RAISED OBJECT ON A PLANAR SURFACE STROKED ACROSS THE FINGERPAD - RESPONSES OF CUTANEOUS MECHANORECEPTORS TO SHAPE AND ORIENTATION

The representations of orientation and shape were studied in the respo nses of cutaneous mechanoreceptors to an isolated, raised object on a planar surface stroked across the fingerpad. The objects were the top portions of a sphere with a 5-mm radius, and two toroids each with a r adius of 5 mm along one axis and differing radii of 1 or 3 mm along th e orthogonal axis. The velocity and direction of stroking were fixed w hile the orientation of the object in the horizontal plane was varied. Each object was stroked along a series of laterally shifted, parallel , linear trajectories over the receptive fields of slowly adapting, ty pe I (SA), and rapidly adapting, type I (RA) mechanoreceptive afferent s innervating the fingerpad of the monkey. ''Spatial event plots'' (SE Ps) of the occurrence of action potentials, as a function of the locat ion of each object on the receptive field, were interpreted as the res ponses of a spatially distributed population of fibers. That portion o f the plot evoked by the curved object (the SEP,) provided a represent ation of the shape and orientation of the two-dimensional outline of t he object in the horizontal plane in contact with the skin. For both S As and RAs, the major vector of the SEP,, obtained by a principal comp onents analysis, was Linearly related to the physical orientation of t he major axis of each toroid. The spatial distribution of discharge ra tes [spatial rate surface profiles (SRSs), after plotting mean instant aneous frequency versus spatial locus within the SEPc] represented obj ect shape in a third dimension, normal to the skin surface. The shape of the SA SRSs, well fitted by Gaussian equations, better represented object shape than that of the RA SRSs. A cross-sectional profile along the minor axis [spatial rate profile (SRP)] was approximately triangu lar for SAs. After normalization for differences in peak height, the f alling slopes of the SA SRPs increased, and the base widths decreased with curvature of the object's minor axis. These curvature-related dif ferences in slopes and widths were invariant with changes in object or ientation. It is hypothesized that circularity in object shape is code d by the constancy of slopes of SA SRPs between peak and base and that the constancy of differences in the widths and falling slopes evoked by different raised objects encodes, respectively, the differences in their sizes and shapes regardless of differences in their orientation on the skin.