40 Hz auditory steady-state response in females: When is it better to entrain?

Auditory steady-state responses (ASSRs) are widely applied to test brain's ability to follow external stimulation in neuropsychiatric disorders. It is known that ASSRs are related to GABAergic transmission. Female sex steroid homones - both estrogens and progesterone - affect functioning of GABAergic system. However, it is not known how these hormones affect brain's ability to entrain. This study was designed to test the ability to synchronize to 40 Hz stimulation during different phases of the menstrual cycle. Twenty-eight healthy females participated in the research during one of the menstrual cycle phases: (1) early follicular; (2) late follicular; (3) and mid-luteal. Auditory 40 Hz trains of 500 ms were delivered binaurally and EEG was recorded. Time-frequency analysis of the data was performed and phase-locking index, evoked amplitude and total intensity measures were extracted and decomposed by non-negative multi-way factorization. Additionally, alpha power of the baseline period was calculated. Parameters of ASSR were increasing in a linear manner with increasing levels of 17β-estradiol and largest estimates of ASSR parameters were obtained in the late follicular phase, smallest - in the mid-luteal phase. Alpha power values were highest in the late follicular phase and lowest in the mid-luteal phase, pointing to lower arousal level in the late follicular phase. We speculate that increased 40 Hz ASSRs during mid-cycle might be related to the level of general arousal and specific GABA-mediated changes during the menstrual cycle. The results suggest that the ability to entrain to 40 Hz stimulation depends on the phase of menstrual cycle. This should be taken into account, particularly when ASSRs are used in clinical practice, comparing patients and healthy subjects.