This article describes new approaches to measure and strengthen functionally specific neural pathways with transcranial magnetic stimulation. These advanced noninvasive brain stimulation methodologies can provide new opportunities for the understanding of brain-behavior relations and development of new therapies to treat brain disorders.
Understanding interactions between brain areas is important for the study of goal-directed behavior. Functional neuroimaging of brain connectivity has provided important insights into fundamental processes of the brain like cognition, learning, and motor control. However, this approach cannot provide causal evidence for the involvement of brain areas of interest. Transcranial magnetic stimulation (TMS) is a powerful, noninvasive tool for studying the human brain that can overcome this limitation by transiently modifying brain activity. Here, we highlight recent advances using a paired-pulse, dual-site TMS method with two coils that causally probes cortico-cortical interactions in the human motor system during different task contexts. Additionally, we describe a dual-site TMS protocol based on cortical paired associative stimulation (cPAS) that transiently enhances synaptic efficiency in two interconnected brain areas by applying repeated pairs of cortical stimuli with two coils. These methods can provide a better understanding of the mechanisms underlying cognitive-motor function as well as a new perspective on manipulating specific neural pathways in a targeted fashion to modulate brain circuits and improve behavior. This approach may prove to be an effective tool to develop more sophisticated models of brain-behavior relations and improve diagnosis and treatment of many neurological and psychiatric disorders.
Noninvasive brain stimulation is a promising assessment tool and treatment for many neurological disorders, such as Parkinson's disease, Alzheimer's disease, and stroke1,2,3,4. There is accumulating evidence establishing the relationship between the behavioral manifestations of neurological diseases and abnormalities of cortical excitability, neuroplasticity, cortico-cortical and cortico-subcortical connectivity5,6. Therefore, basic knowledge about brain network dynamics and plasticity in neurological conditions can provide invaluable insight into disease diagnosis, progression, and response to therapy. Functional magnetic resonance imaging (fMRI) is a useful tool to understand the complex relations between brain and behavior in both healthy and diseased brain networks and has the potential to improve treatment based on a network perspective7,8,9. However, fMRI is correlational in nature and cannot provide a causal link between brain function and behavior, nor manipulate functional connectivity to restore abnormal neural circuits associated with behavioral impairments in patients10,11,12. Transcranial magnetic stimulation (TMS) can both causally measure and modulate human brain function and behavior in health and disease3,13,14,15.
TMS is a safe, noninvasive method to stimulate the human brain16,17 and can be used to induce and measure plasticity18. This method can advance our understanding of causal relationships between individual brain areas and behavior10,11,12,19 and their specific functional interactions with other nodes of a brain network20,21,22,23. To date, most studies have focused on the human motor system, given that TMS to the hand area of the motor cortex (M1) can produce motor evoked potentials (MEPs) as physiological readouts for changes associated with motor behavior24, allowing examination of different inhibitory and excitatory circuits at the system level in the human brain25. Recent advances using a conditioning test TMS approach with two coils show that it is possible to measure functional interactions between different cortical areas. In the motor system, dual-site TMS experiments show that inputs from cortical areas interconnected with M1 can change with task demands, age, or disease14,26. Seminal work by Ferbert and colleagues has found that applying a conditioning stimulus to M1 prior to a test stimulus of the other M1 can result in inhibition of the MEP amplitude, a phenomenon known as short interval interhemispheric inhibition (SIHI)28. A number of TMS studies using this approach have also shown that M1 is strongly interconnected with the contralateral M1, ventral premotor cortex (PMv), dorsal premotor cortex (PMd), supplementary motor area (SMA), pre-SMA, primary sensory cortex (S1), dorsolateral prefrontal cortex (DLPFC), and posterior parietal cortex (PPC) at rest27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42. Interestingly, the effect of stimulation from these cortical areas on motor cortical excitability are anatomically, temporally, and functionally specific to the ongoing brain activity during the preparation of a movement (state- and context-dependent43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,69). However, very few studies using dual-site TMS have characterized patterns of functional cortico-cortical connectivity with motor and cognitive impairments in patients with brain disorders70,71,72. This affords opportunities to develop new methods for assessing and treating motor and cognitive disorders.
Using this technique, it also has been found that repeated pairs of cortical TMS applied to cortical areas interconnected with M1 such as contralateral M168,69,70, PMv76,77,78, SMA71, and PPC80,81,82 can induce changes in synaptic efficiency in specific neural pathways based on the Hebbian principle of associative plasticity83,84,85,86 and enhance behavioral performance72,73,74. Still, few studies have used this approach to study circuit and plasticity dysfunction in neurological disorders2,75,76,77,78,79,80,81,82,83,84,90,91,92,93,94,95,96. It remains to be shown whether strengthening functionally specific neural pathways with TMS can restore activity in dysfunctional circuits, or whether the prospective strengthening of intact circuity can augment resilience97 in brain networks supporting motor and cognitive function across the lifespan and in disease. The lack of fundamental understanding of the neural mechanisms underlying neurological disorders and effects of stimulation on interconnected dysfunctional brain networks limits current treatment.
