Two Types of TMS-Induced Movement Variability After Stimulation of the Primary Motor Cortex

The posterior parietal cortex (PPC) mediates anticipatory motor control

BACKGROUND

Flexible and precisely timed motor control is based on functional interaction within a cortico-subcortical network. The left posterior parietal cortex (PPC) is supposed to be crucial for anticipatory motor control by sensorimotor feedback matching.

OBJECTIVE

Intention of the present study was to disentangle the specific relevance of the left PPC for anticipatory motor control using transcranial direct current stimulation (tDCS) since a causal link remains to be established.

METHODS

Anodal vs. cathodal tDCS was applied for 10 min over the left PPC in 16 right-handed subjects in separate sessions. Left primary motor cortex (M1) tDCS served as control condition and was applied in additional 15 subjects. Prior to and immediately after tDCS, subjects performed three tasks demanding temporal motor precision with respect to an auditory stimulus: sensorimotor synchronization as measure of anticipatory motor control, interval reproduction and simple reaction.

RESULTS

Left PPC tDCS affected right hand synchronization but not simple reaction times. Motor anticipation was deteriorated by anodal tDCS, while cathodal tDCS yielded the reverse effect. The variability of interval reproduction was increased by anodal left M1 tDCS, whereas it was reduced by cathodal tDCS. No significant effects on simple reaction times were found.

CONCLUSION

The present data support the hypothesis that left PPC is causally involved in right hand anticipatory motor control exceeding pure motor implementation as processed by M1 and possibly indicating subjective timing. Since M1 tDCS particularly affects motor implementation, the observed PPC effects are not likely to be explained by alterations of motor-cortical excitability.

Transcranial Magnetic Stimulation Studies of Human Time Perception: A Primer

The study of the neural basis of time perception has seen a resurgence of interest within the past decade. A variety of these studies have included the use of transcranial magnetic stimulation (TMS), a noninvasive technique for stimulating discrete regions of the surface of the brain. Here, the results of these studies are reviewed and their conclusions are interpreted within a context-dependent framework. However, the use of TMS as an investigatory technique has much unexplored potential that may be particularly beneficial to the study of time perception. As such, considerations are made regarding the design of TMS studies of time perception and future directions are outlined that may be utilized to further elucidate the neural basis of timing in the human brain.

The functional role of the ventral premotor cortex in a visually paced finger tapping task: a TMS study

The accurate control of timed actions is a fundamental aspect of our daily activities. Repetitive movements can be either self-paced or synchronized with an external stimulus. Finger tapping (FT) is a suitable task to study the mechanisms of motor timing in both conditions. The neuronal network supporting motor timing in FT tasks comprises the lateral cerebellum, the lateral and mesial premotor areas as well as parietal sites. It has been suggested that lateral premotor cortices (PMC) are involved in time representation and sensorimotor transformations needed for synchronization. Most studies have focused on the dorsal aspect of PMC (dPMC) whereas the ventral PMC (vPMC) function has been poorly investigated. Here we used an online transcranial magnetic stimulation (TMS) protocol to probe the role of vPMC in an FT task, as compared to a functionally relevant site (dPMC) and an unrelated one. According to the synchronization-continuation paradigm, subjects had to synchronize their tapping to a periodic continuous visual stimulus, and then continue without the external pacer. Two different visual pacers were used: a tapping finger and a hinged tilting bar. We show that TMS reduced the synchronization error when delivered to the vPMC. This effect was larger when the more abstract hinged tilting bar was used as a pacer instead of the finger. No effects were observed in the continuation phase. We hereby offer the first online-TMS evidence of the involvement of vPMC in visually cued FT tasks.

The effect of entrainment on the timing of periodic eye movements

We performed an experiment in which eight healthy individuals made periodic eye movements at five pacing interval conditions (500 ms, 750 ms, 1000 ms, 1250 ms, and 1500 ms). Three methods of entrainment were used in the synchronization phase: saccade, continuous pursuit and discontinuous pursuit. The stimulus train was extinguished and in the continuation phase, subjects made saccadic eye movements at the entrained movement frequencies between two static targets. Using the Wing-Kristofferson model, clock and motor variance were extracted from the time series of continuation trials for all three entrainment conditions. Our results revealed a main effect of time interval on total variance clock variance (as predicted by Weber's law) and on motor variance. We also report that the pursuit entrainment conditions resulted in and mean duration and variance to the saccade entrainment. These results suggest that the neural networks recruited to support a periodic motor timing task depend on the method used to establish the temporal reference.

