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Mitra J Hartmann

Publications and source records attributed to Mitra J Hartmann.

6 recordsLinked to original sources
PubMed2006

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.

Animals↗
PubMed2006

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.

Animals↗
PubMed2005

Correlations between purkinje cell single-unit activity and simultaneously recorded field potentials in the immediately underlying granule cell layer.

Evidence from both anatomical and physiological studies suggests that the ascending segment of the granule cell axon provides a large, driving input to overlying Purkinje cells. In the current experiments, we used dual recording electrodes to simultaneously record spike activity of Purkinje cells and multiunit field potential activity in the directly underlying granule cell layer. These dual recordings were performed both during periods of spontaneous ("background") firing and also after peripheral tactile stimulation. The results demonstrate that in the large majority of cases, there is a strong positive correlation between spontaneous Purkinje cell simple spikes and spontaneous activity in the immediately underlying granule cell layer. The strength of this correlation was dependent on both the firing rate of the Purkinje cell as well as on the rate of granule cell layer multiunit activity. In addition, for any given pair of recordings, the correlation seen during spontaneous activity accurately predicted the magnitude and time course of responses evoked by peripheral tactile stimulation. These results provide additional evidence that the synapses associated with the ascending segment of the granule cell axon have a substantial influence on Purkinje cell output. This relationship is considered in the context of our ongoing reevaluation of the physiological relationship between cerebellar granule and Purkinje cells.

Action Potentials↗
PubMed2003

Mechanical characteristics of rat vibrissae: resonant frequencies and damping in isolated whiskers and in the awake behaving animal.

We investigated the natural resonance properties and damping characteristics of rat macrovibrissae (whiskers). Isolated whiskers rigidly fixed at the base showed first-mode resonance peaks between 27 and 260 Hz, principally depending on whisker length. These experimentally measured resonant frequencies were matched using a theoretical model of the whisker as a conical cantilever beam, with Young's modulus as the only free parameter. The best estimate for Young's modulus was approximately 3-4 GPa. Results of both vibration and impulse experiments showed that the whiskers are strongly damped, with damping ratios between 0.11 and 0.17. In the behaving animal, whiskers that deflected past an object were observed to resonate but were damped significantly more than isolated whiskers. The time course of damping varied depending on the individual whisker and the phase of the whisking cycle, which suggests that the rat may modulate biomechanical parameters that affect damping. No resonances were observed for whiskers that did not contact the object or during free whisking in air. Finally, whiskers on the same side of the face were sometimes observed to move in opposite directions over the full duration of a whisk. We discuss the potential roles of resonance during natural exploratory behavior and specifically suggest that resonant oscillations may be important in the rat's tactile detection of object boundaries.

Animals↗
PubMed2003

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

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.

Bionics↗