Evidence for high-fidelity timing-dependent synaptic plasticity of human motor cortex

Personalized and Circuit-Based Transcranial Magnetic Stimulation: Evidence, Controversies, and Opportunities

The development of neuroimaging methodologies to map brain connectivity has transformed our understanding of psychiatric disorders, the distributed effects of brain stimulation, and how transcranial magnetic stimulation can be best employed to target and ameliorate psychiatric symptoms. In parallel, neuroimaging research has revealed that higher-order brain regions such as the prefrontal cortex, which represent the most common therapeutic brain stimulation targets for psychiatric disorders, show some of the highest levels of interindividual variation in brain connectivity. These findings provide the rationale for personalized target site selection based on person-specific brain network architecture. Recent advances have made it possible to determine reproducible personalized targets with millimeter precision in clinically tractable acquisition times. These advances enable the potential advantages of spatially personalized transcranial magnetic stimulation targeting to be evaluated and translated to basic and clinical applications. In this review, we outline the motivation for target site personalization, preliminary support (mostly in depression), convergent evidence from other brain stimulation modalities, and generalizability beyond depression and the prefrontal cortex. We end by detailing methodological recommendations, controversies, and notable alternatives. Overall, while this research area appears highly promising, the value of personalized targeting remains unclear, and dedicated large prospective randomized clinical trials using validated methodology are critical.

Cerebellar transcranial direct current stimulation disrupts neuroplasticity of intracortical motor circuits

While previous research using transcranial magnetic stimulation (TMS) suggest that cerebellum (CB) influences the neuroplastic response of primary motor cortex (M1), the role of different indirect (I) wave inputs in M1 mediating this interaction remains unclear. The aim of this study was therefore to assess how CB influences neuroplasticity of early and late I-wave circuits. 22 young adults (22 ± 2.7 years) participated in 3 sessions in which I-wave periodicity repetitive transcranial magnetic stimulation (iTMS) was applied over M1 during concurrent application of cathodal transcranial direct current stimulation over CB (tDCS_CB). In each session, iTMS either targeted early I-waves (1.5 ms interval; iTMS_1.5), late I-waves (4.5 ms interval; iTMS_4.5), or had no effect (variable interval; iTMS_Sham). Changes due to the intervention were examined with motor evoked potential (MEP) amplitude using TMS protocols measuring corticospinal excitability (MEP_1mV) and the strength of CB-M1 connections (CBI). In addition, we indexed I-wave activity using short-interval intracortical facilitation (SICF) and low-intensity single-pulse TMS applied with posterior-anterior (MEP_PA) and anterior-posterior (MEP_AP) current directions. Following both active iTMS sessions, there was no change in MEP_1mV, CBI or SICF (all P > 0.05), suggesting that tDCS_CB broadly disrupted the excitatory response that is normally seen following iTMS. However, although MEP_AP also failed to facilitate after the intervention (P > 0.05), MEP_PA potentiated following both active iTMS sessions (both P < 0.05). This differential response between current directions could indicate a selective effect of CB on AP-sensitive circuits.

Using Brain Imaging to Improve Spatial Targeting of Transcranial Magnetic Stimulation for Depression

Transcranial magnetic stimulation (TMS) is an effective treatment for depression but is limited in that the optimal therapeutic target remains unknown. Early TMS trials lacked a focal target and thus positioned the TMS coil over the prefrontal cortex using scalp measurements. Over time, it became clear that this method leads to variation in the stimulation site and that this could contribute to heterogeneity in antidepressant response. Newer methods allow for precise positioning of the TMS coil over a specific brain location, but leveraging these precise methods requires a more precise therapeutic target. We review how neuroimaging is being used to identify a more focal therapeutic target for depression. We highlight recent studies showing that more effective TMS targets in the frontal cortex are functionally connected to deep limbic regions such as the subgenual cingulate cortex. We review how connectivity might be used to identify an optimal TMS target for use in all patients and potentially even a personalized target for each individual patient. We address the clinical implications of this emerging field and highlight critical questions for future research.