Interval timing and trajectory in unequal amplitude movements

The Wing-Kristofferson (WK) model of movement timing emphasises the separation of central timer and motor processes. Several studies of repetitive timing have shown that increase in variability at longer intervals is attributable to timer processes; however, relatively little is known about the way motor aspects of timing are affected by task movement constraints. In the present study, we examined timing variability in finger tapping with differences in interval to assess central timer effects, and with differences in movement amplitude to assess motor implementation effects. Then, we investigated whether effects of motor timing observed at the point of response (flexion offset/tap) are also evident in extension, which would suggest that both phases are subject to timing control. Eleven participants performed bimanual simultaneous tapping, at two target intervals (400, 600 ms) with the index finger of each hand performing movements of equal (3 or 6 cm) or unequal amplitude (left hand 3, right hand 6 cm and vice versa). As expected, timer variability increased with the mean interval but showed only small, non-systematic effects with changes in movement amplitude. Motor implementation variability was greater in unequal amplitude conditions. The same pattern of motor variability was observed both at flexion and extension phases of movement. These results suggest that intervals are generated by a central timer, triggering a series of events at the motor output level including flexion and the following extension, which are explicitly represented in the timing system.

Transcranial magnetic stimulation in a finger-tapping task separates motor from timing mechanisms and induces frequency doubling

We study the interplay between motor programs and their timing in the brain by using precise pulses of transcranial magnetic stimulation (TMS) applied to the primary motor cortex. The movement of the finger performing a tapping task is periodically perturbed in synchronization with a metronome. TMS perturbation can profoundly affect both the finger trajectory and its kinematics, but the tapping accuracy itself is surprisingly not affected. The motion of the finger during the TMS perturbation can be categorized into two abnormal behaviors that subjects were unaware of: a doubling of the frequency of the tap and a stalling of the finger for half the period. More stalls occurred as the tapping frequency increased. In addition, an enhancement of the velocity of the finger on its way up was observed. We conclude that the timing process involved in controlling the tapping movement is separate from the motor processes in charge of execution of the motor commands. We speculate that the TMS is causing a release of the motor plan ahead of time into activation mode. The observed doubles and stalls are then the result of an indirect interaction in the brain, making use of an existing motor plan to correct the preactivation and obtain the temporal goal of keeping the beat.

Role of the cerebellum in externally paced rhythmic finger movements

Several studies have suggested that the cerebellum has an important role in timing of subsecond intervals. Previous studies using transcranial magnetic stimulation (TMS) to test this hypothesis directly have produced inconsistent results. Here we used 1-Hz repetitive TMS (rTMS) for 10 min over the right or left cerebellar hemisphere to interfere transiently with cerebellar processing to assess its effect on the performance of a finger-tapping task. Subjects tapped with their right index finger for 1 min (synchronization phase) with an auditory or visual cue at 0.5, 1, or 2 Hz; they continued for a further 1 min at the same rate with no cues (continuation phase). The blocks of trials were performed in a random order. rTMS of the cerebellum ipsilateral to the movement increased the variability of the intertap interval but only for movements at 2 Hz that were made while subjects were synchronizing with an auditory cue. There was no effect on the continuation phase of the task when the cues were no longer present or on synchronization with a visual cue. Similar results were seen after stimulation over the contralateral dorsal premotor cortex but not after rTMS over supplementary motor area. There was no effect after rTMS over the ipsilateral right cervical nerve roots or over the ipsilateral primary motor cortex. The results support the hypothesis of neural network for event-related timing in the subsecond range that involves a cerebellar-premotor network.

Neurophysiological mechanisms involved in transfer of procedural knowledge

Learning to perform a motor task with one hand results in performance improvements in the other hand, a process called intermanual transfer. To gain information on its neural mechanisms, we studied this phenomenon using the serial reaction-time task (SRTT). Sixteen, right-handed volunteers trained a 12-item sequence of key presses repeated without the subjects' knowledge. Blocks with no repeating sequence, called random blocks, were interspersed with sequence-training blocks. Response times improved in random and training blocks in both hands. The former result reflects nonspecific improvement in performance, and the latter represents a sequence-specific improvement. To evaluate changes in the primary motor cortex (M1), we tested resting motor thresholds (RMT), recruitments curves to transcranial magnetic stimulation (RC), short intracortical inhibition (SICI), and interhemispheric inhibition (IHI) from the dominant left (learning) to the nondominant right (transfer) hemisphere, before and after SRTT training. Training resulted in (1) increased RC and decreased SICI but no changes in RMT in the learning hemisphere, (2) decreased SICI and no changes in RC or RMT in the transfer hemisphere, and (3) decreased IHI. The amount in IHI after training correlated with nonspecific performance improvements in the transfer hand but not with sequence-specific performance improvements. Our results indicate that modulation of interhemispheric inhibition between the M1 areas may, as a result of the learning that has occurred in one hemisphere after practice with one hand, contribute to faster, more skilled performance of the opposite hand.