The timing and intensity of transcranial magnetic stimulation, and the scalp site stimulated, as variables influencing motor sequence performance in healthy subjects

Intermittent theta burst stimulation facilitates functional connectivity from the dorsal premotor cortex to primary motor cortex

Background

Motor information in the brain is transmitted from the dorsal premotor cortex (PMd) to the primary motor cortex (M1), where it is further processed and relayed to the spinal cord to eventually generate muscle movement. However, how information from the PMd affects M1 processing and the final output is unclear. Here, we applied intermittent theta burst stimulation (iTBS) to the PMd to alter cortical excitability not only at the application site but also at the PMd projection site of M1. We aimed to determine how PMd iTBS–altered information changed M1 processing and the corticospinal output.

Methods

In total, 16 young, healthy participants underwent PMd iTBS with 600 pulses (iTBS600) or sham-iTBS600. Corticospinal excitability, short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF) were measured using transcranial magnetic stimulation before and up to 60 min after stimulation.

Results

Corticospinal excitability in M1 was significantly greater 15 min after PMd iTBS600 than that after sham-iTBS600 (p = 0.012). Compared with that after sham-iTBS600, at 0 (p = 0.014) and 15 (p = 0.037) min after iTBS600, SICI in M1 was significantly decreased, whereas 15 min after iTBS600, ICF in M1 was significantly increased (p = 0.033).

Conclusion

Our results suggested that projections from the PMd to M1 facilitated M1 corticospinal output and that this facilitation may be attributable in part to decreased intracortical inhibition and increased intracortical facilitation in M1. Such a facilitatory network may inform future understanding of the allocation of resources to achieve optimal motion output.

The reliability and validity of rapid transcranial magnetic stimulation mapping

BACKGROUND

Traditional transcranial magnetic stimulation mapping involves systematically delivering stimuli over a predefined grid. The pseudorandom walk method seeks to improve map acquisition times by abandoning the grid in favour of delivering stimuli randomly over a given area.

OBJECTIVES

To i) determine the minimum interstimulus interval (ISI) required for reliable mapping outcomes within and between sessions using the pseudorandom walk method and ii) assess the validity of the pseudorandom walk method by testing its equivalence with traditional mapping.

METHODS

Maps collected using the pseudorandom walk method at four ISIs (4, 3, 2, and 1s) were compared to maps collected using traditional mapping in twenty healthy individuals. Outcomes included map area, volume, centre of gravity, mean MEP amplitude, and number of discrete peaks.

RESULTS AND CONCLUSIONS

The pseudorandom walk method was valid and reliable with a 2-second ISI for all outcomes except number of discrete peaks, which was less reliable than other measures.

Determining the Optimal Number of Stimuli per Cranial Site during Transcranial Magnetic Stimulation Mapping

The delivery of five stimuli to each cranial site is recommended during transcranial magnetic stimulation (TMS) mapping. However, this time-consuming practice restricts the use of TMS mapping beyond the research environment. While reducing the number of stimuli administered to each cranial site may improve efficiency and decrease physiological demand, doing so may also compromise the procedure's validity. Therefore, the aim of this study was to determine the minimum number of stimuli per cranial site required to obtain valid outcomes during TMS mapping. Map volume and centre of gravity (CoG) recordings obtained using five stimuli per cranial site were retrospectively compared to those obtained using one, two, three, and four stimuli per cranial site. For CoG longitude, one stimulus per cranial site produced valid recordings (ICC = 0.91, 95% CI 0.82 to 0.95). However, this outcome is rarely explored in isolation. As two stimuli per cranial site were required to obtain valid CoG latitude (ICC = 0.99, 95% CI 0.99 to 0.99) and map volume (ICC = 0.99, 95% CI 0.99 to 0.99) recordings, it is recommended that a minimum of two stimuli be delivered to each cranial site during TMS mapping in order to obtain valid outcomes.

