tDCS and Sex Differences
tDCS and Sex Differences
The majority of transcranial direct current stimulation (tDCS) studies have failed to
 consider sex as a modulating factor. This neglect may partly account for the high
 inter-subject variability bemoaned by many tDCS investigators (e.g., approximately 50% of
 participants do not respond to tDCS) and has certainly delayed progress in the field.
 Therefore, research into how sex influences stimulation-related outcomes is vital to fully
 understand the underlying mechanisms of tDCS, which has shown great inconsistency.

 Because of the menstrual cycle, the hormonal levels of women fluctuate considerably more than
 in men. Importantly, these hormonal variations might impact the efficacy of neuromodulatory
 tools, like tDCS. It is suggested that estrogen, which is high in the second follicular
 phase, reinforces excitatory mechanisms in the motor cortex. However, because anodal tDCS
 enhances cortical excitation there is also a possibility of excessive excitability. For
 instance, anodal tDCS may lead to overexcitation and non-optimal performance when it is
 applied in the second follicular phase of the menstrual cycle. Currently, there is a lack of
 knowledge on how the phases of the menstrual cycle affect tDCS performance outcomes in
 healthy young women because no studies have examined if and how the phases of the menstrual
 cycle alter tDCS efficacy.

 This study is critical for determining the optimal time to administer anodal tDCS, and the
 ideal intensity for that administration, to achieve the most beneficial results. Furthermore,
 this investigation will emphasize the need for future tDCS studies to test women during the
 same menstrual cycle phase.
The majority of transcranial direct current stimulation (tDCS) studies have failed to
 consider sex as a modulating factor. This neglect may partly account for the high
 inter-subject variability bemoaned by many tDCS investigators (e.g., approximately 50% of
 participants do not respond to tDCS) and has certainly delayed progress in the field.
 Therefore, research into how sex influences stimulation-related outcomes is vital to fully
 understand the underlying mechanisms of tDCS, which has shown great inconsistency.

 Because of the menstrual cycle, the hormonal levels of women fluctuate considerably more than
 in men. There are two main phases of the menstrual cycle: 1) the follicular phase,
 characterized by low levels of estradiol and progesterone (first follicular phase, days 1-7)
 followed by increased levels of estradiol and low levels of progesterone (second follicular
 phase, days 7-14); and 2) the luteal phase (days 14-28), characterized by moderate estradiol
 and high progesterone levels. Importantly, these hormonal variations might impact the
 efficacy of neuromodulatory tools, like tDCS.

 It is suggested that estrogen, which is high in the second follicular phase, reinforces
 excitatory mechanisms in the motor cortex. Thus, it appears that higher levels of estradiol
 increase cortical excitability. However, because anodal tDCS enhances cortical excitation
 there is also a possibility of excessive excitability. For instance, anodal tDCS may lead to
 overexcitation and nonoptimal performance when it is applied in the second follicular phase
 of the menstrual cycle. Currently, there is a lack of knowledge on how the phases of the
 menstrual cycle affect tDCS performance outcomes in healthy young women because no studies
 have examined if and how the phases of the menstrual cycle alter tDCS efficacy.

 This research will be significant because the changing hormone levels during the different
 phases of menstruation in women is an especially important factor for minimizing response
 variability from tDCS. Thus, this study is critical for determining the optimal time to
 administer anodal tDCS, and the ideal intensity for that administration, to achieve the most
 beneficial results. Furthermore, this investigation will emphasize the need for future tDCS
 studies to test women during the same menstrual cycle phase.
Inclusion Criteria:

 1. Has a regular menstrual cycle (for women only)

 2. Young adult (18-35 years)

 3. Right-side dominant

 4. At least 30 min of moderate-intensity, physical activity on at least 3 days of the
 week for at least the last 3 months

 5. Without chronic neurological, psychiatric, or medical conditions

 6. Not taking any psychoactive medications.

 Exclusion Criteria:

 1. Pregnant

 2. Known holes or fissures in the skull

 3. Metallic objects or implanted devices in the skull (e.g., metal plate)

 4. Women on hormonal contraceptives/supplements.