PERIPHERAL NEURAL MECHANISMS DETERMINING THE ORIENTATION OF CYLINDERSGRASPED BY THE DIGITS

When a human grasps a cylindrical object, feedback on the orientation of the cylinder with respect to the axes of the digits is crucial for successful manipulation of the object. We measured the ability of huma ns to discriminate the orientations of cylinders passively contacting the fingerpad. For a cylinder of curvature of 521 m(-1) (radius, 1.92 mm) subjects were able to discriminate, at the 75% level, orientation differences of 5.4 degrees; for a less curved cylinder (curvature, 172 m(-1); radius, 5.81 mm) the difference limen decreased to 4.2 degrees . The neural mechanisms underlying the determination of tactile orient ation were investigated by recording the responses of single slowly ad apting type I afferents (SAIs) innervating the fingerpads of anestheti zed monkeys. When cylinders were stepped across the receptive field of an SAI, the resulting response profiles were Gaussian in shape; the s hape corresponded to the shape of the cylinder, increasing in height a nd decreasing in width for more curved cylinders. All SAIs had the sam e underlying profile shape except for a multiplicative constant determ ined by the sensitivity of the individual afferent. Thus it was possib le to reconstruct the response of the population of active SAIs in the fingerpad. Changing the orientation of the cylinder resulted in a rot ation of the population response, but the change in angle of the popul ation response was greater than the change in orientation of the cylin der. This discrepancy increased as the orientation of the cylinder mov ed closer to the orientation of the axis of the finger and was more pr onounced for the less curved cylinder. Measured contact areas between the cylinders and the skin were elliptical, with orientations exceedin g those of the cylinder; again the differences were greater for the le ss curved cylinder and for orientations closer to that of the finger a xis. The human discrimination performance could be explained in terms of the SAI population responses.