Functional aspects of tactile directional sensibility

Abstract: Tactile directional sensibility, i.e. the ability to tell the movement direction of a moving tactile stimulus, depends on the parallel processing of spatiotemporal information and information about changes in the pattern of skin tension. Over the last decade, the peripheral mechanisms underlying directional sensibility have been extensively studied. However, knowledge about information processing on the supraspinal level is limited. Directional sensibility provides a sensitive test for disturbances in the peripheral and central nervous systems. The high aptitude of directional sensibility also raises the question of the functional role of this perceptual capacity. In the first part of the study, subjects determined the direction of skin pulls on the lower leg during functional magnetic resonance imaging. We found bilateral activation in second somatosensory (S2) and insular cortices (IC) but no activation in first somatosensory cortex (S1). In addition, four hemispherectomized patients and six healthy control subjects were examined for touch detection thresholds and directional sensibility. For the paretic side, the patients could not tell the direction of movement of a tactile stimulus whereas touch detection was often in the normal range. On the nonparetic side, patients always performed within the normal range.In the second part the hypothesis was tested that directional sensibility of the hairy skin is important for motor control. It was shown that non-supportive tactile contact between the forearm skin and spatially fixed tactile objects could reduce postural sway. It was further found that spatiotemporal information (air-stream stimulus) improved postural control when applied to the fingertip, but to much lesser extent when applied to the forearm. Skin stretch information (glued rod stimulus) reduced sway when applied to both glabrous and hairy skin. Conclusion: The contralateral hemisphere, especially S2 and IC, is important for processing of tactile directional information. Skin stretch information from both glabrous and hairy skin can attenuate postural sway whereas spatiotemporal information is most effective when emanating in the glabrous skin. The present findings will improve the clinical usefulness of the directional sensibility test. The work also demonstrates that the human hairy skin is important for postural control which may lead to new rehabilitation strategies for patients at risk of falling.