Phosphene and motor transcranial magnetic stimulation thresholds are correlated: A meta-analytic investigation

P Phylactou; T N M Pham; N Narskhani; N Diya; D A Seminowicz; S M Schabrun

doi:10.1016/j.pnpbp.2024.111020

Phosphene and motor transcranial magnetic stimulation thresholds are correlated: A meta-analytic investigation

Prog Neuropsychopharmacol Biol Psychiatry. 2024 Apr 29:133:111020. doi: 10.1016/j.pnpbp.2024.111020. Online ahead of print.

Authors

P Phylactou¹, T N M Pham², N Narskhani², N Diya², D A Seminowicz³, S M Schabrun⁴

Affiliations

¹ School of Physical Therapy, Faculty of Health Sciences, University of Western Ontario, London, ON, Canada; The Gray Centre for Mobility and Activity, Parkwood Institute, London, ON, Canada. Electronic address: pphylact@uwo.ca.
² The Gray Centre for Mobility and Activity, Parkwood Institute, London, ON, Canada.
³ Department of Medical Biophysics, Schulich School of Medicine & Dentistry, University of Western Ontario, London, Canada.
⁴ School of Physical Therapy, Faculty of Health Sciences, University of Western Ontario, London, ON, Canada; The Gray Centre for Mobility and Activity, Parkwood Institute, London, ON, Canada.

PMID: 38692474
DOI: 10.1016/j.pnpbp.2024.111020

Abstract

Transcranial magnetic stimulation (TMS) is commonly delivered at an intensity defined by the resting motor threshold (rMT), which is thought to represent cortical excitability, even if the TMS target area falls outside of the motor cortex. This approach rests on the assumption that cortical excitability, as measured through the motor cortex, represents a 'global' measure of excitability. Another common approach to measure cortical excitability relies on the phosphene threshold (PT), measured through the visual cortex of the brain. However, it remains unclear whether either estimate can serve as a singular measure to infer cortical excitability across different brain regions. If PT and rMT can indeed be used to infer cortical excitability across brain regions, they should be correlated. To test this, we systematically identified previous studies that measured PT and rMT to calculate an overall correlation between the two estimates. Our results, based on 16 effect sizes from eight studies, indicated that PT and rMT are correlated (ρ = 0.4), and thus one measure could potentially serve as a measure to infer cortical excitability across brain regions. Three exploratory meta-analyses revealed that the strength of the correlation is affected by different methodologies, and that PT intensities are higher than rMT. Evidence for a PT-rMT correlation remained robust across all analyses. Further research is necessary for an in-depth understanding of how cortical excitability is reflected through TMS.

Keywords: Cortical excitability; Motor threshold; Phosphene threshold; TMS; Transcranial magnetic stimulation; meta-analysis.

Publication types

Review