Eva-Maria Pool

Dose-dependent effects of theta burst rTMS on cortical excitability and resting-state connectivity of the human motor system

Theta burst stimulation (TBS), a specific protocol of repetitive transcranial magnetic stimulation (rTMS), induces changes in cortical excitability that last beyond stimulation. TBS-induced aftereffects, however, vary between subjects, and the mechanisms underlying these aftereffects to date remain poorly understood. Therefore, the purpose of this study was to investigate whether increasing the number of pulses of intermittent TBS (iTBS) (1) increases cortical excitability as measured by motor-evoked potentials (MEPs) and (2) alters functional connectivity measured using resting-state fMRI, in a dose-dependent manner. Sixteen healthy, human subjects received three serially applied iTBS blocks of 600 pulses over the primary motor cortex (M1 stimulation) and the parieto-occipital vertex (sham stimulation) to test for dose-dependent iTBS effects on cortical excitability and functional connectivity (four sessions in total). iTBS over M1 increased MEP amplitudes compared with sham stimulation after each stimulation block. Although the increase in MEP amplitudes did not differ between the first and second block of M1 stimulation, we observed a significant increase after three blocks (1800 pulses). Furthermore, iTBS enhanced resting-state functional connectivity between the stimulated M1 and premotor regions in both hemispheres. Functional connectivity between M1 and ipsilateral dorsal premotor cortex further increased dose-dependently after 1800 pulses of iTBS over M1. However, no correlation between changes in MEP amplitudes and functional connectivity was detected. In summary, our data show that increasing the number of iTBS stimulation blocks results in dose-dependent effects at the local level (cortical excitability) as well as at a systems level (functional connectivity) with a dose-dependent enhancement of dorsal premotor cortex-M1 connectivity.

Network Connectivity and Individual Responses to Brain Stimulation in the Human Motor System

The mechanisms driving cortical plasticity in response to brain stimulation are still incompletely understood. We here explored whether neural activity and connectivity in the motor system relate to the magnitude of cortical plasticity induced by repetitive transcranial magnetic stimulation (rTMS). Twelve right-handed volunteers underwent functional magnetic resonance imaging during rest and while performing a simple hand motor task. Resting-state functional connectivity, task-induced activation, and task-related effective connectivity were assessed for a network of key motor areas. We then investigated the effects of intermittent theta-burst stimulation (iTBS) on motor-evoked potentials (MEP) for up to 25 min after stimulation over left primary motor cortex (M1) or parieto-occipital vertex (for control). ITBS-induced increases in MEP amplitudes correlated negatively with movement-related fMRI activity in left M1. Control iTBS had no effect on M1 excitability. Subjects with better response to M1-iTBS featured stronger preinterventional effective connectivity between left premotor areas and left M1. In contrast, resting-state connectivity did not predict iTBS aftereffects. Plasticity-related changes in M1 following brain stimulation seem to depend not only on local factors but also on interconnected brain regions. Predominantly activity-dependent properties of the cortical motor system are indicative of excitability changes following induction of cortical plasticity with rTMS.

Handedness and effective connectivity of the motor system

Handedness denotes the individual predisposition to consistently use the left or right hand for most types of skilled movements. A putative neurobiological mechanism for handedness consists in hemisphere-specific differences in network dynamics that govern unimanual movements. We, therefore, used functional magnetic resonance imaging and dynamic causal modeling to investigate effective connectivity between key motor areas during fist closures of the dominant or non-dominant hand performed by 18 right- and 18 left-handers. Handedness was assessed employing the Edinburgh-Handedness-Inventory (EHI). The network of interest consisted of key motor regions in both hemispheres including the primary motor cortex (M1), supplementary motor area (SMA), ventral premotor cortex (PMv), motor putamen (Put) and motor cerebellum (Cb). The connectivity analysis revealed that in right-handed subjects movements of the dominant hand were associated with significantly stronger coupling of contralateral (left, i.e., dominant) SMA with ipsilateral SMA, ipsilateral PMv, contralateral motor putamen and contralateral M1 compared to equivalent connections in left-handers. The degree of handedness as indexed by the individual EHI scores also correlated with coupling parameters of these connections. In contrast, we found no differences between right- and left-handers when testing for the effect of movement speed on effective connectivity. In conclusion, the data show that handedness is associated with differences in effective connectivity within the human motor network with a prominent role of SMA in right-handers. Left-handers featured less asymmetry in effective connectivity implying different hemispheric mechanisms underlying hand motor control compared to right-handers.