Comparison of effects of transcranial magnetic stimulation on primary motor cortex and supplementary motor area in motor skill learning (randomized, cross over study)

Motor skills require quick visuomotor reaction time, fast movement time, and accurate performance. Primary motor cortex (M1) and supplementary motor area (SMA) are closely related in learning motor skills. Also, it is well known that high frequency repeated transcranial magnetic stimulation (rTMS) on these sites has a facilitating effect. The aim of this study was to compare the effects of high frequency rTMS activation of these two brain sites on learning of motor skills. Twenty three normal volunteers participated. Subjects were randomly stimulated on either brain area, SMA or M1. The motor task required the learning of sequential finger movements, explicitly or implicitly. It consisted of pressing the keyboard sequentially with their right hand on seeing 7 digits on the monitor explicitly, and then tapping the 7 digits by memorization, implicitly. Subjects were instructed to hit the keyboard as fast and accurately as possible. Using Musical Instrument Digital Interface (MIDI), the keyboard pressing task was measured before and after high frequency rTMS for motor performance, which was measured by response time (RT), movement time, and accuracy (AC). A week later, the same task was repeated by cross-over study design. At this time, rTMS was applied on the other brain area. Two-way ANOVA was used to assess the carry over time effect and stimulation sites (M1 and SMA), as factors. Results indicated that no carry-over effect was observed. The AC and RT were not different between the two stimulating sites (M1 and SMA). But movement time was significantly decreased after rTMS on both SMA and M1. The amount of shortened movement time after rTMS on SMA was significantly increased as compared to the movement time after rTMS on M1 (p < 0.05), especially for implicit learning of motor tasks. The coefficient of variation was lower in implicit trial than in explicit trial. In conclusion, this finding indicated an important role of SMA compared to M1, in implicit motor learning.

Transcranial direct current stimulation over the supplementary motor area modulates the preparatory activation level in the human motor system

Transcranial direct current stimulation (tDCS) is a non-invasive stimulation method that can induce transient polarity-specific neuroplastic changes in cortical excitability lasting up to 1h post-stimulation. While excitability changes with stimulation over the primary motor cortex have been well documented, the functional effects of stimulation over premotor regions are less well understood. In the present experiment, we tested how cathodal and anodal tDCS applied over the region of the supplementary motor area (SMA) affected preparation and initiation of a voluntary movement. Participants performed a simple reaction time (RT) task requiring a targeted wrist-extension in response to a go-signal. In 20% of RT trials a startling acoustic stimulus (SAS) was presented 500 ms prior to the "go" signal in order to probe the state of motor preparation. Following the application of cathodal, anodal, or sham tDCS (separate days) over SMA for 10 min, participants performed blocks of RT trials at 10 min intervals. While sham stimulation did not affect RT or incidence of early release by the SAS, cathodal tDCS led to a significant slowing of RT that peaked 10 min after the end of stimulation and was associated with a marked decrease in the incidence of movement release by the SAS. In contrast, anodal tDCS resulted in faster RTs, but the incidence of release was unchanged. These results are consistent with the SMA playing a role in the pre-planning of movements and that modulating its activity with tDCS can lead to polarity-specific changes in motor behavior.

Influence of rTMS over the left primary motor cortex on initiation and performance of a simple movement executed with the contralateral arm in healthy volunteers

Repetitive transcranial magnetic stimulation (rTMS) affects cortical excitability according to the frequency of stimulation. Few data are available on the influence of rTMS applied over the primary motor cortex (M1) on motor performances in healthy volunteers. The aim of this study was to determine, through kinematic analysis, whether rTMS over the left M1 changes initiation and performance of movement executed with the contralateral arm. Nine healthy males completed a set of motor tasks, consisting of a single-joint rapid movement between two objects performed under three different behavioral conditions (self-initiated; externally triggered known, during which the subject could see where the target was positioned in advance; externally triggered unknown, during which the subject could not see where the target was positioned until he reached it). The tasks were performed in a randomized order in three different sessions, with a seven-day interval between each session: (1) without stimulation (baseline); (2) immediately after 1-Hz rTMS; (3) immediately after 10-Hz rTMS. We measured reaction time, movement time, calculated as the sum of the time taken to reach the target from movement onset (T1) and that taken to reach the target to movement termination (T2), acceleration and deceleration time on the velocity profile, as well as the ratio between them, and maximum speed and maximum acceleration. Reaction time, movement time, and T2 significantly increased after 1-Hz rTMS and decreased after 10-Hz rTMS, while the other parameters remained unchanged. Our results suggest that rTMS may modify both initiation and performance of a voluntary movement.

