Blue magnetophosphene perception under subthreshold magnetic stimulation in a human retinal degeneration model

Introduction Extremely low-frequency magnetic fields (ELF-MF; <300 Hz) can induce visual percepts known as magnetophosphenes through weak, subthreshold electric fields in neural tissue. Although magnetophosphene perception has been characterized in healthy observers, the anatomical locus of interaction (retinal vs. cortical) remains incompletely constrained, particularly under conditions of severe retinal dysfunction. Methods Here, we employed a controlled single-subject perturbation framework to mechanistically dissociate candidate sites of ELF-MF interaction using systematic variation of stimulation geometry, sham conditions, and independent suprathreshold transcranial magnetic stimulation (TMS) of the visual cortex. Results ELF-MF exposure at 20 Hz and 100 millitesla root mean square (mT RMS) evoked reproducible, spatially uniform, blue-tinted visual percepts only when stimulation geometry favored ocular tissues, whereas occipital ELF-MF placement and all sham conditions produced no percept. In contrast, direct cortical stimulation with TMS generated qualitatively distinct, achromatic, retinotopically organized phosphenes and never reproduced the ELF-MF-evoked phenomenology. Discussion Given the large disparity in induced electric field strengths between ELF-MF and TMS, these contrasts constrain plausible mechanisms to retinal or post-receptoral sites operating in a subthreshold modulation regime and argue against primary cortical initiation. While limited to a single constrained biological model, these findings refine current biophysical models of human magnetophosphene perception and motivate future studies combining replication, quantitative psychophysics, and individualized dosimetry to further delineate subthreshold electromagnetic interactions with the visual system.