Intracortical Inhibition Within the Primary Motor Cortex Can Be Modulated by Changing the Focus of Attention

Abstract

It is well recognized that an external focus (EF) compared with an internal focus (IF) of attention improves motor learning and performance. Studies have indicated benefits in accuracy, balance, force production, jumping performance, movement speed, oxygen consumption, and fatiguing task. Although behavioral outcomes of using an EF strategy are well explored, the underlying neural mechanisms remain unknown. A recent TMS study compared the activity of the primary motor cortex (M1) between an EF and an IF. More precisely, this study showed that, when adopting an EF, the activity of intracortical inhibitory circuits is enhanced. On the behavioral level, the present protocol tests the influence of attentional foci on the time to task failure (TTF) when performing submaximal contractions of the first dorsal interosseous (FDI). Additionally, the current paper describes two TMS protocols to assess the influence of attentional conditions on the activity of cortical inhibitory circuits within the M1. Thus, the present article describes how to use single-pulse TMS at intensities below the motor threshold (subTMS) and paired-pulse TMS, inducing short-interval intracortical inhibition (SICI) when applied to the M1. As these methods are assumed to reflect the responsiveness of GABAergic inhibitory neurons, without being affected by spinal reflex circuitries, they are well suited to measuring the activity of intracortical inhibitory circuits within the M1. The results show that directing attention externally improves motor performance, as participants were able to prolong the time to task failure. Moreover, the results were accompanied by a larger subTMS-induced electromyography suppression and SICI when adopting an EF compared to an IF. As the level of cortical inhibition within the M1 was previously demonstrated to influence motor performance, the enhanced inhibition with an EF might contribute to the better movement efficiency observed in the behavioral task, indicated by a prolonged TTF with an EF.

Human motor fatigability as evoked by repetitive movements results from a gradual breakdown of surround inhibition

“…In line with this proposal, it has been shown that isometric contractions can be maintained longer when an external focus of attention is adopted (Kuhn et al, 2017a). Interestingly, this improvement of performance was accompanied by an increase in SICI (Kuhn et al, 2017a; Kuhn et al, 2017b) and an increase in surround inhibition (Kuhn et al, 2018).…”

Sex differences in Parkinson's disease: A transcranial magnetic stimulation study

“…Motor inhibition is essential for quality of motor execution and particularly for modulating motor cortex output during fine movements, and the increased prominence of SICI in female PD patients may reflect better preserved capacity for movement control at level of the motor cortex. It has been suggested that decreased SICI in PD reflects mainly decreased threshold of excitatory neurons mediating short‐interval intracortical facilitation, with only a minor contribution of impaired synaptic inhibition .…”

Effects of an external compared to an internal focus of attention on the excitability of fast and slow(er) motor pathways

“…For example, in a study with elaborated TMS protocols (paired-pulse and subthreshold stimulation paradigms), Kuhn and co-workers 27 demonstrated that adopting an EF contrasted to an IF enhances cortical inhibition within M1. The enhanced activity of inhibitory interneurons within M1 may indicate a reduced motor-cortical activation 11 , 27 , 28 , which could be interpreted as a more efficient and more automatic mode of motor control when adopting an EF contrasted with an IF.These data might therefore be considered as a more efficient neural strategy. Besides the effects found in the prime-mover, there is also evidence that adjacent muscles are activated differently when participants use an EF versus an IF.…”

“…For example, Chow and colleagues 24 demonstrated that the decreased balance performance (bigger sway path) when using an IF over an EF in young adults was accompanied by a higher T3 (verbal-analytical) and Fz (motor-planning) coherence which reflects increased verbal-analytical involvement during motor planning and control 25 , 26 . Moreover, using different TMS protocols, recent studies 11 , 27 , 28 demonstrated altered neural activation in M1 for the muscle directly involved in the motor task (prime mover) but also the surrounding and/or antagonistic muscles. For example, in a study with elaborated TMS protocols (paired-pulse and subthreshold stimulation paradigms), Kuhn and co-workers 27 demonstrated that adopting an EF contrasted to an IF enhances cortical inhibition within M1.…”