| The long term goal of this research is to understand the neural representation of tactile motion. To this end, we present tactile stimuli whose perceptual properties of their visual counterparts have been well established, namely scanning bars, dot pattern, random dot displays, gratings, plaids, barber-poles, and bar-fields, to comprehensively characterize neural mechanisms underlying the processing of tactile motion. While the responses of individual peripheral afferent showed no direction selectivity and the direction tuning emerged in area 3b which is the first cortical processing level. We then identified neurons in area 1 that showed strong direction selectivity and, most importantly, invariance in their representations of motion direction relative to other stimulus properties. Further, we showed that the motion signals conveyed by individual neurons in area 1 can account for the ability of human observers to discriminate the direction of motion of these stimuli, as measured in paired psychophysical experiments. We then assessed how local motion cues are integrated to form a global motion percept. Indeed, the speed and direction of motion of individual (one-dimensional) edges is ambiguous because information about the motion component parallel to their orientation is not available, a predicament known as the aperture problem. To acquire a veridical percept of an object's direction of motion, it is necessary to integrate motion information across multiple stimulus contours that differ in orientation or to rely on terminators (end points, corners, and intersections) whose direction of motion is unambiguous. In human psychophysical experiments, we first showed that, analogous to its visual counterpart, the somatosensory system integrates local information in stimuli that comprise ambiguous local motion cues. We presented gratings and plaids and recorded the responses of neurons in primary somatosensory cortex of macaques. The responses of neurons in area 1 to plaids exhibited properties characteristic of motion integrators. Surprisingly, the perceived directions of a subset of plaids, namely type 2 plaids, are very different when these are presented tactually and visually. The results suggest that the somatosensory system uses a distinctive motion integration mechanism whereby the perceived direction and neuronal responses are determined by the vector average of motion signals emanating from edges and terminators. We conclude that area 1 contains a robust representation of motion and adopt vector average integration mechanism. |