Influence of BDNF Val66Met polymorphism on excitatory-inhibitory balance and plasticity in human motor cortex

OBJECTIVE

While previous studies showed that the single nucleotide polymorphism (Val66Met) of brain-derived neurotrophic factor (BDNF) can impact neuroplasticity, the influence of BDNF genotype on cortical circuitry and relationship to neuroplasticity remain relatively unexplored in human.

METHODS

Using individualised transcranial magnetic stimulation (TMS) parameters, we explored the influence of the BDNF Val66Met polymorphism on excitatory and inhibitory neural circuitry, its relation to I-wave TMS (ITMS) plasticity and effect on the excitatory/inhibitory (E/I) balance in 18 healthy individuals.

RESULTS

Excitatory and inhibitory indexes of neurotransmission were reduced in Met allele carriers. An E/I balance was evident, which was influenced by BDNF with higher E/I ratios in Val/Val homozygotes. Both long-term potentiation (LTP-) and depression (LTD-) like ITMS plasticity were greater in Val/Val homozygotes. LTP- but not LTD-like effects were restored in Met allele carriers by increasing stimulus intensity to compensate for reduced excitatory transmission.

CONCLUSIONS

The influence of BDNF genotype may extend beyond neuroplasticity to neurotransmission. The E/I balance was evident in human motor cortex, modulated by BDNF and measurable using TMS. Given the limited sample, these preliminary findings warrant further investigation.

SIGNIFICANCE

These novel findings suggest a broader role of BDNF genotype on neurocircuitry in human motor cortex.

Double-Sine-Wave Quadri-Pulse Theta Burst Stimulation of Precentral Motor Hand Representation Induces Bidirectional Changes in Corticomotor Excitability

Neuronal plasticity is considered to be the neurophysiological correlate of learning and memory and changes in corticospinal excitability play a key role in the normal development of the central nervous system as well as in developmental disorders. In a previous study, it was shown that quadri-pulse theta burst stimulation (qTBS) can induce bidirectional changes in corticospinal excitability (1). There, a quadruple burst consisted of four single-sine-wave (SSW) pulses with a duration of 160 μs and inter-pulse intervals of 1.5 ms to match I-wave periodicity (666 Hz). In the present study, the pulse shape was modified applying double-sine-waves (DSW) rather than SSW pulses, while keeping the pulse duration at 160 μs. In two separate sessions, we reversed the current direction of the DSW pulse, so that its second component elicited either a mainly posterior-to-anterior (DSW PA-qTBS) or anterior-to-posterior (DSW AP-qTBS) directed current in the precentral gyrus. The after-effects of DSW qTBS on corticospinal excitability were examined in healthy individuals (n = 10) with single SSW TMS pulses. For single-pulse SSW TMS, the second component produced the same preferential current direction as DSW qTBS but had a suprathreshold intensity, thus eliciting motor evoked potentials (PA-MEP or AP-MEP). Single-pulse SSW TMS revealed bidirectional changes in corticospinal excitability after DSW qTBS, which depended on the preferentially induced current direction. DSW PA-qTBS at 666 Hz caused a stable increase in PA-MEP, whereas AP-qTBS at 666 Hz induced a transient decrease in AP-MEP. The sign of excitability following DSW qTBS at I-wave periodicity was opposite to the bidirectional changes after SSW qTBS. The results show that the pulse configuration and induced current direction determine the plasticity-effects of ultra-high frequency SSW and DSW qTBS at I-wave periodicity. These findings may offer new opportunities for short non-invasive brain stimulation protocols that are especially suited for stimulation in children and patients with neurological or neurodevelopmental disorders.