Repetitive TMS suggests a role of the human dorsal premotor cortex in action prediction

Predicting the actions of other individuals is crucial for our daily interactions. Recent evidence suggests that the prediction of object-directed arm and full-body actions employs the dorsal premotor cortex (PMd). Thus, the neural substrate involved in action control may also be essential for action prediction. Here, we aimed to address this issue and hypothesized that disrupting the PMd impairs action prediction. Using fMRI-guided coil navigation, rTMS (five pulses, 10 Hz) was applied over the left PMd and over the vertex (control region) while participants observed everyday actions in video clips that were transiently occluded for 1 s. The participants detected manipulations in the time course of occluded actions, which required them to internally predict the actions during occlusion. To differentiate between functional roles that the PMd could play in prediction, rTMS was either delivered at occluder-onset (TMS-early), affecting the initiation of action prediction, or 300 ms later during occlusion (TMS-late), affecting the maintenance of an ongoing prediction. TMS-early over the left PMd produced more prediction errors than TMS-early over the vertex. TMS-late had no effect on prediction performance, suggesting that the left PMd might be involved particularly during the initiation of internally guided action prediction but may play a subordinate role in maintaining ongoing prediction. These findings open a new perspective on the role of the left PMd in action prediction which is in line with its functions in action control and in cognitive tasks. In the discussion, the relevance of the left PMd for integrating external action parameters with the observer’s motor repertoire is emphasized. Overall, the results are in line with the notion that premotor functions are employed in both action control and action observation.

Suppression of beta oscillations in the subthalamic nucleus following cortical stimulation in humans

It is unclear how subthalamic nucleus activity is modulated by the cerebral cortex. Here we investigate the effect of transcranial magnetic stimulation (TMS) of the cortex on oscillatory subthalamic local field potential activity in the 8-35 Hz (alpha/beta) band, as exaggerated synchronization in this band is implicated in the pathophysiology of parkinsonism. We studied nine patients with Parkinson's disease (PD) to test whether cortical stimulation can modulate synchronized oscillations in the human subthalamic nucleus. With patients at rest, single-pulse TMS was delivered every 5 s over each primary motor area and supplementary motor area at intensities of 85-115% resting motor threshold. Subthalamic local field potentials were recorded from deep brain stimulation electrodes implanted into this nucleus for the treatment of PD. Motor cortical stimulation suppressed beta activity in the subthalamic nucleus from approximately 0.2 to 0.6 s after TMS (repeated measures anova; main effect of time, P < 0.01; main effect of side, P = 0.03), regardless of intensity. TMS over the supplementary motor area also reduced subthalamic beta activity at 95% (P = 0.05) and 115% resting motor threshold (P = 0.01). The oscillatory activity decreased to 80 +/- 26% of baseline (averaged across sites and stimulation intensities). Suppression with subthreshold stimuli confirmed that these changes were centrally driven and not due to peripheral afference. The results may have implications for mechanisms underlying the reported therapeutic benefits of cortical stimulation.

Distributed representations of the "preparatory set" in the frontal oculomotor system: a TMS study

Background

The generation of saccades is influenced by the level of "preparatory set activity" in cortical oculomotor areas. This preparatory activity can be examined using the gap-paradigm in which a temporal gap is introduced between the disappearance of a central fixation target and the appearance of an eccentric target.

Methods

Ten healthy subjects made horizontal pro- or antisaccades in response to lateralized cues after a gap period of 200 ms. Single-pulse transcranial magnetic stimulation (TMS) was applied to the dorsolateral prefrontal cortex (DLPFC), frontal eye field (FEF), or supplementary eye field (SEF) of the right hemisphere 100 or 200 ms after the disappearance of the fixation point. Saccade latencies were measured to probe the disruptive effect of TMS on saccade preparation. In six individuals, we gave realistic sham TMS during the gap period to mimic auditory and somatosensory stimulation without stimulating the cortex.

