Dynamic context-dependent somatotopy of the primary motor representation

Background and Objective: The methodological concept of the „motor hotspot“ is a typical notion in transcranial brain stimulation techniques. Yet, constraints on the conception of a discrete somatotopy of the human primary motor cortex (M1), e.g. the diversity of representational clusters (Schieber 2001), overlapping representations (Sanes et al. 1995) as well as dependence on hand posture (Wassermann et al. 1998) and use- dependent adaptability (Classen et at. 1998) argue that the notion of a static hotspot lacks plausibility. To test the alternative hypothesis of dynamic context- dependent somatotopy we investigated the effect of highly focal (<5mm) stimulation over an arguably static hotspot with navigated TMS (nTMS) during differenzial allocation of attention to three digits within the dominant primary motor representation.

Methods: We studied eight healthy volunteers applying nTMS always over the dominant FDI- hotspot. To allocate attention to the individual digits including the contralateral FDI as a control, we used a simple imperative visual cue paradigm for motor attention. In advance, participants underwent a training to familiarize them with the experimental setting and task set. Cortico- spinal excitability (CE) was probed with motor evoked potentials for cued, false- cued and cue- free baseline conditions.

Results: We found, as expected, that CE is significantly higher for the cued- condition versus baseline condition in all digits (p<0.05). The range was 31.85%, 7.53% or 19,34% for APB, FDI and ADM, respectively. Remarkably, we also found a strong decrease in excitability when comparing the false- cued versus the baseline condition (APB: –5.82%, FDI: –28.70%, ADM: –18.09%). We did not find that the CE of the referential FDI was always higher than the other digits independent of the contextual modification, which would have supported the notion of a static hotspot.

Discussion: We conclude that our findings strongly support the notion of context- dependent adaptive processes in the primary motor cortex within a subregion smaller than 5mm („microplasticity“). These findings are in line with short- term use- dependent plasticity (Classen et al. 1998) and thus likely related to LTP- like effects (Bütefisch et al. 2000) and motor learning (Rioult- Pedotti et al. 1998).