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キーワード:
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要旨:
Method:
In 3 subjects, VEPs were reliably induced by a small checkerboard stimulus briefly presented in the parafoveal lower right quadrant (Fig. 1B). Subjects S1 and S2 had classical VEP patterns, in S3 the P100 was missing. A random quadrant of the checkerboard was shown at reduced contrast. Subjects had to identify and report it by a button-press. TMS was applied to the left occ. pole (MagVenture MagPro X100; MC-B70 coil). The best TMS timing and coil position were determined in pretests. The timing with the strongest suppression corresponded to the N80 (S1S2) and the missing P100 (S3), respectively.
We tested 3 conditions: Combined “TMSvisual” stimulation, “Visualonly” and “TMSonly”. An experimental run (∼4min) contained trials of all conditions in a randomized order. 5 TMS intensities were tested, ranging from phosphene threshold to the intensity evoking chance level performance (or maximally 85 of max. stimulator output; Fig.1A). The 5 intensities were tested in separate runs in a randomized order. Using several sessions, ∼120 trials were acquired for each condition at each intensity.
EEG was recorded using a BrainAmp MR plus amplifier (Brain Products, Germany; 32 channels; impedances <5 kOhm) and analyzed using EEGLAB 6.01 (Delorme Makeig, 2004). Pre-processing involved TMS artifact removal using polynomial interpolation, band-pass filtering (cutoff 0.1 50 Hz), baseline correction and eye blink rejection. The mean of the TMSonly trials was subtracted from the mean of the TMSvisual trials to determine the TMS effect on the VEPs. Analysis concentrated on a region-of-interest of 7 electrodes (Fig. 1C).
Result:
In S1 and S2, the P100 increased monotonically for the 3 lower TMS intensities (Fig. 1CD) and leveled off for the 2 highest intensities, at which visual suppression occurred (Fig. 1A). In S3, the N150 increased for the first 4 intensities, and then decreased. Similar modulations occured for the N150 in S1 and S2 and the “P200” in S3 (data not shown).
Conclusion:
The VEP modulation patterns hint towards a saturation effect taking place when TMS is strong enough to induce robust suppression. Future work involves testing a further subject to confirm the modulation effects, and the systematic variation of the TMS SOA.
