Probing V5/MT excitability with transcranial magnetic stimulation following visual motion adaptation to random and coherent motion

The response to stimulating the visual cortex with transcranial magnetic stimulation (TMS) depends on its initial activation state, for example, visual motion adaptation biases perceived TMS-induced phosphene characteristics (e.g., color). We quantified this state dependence by assessing the probability of reporting a phosphene (P-lambda) with "threshold" TMS (i.e., the TMS intensity producing P-lambda = 0.5 at baseline) following visual motion adaptation to a random dot motion display. Postadaptation, P-lambda was increased, and this effect was confined to the adapted neuronal population. We then adapted subjects using a population of moving dots of fixed average motion direction with standard deviations (SD) ranging from 1 degrees to 128 degrees (SD fixed for a given trial). P-lambda was significantly increased at all dot motion SDs except SD = 1 degrees. Neuronal adaptation increases the susceptibility of the neuronal population to activation by threshold intensity TMS. Thus the process of neuronal adaption is not necessarily synonymous with a downmodulation of neuronal excitability.