Effects of transcranial magnetic stimulation coil orientation and pulse width on short-latency afferent inhibition

Purpose We used a controllable pulse parameter transcranial magnetic stimulation (cTMS) device to assess whether adjusting pulse width and coil orientation would allow more selective stimulation of different neuronal populations. Methods Young healthy subjects participated in experiments involving single pulse stimulation over the hand motor area elicited by a cTMS device connected to a figure-of-eight coil. Experiment 1 (n=10) evaluated the effect of coil orientation (posterior-anterior, PA; anterior-posterior, AP) and pulse width (30, 60 and 120 μs) on the strength-duration curve, the input-output (IO) curve and the latency of the motor evoked potentials (MEPs) in the first dorsal interosseous muscle. Experiment 2 (n=12) evaluated the effect of coil orientations (PA, AP) and pulse width (30 and 120 μs) on short-latency afferent inhibition (SAI), tested with electrical median nerve stimulation at the wrist prior to TMS (inter-stimulus intervals: N20 latency +2 and +4 ms). All tests were completed during background contraction (∼10% maximum). Results The mean strength-duration time constants were shorter for PA than AP directed currents when estimated using motor threshold data (231 vs. 294 μs; t-test, p = 0.008) and IO data (252 vs. 296 μs; t-test, p < 0.001). ANOVA revealed an interaction of pulse width and orientation on MEP latencies (p = 0.001), due mainly to the increase in latencies with short duration AP stimuli. A similar pulse width and orientation interaction was observed for SAI (p = 0.011), resulting from the stronger inhibition with AP stimuli of short duration. Conclusion PA and AP oriented pulses appear to activate neural populations with different time constants. The AP-sensitive neural populations that elicit the longest latency MEPs are more readily stimulated by short than by long duration pulses, and appear more sensitive to SAI. Manipulating pulse width may improve the selectivity of AP stimulation

Evidence for a role of a cortico-subcortical network for automatic and unconscious motor inhibition of manual responses

It is now clear that non-consciously perceived stimuli can bias our decisions. Although previous researches highlighted the importance of automatic and unconscious processes involved in voluntary action, the neural correlates of such processes remain unclear. Basal ganglia dysfunctions have long been associated with impairment in automatic motor control. In addition, a key role of the medial frontal cortex has been suggested by administrating a subliminal masked prime task to a patient with a small lesion restricted to the supplementary motor area (SMA). In this task, invisible masked arrows stimuli were followed by visible arrow targets for a left or right hand response at different interstimuli intervals (ISI), producing a traditional facilitation effect for compatible trials at short ISI and a reversal inhibitory effect at longer ISI. Here, by using fast event-related fMRI and a weighted parametric analysis, we showed BOLD related activity changes in a cortico-subcortical network, especially in the SMA and the striatum, directly linked to the individual behavioral pattern. This new imaging result corroborates previous works on subliminal priming using lesional approaches. This finding implies that one of the roles of these regions was to suppress a partially activated movement below the threshold of awareness

Modulation of brain activity during a Stroop inhibitory task by the kind of cognitive control required

This study used a proportion congruency manipulation in the Stroop task in order to investigate, at the behavioral and brain substrate levels, the predictions derived from the Dual Mechanisms of Control (DMC) account of two distinct modes of cognitive control depending on the task context. Three experimental conditions were created that varied the proportion congruency: mostly incongruent (MI), mostly congruent (MC), and mostly neutral (MN) contexts. A reactive control strategy, which corresponds to transient interference resolution processes after conflict detection, was expected for the rare conflicting stimuli in the MC context, and a proactive strategy, characterized by a sustained task-relevant focus prior to the occurrence of conflict, was expected in the MI context. Results at the behavioral level supported the proactive/reactive distinction, with the replication of the classic proportion congruent effect (i.e., less interference and facilitation effects in the MI context). fMRI data only partially supported our predictions. Whereas reactive control for incongruent trials in the MC context engaged the expected fronto-parietal network including dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex, proactive control in the MI context was not associated with any sustained lateral prefrontal cortex activations, contrary to our hypothesis. Surprisingly, incongruent trials in the MI context elicited transient activation in common with incongruent trials in the MC context, especially in DLPFC, superior parietal lobe, and insula. This lack of sustained activity in MI is discussed in reference to the possible involvement of item-specific rather than list-wide mechanisms of control in the implementation of a high task-relevant focus

Scanning conditions in functional connectivity magnetic resonance imaging: how to standardise resting-state for optimal data acquisition and visualisation?

Functional connectivity magnetic resonance imaging (fcMRI), performed during resting wakefulness without tasks or stimulation, is a non-invasive technique to assess and visualise functional brain networks in vivo. Acquisition of resting-state imaging data has become increasingly common in longitudinal studies to investigate brain health and disease. However, the scanning protocols vary considerably across different institutions creating challenges for comparability especially for the interpretation of findings in patient cohorts and establishment of diagnostic or prognostic imaging biomarkers. The aim of this chapter is to discuss the effect of two experimental conditions (i.e. a low cognitive demand paradigm and a pure resting-state fcMRI) on the reproducibility of brain networks between a baseline and a follow-up session, 30 (±5) days later, acquired from 12 right-handed volunteers (29 ± 5 yrs). A novel method was developed and used for a direct statistical comparison of the test-retest reliability using 28 well-established functional brain networks. Overall, both scanning conditions produced good levels of test-retest reliability. While the pure resting-state condition showed higher test-retest reliability for 18 of the 28 analysed networks, the low cognitive demand paradigm produced higher test-retest reliability for 8 of the 28 brain networks (i.e. visual, sensorimotor and frontal areas); in 2 of the 28 brain networks no significant changes could be detected. These results are relevant to planning of longitudinal studies, as higher test-retest reliability generally increases statistical power. This work also makes an important contribution to neuroimaging where optimising fcMRI experimental scanning conditions, and hence data visualisation of brain function, remains an on-going topic of interest. In this chapter, we provide a full methodological explanation of the two paradigms and our analysis so that readers can apply them to their own scanning protocols

Effects of high-frequency transcranial magnetic stimulation on functional performance in individuals with incomplete spinal cord injury: study protocol for a randomized controlled trial

Background: Repetitive transcranial magnetic stimulation (rTMS) has been investigated as a new tool in neurological rehabilitation of individuals with spinal cord injury (SCI). However, due to the inconsistent results regarding the effects of rTMS in people with SCI, a randomized controlled double-blind crossover trial is needed to clarify the clinical utility and to assess the effect size of rTMS intervention in this population. Therefore, this paper describes a study protocol designed to investigate whether the use of rTMS can improve the motor and sensory function, as well as reduce spasticity in patients with incomplete SCI. Methods: A double-blind randomized sham-controlled crossover trial will be performed by enrolling 20 individuals with incomplete SCI. Patients who are at least six months post incomplete SCI (aged 18–60 years) will be recruited through referral by medical practitioners or therapists. Individuals will be randomly assigned to either group 1 or group 2 in a 1:1 ratio, with ten individuals in each group. The rTMS protocol will include ten sessions of high-frequency rTMS (5 Hz) over the bilateral lower-limb motor area positioned at the vertex (Cz). Clinical evaluations will be performed at baseline and after rTMS active and sham. Discussion: rTMS has produced positive results in treating individuals with physical impairments; thus, it might be promising in the SCI population. The results of this study may provide new insights to motor rehabilitation thereby contributing towards the better usage of rTMS in the SCI population. Trial registration: ClinicalTrials.gov, NCT02899637 . Registered on 25 August 2016.Medicine, Faculty ofNon UBCReviewedFacult