Results

TMS to DLPFC, FEF, or SEF increased the latencies of contraversive pro- and antisaccades. This TMS-induced delay of saccade initiation was particularly evident in conditions with a relatively high level of preparatory set activity: The increase in saccade latency was more pronounced at the end of the gap period and when participants prepared for prosaccades rather than antisaccades. Although the "lesion effect" of TMS was stronger with prefrontal TMS, TMS to FEF or SEF also interfered with the initiation of saccades. The delay in saccade onset induced by real TMS was not caused by non-specific effects because sham stimulation shortened the latencies of contra- and ipsiversive anti-saccades, presumably due to intersensory facilitation.

Conclusion

Our results are compatible with the view that the "preparatory set" for contraversive saccades is represented in a distributed cortical network, including the contralateral DLPFC, FEF and SEF.

Muscles in "concert": study of primary motor cortex upper limb functional topography

Background

Previous studies with Transcranial Magnetic Stimulation (TMS) have focused on the cortical representation of limited group of muscles. No attempts have been carried out so far to get simultaneous recordings from hand, forearm and arm with TMS in order to disentangle a ‘functional’ map providing information on the rules orchestrating muscle coupling and overlap. The aim of the present study is to disentangle functional associations between 12 upper limb muscles using two measures: cortical overlapping and cortical covariation of each pair of muscles. Interhemispheric differences and the influence of posture were evaluated as well.

Methodology/Principal Findings

TMS mapping studies of 12 muscles belonging to hand, forearm and arm were performed. Findings demonstrate significant differences between the 66 pairs of muscles in terms of cortical overlapping: extremely high for hand-forearm muscles and very low for arm vs hand/forearm muscles. When right and left hemispheres were compared, overlapping between all possible pairs of muscles in the left hemisphere (62.5%) was significantly higher than in the right one (53.5% ).

The arm/hand posture influenced both measures of cortical association, the effect of Position being significant [p = .021] on overlapping, resulting in 59.5% with prone vs 53.2% with supine hand, but only for pairs of muscles belonging to hand and forearm, while no changes occurred in the overlapping of proximal muscles with those of more distal districts.

Conclusions/Significance

Larger overlapping in the left hemisphere could be related to its lifetime higher training of all twelve muscles studied with respect to the right hemisphere, resulting in larger intra-cortical connectivity within primary motor cortex. Altogether, findings with prone hand might be ascribed to mechanisms facilitating coupling of muscles for object grasping and lifting -with more proximal involvement for joint stabilization- compared to supine hand facilitating actions like catching. TMS multiple-muscle mapping studies permit a better understanding of motor control and ‘plastic’ reorganization of motor system.

Parallel input makes the brain run faster

In serial sensory processing, information flows from the thalamus via primary sensory cortices to higher-order association areas. However, association cortices also receive, albeit weak, direct thalamocortical sensory inputs of unknown function. For example, while information proceeds from primary (SI) to secondary (SII) somatosensory cortex in a serial fashion, both areas are known to receive direct thalamocortical sensory input. The present study examines the potential roles of such parallel input arrangements. The subjects were presented with median nerve somatosensory stimuli with the instruction to respond with the contralateral hand. The locations and time courses of the activated brain areas were first identified with magnetoencephalography (MEG). In a subsequent session, these brain areas were modulated with single-pulse transcranial magnetic stimulation (TMS) at 15-210 ms after the somatosensory stimulus while electroencephalography (EEG) was recorded. TMS pulses at 15-40 ms post-stimulus significantly speeded up reaction times and somatosensory-evoked responses, with largest facilitatory effects when the TMS pulse was given to contralateral SII at about 20 ms. To explain the results, we propose that the early somatosensory-evoked physiological SII activation exerts an SII-->SI influence that facilitates the reciprocal SI-->SII pathway - with TMS to SII we apparently amplified this mechanism. The results suggest that the human brain may utilize parallel inputs to facilitate long-distance cortico-cortical connections, resulting in accelerated processing and speeded reaction times. This arrangement could also allow very early top-down modulation of the bottom-up stream of sensory information.