A specific phase of transcranial alternating current stimulation at the β frequency boosts repetitive paired-pulse TMS-induced plasticity

Transcranial alternating current stimulation (tACS) at 20 Hz (β) has been shown to modulate motor evoked potentials (MEPs) when paired with transcranial magnetic stimulation (TMS) in a phase-dependent manner. Repetitive paired-pulse TMS (rPPS) with I-wave periodicity (1.5 ms) induced short-lived facilitation of MEPs. We hypothesized that tACS would modulate the facilitatory effects of rPPS in a frequency- and phase-dependent manner. To test our hypothesis, we investigated the effects of combined tACS and rPPS. We applied rPPS in combination with peak or trough phase tACS at 10 Hz (α) or β, or sham tACS (rPPS alone). The facilitatory effects of rPPS in the sham condition were temporary and variable among participants. In the β tACS peak condition, significant increases in single-pulse MEPs persisted for over 30 min after the stimulation, and this effect was stable across participants. In contrast, β tACS in the trough condition did not modulate MEPs. Further, α tACS parameters did not affect single-pulse MEPs after the intervention. These results suggest that a rPPS-induced increase in trans-synaptic efficacy could be strengthened depending on the β tACS phase, and that this technique could produce long-lasting plasticity with respect to cortical excitability.

Modulation of Motor Cortex Plasticity by Repetitive Paired-Pulse TMS at Late I-Wave Intervals Is Influenced by Intracortical Excitability

The late indirect (I)-waves recruited by transcranial magnetic stimulation (TMS) over primary motor cortex (M1) can be modulated using I-wave periodicity repetitive TMS (iTMS). The purpose of this study was to determine if the response to iTMS is influenced by different interstimulus intervals (ISIs) targeting late I-waves, and whether these responses were associated with individual variations in intracortical excitability. Seventeen young (27.2 ± 6.4 years, 12 females) healthy adults received iTMS at late I-wave intervals (4.0, 4.5, and 5.0 ms) in three separate sessions. Changes due to each intervention were examined with motor evoked potential (MEP) amplitudes and short-interval intracortical facilitation (SICF) using both posterior-anterior (PA) and anterior-posterior (AP) TMS current directions. Changes in MEP amplitude and SICF were influenced by iTMS ISI, with the greatest facilitation for ISIs at 4 and 5 ms with PA TMS, and 4 ms with AP TMS. Maximum SICF at baseline (irrespective of ISI) was associated with increased iTMS response, but only for PA stimulation. These results suggest that modifying iTMS parameters targeting late I-waves can influence M1 plasticity. They also suggest that maximum SICF may be a means by which responders to iTMS targeting the late I-waves could be identified.

Characterising the influence of cerebellum on the neuroplastic modulation of intracortical motor circuits

The cerebellum (CB) has extensive connections with both cortical and subcortical areas of the brain, and is known to strongly influence function in areas it projects to. In particular, research using non-invasive brain stimulation (NIBS) has shown that CB projections to primary motor cortex (M1) are likely important for facilitating the learning of new motor skills, and that this process may involve modulation of late indirect (I) wave inputs in M1. However, the nature of this relationship remains unclear, particularly in regards to how CB influences the contribution of the I-wave circuits to neuroplastic changes in M1. Within the proposed research, we will therefore investigate how CB effects neuroplasticity of the I-wave generating circuits. This will be achieved by downregulating CB excitability while concurrently applying a neuroplastic intervention that specifically targets the I-wave circuitry. The outcomes of this study will provide valuable neurophysiological insight into key aspects of the motor network, and may inform the development of optimized interventions for modifying motor learning in a targeted way.

Combined endogenous and exogenous disinhibition of intracortical circuits augments plasticity induction in the human motor cortex

BACKGROUND

Motor imagery (MI) engages cortical areas in the human brain similar to motor practice. Corticospinal excitability (CSE) is facilitated during but not after MI practice. We hypothesized that lasting CSE changes could be achieved by associatively pairing this endogenous modulation with exogenous stimulation of the same intracortical circuits.