Inter-individual variability in cortical excitability and motor network connectivity following multiple blocks of rTMS

The responsiveness to non-invasive neuromodulation protocols shows high inter-individual variability, the reasons of which remain poorly understood. We here tested whether the response to intermittent theta-burst stimulation (iTBS) - an effective repetitive transcranial magnetic stimulation (rTMS) protocol for increasing cortical excitability - depends on network properties of the cortical motor system. We furthermore investigated whether the responsiveness to iTBS is dose-dependent. To this end, we used a sham-stimulation controlled, single-blinded within-subject design testing for the relationship between iTBS aftereffects and (i) motor-evoked potentials (MEPs) as well as (ii) resting-state functional connectivity (rsFC) in 16 healthy subjects. In each session, three blocks of iTBS were applied, separated by 15min. We found that non-responders (subjects not showing an MEP increase of ≥10% after one iTBS block) featured stronger rsFC between the stimulated primary motor cortex (M1) and premotor areas before stimulation compared to responders. However, only the group of responders showed increases in rsFC and MEPs, while most non-responders remained close to baseline levels after all three blocks of iTBS. Importantly, there was still a large amount of variability in both groups. Our data suggest that responsiveness to iTBS at the local level (i.e., M1 excitability) depends upon the pre-interventional network connectivity of the stimulated region. Of note, increasing iTBS dose did not turn non-responders into responders. The finding that higher levels of pre-interventional connectivity precluded a response to iTBS could reflect a ceiling effect underlying non-responsiveness to iTBS at the systems level.

Network dynamics engaged in the modulation of motor behavior in healthy subjects.

Motor skills are mediated by a dynamic and finely regulated interplay of the primary motor cortex (M1) with various cortical and subcortical regions engaged in movement preparation and execution. To date, data elucidating the dynamics in the motor network that enable movements at different levels of behavioral performance remain scarce. We here used functional magnetic resonance imaging (fMRI) and dynamic causal modeling (DCM) to investigate effective connectivity of key motor areas at different movement frequencies performed by right-handed subjects (n=36) with the left or right hand. The network of interest consisted of motor regions in both hemispheres including M1, supplementary motor area (SMA), ventral premotor cortex (PMv), motor putamen, and motor cerebellum. The connectivity analysis showed that performing hand movements at higher frequencies was associated with a linear increase in neural coupling strength from premotor areas (SMA, PMv) contralateral to the moving hand and ipsilateral cerebellum towards contralateral, active M1. In addition, we found hemispheric differences in the amount by which the coupling of premotor areas and M1 was modulated, depending on which hand was moved. Other connections were not modulated by changes in motor performance. The results suggest that a stronger coupling, especially between contralateral premotor areas and M1, enables increased motor performance of simple unilateral hand movements.

Differential modulation of motor network connectivity during movements of the upper and lower limbs

Voluntary movements depend on a well-regulated interplay between the primary motor cortex (M1) and premotor areas. While to date the neural underpinnings of hand movements are relatively well understood, we only have rather limited knowledge on the cortical control of lower-limb movements. Given that our hands and feet have different roles for activities of daily living, with hand movements being more frequently used in a lateralized fashion, we hypothesized that such behavioral differences also impact onto network dynamics underlying upper and lower limb movements. We, therefore, used functional magnetic resonance imaging (fMRI) and dynamic causal modeling (DCM) to investigate differences in effective connectivity underlying isolated movements of the hands or feet in 16 healthy subjects. The connectivity analyses revealed that both movements of the hand and feet were accompanied by strong facilitatory coupling of the respective contralateral M1 representations with premotor areas of both hemispheres. However, excitatory influences were significantly lower for movements of the feet compared to hand movements. During hand movements, the M1(hand) representation ipsilateral to the movement was strongly inhibited by premotor regions and the contralateral M1 homologue. In contrast, interhemispheric inhibition was absent between the M1(foot) representations during foot movements. Furthermore, M1(foot) ipsilateral to the moving foot exerted promoting influences onto contralateral M1(foot). In conclusion, the generally stronger and more lateralized coupling pattern associated with hand movements suggests distinct fine-tuning of cortical control to underlie voluntary movements with the upper compared to the lower limb.

