ENCODING OF SHAPE AND ORIENTATION OF OBJECTS INDENTED INTO THE MONKEYFINGERPAD BY POPULATIONS OF SLOWLY AND RAPIDLY ADAPTING MECHANORECEPTORS

The peripheral neural representation of object shape and orientation w as studied by recording the responses of a spatially distributed popul ation of rapidly and slowly adapting type I mechanoreceptors (RAs and SAs, respectively) to objects of different shapes and orientations ind ented 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 fingerp ad for 4 s at orientations of 0, 45, 90, and 135 degrees using a conta ct force of 15 gwt. Estimations of the population responses (PRs) were constructed by combining the responses of 91 SA and 97 RA single affe rents at discrete times during the indentation. The PR was composed of the neural discharge rates (z coordinate) plotted at x and y coordina tes of the most sensitive spot of the receptive field. The shapes of t he PRs were related to the shapes of the objects by fitting the PRs wi th Gaussian surfaces. The orientations of the PRs were determined from weighted principal component analyses. The SA PR encoded both the ori entation 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 R A PR encoded orientation only for the object with the highest curvatur e but did so ambiguously. Only the SA PR was well fit by a Gaussian su rface. The shape of the object was discriminated by the SA PR within t he first 500 ms of contact, and the form of the SA PR remained constan t 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 a t 1 s. Thus the shape and orientation of each object were coded by the shape and orientation of the SA PR.