Broadband gamma-band EEG changes during magnetophosphene perception induced by 20 Hz magnetic field stimulation
ObjectiveMagnetophosphenes are visual percepts induced by extremely low-frequency magnetic fields (ELF-MF; <300 Hz), yet the EEG expression of suprathreshold magnetophosphene-inducing stimulation remains poorly characterized and is not reliably captured by classical low-frequency markers. We tested whether suprathreshold 20 Hz tAMS is accompanied by broadband high-frequency EEG changes rather than focal oscillatory effects. ApproachEEG 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 gamma power. Main resultsPerception 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 occipito-parietal 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. No focal power change over primary occipital electrodes remained significant after Bonferroni correction. SignificanceSuprathreshold magnetophosphene-inducing stimulation is not reliably captured by focal low-frequency EEG markers but is accompanied by distributed broadband high-frequency activity. Because stimulation intensity and perceptual reports were strongly coupled, these effects should be interpreted as EEG correlates of a suprathreshold stimulation-perception state rather than as isolated markers of perception.