The effect of task-irrelevant visual backgrounds on human transcranial magnetic stimulation-evoked electroencephalography responses and cortical alpha activity.

Brain responses evoked by transcranial magnetic stimulation (TMS) in task-free experimental contexts are known to depend on psychophysiological states such as sleep, vegetative state and caffeine-induced arousal. Much less is known about how TMS-evoked responses depend on task-irrelevant steady perceptual input. Here, we examined ongoing alpha activity and the mean amplitude of EEG potentials in response to occipitally applied TMS as a function of task-irrelevant visual backgrounds. Responses to TMS were robustly modulated by photographs of natural scenes and man-made environments. These effects began as early as during the N100 and continued for several hundred milliseconds after the stimulation. There was also a more general effect of background along with other stimuli, such as blank backgrounds, sinusoidal gratings and moving dot-patterns. This effect was observable from ongoing alpha activity as well. Based on these results we conclude that different types of steady perceptual input modulate visual cortex reactivity and/or connectivity and it is possible to measure these modulations by combining TMS with electroencephalography.