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To address several fundamental questions regarding how multiwhisker tactile stimuli are integrated and processed by the trigeminal somatosensory system, a novel behavioral task was developed that required rats to discriminate the width of either a wide or narrow aperture using only their large mystacial vibrissae. Rats quickly acquired this task and could accurately discriminate between apertures of very similar width. Accurate discriminations required a large number of intact facial whiskers. Systematic removal of individual whiskers caused a decrease in performance that was directly proportional to the number of whiskers removed, indicating that tactile information from multiple whiskers is integrated as rats gauge aperture width. In different groups of rats, different sets of whiskers were removed in patterns that preferentially left whisker rows or whisker arcs intact. These different whisker removals caused similar decreases in performance, indicating that individual whiskers within the vibrissal array are functionally equivalent during performance of this task. Lesions of the barrel cortex abolished the ability of rats to discriminate, demonstrating that this region is critically involved in this tactile behavior. Interestingly, sectioning the facial nerve, which abolished whisker movements, did not affect the ability to perform accurate discriminations, indicating that active whisker movements are not necessary for accurate performance of the task. Collectively, these results indicate that the trigeminal somatosensory system forms internal representations of external stimuli (in this case, aperture width) by integrating tactile input from many functionally equivalent facial whiskers and that the vibrissal array can function as a fine-grained distance detector without active whisker movements.

A system is described that delivers complex, biologically realistic, tactile stimuli to the rat's facial whisker pad by independently stimulating up to 16 individual facial whiskers in a flexible yet highly controlled and repeatable manner. The system is technically simple and inexpensive to construct. The system consists of an array of 16 miniature-solenoid driven actuators that are attached to 16 individual facial whiskers via very small (130 microm dia.) Teflon-coated stainless steel wires. When individual solenoids are energized, the wire is rapidly retracted, resulting in a deflection of individual whiskers. The rise time of deflection is approx. 1 mm/ms. Repeatable stimulation of individual whiskers can be achieved without touching adjacent whiskers, thereby allowing a very high density of stimulators to be attached within the spatially restricted region of the facial whisker pad. Complex patterns of whisker stimulation (designed to mimic biologically realistic stimuli) are delivered to the whisker pad by activating individual solenoid actuators in precisely controlled temporal patterns. These stimulations can be combined with multi-electrode single-unit ensemble recordings at multiple sites within the rat trigeminal somatosensory system. Analysis of neuronal population responses to these complex stimuli is intended to examine how the trigeminal somatosensory system encodes and processes spatiotemporally complex stimuli.