Force steadiness during voluntary contractions depends on integration of internal models and peripheral proprioceptive feedback. When visual feedback is removed, the central nervous system must rely more heavily on these internal and sensory signals to maintain stable output. The dorsolateral prefrontal cortex (DLPFC) is implicated in motor control and sensorimotor monitoring, but its causal role in modulating the qualitative stability of motor output under limited-feedback conditions has been unclear.
The study aimed to determine whether modulating left DLPFC activity with theta-burst transcranial stimulation (TBS) alters force steadiness during submaximal isometric contractions performed without visual feedback, and whether any observed changes depend on alterations in corticospinal excitability.
Twenty-eight healthy volunteers were randomized to one of two TBS conditions applied over the left DLPFC: inhibitory continuous TBS (cTBS) or facilitatory intermittent TBS (iTBS). The motor task consisted of maintaining a 35% submaximal isometric wrist flexion without visual feedback. Measurements were taken at three time points: prestimulation (baseline), 15 minutes poststimulation, and 30 minutes poststimulation.
Primary outcome:
Secondary outcomes and physiological measures:
Random assignment and the two stimulation paradigms (cTBS vs iTBS) allowed comparison of effects due to transient inhibition versus facilitation of the left DLPFC.
The trial focused on whether TBS over the DLPFC changed the variability of force output (coefficient of variation) during the 35% isometric task without visual feedback. Secondary analyses assessed whether any changes in force steadiness coincided with measurable changes in MEP amplitudes, MVC, or mean submaximal force.
Following inhibitory cTBS over the left DLPFC, the coefficient of variation of submaximal force decreased significantly at 15 minutes poststimulation (P = 0.013), indicating improved force steadiness during the task without visual feedback. This improvement in steadiness was observed without significant changes in MEP amplitudes, maximal voluntary contraction force, or the mean level of submaximal force production at the assessed poststimulation time points. The facilitatory intermittent TBS condition did not produce the same reported decrease in force variability.
Importantly, the improvement in steadiness occurred in the absence of detectable changes in resting corticospinal excitability as indexed by MEP amplitude measurements at the recorded time points.
The authors interpret these findings to indicate that transient inhibition of the left DLPFC can enhance the qualitative stability of motor output when visual feedback is absent. Because MEP amplitudes and MVC did not change, the effect on steadiness does not appear to be mediated by measurable alterations in resting corticospinal excitability or maximal force-generating capacity. Instead, the DLPFC may contribute to optimizing sensorimotor integration or reducing maladaptive monitoring of internal feedback under limited-feedback situations, thereby producing more stable force output.
These results support a causal role for the left DLPFC in modulating motor output stability during submaximal tasks performed without visual guidance. The finding that inhibitory cTBS improved steadiness without changing MEPs suggests behavioral improvements can arise from cortical network-level changes not captured by resting MEP amplitude alone. Future research could explore whether similar modulation applies to different muscles, intensities, age groups, or patient populations in which force steadiness is clinically relevant. The study provides a basis for investigating noninvasive neuromodulation of prefrontal circuits to influence sensorimotor control.
Keywords reported by the authors included: dorsolateral prefrontal cortex; force steadiness; theta burst transcranial stimulation. The randomized controlled trial was published in Neuroreport, with PMID 42474339 and DOI 10.1097/WNR.0000000000002290. Details such as participant demographics beyond the sample size and any additional secondary analyses were not reported in the abstract.