Despite its capability, TMS has yet to become a standard part of the armamentarium of neuroscience and clinical tools for understanding brain-behavior relations, pathophysiology of brain disorders, and the effectiveness of treatment. Therefore, to realize its potential and support its large-scale application, standardizing TMS methods is important because it is more likely to increase the rigor of future TMS experiments and reproducibility across independent laboratories. This article outlines how TMS can be used to both measure and manipulate functional interactions. Here, we describe this technique in the motor system (e.g., parieto-motor pathway44) by measuring TMS-based output measures (e.g., MEPs), where the method is best understood. However, it is important to note that this protocol also can be adapted to target functional coupling of other subcortical85, cerebellar86,87, and cortical areas.73,74,88 In addition, neuroimaging techniques such as EEG89,90,91 and fMRI92,93 can be used to assess the TMS-induced changes in activity and connectivity26,94. We conclude by proposing that the study of the functional involvement of circuit-level cortical connectivity with these TMS methods in both health and disease makes it possible to develop targeted diagnoses and innovative therapies based on more sophisticated network models of brain-behavior relations.
The following three TMS methods are described below. First, two methods are described to measure cortico-cortical connectivity using dual-site transcranial magnetic stimulation (dsTMS) while participants are either 1) at rest (resting state) or 2) performing an object-directed reach-to-grasp movement (task-dependent). Second, a cortical paired associative stimulation (cPAS) method is described to modulate the interplay between two brain areas in a controlled manner by pairing cortical stimuli (e.g., posterior parietal and primary motor cortices) to strengthen functional specific neural pathways with TMS and induce changes in cortical excitability. A representative data set is provided for each method. All the methods described in this protocol were approved by the University of Michigan Institutional Review Board in accordance with the Declaration of Helsinki.
1. Participant Recruitment
2. Electromyography (EMG) Electrode Placement
3. Localizing Brain Areas for Targeted TMS
4. Localizing Optimal TMS Coil Position and Determining Thresholds
NOTE: In this experiment, CoilM1 refers to the coil used to deliver stimulation to M1, while CoilTwo refers to the coil used to deliver stimulation to the other cortical area of interest (e.g., posterior parietal cortex). Thresholding over M1 must be determined for CoilTwo to calculate the maximum stimulator output (MSO) used over nonmotor areas. Motor threshold values should be reported to allow for comparisons and reproducibility across experiments.
5. Dual-site TMS (Resting State)
6. Dual-site TMS (Task Context)
NOTE: Dual-site TMS also can be used to test whether functional connectivity at rest can be modulated by different task contexts.
7. Cortical Paired Associative Stimulation (cPAS)
NOTE: This protocol involves delivering pairs of monophasic pulses to two different cortical areas over short periods to induce pathway-specific changes in synaptic strength between connections within the human brain. This approach is based on Hebbian principles of spike timing dependent plasticity107,108,109,110. Similar to dual-site TMS methods, cPAS is delivered with two TMS machines connected to two individual TMS coils over two different cortical areas (e.g., PPC and M1).
8. Data Processing and Analysis
Figure 5 shows the size of an exemplar MEP response elicited in the FDI muscle by TMS for an unconditioned test stimuli (TS alone to M1, blue trace) or conditioned stimuli from PPC (CS-TS, red trace) while the participant was at rest (top panel) or planning a goal-directed grasping action to an object (bottom panel). At rest, the PPC exerts an inhibitory influence on ipsilateral M1, as shown by the decrease in MEP amplitudes potentiated by a subthreshold CS delivered over PPC 5 ms before a suprathreshold TS over M1 (top panel). During the preparation of a grasp action, this net inhibitory drive at rest from PPC switched to facilitation (a release of inhibition). To directly compare PPC-M1 interactions during rest versus task demands, the MEP amplitudes were normalized to TS alone trials for each condition and plotted as a ratio for MEP amplitude. The PPC-M1 interaction was facilitated from rest when planning an object-directed grasp (purple bars).
The top panel in Figure 6 shows changes in MEP amplitudes during the administration of the cPAS protocol. MEP amplitudes induced by paired stimulation of PPC and M1 gradually increased over time during the stimulation protocol, suggesting plastic effects at the level of the parieto-motor connection, M1 corticospinal neurons, or both. The bottom panel of Figure 6 shows changes in MEP amplitudes elicited in the resting FDI muscle by single-pulse TMS over M1 before and after the cPAS protocol. The size of the MEP amplitudes increased 10 min after the cPAS protocol, suggesting motor excitability aftereffects were induced after the administration of the repeated pairs of cortical stimuli over PPC and M1.
