Assessing the Effects of Hormones on Noninvasive Transcranial Stimulation

Motor Evoked Potential Amplitude Change

Motor evoked potential (MEP) amplitude is recorded via surface EMG electrodes placed on the upper or lower limb musculature in response to single-pulse diagnostic TMS delivered to the primary motor cortex (M1). MEP amplitude is measured as the peak-to-peak voltage of the evoked muscle response in millivolts. The primary outcome is the change in trial-averaged MEP amplitude from before to after a train of up to 2,000 pulses of TMS, TES, or combined TMS+TES. This delta is compared across two hormonal phases of the menstrual cycle - estrogen nadir (days 1-3) and the late follicular phase (days 10-14) - and across the three intervention arms. A larger positive delta indicates greater corticospinal excitability / synpatic potentiation following stimulation, while a negative delta indicates suppression of corticospinal excitability / synpatic potentiation.

Numerical Rating Scale

A numerical rating scale (NRS) uses numbers to quantify subjective experiences of pain. NRS scores will be self-reported on a scale of 0-10 with 0 being no pain and 10 being worst pain imaginable. NRS will be presented to patients during calibration phases of Transcranial Electrical Stimulation (TES), during heat pain thresholding, and during pressure pain thresholding in the form of a visual scale and be asked to vocalize a number between 0 and 10 in response to a pain stimulus.

Menstrual Phase Identification Questionnaire

The Menstrual Phase Identification Questionnaire (MPIQ) is a short self-reported survey used to identify the current stage of the participant's menstrual cycle. It is administered at baseline and periodically throughout the study, including at home via Qualtrics up to once per day when necessary, to confirm whether the participant is at estrogen nadir (days 1-3) or in the late follicular phase (days 10-14) prior to each stimulation session. This measure is used to verify accurate scheduling of stimulation sessions relative to hormonal phase and to serve as a proxy for circulating estrogen levels, which are expected to be low at estrogen nadir and elevated during the late follicular phase. Accurate phase identification is critical to the study design as the primary outcome measure - change in MEP amplitude - is compared across these two hormonal phases.

Paired Pulse Responses

Paired-pulse EMG responses are recorded via surface EMG electrodes placed on the upper or lower limb musculature in response to paired-pulse diagnostic TMS delivered to the primary motor cortex (M1). Three paired-pulse paradigms are employed: Short-Interval Intracortical Inhibition (SICI), measured at an interstimulus interval (ISI) of 1-5 ms, which reflects GABAergic inhibitory interneuron activity and is expressed as a reduction in MEP amplitude relative to an unconditioned test stimulus; Intracortical Facilitation (ICF), measured at an ISI of 7-20 ms, which reflects glutamatergic excitatory interneuron activity and is expressed as an increase in MEP amplitude relative to an unconditioned test stimulus; and Long-Interval Intracortical Inhibition (LICI), measured at an ISI of 100-200 ms, which reflects GABA-B mediated inhibition and is expressed as a reduction in MEP amplitude relative to an unconditioned test stimulus. The outcome is the change in each of measure from baseline.

EMG responses

In addition to peak-to-peak MEP amplitude and paired-pulse responses, a range of additional quantitative metrics derived from the surface EMG signal are recorded before and after each repetitive stimulation train. These include but are not limited to MEP latency, MEP duration, MEP waveform morphology, and trial-to-trial variability of evoked responses. EMG is recorded via HD-EMG grids and/or bipolar electrode configurations placed on the upper and lower limbs, capturing muscle activity evoked by single-pulse diagnostic TMS delivered to the primary motor cortex (M1). These measures provide supplementary characterization of corticospinal excitability and neuromuscular response properties beyond MEP amplitude alone, and are compared across hormonal phases and intervention arms to gain a broader understanding of how repetitive TMS, TES, and combined TMS+TES modulate cortical output.