Effect of Pulse Duration and Direction on Plasticity Induced by 5 Hz Repetitive Transcranial Magnetic Stimulation in Correlation With Neuronal Depolarization

Height-dependent variation in corticospinal excitability modulation after active but not sham intermittent theta burst stimulation

Poor reproducibility and high inter-individual variability in responses to intermittent theta burst stimulation (iTBS) of the human motor cortex (M1) are matters of concern. Here we recruited 17 healthy young adults in a randomized, sham-controlled, crossover study. Transcranial magnetic stimulation (TMS)-elicited motor evoked potentials (MEPs) were measured pre-iTBS (T0) and post-iTBS at 4-7 (T1), 9-12 (T2), 17-20 (T3), and 27-30 minutes (T4) from the right first dorsal interosseous muscle. MEP grand average (MEPGA) was defined as the mean of the normalized-to-baseline MEPs at all timepoints post-iTBS. As secondary objectives, we measured blood pressure, heart rate, and capillary blood glucose pre-iTBS, and at 0 and 30 minutes post-iTBS. The TMSens_Q structured questionnaire was filled out at the end of each session. Two-way repeated ANOVA did not show a significant TIME×INTERVENTION interaction effect on MEP amplitude, MEP latency, blood pressure, heart rate, and blood glucose (p > 0.05). Sleepiness was the most reported TMSens_Q sensation (82.3 %) in both groups. Surprisingly, the subjects' height negatively correlated with the normalized MEP amplitudes at T3 (r = -0.65, p = 0.005), T4 (r = -0.66, p = 0.004), and MEPGA (r = -0.68, p = 0.003), with a trend correlation at T1 (r = -0.46, p = 0.062) and T2 (r = -0.46, p = 0.065) in the active but not sham group. In view of this, we urge future studies to delve deeper into the influence of height on neuroplasticity induction of the M1 representation of peripheral muscles. In the end, we highlight unique methodological considerations in our study protocol and future recommendations for M1-iTBS studies.

Intermittent Theta Burst Stimulation Over Cerebellum Facilitates Neurological Recovery in Poststroke Depression via the cAMP/PKA/CREB Pathway

BACKGROUND

Stroke causes somatic dysfunction and psychological disorders, leading to poststroke depression (PSD). This study investigates mood alterations in PSD models via cerebellar intermittent theta burst stimulation (iTBS).

METHODS

PSD animal models were developed using middle cerebral artery occlusion and chronic unpredictable mild stress procedures. PSD models underwent cerebellar iTBS with different pulse numbers. Neurological recovery was evaluated using open-field test, sucrose preference test, forced swimming test, and balance beam test. Golgi and hematoxylin-eosin staining assessed neuronal repair, while quantitative real-time polymerase chain reaction, enzyme-linked immunosorbent assay (ELISA), immunofluorescence, and Western blotting evaluated effects on BDNF (brain-derived neurotrophic factor), hypothalamic-pituitary-adrenal axis factors, and the cAMP/PKA (protein kinase A)/CREB (cAMP-response element-binding protein) pathway. The study first determined the effects of different intensities of iTBS stimulation on neurological recovery in PSD rats. Second, the effects of iTBS stimulation on the cAMP/PKA/CREB pathway were verified using adenoviral blockade of PKA and CREB at iTBS-1800.

RESULTS

PSD models showed decreased vertical movement, locomotor distance, and sucrose preference and increased immobility time and balance beam test score, which were reversed by iTBS. iTBS increased dendritic length and spine density in Purkinje cells, alleviated neuronal damage in multiple brain regions, and enhanced BDNF synthesis. It also regulated adrenocorticotropic hormone, cortisol, and GR (glucocorticoid receptor) expression, and activated the cAMP/PKA/CREB pathway.

CONCLUSIONS

Cerebellar iTBS improves PSD by activating the cAMP-PKA/CREB pathway, increasing BDNF, and reducing hypothalamic-pituitary-adrenal axis hyperactivity, suggesting potential for human PSD treatment.

Optimized monophasic pulses with equivalent electric field for rapid-rate transcranial magnetic stimulation