Figure 1: Three-dimensional reconstruction of a typical participant's anatomical MRI with marked cortical sites over the primary motor cortex (M1, blue symbol) and posterior parietal cortex (PPC, red symbol) in the left hemisphere. Neuronavigation software for TMS was employed to target individually determined cortical areas with each figure-8 TMS coil. Please click here to view a larger version of this figure.
Figure 2: Schematic representation of the dual-site, paired-pulse transcranial magnetic stimulation with two coils (dsTMS) used to probe functional interactions between the posterior parietal cortex (PPC) and primary motor cortex (M1) at rest (resting state). A CS was applied to the PPC to examine its effect on a subsequent suprathreshold TS to M1. Any change in the amplitude of the right-hand muscle response to TMS is measured with EMG. For this experiment, the CS intensity was 90% of RMT. The intensity of TS was adjusted to elicit a MEP of ~1 mV peak-to-peak in the relaxed FDI and ADM. The ISI between pulses was 5 ms. Please click here to view a larger version of this figure.
Figure 3: The dsTMS approach used to probe functional interactions between PPC and M1 during a reach-to-grasp movement (task context). The illumination of an LED instructed the participant to plan one of two possible rightward hand actions on the target object: 1) grasp the smaller top cylinder or 2) grasp the larger bottom cylinder. TS alone or CS–TS at the specified ISI (e.g., 5 ms) was delivered 300 ms after the 'GO' cue (e.g., LED onset) during the reaction time period (plan phase) such that MEP recordings were collected before actual movement initiation (dotted black line). Please click here to view a larger version of this figure.
Figure 4: Schematic of cortical paired associative stimulation protocol (cPAS) used to strengthen functionally specific neural pathways. The first stimulus was applied to the area of interest with CoilTwo (e.g., PPC, red coil) 5 ms before the second stimulus was delivered to M1 (blue coil) with CoilM1. The pairs of cortical stimuli were delivered at a frequency of 0.2 Hz (once every 5 s) and repeated for 100 trials (~8.3 min). Please click here to view a larger version of this figure.
Figure 5: Exemplar MEP traces for an unconditioned test stimulus (TS alone, blue trace) or conditioned stimulus (CS-TS, red trace) for the resting state (top panel) and context-dependent (bottom panel) condition. Bar graphs show the MEP amplitudes from the dsTMS protocol while the participant is at rest or performing a grasping task (action). When the participant was at rest (top panel), CS-TS (red bar) decreased the mean amplitude of MEPs (inhibition) compared to the unconditioned TS alone (blue bar). In contrast, when the participant planned the reach-to-grasp task (bottom panel), the mean MEP amplitude increased (facilitation) for CS-TS (red bar) trials compared to the TS alone (blue bar) trials. To directly compare the PPC-M1 interaction for rest versus action condition, the mean MEP amplitude elicited by paired-pulse stimulation (CS–TS) was normalized by calculating the ratio of the amplitude relative to the mean unconditioned MEP amplitude (TS alone). Purple bars represent the normalized MEP amplitude for each condition. Y = 1 indicates no effect of CS on M1 excitability (dotted black line), whereas ratios higher than 1 indicate increased M1 excitability and ratios lower than 1 indicate decreased M1 excitability because of conditioned stimuli (CS-TS). Error bars represent SEM. Please click here to view a larger version of this figure.
Figure 6: MEPs during cPAS. Top panel shows that MEP amplitudes increased during the administration of cPAS. The bottom panel shows the effect of cPAS protocol on MEP amplitude. After the cPAS intervention (red bar) corticospinal excitability increased after 10 min (dark grey bar) compared to baseline (light grey bar), as assessed by MEPs in the quiescent hand muscles. The red bar represents the paired stimulation intervention, cPAS (100 pairs at 0.2 Hz, ~8.3 min). This suggests that modulating parieto-motor interactions with cPAS can induce transient changes in motor plasticity. Error bars represent SEM. Please click here to view a larger version of this figure.
The dual-site TMS method described here can be employed to investigate functional interactions between different cortical areas interconnected with the primary motor cortex while a participant is at rest or planning a goal-directed action. While brain imaging is correlative, basic knowledge from dual-site TMS methods can reveal causal brain-behavior relations associated with changes in cortico-cortical circuits. In addition, cortical paired associative stimulation with two TMS coils applied in areas interconnected with M1 can be employed to strengthen functionally specific connectivity for movement control and increase the efficiency of inducing plasticity. Taken together, these methods demonstrate that these TMS protocols can both measure and manipulate neural activity underlying information flow between brain areas in an anatomical-, task-, and time-dependent manner within the motor system. This affords opportunities to test different hypotheses related to the causal contribution of cortical areas to motor function.
