Phivos Phylactou¹, David A. Seminowicz¹, Siobhan M. Schabrun¹; ¹University of Western Ontario

The application of transcranial magnetic stimulation (TMS) most commonly relies on the estimation of the resting motor threshold (rMT), which serves as a proxy measure of cortical excitability. However, the rMT cannot always be estimated, as it relies on an intact pathway, travelling from the primary motor cortex (M1) to the periphery. In order to broaden the application of TMS, as well as to better understand the effects of TMS on the nervous system, there is an evident need for additional measures of cortical excitability. In this preliminary study, we provide evidence that pupil size dilation may serve as a potential measure of cortical excitability. In detail, 11 participants received 200 single pulses of either active or sham TMS at various intensities, while their pupil size was recorded. Bayesian evidence from a repeated measures ANOVA suggested that the maximum pupil size dilation as a response to TMS, was dissociable across intensities and between sham and active TMS (BF > 6). Post-hoc Bayesian paired t-tests provided further evidence of the different pupil responses between the various intensities and in sham versus active TMS. These findings show promise for the introduction of a new potential estimate of cortical excitability through measuring pupil responses.

Acknowledgements: This work is supported by the Mary Elizabeth Horney Fellowship in Rehabilitation by the St. Joseph’s Health Care Foundation