Adaptation of motor preparation to lapses in attention: a transcranial magnetic stimulation study

Sustained attention tasks are characterized by a gradual decrease in attentional resources across time usually investigated through an increase in reaction time (RT). Weissman et al. (2006) depicted by means of functional magnetic resonance imaging (fMRI) that neural adaptations to lapses in attention involve an increase in the activity of the fronto-parietal network. Specifically, the primary motor cortex (M1) exhibits an increase in stimulus-related fMRI response once attentional lapses occur (Weissman et al., 2006; Yarkoni et al., 2009). In the present study, we investigated whether pre-stimulus neural activity increases when attentional lapses occur. We hypothesized that the increased stimulus-related fMRI response previously found once attentional lapses occur was related to an increase in motor preparation occurring during inter-stimuli intervals. To examine this, the time course of corticospinal excitability was investigated by means of single-pulse transcranial magnetic stimulation (TMS) throughout a sustained attention RT task of 32 min duration. Single-pulse TMS was applied at 5 min intervals, while subjects were preparing for the upcoming stimulus. Attentional lapses were observed as revealed by a significant increase in RT (p < .001) after 9 min of sustained attention. Friedman ANOVA showed that motor-evoked potential amplitude, reflecting the level of corticospinal excitability, (i) significantly increased between a period 15 min into the task compared to pre-task resting state values (p < .01) and (ii) returned to the pre-task resting state values once the task finished. In accordance with the interpretation given by Weissman et al. (2006), we propose that the motor structures become increasingly active once lapses in attention occurs to cope with the increasing task demand over time. This over-engagement is not reflected by an increase in stimulus-related neural response of the M1 area only but also involves an increase of the corticospinal excitability during the motor preparation phase