In this light, the approach also can provide an essential foundation for understanding network connectivity at a systems-level in neurological and psychiatric patients with similar symptomology and enable its use as both a tool to diagnose and treat circuit dysfunction. Therefore, it is important for more studies to explore other cortical areas outside the motor system to test its generalizability across brain networks in both healthy and diseased brains. This is an important factor given that one cannot assume that the response to TMS in one brain region will produce the same physiological effect when applied to another region. It is also advantageous that these procedures can be extended to more complex movements, and other domains outside of movement such as cognition, perception, and mood. Indeed, several studies using dual-site TMS and cPAS have begun to examine the effects and feasibility of study in the visual and cognitive systems73,74,88. Importantly, this will afford opportunities to develop a more sophisticated understanding of the neural underpinnings linking brain activity to motor, cognitive, and affective function. As a result, it is critical that a solid mechanistic knowledge about neural circuit dynamic in patient populations is investigated before determining the usefulness of applying these protocols in future clinical settings.
Although growing evidence suggests that TMS is a novel approach capable of characterizing synaptic dysfunction and plasticity in neurological and psychiatric disorders such as Parkinson's disease, Alzheimer's disease, and stroke, the clinical utility of these assessments needs to be established on a larger scale. Moreover, to date all work in patient populations has focused only on the functional circuits while the participants are at rest. It is vital that future studies with dual-site TMS consider state- and task-dependent effects, particularly when the patient is challenged, to fill knowledge gaps in understanding how altered brain dynamics contribute to specific motor, cognitive, and affective dysfunctions. Importantly, this setting allows for unprecedented opportunities to comprehensively study functional brain circuits and plasticity noninvasively by both recording and manipulating neural activity. This can eventually be translated to novel clinical therapies for brain disorders.
Awaiting these clinical advances, a critical first step is to increase the rigor and reproducibility of TMS experiments across independent laboratories by providing well-defined methodological procedures that are easily deployable and shareable. The following guidelines for the TMS procedures described above can help standardize the design, implementation, and conclusiveness of findings. First, stimulation parameters such as the intensity, duration, ISI, timing, coil position, and anatomical locations should be carefully documented and repeated in the same task context across multiple independent laboratories to encourage large-scale testing and application. Second, brain targets should be precisely defined based on clear anatomical and functional criteria that capture brain activity within brain circuits associated with behavior. Third, neuronavigation should be used to guide the TMS coil placement when targeting said brain circuits. It also is recommended that experiments be hypothesis-driven and use both a control task to ensure changes are related selectively to the task context and a control brain site outside the putative targeted network to rule out the nonspecific effect of stimulation. Fourth, to better inform the diagnostic accuracy and therapeutic effectiveness of these methods in future clinical settings, basic research will need to use a multimodal approach combining TMS measures and manipulations with neuroimaging and behavioral measures to better characterize the underlying pathological changes and effect of treatment. Fifth, variability of individual responses using dual-site TMS methods need to be reported because it could provide important information about how interventions can be optimized for different brain areas, leading to new treatments based on individual pathophysiological mechanisms. Finally, researchers need to be transparent when reporting findings by including negative results42 and make data publicly available for interpretation to increase sample sizes and promote more efficient science. This comprehensive approach will increase rigor and reproducibility in both the collection and analysis of data that can guide future basic neuroscience and clinical studies. Ultimately, this will enable improvements in experimental design and optimize targeted therapies, thereby reducing morbidity and impairments in neurological and psychiatric disorders.
The authors have nothing to disclose.
This work was supported by the University of Michigan: MCubed Scholars Program and School of Kinesiology.
Alpha B.I. D50 coil (coated) | Magstim | 50mm coil | |
BrainSight 2.0 Software | Rogue Research | Neuronavigation software | |
BrainSight frameless Stereotactic System | Rogue Research | Neuronavigation equiptment | |
D702 Coil | Magstim | 70mm coil | |
Discovery MR750 | General Electric | 3.0T MRI machine | |
Disposable Earplugs | 3M | Foam earplugs | |
ECG Electrodes 30mm x 24mm | Coviden-Kendall | H124SG | Disposable electrodes |
Four Channel Isolated Amplifier | Intronix Technologies Corporation | 2024F | EMG amplifier |
gGAMMAcap | g.tec Medical Engineering | EEG head cap | |
Micro1401-3 | Cambridge Electronic Design | Scientific data recorder and processing machine | |
Nuprep Skin Prep Gel | Weaver and Company | Skin prep abrasive gel | |
Signal v.7 | Cambridge Electronic Design | Data acquisition and analysis software | |
The Magstim BiStim2 | Magstim | Transcranial magnetic stimulator (two 2002 units) |