This study is investigating how two types of non-invasive brain stimulation, transcranial electrical stimulation (TES) and transcranial magnetic stimulation (TMS), affect brain activity, and whether combining them produces stronger or more consistent effects than either one used alone. The motivation for this comes from the observation that TMS, which is FDA-approved for treating depression, tends to work less well in postmenopausal women because lower estrogen levels reduce the brain's ability to respond to stimulation. The research team believes that pairing TMS with TES, which targets a different set of brain cells, may be able to overcome this hormonal barrier and make stimulation more effective. Participants will come into the lab for up to 24 visits over 8 months, beginning with an initial visit to establish the right stimulation settings, followed by a series of stimulation sessions in which brain activity is measured before and after receiving either TES, TMS, or both together. For female participants, sessions will be scheduled at specific points in the menstrual cycle to capture the natural monthly rise and fall of estrogen, while male participants will be scheduled on a comparable fixed interval as a comparison group. The insights gained from this study could directly inform the development of better, more reliable brain stimulation treatments for women with depression.

TMS is well understood to have variable effects depending on hormonal levels such as estrogen, progesterone, testosterone, and thyroid hormone. A leading theory for this variability is that these hormones, particularly estrogen, exert neuroprotective effects on NMDA receptors in the brain. These NMDA receptors are ligand/voltage gated ion channels present in neurons in the cortex. Conventional TMS produces electric fields parallel to the skull surface and is therefore more likely to excite Layer 2/3 (L2/3) pyramidal neurons. Under normal circumstances, these neurons excite downstream projections, and this synchronized activation opens NMDA channels, initiating the synaptic plasticity cascade. In postmenopausal patients, however, reduced estrogen leads to diminished NMDA receptor expression, lower glutamate availability, and a corresponding reduction in NMDA current, all of which attenuate the effects of TMS. This represents a significant gap in current treatment capability, as postmenopausal patients with depression may derive substantially less benefit from TMS due to these hormonal influences on cortical excitability. A therapeutic approach capable of rescuing NMDA currents across varying hormonal brain states is therefore needed. The purpose of this protocol is twofold: to validate that TMS efficacy does fluctuate as a function of baseline hormonal levels (in this case estrogen), and to determine whether diminished efficacy can be rescued with the use of concurrent pulsed-TES (pTES). Tracking hormonal levels in perimenopausal and postmenopausal women, however, is not possible without regular blood samples due to the irregularity of hormonal dynamics during this stage of life. In order to circumvent this, this study will follow premenopausal women across the menstrual cycle. Leveraging the natural fluctuations in estrogen levels across the menstrual cycle will allow us to get a crude approximation of hormonal levels without having to do any blood samples. Group average differences in EMG responses as a result of stimulation will help elucidate how TMS responsiveness varies with hormonal levels and directly inform future research on noninvasive neuromodulation for refractory symptoms in postmenopausal and perimenopausal women.

This study will be conducted across at most 24 separate <4-hour sessions across 8 months (that need not be consecutive). At the baseline study visit, prior to stimulation, informed consent will be reviewed and obtained, the participant will complete a Demographic and Medical History and Menstrual Phase Identification Questionnaire (MPIQ) survey and a qualitative history statement will be obtained. Thereafter, the participant will begin the motor mapping wherein TES, TMS, and joint TES+TMS parameters will be adjusted such that motor responses are seen in EMG at a suitable pain (<6 on numerical rating scale). Thereafter, repetitive stimulation with either TMS, TES, or joint TES+TMS will be conducted across at most 16 days over the span of at most 6 months. These 6 months need not be consecutive. At most two stimulation sessions will be conducted per menstrual cycle for premenopausal women. For each intervention arm (TES, TMS, TES+TMS), repetitive stimulation will be scheduled once at estrogen nadir which corresponds to days 1-3 of the menstrual cycle and the late follicular phase (days 10-14). This, however, assumes a 28-day cycle and flexibility in the participants schedule so the exact scheduling of these two sessions will vary and depend on input from the MPIQ and qualitative statement. For each month, a single intervention arm will be used (e.g. if TES is used for estrogen nadir, TES will be used during the late follicular stage). The ordering of each intervention may be randomized (however, there is not a limit on the number of days dedicated to each intervention e.g. days 9-24 may solely be dedicated to TES+TMS). For men (control participants), stimulation sessions will be scheduled such that the first and second session per month are roughly 9 days apart (analogous to the scheduling in the 28-day cycle).

TMS evoked EMG responses will be recorded before and after >2000 pulses of either TMS, TES, or combined TES and TMS during these sessions. The delta in trail averaged EMG responses between estrogen nadir and the late follicular phase will be compared between these three techniques. Short-Interval Intracortical Inhibition (SICI), Long-Interval Intracortical Inhibition (LICI), and Intracortical Facilitation (ICF) responses will also be recorded to gain a better insight into the modulation of cortical activity as a function of either/both the intervention or hormonal levels. The insights gained from this study could directly inform the development of better, more reliable brain stimulation treatments for women with depression, especially those undergoing perimenopause/postmenopause.

Inclusion Criteria:

• Age between 18 and 40 years old
• Female subjects must have regular menstrual cycles (defined as a cycle that occurs at predictable intervals, typically between 24 and 38-days length) via self-report

Exclusion Criteria:

• Allergies to henna
• Allergies to gel used for electrodes
• History of seizures or epilepsy
• Subjects must not have any serious disease, disorder, infection, or cognitive impairments that could affect their ability to participate in this study.
• Female subjects of child-bearing potential must not be pregnant
• Subjects must not have any implanted stimulators or pulse generators
• Subjects must not have heart disease, including known arrhythmia
• Subjects must not have any metal implants in their head

While all genders are able to enroll in this study, men will only serve as a comparator/control group. Only females with regular menstrual periods will be used to directly analyze changes in motor evoked potentials.