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 shape is represented in the activity of cutaneous mechanoreceptive peripheral nerve fibers. Cylindrical bars of varying curvature were indented into the receptive fields of slowly and rapidly adapting mechanoreceptive afferent nerve fibers (SAs and RAs respectively) supplying the fingerpad of the anesthetized monkey. The evoked pattern of nerve impulses in single nerve fibers was recorded electrophysiologically. SAs responded to differences in the curvature, both during the ramp and static phases of the skin indentation. RAs responded only during the ramp phase of the indentation, but their responses were not modulated by differences 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 responses rates plotted over the skin surface) of primarily SAs in a spatially distributed population of fibers govern the recognition of the overall object shape as a distribution of curvatures; Intensive parameters (such as the magnitude of discharge 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.