CUTANEOUS NEURAL CODES FOR SHAPE

In the pursuit of peripheral neural representations of shape for the s ense of touch, a series of two- and three-dimensional objects were str oked across the fingerpad of the anesthetized monkey and responses evo ked in cutaneous mechanoreceptive primary afferent nerve fibers record ed. Responses of slowly adapting fibers (SAs) and rapidly adapting fib ers (RAs) were recorded to the stroking of a cylinder, a sphere, sever al ellipsoids, and a pattern of alternating convex and concave cylindr ical bars. The compressional force was maintained constant during a st roke, and the stroke velocities as well as orientations of the objects and stroke trajectories were varied between separate sets of trials. The major geometrical properties of the shapes were well represented i n the spatiotemporal responses of the afferent fiber populations, part icularly those of the SAs. Intensive parameters of shapes, such as the magnitude of change in skin curvature produced as a result of contact with the object surface, were encoded in the discharge rates of SAs a nd RAs, but this neural code was also influenced by changes in stroke velocity. Spatial parameters of shapes such as the extent of contact a nd the changes in contour that characterize a shape as belonging to a particular category (such as a sphere as opposed to a cylinder) are en coded in the spatially distributed discharge rates of the SA populatio n. This spatial response profile provides a neural code that is probab ly invariant with moderate changes in the way the object comes in cont act with the skin, such as the contact force or the orientation of the object.