RESPONSES OF CUTANEOUS MECHANORECEPTORS TO THE SHAPE OF OBJECTS APPLIED TO THE PRIMATE FINGERPAD

The Present study is one of a series whose aim is to determine how sha pe is represented in the activity of cutaneous mechanoreceptive periph eral nerve fibers. Cylindrical bars of varying curvature were indented into the receptive fields of slowly and rapidly adapting mechanorecep tive afferent nerve fibers (SAs and RAs respectively) supplying the fi ngerpad of the anesthetized monkey. The evoked pattern of nerve impuls es in single nerve fibers was recorded electrophysiologically. SAs res ponded to differences in the curvature, both during the ramp and stati c phases of the skin indentation. RAs responded only during the ramp p hase of the indentation, but their responses were not modulated by dif ferences in curvature. Evidence from the present and previous studies is used to support the following hypotheses: Spatial parameters (such as the 'shapes' or 'widths' of response rates plotted over the skin su rface) of primarily SAs in a spatially distributed population of fiber s govern the recognition of the overall object shape as a distribution of curvatures; Intensive parameters (such as the magnitude of dischar ge rates) of only SAs under static indentations, and both SAs and RAs under stroking, are important for discriminations of small differences in curvatures of objects belonging to the same category of shape.