The response of single guard and down hair mechanoreceptors to moving air-jet stimulation.

The response properties of single guard (G) and down (D) hair afferent nerve fibers innervating the hairy skin of the hindlimb were studied in acute barbiturate-anesthetized cats. The purpose of the study was to identify and analyze the relative contribution of those stimulus features determining the discharge patterns evoked in single afferents by a fine air-jet stimulus moving across the skin and varying in force, velocity, position, direction and orientation. The response of single G hair afferents to moving air-jet stimuli reveals that the responsiveness of each fiber to stimuli with arbitrary orientation, direction and position within the receptive field (RF) displays an optimum velocity sensitivity which is not predictable from punctate data. Although the response pattern is remarkably consistent for each moving stimulus condition, there are significant differences in response as a function of stimulus orientation, direction and velocity. RF 'maps' constructed from the responses evoked as the air-jet traverses the skin reveal multiple zones of high and low sensitivity. The distribution of sensitive zones is remarkably consistent for maps constructed with stimuli varying in orientation, direction and velocity. It is apparent that the principal determinant of the response for a given stimulus traverse is the spatial distribution of sensitive spots throughout the RF. Although noticeably more uniform in sensitivity, the RFs of D hair afferents demonstrate similar properties. These findings indicate that G and D hair afferent nerve fibers respond more vigorously to moving stimuli than to stationary displacement and display complex RF inhomogeneities which must be taken into account for the study of central neuronal information processing and feature extraction.