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Max-Planck-Gesellschaft Tübingen: Reduced sensitivity during limb movement - ‘tactile suppression’ MPS investigated the timeline of a phenomenon called “tactile suppression” where tactile sensitivity is decreased during an active limb-movement. Tactile suppression can be seen as analogous to the visual phenomenon of saccadic suppression, where the visual sensitivity is strongly reduced during ballistic eye movements. In the first year of IMMERSENCE the researcher of MPS were able to show that the tactile discrimination performance drops significantly when participants actively moved the arm that had been stimulated compared to the condition where participants were asked to perform the discrimination task without an arm movement. However, it is still an open question whether tactile suppression is an active process, whereby tactile sensitivity is actively reduced, or whether it is a passive process, whereby sensory noise, i.e. from other cutaneous areas, is added to the sensory system. To answer this question MPS used an integrated kinesthetic and tactile display (figure below), consisting of a force-feedback robotic arm (DekiFeD4) and a commercially available computer mouse for visually handicapped people (VirTOUCH-Mouse). Using psychophysical methods, the researchers compared tactile discrimination performance during an “active” (participants move their arm) and “passive” (DekiFeD4 moves participant’s arm) movement condition to investigate the timeline of tactile suppression and to infer the role of a descending motor command. Tactile stimuli were presented randomly before and after arm movement onset, respectively.
Experimental setup consisting of a force-feedback robotic arm (DekiFeD4) and a VirTOUCH-Mouse. Results indicate that the timeline of tactile suppression differs in the “active” and “passive” movement condition. In the “active” condition a significant drop in sensitivity arises in the time range of 120 to 70 ms before the onset of the arm movement, whereas tactile sensitivity drops from the beginning of the arm movement in the “passive” condition (see figure below). This difference agrees with the findings of Seki et al. [Nature Neuroscience, 2003], claiming that a descending motor command (present in the “active” but not in the “passive” condition) leads to a blocking of interneurons. Consequently, afferent information would be limited due to this gating effect. Furthermore, the results show strong evidence that tactile sensitivity is recovering in both movement conditions in a time range of 100 to 150 ms after the movement has begun. This seems to be a mechanism that masks the transition from steady state to a limb movement, which could serve to prevent from low tactile sensitivity.
Proportion of correct answers against tactile stimulus presentation time. Negative time stamp means that pin was presented before the onset of the arm movement. Arm movement starts at t = 0 ms. These limitations of the tactile modality can be used to shortcut the development of tactile devices by using stimuli that do not contain all the physically correct parameters. Our results can be used to simplify the rendering processes in haptically enhanced virtual environments. Knowing that active movements cause a decrease in tactile performance can help to tailor tactile and haptic stimuli to specific demands in the each phase of limb movements. When combining real and virtual cues, imperfect synchronization will greatly diminish a user’s feeling of presence. Since perfect synchronization is not achievable, MPS planned to act on the human perceptual system in order to accustom it to these limitations. Given the correct conditions, perceived synchrony can be an adaptive process. Until now, the flexibility of the perceptual system has been studied only for audio-video interfaces, and the knowledge applied to television broadcasting and movie theatres. Here the researchers investigated adaptation to multimodal information, including haptics. |
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