Encoding of shape and orientation of objects indented into the monkey fingerpad by populations of slowly and rapidly adapting mechanoreceptors

The peripheral neural representation of object shape and orientation was studied by recording the responses of a spatially distributed population of rapidly and slowly adapting type I mechanoreceptors (RAs and SAs, respectively) to objects of different shapes and orientations indented at a fixed location on the fingerpad of the anesthetized monkey. The toroidal objects had a radius of 5 mm on the major axis, and 1, 3, or 5 mm on the minor axis. Each object was indented into the fingerpad for 4 s at orientations of 0, 45, 90, and 135 degrees using a contact force of 15 gwt. Estimations of the population responses (PRs) were constructed by combining the responses of 91 SA and 97 RA single afferents at discrete times during the indentation. The PR was composed of the neural discharge rates (z coordinate) plotted at x and y coordinates of the most sensitive spot of the receptive field. The shapes of the PRs were related to the shapes of the objects by fitting the PRs with Gaussian surfaces. The orientations of the PRs were determined from weighted principal component analyses. The SA PR encoded both the orientation and shape of the objects, whereas the RA PR did neither. The SA PR orientation was biased toward the long axis of the finger. The RA PR encoded orientation only for the object with the highest curvature but did so ambiguously. Only the SA PR was well fit by a Gaussian surface. The shape of the object was discriminated by the SA PR within the first 500 ms of contact, and the form of the SA PR remained constant during the subsequent 3.5 s. This was manifested by constant widths of the PR along the major and minor axes despite a peak response that decreased from its maximum at 200 ms to an asymptotic value starting at 1 s. Thus the shape and orientation of each object were coded by the shape and orientation of the SA PR.