Responses of human mechanoreceptive afferents to embossed dot arrays scanned across fingerpad skin.

The spatial resolving capacities of the four classes of mechanoreceptive afferents innervating human fingerpad skin were investigated to determine which class sets the limit of tactile spatial resolution for scanning stimuli. The stimulus consisted of an array of embossed dots (0.7 mm diameter, 0.5 mm high) arranged in a tetragonal pattern with dot spacing decreasing linearly from 6.4 mm at one end of the array to 0.87 mm at the other. The pattern was wrapped around a drum and repeatedly scanned across the receptive field of single afferents by continuously rotating the drum. Responses to many closely spaced scans were obtained by imposing a lateral shift of the pattern between each revolution. Impulses were recorded microneurographically. Responses were plotted in raster form to produce a neural image of the pattern. Responses of rapidly and slowly adapting type I (FAI and SAI) afferents resolved dots down to a spacing of about 1.5 mm. Responses of type II (FAII and SAII) afferents resolved dots down to a spacing of about 3.5 mm. Variation in scanning speed (range, 20-90 mm/sec) and contact force (range, 0.4-1.0 N) had minimal effects on spatial resolution of all afferents. The response clusters associated with individual widely spaced dots were used to investigate receptive field structure. FAI and SAI fields (mean areas, 6.1 and 4.8 mm2, respectively) each contained several zones of maximal sensitivity. FAI fields had five to eight such zones, whereas SAI fields had three to five such zones. As dot spacing decreased, neighboring dots interacted to affect the responses associated with the individual zones within a field. Initially, one or more zones were deactivated, effectively reducing receptive field size and allowing representation of finer spatial detail than would be predicted from the overall area of the receptive field. At very close dot spacings responses were only obtained when more than one sensitive zone within a field were simultaneously activated by different dots.