Optoelectronic monitoring of individual whisker movements in rats

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Abstract

We describe two systems for the real-time recording and display of individual vibrissa movements in head-fixed rats. Both systems utilize high-speed, linear image sensors, each composed of an array of light sensitive elements (CCDs). Uniform illumination of the array generates a constant baseline voltage in each element. The shadow produced by the movement of a whisker interposed between the light source and the sensors produces a voltage shift in a subset of elements. The successive position of the shift is linearly related to the momentary whisker position. Associated software/hardware scans the array at regular intervals to identify the successive positions of voltages above a preset threshold and outputs the data to a microprocessor for computation of the whisker movement trajectory. In both systems, movements of a single whisker may be monitored ‘on-line’ with high spatial and temporal resolution; in one case with, in the other without the presence of neighboring whiskers. Optoelectronic monitoring facilitates rapid and efficient (computer-assisted) acquisition and analysis of data on rodent whisking behavior.

Introduction

The rat’s mystacial vibrissae comprise a set of sensory hairs arranged in an invariant species/strain-specific grid pattern and embedded in mystacial pads on the rat’s snout. The spatial organization of the vibrissae representation at various brain levels replicates, and is functionally isomorphic with, the pattern of individual whiskers on the face. These ‘modular’ properties have made the rat’s mystacial vibrissa a widely used ‘model system’ for studies of sensory processing, development and neuronal plasticity (Ebner and Armstrong-James, 1990; Woolsey, 1990; Chalupa, 1995; Jones and Diamond, 1995). Each individual vibrissa is a miniature somatosensorimotor system associated with a trigeminally innervated receptor complex and controlled by facial nerve efferents (Dörfl, 1982, Dörfl, 1985; Rice et al., 1986). Whisking generates patterns of somatosensory input which are used to guide the movements of diverse effectors (neck, jaw, limbs), and, recursively, to control movements of the vibrissae themselves. During exploratory and discriminative behaviors, individual vibrissae function as elements of a receptive array scanned across object surfaces in repetitive, ‘saccade’-like whisks. The rat’s discrimination performance depends upon its ability to modulate vibrissa movement parameters to meet the functional requirements of the discriminative task (Carvell and Simons, 1995).

Videographic analysis of vibrissa movements during texture discriminations (Carvell and Simons, 1990) suggests that whisking movements occur at frequencies between 1 and 25 Hz, across a velocity range from <500 to >1000°/s, and over a range of amplitudes from microns to millimeters. The monitoring of whisker movement trajectories thus requires a high level of spatial and temporal resolution. However, both the spatial and temporal resolution of videography are relatively low, and quantitative analysis of videographic data is extremely time-consuming. It is not surprising, therefore, that the massive literature on vibrissal anatomy and physiology (e.g. Jones and Diamond, 1995) contrasts markedly with the paucity of neurobehavioral studies of whisking behavior.

We have previously described a technique for the operant conditioning of vibrissa movements in the head-fixed rat, which provides a high level of stimulus and behavioral control of whisking (Bermejo et al., 1996a). However, the technique provides no information about the kinematics of vibrissa movements. We now describe optoelectronic methods which permit ‘on-line’ tracking of individual whisker movement trajectories in the head-fixed rat, with appropriate spatio-temporal resolution. A preliminary account of this work was presented at a meeting of the Society for Neuroscience, Washington, D.C., 1996 (Bermejo et al., 1996b).

Section snippets

Subjects and head fixation

Subjects were female Long–Evans and Sprague–Dawley rats, 90–120 days old. Anesthetized rats were fitted with a platform of dental cement, which had a mounting screw (Small Parts # Q-TSB-632-12) embedded in its central portion. They were maintained on a 23.5 h water deprivation schedule, with food available in the home cage ad lib, and handled for a brief period each day for about a week. For testing, the rat was placed in a restraining device which completely enclosed its body, while its head

Kinematic analysis of whisker movements

In several rats, all but a single pair of bilaterally homologous whiskers (C-1) were clipped to the base, prior to testing, and movements of these vibrissae were monitored using the CCD system. The whisking data sets were stored as individual files and analysed ‘off-line’. Individual vibrissa movements were displayed on a computer monitor by specially written software. For kinematic analysis a single waveform was isolated and software programs were used to identify ‘critical points’ in the

Discussion

Instrumentation for the measurement of whisking movements should detect movements of an individual whisker (a) in the presence of neighboring whiskers, (b) with adequate spatial and temporal resolution, (c) without biasing the selection of movement topographies or (d) altering movement kinematics. Both systems described in this report track the movements of a single whisker with adequate spatio-temporal resolution. The biometric data obtained with both systems are consistent with those obtained

Conclusions

These optoelectronic systems do not provide information on the duration or topography of whisker contacts, and can access only movements of a single vibrissa. It will therefore be important to interpret optoelectronically-derived data within the context provided by videographic analysis. However, optoelectronic systems make possible ‘real-time’ tracking of whisker movement trajectories with spatial and temporal resolutions significantly higher than videographic methods. Both systems described

Acknowledgements

Supported by NSF Grant IBN-93-19486, Grant MH-00836 and Research Scientist Award (K05-MH00320) from the National Institutes of Mental Health.

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