Ever-ready for action: Spatial effects on motor system excitability

Abstract

Modulation of excitability in the motor system can be observed before overt movements but also in response to covert invitations to act. We asked whether such changes can be induced in the absence of even covert motor instructions, namely, as a function of the location of the hand with reference to the body. Participants received single-pulse TMS over the motor cortex while they placed their contralateral hand (right hand in Experiment 1, left hand in Experiment 2) to the right or left of their body midline, and looked either at or away from their hand. In both experiments, greater excitability was observed when gaze was directed to the right. This finding is interpreted as a consequence of left brain lateralization of motor attention. Contrary to our expectations, we furthermore consistently observed greater excitability when gaze was directed away from the hand. To account for this finding, we introduce the concept of "surveillance attention" which, we speculate, modulates cortical gain, and thereby cortical excitability. Its function is to increase readiness to act in non-foveated regions of space. Such a process confers an advantage in environments, like those in which humans evolved, in which threatening stimuli may appear unexpectedly, and at any time.

Keywords: Action readiness; Cortical gain; Hemispace; Motor attention; Motor system; Spatial attention; Surveillance attention; TMS.

Fig. 1 –

Schematic drawing of the experimental setup. In this example the participant is looking at the right hemispace, while the hand is located in the left hemispace. TMS is delivered to left M1 and MEPs are recorded in the right FDI.

Fig. 2 –

Results of Experiment 1. MEPs recorded in the right FDI. The left and right panels display the estimated mean MEP ratio (large filled circle) and observed individual mean MEP ratios (small open circles) when gaze was located in the left and right hemispace (factor Gaze Location); within each panel, the leftmost and rightmost markers correspond to conditions in which gaze and hand location were congruent and incongruent, respectively (factor Gaze-Hand Congruence). For example, the leftmost markers in the left panel display the MEP ratios when gaze and hand were both located in the left hemispace. Observations are connected via lines to show the degree of consistency across individuals. There was an effect of Gaze Location and an effect of Gaze-Hand Congruence.

Fig. 3 –

Results of Experiment 2. MEPs recorded in the left FDI. Plotting conventions are the same as in Fig. 2. There was again an effect of Gaze Location (left vs. right panel) and an effect of Gaze-Hand Congruence (leftmost versus rightmost markers within each panel), extending the results of Experiment 1 to the left hand of right-handers.