Interindividual differences in motor network connectivity and behavioral response to iTBS in stroke patients

Cerebral plasticity-inducing approaches like repetitive transcranial magnetic stimulation (rTMS) are of high interest in situations where reorganization of neural networks can be observed, e.g., after stroke. However, an increasing number of studies suggest that improvements in motor performance of the stroke-affected hand following modulation of primary motor cortex (M1) excitability by rTMS shows a high interindividual variability. We here tested the hypothesis that in stroke patients the interindividual variability of behavioral response to excitatory rTMS is related to interindividual differences in network connectivity of the stimulated region. Chronic stroke patients (n = 14) and healthy controls (n = 12) were scanned with functional magnetic resonance imaging (fMRI) while performing a simple hand motor task. Dynamic causal modeling (DCM) was used to investigate effective connectivity of key motor regions. On two different days after the fMRI experiment, patients received either intermittent theta-burst stimulation (iTBS) over ipsilesional M1 or control stimulation over the parieto-occipital cortex. Motor performance and TMS parameters of cortical excitability were measured before and after iTBS. Our results revealed that patients with better motor performance of the affected hand showed stronger endogenous coupling between supplemental motor area (SMA) and M1 before starting the iTBS intervention. Applying iTBS to ipsilesional M1 significantly increased ipsilesional M1 excitability and decreased contralesional M1 excitability as compared to control stimulation. Individual behavioral improvements following iTBS specifically correlated with neural coupling strengths in the stimulated hemisphere prior to stimulation, especially for connections targeting the stimulated M1. Combining endogenous connectivity and behavioral parameters explained 82% of the variance in hand motor performance observed after iTBS. In conclusion, the data suggest that the individual susceptibility to iTBS after stroke is influenced by interindividual differences in motor network connectivity of the lesioned hemisphere.

Network dynamics engaged in the modulation of motor behavior in stroke patients

Stroke patients with motor deficits typically feature enhanced neural activity in several cortical areas when moving their affected hand. However, also healthy subjects may show higher levels of neural activity in tasks with higher motor demands. Therefore, the question arises to what extent stroke‐related overactivity reflects performance‐level‐associated recruitment of neural resources rather than stroke‐induced neural reorganization. We here investigated which areas in the lesioned brain enable the flexible adaption to varying motor demands compared to healthy subjects. Accordingly, eleven well‐recovered left‐hemispheric chronic stroke patients were scanned using functional magnetic resonance imaging. Motor system activity was assessed for fist closures at increasing movement frequencies performed with the affected/right or unaffected/left hand. In patients, an increasing movement rate of the affected hand was associated with stronger neural activity in ipsilesional/left primary motor cortex (M1) but unlike in healthy controls also in contralesional/right dorsolateral premotor cortex (PMd) and contralesional/right superior parietal lobule (SPL). Connectivity analyses using dynamic causal modeling revealed stronger coupling of right SPL onto affected/left M1 in patients but not in controls when moving the affected/right hand independent of the movement speed. Furthermore, coupling of right SPL was positively coupled with the “active” ipsilesional/left M1 when stroke patients moved their affected/right hand with increasing movement frequency. In summary, these findings are compatible with a supportive role of right SPL with respect to motor function of the paretic hand in the reorganized brain.

