Task-dependent changes of motor cortical network excitability during precision grip compared to isolated finger contraction

Abstract

The purpose of this study was to determine whether task-dependent differences in corticospinal pathway excitability occur in going from isolated contractions of the index finger to its coordinated activity with the thumb. Focal transcranial magnetic stimulation (TMS) was used to measure input-output (I/O) curves--a measure of corticospinal pathway excitability--of the contralateral first dorsal interosseus (FDI) muscle in 21 healthy subjects performing two isometric motor tasks: index abduction and precision grip. The level of FDI electromyographic (EMG) activity was kept constant across tasks. The amplitude of the FDI motor evoked potentials (MEPs) and the duration of FDI silent period (SP) were plotted against TMS stimulus intensity and fitted, respectively, to a Boltzmann sigmoidal function. The plateau level of the FDI MEP amplitude I/O curve increased by an average of 40% during the precision grip compared with index abduction. Likewise, the steepness of the curve, as measured by the value of the maximum slope, increased by nearly 70%. By contrast, all I/O curve parameters [plateau, stimulus intensity required to obtain 50% of maximum response (S(50)), and slope] of SP duration were similar between the two tasks. Short- and long-latency intracortical inhibitions (SICI and LICI, respectively) were also measured in each task. Both measures of inhibition decreased during precision grip compared with the isolated contraction. The results demonstrate that the motor cortical circuits controlling index and thumb muscles become functionally coupled when the muscles are used synergistically and this may be due, at least in part, to a decrease of intracortical inhibition and an increase of recurrent excitation.

Investigating TMS–EEG Indices of Long-Interval Intracortical Inhibition at Different Interstimulus Intervals

“…Furthermore, separate profiles of inhibitory recruitment have been reported at different intervals [16]; the inhibitory effect of LICI when applied 100 ms prior to SICI is reduced [17] or absent [18] when LICI precedes SICI by 150 ms, and LICI at 100 ms and 150 ms show differential task-related variations in inhibitory tone [17]. As changes in LICI have been associated with several different pathologies [6], and may be involved with aspects of motor control [17,19,20], it is important to gain a better understanding of the mechanisms reflected by this measurement at different ISIs.…”

Age-related Differences in Pre- and Post-synaptic Motor Cortex Inhibition are Task Dependent

“…Furthermore, the nature of this modulation is thought to be task-dependent [17,18]. In contrast, task-related changes in inhibition in older adults have been limited to measurements made during tonic contractions [19,20] or in the period prior to contraction [21][22][23].…”

“…However, some evidence suggests that the task-dependency of inhibitory tone in M1 is modified by age [24]. Interestingly, task-dependent changes in SICI and LICI have been suggested to be mediated by pre-synaptic mechanisms [18], suggesting that effects of age on the task-dependent modulation of inhibition may be influenced by changes in pre-synaptic motor cortex inhibition.…”

“…The aim of the current study was therefore to compare the magnitude of SICI, LICI and LICI-SICI interactions between young and old subjects during relaxation, index finger abduction and precision grip (between the index finger and thumb) -tasks that have previously produced specific changes in intracortical inhibition in young subjects [18]. Also, as effects of age on pre-synaptic motor cortex inhibition were expected to be timing-dependent, LICI-SICI interactions were assessed using two ISIs of 100 ms and 150 ms. Based on previous studies [25,26], we expected that old subjects would show reduced pre-synaptic M1 inhibition at 150 ms.…”

Postural challenge affects motor cortical activity in young and old adults

“…between index finger and thumb muscle activation when healthy adults used these muscles synergistically, mediated at least in part by a decrease of SICI and an increase of recurrent excitation (Kouchtir- Devanne et al, 2012).…”