Abstract
Transcranial magnetic stimulation (TMS) is increasingly used as a method to modify and study functional brain activity. However, results from various studies have produced conflicting theories on how TMS of cortical tissue influences ongoing visual processing. To investigate this issue, single pulse TMS was applied over left V1 in five healthy subjects during an orientation discrimination task (vertical vs. horizontal) using a Gabor patch (2 c/deg, presented 6° in the right visual field). Stimulus contrast was set to each individual's threshold, measured in the absence of TMS. When TMS was applied over V1 performance decreased in all observers (by 1.2–8.7%) compared to accuracy levels obtained during stimulation of a control site (Cz). Crucially, accuracy levels during V1 stimulation gradually improved across blocks of 200 trials in some subjects, whereas performance remained stable during control site stimulation. In contrast, this pattern of recovery was not found in an analogous backward masking paradigm, using a brief visual noise mask instead of a TMS pulse. These results show that that the magnitude of TMS disruption can dissipate with repeated stimulation. This suggests that future studies using this technique should minimise the length of TMS exposure within each session to maximise its effectiveness. Our results show that the visual system can adapt dynamically to increased internal noise levels, minimising the impact of TMS induced cortical activity on sensory judgments.