Scotomas induced by multiple, spatially invariant TMS pulses have stable size and subjective contrast

Transcranial magnetic stimulation (TMS) can be used for studying causal effects on visual phenomenology. Occipitally delivered TMS pulses when applied after a brief spatially extended visual reference stimulus induce a localized degrading effect on the visual quality of the reference, a subjective darkening called scotoma. The stability of the subjective characteristics of artificial scotomas has not been studied with advanced neuronavigation of TMS. In 3 experiments we studied the size and relative contrast of TMS-induced scotomas and looked for possible adaptation effects to TMS delivered to the same cortical location for many successive trials. MRI-based neuro-navigated biphasic single-pulse stimulation was used to show that (i) ISI values leading to scotomas in all individual subjects extend over a wide range of time intervals from 35 ms to 199 ms, (ii) the size of and relative decrease of contrast of scotoma area remained stable over multiple stimulations, and (iii) TMS effect on scotomas was location-specific so that carry-over effects from temporarily changed TMS location to another hemisphere were absent — returning back with stimulation to the original site from a temporarily changed site led to the previous value of scotoma expression.