P 82. Dose-dependent effects of theta-burst rTMS on the cortical excitability and fMRI-connectivity of the primary motor cortex

Introduction Theta Burst Stimulation (TBS) is an effective rTMS-protocol to modulate the excitability of cortical motor regions (Huang et al., 2005). However, the effects between subjects are rather variable (Hamda et al., 2012). The reason for this variability is still unclear (Thickbroom, 2007). Recently, animal studies showed that there are dose-dependent effects of TBS on the expression of cellular proteins (Volz et al., 2010). In contrast, studies with human subjects did not find a consistent dose–effect after applying two TBS sessions serially at different intersession intervals (Gamboa et al., 2010, 2011). Objectives The aim of our study was to investigate the effect of a triple TBS session on the cortical excitability compared to a control-stimulation. By combining TBS with functional magnetic resonance imaging (fMRI) measurements we sought to reveal stimulation effects on cortical connectivity. Methods 15 healthy subjects received three stimulations according to the iTBS-protocol (600 pulses per stimulation, (Huang et al., 2005)). iTBS sessions were applied in a serial fashion spaced by intervals of 15min. Two different stimulation sites were tested at different days: primary motor cortex (M1) and the parieto-occipital cortex (control). Stimulation after-effects on cortical excitability were tested via stimulus–response curves. In separate stimulation sessions, the iTBS effects on fMRI-connectivity were tested for two conditions: (i) resting-state measurements and (ii) during thumb movements. The following motor areas were included in the network analysis: M1, supplementary motor area (SMA), dorsal and ventral premotor cortex (dPMC, vPMC), anterior intraparietal cortex, putamen, thalamus and cerebellum. Results We found a dose-dependent effect of iTBS on the height of the stimulus–response-curve with significantly higher MEPs after applying iTBS over M1 compared to the control-stimulation. The connectivity-analyses revealed that after M1 stimulation with 1800 pulses the effective connectivity of the ipsilateral dPMC to the stimulated M1 was significantly enhanced while the control-stimulation had no differential effect on cortical connectivity ( p Conclusions Our results suggest that the after-effects of iTBS are dose-dependent. Furthermore, our data show that iTBS of M1 leads to a higher integration of the stimulated area with premotor areas.

P82. Motor network connectivity predicts responsiveness to theta-burst stimulation

Introduction Intermittent theta-burst stimulation (iTBS) effectively increases cortical excitability within the human brain (Huang et al., 2005). However, individual after-effects of iTBS vary between subjects, with a large proportion not responding at all in terms of changes in excitability (Ridding and Ziemann, 2010; Hamada et al., 2013). We here investigated whether subjects responding to iTBS show differential changes in resting-state functional connectivity (rsFC) within the cortical motor system compared to subjects with no response and whether the application of multiple iTBS-blocks can alter responsiveness. Methods We used a sham-stimulation controlled, single-blinded within-subject design to test for iTBS after-effects on (i) motor evoked potentials (MEPs) and (ii) resting-state functional connectivity (rsFC) in 16 healthy, right-handed subjects ( m =7, 27±3years). iTBS was applied over the left primary motor cortex (M1-iTBS) and over the parieto-occipital vertex (sham-iTBS) in separate sessions. In each stimulation session three iTBS blocks were applied, separated by 15min. Seed-based whole-brain rsFC was computed for the stimulated M1. Results Subjects were divided into groups of responders ( n =7) and non-responders ( n =9) according to iTBS-induced changes in MEPs (criterion: increase of at least 10% compared to baseline; Hinder et al., 2014). Increases in MEP-amplitudes following all three M1-iTBS blocks compared to sham-iTBS could exclusively be found for responders. Likewise, rsFC between M1 and premotor areas was significantly higher in responders after all three iTBS blocks ( p ⩽0.05, cluster-level FWE-corrected), whereas no significant increases could be found for the non-responder group. Dose-dependent increases in MEP-amplitudes and rsFC could also only be found for responders. Importantly, non-responders featured higher levels of pre-interventional rsFC compared to responders ( p ⩽0.01, cluster-level FWE-corrected). Individual changes in MEPs and rsFC did not correlate. Discussion Significant iTBS-induced modulations of rsFC and MEP-amplitudes were exclusively found for the responder group, suggesting that responsiveness to iTBS is paralleled by differential changes in motor network connectivity. However, there was no linear correlation between changes in MEP-amplitudes and rsFC. Additional iTBS blocks did not cause a conversion from non-responders to responders, but rather enhanced cortical excitability and rsFC in responders. Furthermore, lower levels of pre-interventional rsFC might contribute to better effectiveness of iTBS.