When transcranial magnetic stimulation (TMS) is applied over occipital cortex approximately 80-100 ms after the onset of a stimulus its visibility is decreased. The location of the occipital stimulation is typically selected by first determining where on the scalp TMS results in an experience of a phosphene. Recently it has been shown that phosphene sensations can also be elicited with parietal TMS (Marzi, Mancini, & Savazzi 2009). Our current study investigated whether TMS to parietal regions that elicit sensations of phosphenes also produce visual suppression, akin to that produced by occipital TMS, and if so, how the effects of suppression compare across the two areas. Occipital and parietal stimulation sites within the same hemisphere were selected according to whether they elicited phosphenes. Then, TMS was randomly applied at 0 to 130 ms after the onset of the stimulus (SOA) in steps of 10 ms to these areas. Participants responded to the orientation of the line stimulus and rated its visibility on every trial. Our occipital TMS data replicate previous reports of visual suppression around the classical 80-100 ms window both in the objective line orientation responses and subjective visibility ratings. TMS to the parietal regions resulted in some suppression of visual information in a similar time frame, but the effects were less pronounced and more intermittent than with occipital TMS. Together, these data suggest that both the occipital and the parietal cortex may play an important role in stimulus visibility.
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