of these results for the control of whisker motion, horizontal object localization, and processing in the thalamus and cortex of the rat vibrissal system.

Toward biomorphic control using custom aVLSI CPG chips

The locomotor controller for walking, running, swimming, and flying animals is based on a central pattern generator (CPG). Models of CPGs as systems of coupled nonlinear oscillators have been proposed and have been used for the control of robots. In this paper we describe the implementation of an adaptive CPG model in a compact, custom analog VLSI circuit. We demonstrate the function of the chip by controlling an underactuated, running robotic leg. This circuit has adaptive properties that allow it to tune its behavior based on sensory feedback. To our knowledge this is the first instance of an adaptive CPG chip. This approach supports the construction of extremely inexpensive, low power and compact controllers for walking, flying and swimming machines.

Mechanical signals at the base of a rat vibrissa: the effect of intrinsic vibrissa curvature and implications for tactile exploration.

Rats actively tap and sweep their large mystacial vibrissae (whiskers) against objects to tactually explore their surroundings. When a vibrissa makes contact with an object, it bends, and this bending generates forces and bending moments at the vibrissa base. Researchers have only recently begun to quantify these mechanical variables. The present study quantifies the forces and bending moments at the vibrissa base with a quasi-static model of vibrissa deflection. The model was validated with experiments on real vibrissae. Initial simulations demonstrated that almost all vibrissa-object collisions during natural behavior will occur with the concave side of the vibrissa facing the object, and we therefore paid particular attention to the role of the vibrissas intrinsic curvature in shaping the forces at the base. Both simulations and experiments showed that vibrissae with larger intrinsic curvatures will generate larger axial forces. Simulations also demonstrated that the range of forces and moments at the vibrissal base vary over approximately three orders of magnitude, depending on the location along the vibrissa at which object contact is made. Both simulations and experiments demonstrated that collisions in which the concave side of the vibrissa faces the object generate longer-duration contacts and larger net forces than collisions with the convex side. These results suggest that the orientation of the vibrissas intrinsic curvature on the mystacial pad may increase forces during object contact and provide increased sensitivity to detailed surface features.

Variation in Young's modulus along the length of a rat vibrissa

Rats use specialized tactile hairs on their snout, called vibrissae (whiskers), to explore their surroundings. Vibrissae have no sensors along their length, but instead transmit mechanical information to receptors embedded in the follicle at the vibrissa base. The transmission of mechanical information along the vibrissa, and thus the tactile information ultimately received by the nervous system, depends critically on the mechanical properties of the vibrissa. In particular, transmission depends on the bending stiffness of the vibrissa, defined as the product of the area moment of inertia and Youngs modulus. To date, Youngs modulus of the rat vibrissa has not been measured in a uniaxial tensile test. We performed tensile tests on 22 vibrissae cut into two halves: a tip-segment and a base-segment. The average Youngs modulus across all segments was 3.34±1.48GPa. The average modulus of a tip-segment was 3.96±1.60GPa, and the average modulus of a base-segment was 2.90±1.25GPa. Thus, on average, tip-segments had a higher Youngs modulus than base-segments. High-resolution images of vibrissae were taken to seek structural correlates of this trend. The fraction of the cross-sectional area occupied by the vibrissa cuticle was found to increase along the vibrissa length, and may be responsible for the increase in Youngs modulus near the tip.