METHODS

We combined MI with a disinhibition protocol (DIS) targeting intracortical circuits by paired-pulse repetitive transcranial magnetic stimulation in one main and three subsequent experiments. The follow-up experiments were applied to increase effects, e.g., by individualizing inter-stimulus intervals, adding neuromuscular stimulation and expanding the intervention period. CSE was captured during (online) and after (offline) the interventions via input-output changes and cortical maps of motor evoked potentials. A total of 35 healthy subjects (mean age 26.1 ± 2.6 years, 20 females) participated in this study.

RESULTS

A short intervention (48 stimuli within ∼90s) increased CSE. This plasticity developed rapidly, was associative (with MI_on, but not MI_off or REST) and persisted beyond the intervention period. Follow-up experiments revealed the relevance of individualizing inter-stimulus intervals and of consistent inter-burst periods for online and offline effects, respectively. Expanding this combined MI/DIS intervention to 480 stimuli amplified the sustainability of CSE changes. When concurrent neuromuscular electrical stimulation was applied, the plasticity induction was cancelled.

CONCLUSIONS

This novel associative stimulation protocol augmented plasticity induction in the human motor cortex within a remarkably short period of time and in the absence of active movements. The combination of endogenous and exogenous disinhibition of intracortical circuits may provide a therapeutic backdoor when active movements are no longer possible, e.g., for hand paralysis after stroke.

Age-related changes in late I-waves influence motor cortex plasticity induction in older adults

KEY POINTS

The response to neuroplasticity interventions using transcranial magnetic stimulation (TMS) is reduced in older adults, which may be due, in part, to age-related alterations in interneuronal (I-wave) circuitry. The current study investigated age-related changes in interneuronal characteristics and whether they influence motor cortical plasticity in older adults. While I-wave recruitment was unaffected by age, there was a shift in the temporal characteristics of the late, but not the early I-waves. Using I-wave periodicity repetitive TMS (iTMS), we showed that these differences in I-wave characteristics influence the induction of cortical plasticity in older adults.

ABSTRACT

Previous research shows that neuroplasticity assessed using transcranial magnetic stimulation (TMS) is reduced in older adults. While this deficit is often assumed to represent altered synaptic modification processes, age-related changes in the interneuronal circuits activated by TMS may also contribute. Here we assessed age-related differences in the characteristics of the corticospinal indirect (I) waves and how they influence plasticity induction in primary motor cortex. Twenty young (23.7 ± 3.4 years) and 19 older adults (70.6 ± 6.0 years) participated in these studies. I-wave recruitment was assessed by changing the direction of the current used to activate the motor cortex, whereas short-interval intracortical facilitation (SICF) was recorded to assess facilitatory I-wave interactions. In a separate study, I-wave periodicity TMS (iTMS) was used to examine the effect of I-wave latency on motor cortex plasticity. Data from the motor-evoked potential (MEP) onset latency produced using different coil orientations suggested that there were no age-related differences in preferential I-wave recruitment (P = 0.6). However, older adults demonstrated significant reductions in MEP facilitation at all 3 SICF peaks (all P values < 0.05) and a delayed latency of the second and third SICF peaks (all P values < 0.05). Using I-wave intervals that were optimal for young and older adults, these changes in the late I-waves were shown to influence the plasticity response in older adults after iTMS. These findings suggest that temporal characteristics are delayed for the late I-waves in older adults, and that optimising TMS interventions based on I-wave characteristics may improve the plasticity response in older adults.

Protocols of non-invasive brain stimulation for neuroplasticity induction

Transcranial non-invasive brain stimulation (NIBS) has been widely applied in basic research and clinical intervention in the past few decades. It modulates cortical excitability through varies combinations of current form, stimulation position, strength, frequency, duration and intervals. In this review, protocols of different types of NIBS and their aftereffect are introduced. Moreover, evidences in physiology, pharmacology and behavior response are provided to support the effects of NIBS are plasticity-like effects because of their common mechanisms of synaptic plasticity. This is further confirmed by experiments on small animals at the cellular level.

