Tactile detection of a dot on a smooth surface: peripheral neural events

The capacities of humans to detect the presence of a single raised dot of 550 micron diameter on a smooth plate and to judge the magnitude of evoked sensation were determined for dots of different heights, stroked at different velocities across the passive fingerpad. Evoked responses to the same stimuli were recorded from single, slowly adapting (SA), rapidly adapting (RA), and Pacinian (PC) mechanoreceptive peripheral nerve fibers innervating the fingerpad of anesthetized macaque monkeys. When the stroke velocity was 10 mm/s, dot height detection thresholds, as determined from measurements of detection sensitivity were between 1 and 3 microns for all human observers. From fiber recordings in monkeys, the RAs had dot height thresholds of 2-4 microns, i.e., within the range of human detection thresholds. The dot height thresholds were 8 microns or greater for SAs and 21 micron or greater for PCs. In contrast, force thresholds for punctate von Frey filaments did not differ for RAs and SAs and were lowest for PCs. The magnitude of sensation evoked in human increased with increases in dot height above threshold. Similarly, the number of nerve impulses evoked in monkey RAs increased with dot height as did the widths of RA receptive fields. Neither changes in stroke velocity from 10 to 40 mm/s nor changes in vertical force applied by the dot plate to the skin altered sensory magnitude evoked by a 15-microns high dot or the number of impulses evoked in RAs. However, a decrease in stroke velocity from 10 to 1.5 mm/s elevated sensory detection thresholds and, for the 15-microns high dot, decreased sensory magnitude, the number of impulses in RAs, and the widths of RA receptive fields. It was hypothesized that the mechanical event responsible for activating the RA was the lateral deformation of elevated regions of skin. In support of this, the number of impulses evoked in RAs by a dot was greater when the dot was stroked across, as opposed to along, the papillary ridges. Also, under certain stimulus conditions, a correspondence was observed between the occurrence of each action potential in an RA and the passage of the leading edge of the dot across the peak of a papillary ridge. It is concluded that the responses of RAs alone account for the sensory capacity to detect a dot of minimal height on a smooth surface with the fingerpad.(ABSTRACT TRUNCATED AT 400 WORDS)