NEURAL ENCODING OF SHAPE - RESPONSES OF CUTANEOUS MECHANORECEPTORS TOA WAVY SURFACE STROKED ACROSS THE MONKEY FINGERPAD

1. The role of cutaneous mechanoreceptors in the tactile perception of shape was investigated. Objects whose surfaces were shaped as a patte rn of smooth, alternating convex and concave cylindrical surfaces of d iffering radii of curvature were constructed such that there were no d iscontinuities in the slope of the surface. These ''wavy surfaces'' we re stroked across the fingerpad of the anesthetized monkey and electro physiological responses of slowly adapting type I mechanoreceptive aff erents (SAs) and rapidly adapting type I mechanoreceptive afferents (R As) were recorded. 2. For both SAs and RAs, each convexity indenting t he skin evoked a burst of impulses and each concavity of the same curv ature that followed elicited a pause in response. ''Spatial event plot s'' (SEPs) of the occurrence of action potentials as a function of the location of the object on the receptive field were obtained and inter preted as the responses of a spatially distributed population of fiber s. With increasing magnitude of curvature (equivalently, decreasing ra dius of curvature) of convexity, the mean width of the burst in the SE Ps for each fiber type (representing the width of a region of skin con taining active fibers) decreased and the mean discharge rate during th e burst increased. Over a range of velocities of stroking from 1 to 40 mm/s, the number of RAs activated increased with velocity, whereas SA s were active at all velocities. For both SAs and RAs, the burst rates increased with velocity, whereas the widths of the bursts and pauses remained approximately invariant. Thus the spatial measures of burst o r pause width provide a robust representation of the size of a feature on the object surface. 3. For a given velocity of stroking, the spati ally distributed pattern of averaged discharge rates (spatial rate pro file, SRP) provided a representation of the shape of the wavy surface. The distance between neighboring peaks in the SRP for individual RAs and SAs was approximately the same as the distance between the peaks o f the wavy surface. The averaged SRP for a population of SAs provided a better representation of shape than that for RAs. Whereas active reg ions in the SEP can be isomorphic to the two dimensional form of the s timulus ''footprint'' in contact with the skin surface, the SRP, which in addition encodes the features of the stimulus in the third dimensi on normal to the skin surface, is not isomorphic to the stimulus shape . 4. When the sizes as well as the shapes of objects are varied, it is hypothesized that a central processing mechanism extracts the invaria nt property of shape from the slopes of the rising and falling phases of an SRP that has been normalized for overall differences in discharg e rates. These differences would be expected to occur with variations in the parameters of stimulation such as compressional force, stroke t rajectory, and stroke velocity. It was shown that a common feature of the mean SRP for SAs evoked by each wavy surface convexity, regardless of its radius, was the constancy of the slope from the base to the pe ak and from the peak to the base. Thus a possible code for the constan t curvature of a cylinder of the triangular-shaped spatial response pr ofile evoked in the SA population by the cylindrical convexity.