Impact of different intensities of intermittent theta burst stimulation on the cortical properties during TMS-EEG and working memory performance

Intermittent theta burst stimulation (iTBS) is a noninvasive brain stimulation technique capable of increasing cortical excitability beyond the stimulation period. Due to the rapid induction of modulatory effects, prefrontal application of iTBS is gaining popularity as a therapeutic tool for psychiatric disorders such as depression. In an attempt to increase efficacy, higher than conventional intensities are currently being applied. The assumption that this increases neuromodulatory may be mechanistically false for iTBS. This study examined the influence of intensity on the neurophysiological and behavioural effects of iTBS in the prefrontal cortex. Sixteen healthy participants received iTBS over prefrontal cortex at either 50, 75 or 100% resting motor threshold in separate sessions. Single-pulse TMS and concurrent electroencephalography (EEG) was used to assess changes in cortical reactivity measured as TMS-evoked potentials and oscillations. The n-back task was used to assess changes in working memory performance. The data can be summarised as an inverse U-shape relationship between intensity and iTBS plastic effects, where 75% iTBS yielded the largest neurophysiological changes. Improvement in reaction time in the 3-back task was supported by the change in alpha power, however, comparison between conditions revealed no significant differences. The assumption that higher intensity results in greater neuromodulatory effects may be false, at least in healthy individuals, and should be carefully considered for clinical populations. Neurophysiological changes associated with working memory following iTBS suggest functional relevance. However, the effects of different intensities on behavioural performance remain elusive in the present healthy sample.

Noninvasive Stimulation of the Human Brain: Activation of Multiple Cortical Circuits

Noninvasive brain stimulation methods, such as transcranial electric stimulation and transcranial magnetic stimulation are widely used tools for both basic research and clinical applications. However, the cortical circuits underlying their effects are poorly defined. Here we review the current knowledge based on data mostly coming from experiments performed on human subjects, and also to a lesser extent on rodent or primate models. The data suggest that multiple mechanisms are likely to be involved, such as the direct activation of layer V pyramidal neurons, but also of different types of GABAergic interneurons. In this regard, we propose a key role for a specific type of interneuron known as neurogliaform cell.

Quadri-Pulse Theta Burst Stimulation using Ultra-High Frequency Bursts - A New Protocol to Induce Changes in Cortico-Spinal Excitability in Human Motor Cortex

Patterned transcranial magnetic stimulation (TMS) such as theta burst stimulation (TBS) or quadri-pulse stimulation (QPS) can induce changes in cortico-spinal excitability, commonly referred to as long-term potentiation (LTP)-like and long-term depression (LTD)-like effects in human motor cortex (M1). Here, we aimed to test the plasticity-inducing capabilities of a novel protocol that merged TBS and QPS. 360 bursts of quadri-pulse TBS (qTBS) were continuously given to M1 at 90% of active motor threshold (1440 full-sine pulses). In a first experiment, stimulation frequency of each burst was set to 666 Hz to mimic the rhythmicity of the descending cortico-spinal volleys that are elicited by TMS (i.e., I-wave periodicity). In a second experiment, burst frequency was set to 200 Hz to maximize postsynaptic Ca²⁺ influx using a temporal pattern unrelated to I-wave periodicity. The second phase of sinusoidal TMS pulses elicited either a posterior-anterior (PA) or anterior-posterior (AP) directed current in M1. Motor evoked potentials (MEPs) were recorded before and after qTBS to probe changes in cortico-spinal excitability. PA-qTBS at 666 Hz caused a decrease in PA-MEP amplitudes, whereas AP-qTBS at 666 Hz induced an increase in mean AP-MEP amplitudes. At a burst frequency of 200 Hz, PA-qTBS and AP-qTBS produced an increase in cortico-spinal excitability outlasting for at least 60 minutes in PA- and AP-MEP amplitudes, respectively. Continuous qTBS at 666 Hz or 200 Hz can induce lasting changes in cortico-spinal excitability. Induced current direction in the brain appears to be relevant when qTBS targets I-wave periodicity, corroborating that high-fidelity spike timing mechanisms are critical for inducing bi-directional plasticity in human M1.

