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Abstract:
Transcranial magnetic stimulation (TMS) applied over the occipital pole is able to suppress the perception of visual stimuli. This suppression is based on a shift of the perception threshold. Here we compared the modulatory effect of a light flash with the suppression effect of TMS.
Subjects had to report the orientation of a flashed hook. At two different adaptation levels (0.3 and 3 cd m-2) we determined contrast thresholds by the method of constant stimuli. A light flash (15 or 150 cd m-2 for the different adaptation levels) was applied with different SOAs after the appearance of the hook, and threshold shifts were measured. Similarly, threshold shifts due to focal TMS with SOAs from -25 to +195 ms were measured.
With high background intensity, TMS shifted thresholds up to 1.2 Weber contrast units (WC), and with low background intensity the shift was up to 6.6 WC. Latency for the maximal effect was shorter at the high background level (85 ms SOA) than at the lower level (115 ms SOA). We found similar effects on threshold shifts with light flashes at SOAs from 20 to 80 ms. Our results suggest that the masking effect of TMS might share similar mechanisms with a masking effect by light.
