Visual motion adaptation increases the susceptibility of area V5/MT to phosphene induction by transcranial magnetic stimulation

Abstract Objectives The effects of visual cortical transcranial magnetic stimulation (TMS) depends upon the initial state of the stimulated region. Thus TMS perceptually facilitates the attributes encoded by adapted neuronal populations. These reports however, relied upon subjects’ description of phosphene qualia and were not quantified. We aimed to: (1) quantify the effect of visual motion adaptation on cortical excitability; (2) assess whether the effect on neuronal excitability was limited to the neuronal population undergoing adaptation or whether there was a generalised modulation of visual cortical excitability. Methods Visual motion adaptation was induced using a random dot kinematogram display. The frequency of induced phosphenes, using baseline threshold TMS intensity, was used to probe visual cortical excitability. Results Adaptation to visual motion increased the frequency of V5/MT-stimulated phosphene reports. The effect was only observed when the adapting stimulus and the phosphene spatially overlapped. Conclusions Neuronal adaptation increases the susceptibility to threshold intensity TMS-induced facilitation of neuronal activation. Significance Our data imply that the process of neuronal adaption is not synonymous with down-modulation of neuronal excitability depends upon the relative intensity of the stimulus probing neuronal function.