Cutaneous neural codes for shape

In the pursuit of peripheral neural representations of shape for the sense of touch, a series of two- and three-dimensional objects were stroked across the fingerpad of the anesthetized monkey and responses evoked in cutaneous mechanoreceptive primary afferent nerve fibers recorded. Responses of slowly adapting fibers (SAs) and rapidly adapting fibers (RAs) were recorded to the stroking of a cylinder, a sphere, several ellipsoids, and a pattern of alternating convex and concave cylindrical bars. The compressional force was maintained constant during a stroke, 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 in the spatiotemporal responses of the afferent fiber populations, particularly 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 and RAs, but this neural code was also influenced by changes in stroke velocity. Spatial parameters of shapes such as the extent of contact and the changes in contour that characterize a shape as belonging to a particular category (such as a sphere as opposed to a cylinder) are encoded in the spatially distributed discharge rates of the SA population. This spatial response profile provides a neural code that is probably invariant with moderate changes in the way the object comes in contact with the skin, such as the contact force or the orientation of the object.