Modulation of phosphene perception during saccadic eye movements: a transcranial magnetic stimulation study of the human visual cortex

Saccadic suppression allows for perceptual stability during rapid movements of the eyes. One of the neural mechanisms may involve saccade-related modulation of neural activity in the visual cortex. Using the perception of phosphenes induced by transcranial magnetic stimulation (TMS) applied over the visual cortex (VC) as an index of cortical excitability, we sought to determine if VC excitability was modulated at varying times relative to saccade onset. We used two measures of excitability: (1) stimulator intensity required to induce phosphenes in 50% of trials, also called the phosphene threshold (PT), and (2) the subjective intensity of the phosphene. We found that there was no change in PT for different saccade-TMS onset asynchronies while there was an increase in perceived phosphene-intensity near the time of saccade onset (F(7,42)=4.34, P=0.001). Contrary to what would be expected from a saccadic suppression model, our results suggest that excitability of the visual cortex is slightly enhanced at the time of saccade onset.