Remy Cohan^1,2,3 (), Stefania S. Moro^1,2,3, Jennifer K. E. Steeves^1,2,3; ¹Centre for Vision Research, ²Centre for Integrative and Applied Neuroscience, ³Department of Psychology, York University, Toronto, Canada

Visual phosphenes are perceived flashes of light in the absence of retinal input and can be evoked by transcranial magnetic stimulation (TMS) to primary visual cortex (V1). Previous dose-response studies using direct electrical stimulation (DES) of the visual cortex have shown that higher intensities produce larger phosphenes, suggesting increased stimulation intensity affects a larger cortical area. Unlike DES, in TMS intervening tissues such as scalp, skull, and cerebrospinal fluid can attenuate induced electric fields. Prior studies have examined stimulation intensity, but few have investigated how individual differences in phosphene size relate to biophysical factors. The current study examines TMS-induced phosphene size and its relation to phosphene thresholds, scalp-to-cortex distance, and modelled electric field strength measured using MRI-guided stereotaxic neuronavigation and our computer-based phosphene reporting tool. V1 stimulation evoked phosphenes in quadrants of the visual field corresponding to retinotopic region. Perceived phosphene size was negatively correlated with phosphene thresholds, indicating that higher phosphene thresholds led to smaller perceived phosphenes. Age and scalp-to-cortex distance were not correlated with phosphene size, consistent with the notion that scalp-to-cortex distance does not account for different intervening tissue types and corresponding properties. Phosphene size, however, was negatively correlated with electric field strength, indicating that intervening tissue properties may attenuate TMS intensity. These findings suggest that variability in phosphene perception may reflect biophysical factors, such as intervening tissue properties and highlight the importance of accounting for biophysical factors and electric field modelling to better understand variability in TMS-evoked phosphene perception. In addition, using a standardised method such as a phosphene mapping tool to quantify individual differences in TMS response is valuable to facilitate standardization of methods for non-invasive brain stimulation to advance TMS research and optimize its application in clinical settings.

Acknowledgements: This research was funded by the Natural Science and Engineering Research Council of Canada, Canada First Research Excellence Fund (CFREF), Vision Science to Applications (VISTA), and Connected Minds.