Tactile co-activation improves detection of afferent spatial modulation

Tactile co-activation, i.e., synchronous stimulation of a region of skin, has been reported to improve tactile spatial acuity and expand the corresponding somatosensory cortical representation. The current study aimed to clarify the nature of the changes resulting from tactile co-activation, using three measures of tactile sensitivity obtained with controlled mechanical stimulation. One was the grating orientation (GR/OR) discrimination task, where acuity is indexed by the threshold groove width required for 75% correct discrimination between two orthogonal orientations of a grating on the fingerpad. Since this task may be susceptible to intensity cues due to tactile anisotropy, another acuity measure, the 3-dot task, was also used. In this task, the acuity threshold corresponds to 75% correct discrimination of the direction of offset of the central dot in a 3-dot array. In Experiment 1, co-activation failed to induce significant improvement in acuity with either of these measures. Experiment 2 employed both the GR/OR task, and a third measure based on discriminating a grooved from a smooth surface (SM/GV). While the former task demands detailed spatial resolution, the latter requires only that spatial modulation in the afferent population be detected. This experiment also included a control group. GR/OR performance did not significantly improve for either the control or experimental groups. There was, however, a significant improvement in SM/GV performance following co-activation for the experimental but not the control group. These findings indicate that the SM/GV task may be better suited than the GR/OR or 3-dot tasks for measuring changes in tactile sensitivity following co-activation.