Task-specific increase in corticomotor excitability during tactile discrimination

Task-dependant changes in corticomotor excitability have been described mainly in the context of grasp-oriented actions, neglecting the sensory aspects of hand function. Here, we contrasted task-dependant facilitation in small hand muscles [i.e., first dorsal interosseous (FDI) and abductor digiti minimi (ADM)] in the context of finger movements involving either discrimination or non discrimination (ND) of tactile features. Healthy young individuals (n = 16) were trained to produce rhythmic to and fro movements at the sound of metronome ticks (0.8 Hz frequency, 5 s total duration) with either the index or the little finger of the right hand. In the tactile discrimination (TD) condition, participants were asked to attend to the location of two different 2-D tactile shapes disposed on the explored surface, whereas in the ND condition, the finger was moved over a blank surface. In both conditions, a transcranial magnetic stimulation (TMS) pulse was delivered at a specific time point in the course of the finger movement. Corticomotor excitability was assessed by monitoring changes in the amplitude and latency of motor evoked potentials (MEPs) in the FDI and ADM. Changes in the duration of the silent period were also assessed. The analysis revealed a significant large effect of task conditions (P < 0.001) on MEP amplitude, owing to the increase in MEP size observed during the TD, as compared to the ND condition. No interaction between "Task" and "Muscle" was detected, however, indicating that MEPs in the two muscles were equally affected by the task conditions. No significant changes were detected for variations in MEP latency or in the SP duration. An additional control experiment performed in a subset of the participants (n = 9) showed that MEP facilitation was substantially reduced when attention to sensations arising from finger contact with the shapes was diverted away by completion of a concurrent cognitive task (counting backward by three). These findings provide further insights into the factors influencing task-dependant changes in corticomotor excitability during hand actions. Our results highlight the importance of behavioral context and attention, in particular, in leading to further enhancement in corticomotor excitability when the finger is actively engaged in TD.