Peripheral neural mechanisms determining the orientation of cylinders grasped 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 humans 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 orientation were investigated by recording the responses of single slowly adapting type I afferents (SAIs) innervating the fingerpads of anesthetized monkeys. When cylinders were stepped across the receptive field of an SAI, the resulting response profiles were Gaussian in shape; the shape corresponded to the shape of the cylinder, increasing in height and decreasing in width for more curved cylinders. All SAIs had the same underlying profile shape except for a multiplicative constant determined by the sensitivity of the individual afferent. Thus it was possible to reconstruct the response of the population of active SAIs in the fingerpad. Changing the orientation of the cylinder resulted in a rotation of the population response, but the change in angle of the population response was greater than the change in orientation of the cylinder. This discrepancy increased as the orientation of the cylinder moved closer to the orientation of the axis of the finger and was more pronounced for the less curved cylinder. Measured contact areas between the cylinders and the skin were elliptical, with orientations exceeding those of the cylinder; again the differences were greater for the less curved cylinder and for orientations closer to that of the finger axis. The human discrimination performance could be explained in terms of the SAI population responses.