Objective.Transcranial magnetic stimulation (TMS) with monophasic pulses achieves greater changes in neuronal excitability but requires higher energy and generates more coil heating than TMS with biphasic pulses, and this limits the use of monophasic pulses in rapid-rate protocols. We sought to design a stimulation waveform that retains the characteristics of monophasic TMS but significantly reduces coil heating, thereby enabling higher pulse rates and increased neuromodulation effectiveness.Approach.A two-step optimization method was developed that uses the temporal relationship between the electric field (E-field) and coil current waveforms. The model-free optimization step reduced the ohmic losses of the coil current and constrained the error of the E-field waveform compared to a template monophasic pulse, with pulse duration as a second constraint. The second, amplitude adjustment step scaled the candidate waveforms based on simulated neural activation to account for differences in stimulation thresholds. The optimized waveforms were implemented to validate the changes in coil heating.Main results.Depending on the pulse duration and E-field matching constraints, the optimized waveforms produced 12%-75% less heating than the original monophasic pulse. The reduction in coil heating was robust across a range of neural models. The changes in the measured ohmic losses of the optimized pulses compared to the original pulse agreed with numeric predictions.Significance.The first step of the optimization approach was independent of any potentially inaccurate or incorrect model and exhibited robust performance by avoiding the highly nonlinear behavior of neural responses, whereas neural simulations were only run once for amplitude scaling in the second step. This significantly reduced computational cost compared to iterative methods using large populations of candidate solutions and more importantly reduced the sensitivity to the choice of neural model. The reduced coil heating and power losses of the optimized pulses can enable rapid-rate monophasic TMS protocols.

Dependence of cortical neuronal strength-duration properties on TMS pulse shape

OBJECTIVE

The aim of present study was to explore the effects of different combinations of transcranial magnetic stimulation (TMS) pulse width and pulse shape on cortical strength-duration time constant (SDTC) and rheobase measurements.

METHODS

Resting motor thresholds (RMT) at pulse widths (PW) of 30, 45, 60, 90 and 120 µs and M-ratios of 0.2, 0.1 and 0.025 were determined using figure-of-eight coil with initial posterior-to-anterior induced current. The M-ratio indicates the relative phases of the induced current with lower values signifying a more unidirectional stimulus. Strength-duration time constant (SDTC) and rheobase were estimated for each M-ratio and various PW combinations. Simulations of biophysically realistic cortical neuron models assessed underlying neuronal populations and physiological mechanisms mediating pulse shape effects on strength-duration properties.

RESULTS

The M-ratio exerted significant effect on SDTC (F(2,44) = 4.386, P = 0.021), which was longer for M-ratio of 0.2 (243.4 ± 61.2 µs) compared to 0.025 (186.7 ± 52.5 µs, P = 0.034). Rheobase was significantly smaller when assessed with M-ratio 0.2 compared to 0.025 (P = 0.026). SDTC and rheobase values were most consistent with pulse width sets of 30/45/60/90/120 µs, 30/60/90/120 µs, and 30/60/120 µs. Simulation studies indicated that isolated pyramidal neurons in layers 2/3, 5, and large basket-cells in layer 4 exhibited SDTCs comparable to experimental results. Further, simulation studies indicated that reducing transient Na+ channel conductance increased SDTC with larger increases for higher M-ratios.

CONCLUSIONS

Cortical strength-duration curve properties vary with pulse shape, and the modulating effect of the hyperpolarising pulse phase on cortical axonal transient Na+ conductances could account for these changes, although a shift in the recruited neuronal populations may contribute as well.

SIGNIFICANCE

The dependence of the cortical strength-duration curve properties on the TMS pulse shape and pulse width selection underscores the need for consistent measurement methods across studies and the potential to extract information about pathophysiological processes.

Closed-loop optimal and automatic tuning of pulse amplitude and width in EMG-guided controllable transcranial magnetic stimulation

This paper proposes an efficient algorithm for automatic and optimal tuning of pulse amplitude and width for sequential parameter estimation (SPE) of the neural membrane time constant and input-output (IO) curve parameters in closed-loop electromyography-guided (EMG-guided) controllable transcranial magnetic stimulation (cTMS). The proposed SPE is performed by administering a train of optimally tuned TMS pulses and updating the estimations until a stopping rule is satisfied or the maximum number of pulses is reached. The pulse amplitude is computed by the Fisher information maximization. The pulse width is chosen by maximizing a normalized depolarization factor, which is defined to separate the optimization and tuning of the pulse amplitude and width. The normalized depolarization factor maximization identifies the critical pulse width, which is an important parameter in the identifiability analysis, without any prior neurophysiological or anatomical knowledge of the neural membrane. The effectiveness of the proposed algorithm is evaluated through simulation. The results confirm satisfactory estimation of the membrane time constant and IO curve parameters for the simulation case. By defining the stopping rule based on the satisfaction of the convergence criterion with tolerance of 0.01 for 5 consecutive times for all parameters, the IO curve parameters are estimated with 52 TMS pulses, with absolute relative estimation errors (AREs) of less than 7%. The membrane time constant is estimated with 0.67% ARE, and the pulse width value tends to the critical pulse width with 0.16% ARE with 52 TMS pulses. The results confirm that the pulse width and amplitude can be tuned optimally and automatically to estimate the membrane time constant and IO curve parameters in real-time with closed-loop EMG-guided cTMS.