Occipital Transcranial magnetic stimulation has opposing effects on visual and auditory stimulus detection: Implications for multisensory interactions

Multisensory interactions occur early in time and in low-level cortical areas, including primary cortices. To test current models of early auditory-visual ( AV) convergence in unisensory visual brain areas, we studied the effect of transcranial magnetic stimulation (TMS) of visual cortex on behavioral responses to unisensory ( auditory or visual) or multisensory ( simultaneous auditory-visual) stimulus presentation. Single-pulse TMS was applied over the occipital pole at short delays (30-150 ms) after external stimulus onset. Relative to TMS over a control site, reactions times ( RTs) to unisensory visual stimuli were prolonged by TMS at 60-75ms poststimulus onset ( visual suppression effect), confirming stimulation of functional visual cortex. Conversely, RTs to unisensory auditory stimuli were significantly shortened when visual cortex was stimulated by TMS at the same delays ( beneficial interaction effect of auditory stimulation and occipital TMS). No TMS-effect on RTs was observed for AV stimulation. The beneficial interaction effect of combined unisensory auditory and TMS-induced visual cortex stimulation matched and was correlated with the RT-facilitation after external multisensory AV stimulation without TMS, suggestive of multisensory interactions between the stimulus-evoked auditory and TMS- induced visual cortex activities. A follow-up experiment showed that auditory input enhances excitability within visual cortex itself ( using phosphene-induction via TMS as a measure) over a similarly early time-window (75-120 ms). The collective data support a mechanism of early auditory-visual interactions that is mediated by auditory-driven sensitivity changes in visual neurons that coincide in time with the initial volleys of visual input.