Habituation behavior of the medium-latency reflex over the anterior tibial muscle after electrical stimulation of the sural nerve

Over human leg muscles, three motor responses (MR) can commonly be elicited, namely short-latency reflex (SLR), medium-latency reflex (MLR), and long-latency reflex (LLR). The MLR is less well understood than SLR and LLR. As the response to subsequent stimuli may be used to characterize central influences of an MR, we were interested, whether the MLR differs from SLR and LLR with respect to its habituation and facilitation behavior. MR were examined over the anterior tibial (TA) muscle at different contraction levels after electrical single or train stimuli (time intervals of 3 ms) over the ipsilateral sural nerve. Furthermore, MR were selectively averaged after each of four subsequent stimuli (1Hz, 0.4 Hz, trains-of-3). After single stimuli, the peak latency values were 46.2±2.3 ms, 88.0±5.8 ms (MLR), and 131.7±22.2 ms (LLR). All three MR gained similarly strong and significantly in amplitude when up to 10 kg of weight was loaded compared with no weight load. After train stimuli, the LLR but not SLR and MLR gained significantly in amplitude as compared with single stimuli. Different to SLR and LLR, the MLR showed significant habituation behavior at a stimulus repetition rate of 1Hz but not of 0.4 Hz. Thus, inhibitory interneurons seem to be involved in the MLR pathway.

Augmenting Plasticity Induction in Human Motor Cortex by Disinhibition Stimulation

Cellular studies showed that disinhibition, evoked pharmacologically or by a suitably timed priming stimulus, can augment long-term plasticity (LTP) induction. We demonstrated previously that transcranial magnetic stimulation evokes a period of presumably GABA(B)ergic late cortical disinhibition (LCD) in human primary motor cortex (M1). Here, we hypothesized that, in keeping with cellular studies, LCD can augment LTP-like plasticity in humans. In Experiment 1, patterned repetitive TMS was applied to left M1, consisting of 6 trains (intertrain interval, 8 s) of 4 doublets (interpulse interval equal to individual peak I-wave facilitation, 1.3-1.5 ms) spaced by the individual peak LCD (interdoublet interval (IDI), 200-250 ms). This intervention (total of 48 pulses applied over ∼45 s) increased motor-evoked potential amplitude, a marker of corticospinal excitability, in a right hand muscle by 147% ± 4%. Control experiments showed that IDIs shorter or longer than LCD did not result in LTP-like plasticity. Experiment 2 indicated topographic specificity to the M1 hand region stimulated by TMS and duration of the LTP-like plasticity of 60 min. In conclusion, GABA(B)ergic LCD offers a powerful new approach for augmenting LTP-like plasticity induction in human cortex. We refer to this protocol as disinhibition stimulation (DIS).

Changes in corticomotor excitability and intracortical inhibition of the primary motor cortex forearm area induced by anodal tDCS

OBJECTIVE

Previous studies have investigated how tDCS over the primary motor cortex modulates excitability in the intrinsic hand muscles. Here, we tested if tDCS changes corticomotor excitability and/or cortical inhibition when measured in the extensor carpi radialis (ECR) and if these aftereffects can be successfully assessed during controlled muscle contraction.

METHODS

We implemented a double blind cross-over design in which participants (n = 16) completed two sessions where the aftereffects of 20 min of 1 mA (0.04 mA/cm2) anodal vs sham tDCS were tested in a resting muscle, and two more sessions where the aftereffects of anodal vs sham tDCS were tested in an active muscle.

RESULTS

Anodal tDCS increased corticomotor excitability in ECR when aftereffects were measured with a low-level controlled muscle contraction. Furthermore, anodal tDCS decreased short interval intracortical inhibition but only when measured at rest and after non-responders (n = 2) were removed. We found no changes in the cortical silent period.

CONCLUSION

These findings suggest that targeting more proximal muscles in the upper limb with anodal tDCS is achievable and corticomotor excitability can be assessed in the presence of a low-level controlled contraction of the target muscle.