Broadband gamma-band EEG changes during magnetophosphene perception induced by 20 Hz magnetic field stimulation

Objective. Magnetophosphenes are visual percepts induced by extremely low-frequency magnetic fields (ELF-MF; <300 Hz), yet their EEG correlates remain poorly characterized and are not reliably captured by classical low-frequency markers. We tested whether magnetophosphene perception is associated with broadband high-frequency EEG changes rather than focal oscillatory effects. Approach. EEG was recorded in N=13 healthy volunteers during 20 Hz sinusoidal magnetic-field exposure delivered using transcranial alternating magnetic stimulation (tAMS) in a global-head configuration. Three conditions were analyzed: no exposure (0 mT), subthreshold (5 mT), and suprathreshold (50 mT). Gamma-band activity (30-80 Hz) was quantified using complementary spectral approaches, including aperiodic-adjusted measures. Main results. Perception reports sharply dissociated the three conditions, with frequent perception at 50 mT only. Suprathreshold stimulation was associated with spatially distributed increases in gamma-band activity over frontal and occipital electrodes. These effects persisted after aperiodic correction using two independent parameterization methods and did not exhibit a consistent narrowband peak, indicating broadband high-frequency changes. Significance. Magnetophosphene perception is not reliably captured by focal low-frequency EEG markers but is instead associated with distributed broadband high-frequency activity. These findings challenge standard assumptions derived from classical visual paradigms and suggest that perception under magnetic stimulation reflects large-scale, state-dependent neural dynamics.