Automated tracking of head and vibrissae movements in freely-moving and anesthetized rats using videographic methods
Rats move their head and body in order to orient their vibrissae towards areas of interest. The vibrissae are then used to scan the environment and to touch objects in a rhythmical fashion (whisking) at rates of 5-25 Hz. Neural and mechanical events, however, occur at much faster timescales. In order to infer relations between these events, such as that between object touch and spike generation, one must follow whisker movements at high temporal resolution. Here, we describe an automated tracking method for use with both freely- moving and anesthetized rats, based on ultra-fast videography and simple image processing techniques. The position of individual whiskers were described by a three-point spline, which allowed us to describe a whisker in terms of its angular position and its derivates, as well as the degree of bending and moments of object touch and release. These events were then correlated with electrophysiological signals (spikes and LFPs). In the case of anesthetized rats, whisking was elicited by electrical stimulation of the peripheral buccal branch of the facial motor nerve. We show that this method results in a negligible failure rate, and consistent pro/retraction profiles similar to those of behaving animals. Other rats were trained to position their head within a discrimination zone as part of a bilateral discrimination task. Head-movements were accurately followed using the corneal reflection, allowing whiskers to be tracked both relative to features in the immediate environment as well as in a strict head-centered coordinate system. Our method complements existing automated techniques for estimating whisker position, and is unique in its capability of tracking (i) the change in shape of the whisker, and (ii) multiple whiskers simultaneously in freely moving, whisking rats. Support Contributed By: BSF grant 2000299, MINERVA Foundation
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