22 but rather to take a more conservative approach (see section on “Analysis”). Biphasic TMS pulses were administered with a Wgure-8 coil (MC-B70), over early occipital cortex. The coil handle was oriented laterally to the right, with initial current direc- tion going away from the handle. Target site localization was based initially on phosphene induction. If a TMS- induced phosphene (nearly) overlapped with the visual Weld location that corresponded to where visual stimuli would later be presented, the coil was Wxed. Stimulation intensity was individually calibrated initially at »10% above phos- phene threshold. When in initial masking trials the partici- pants indicated rarely to subjectively perceive the stimulus orientation, TMS intensity was decreased to the point where participants indicated sometimes to perceive the stimulus, and sometimes not. The average intensity Wnally used was 58% machine output (MagPro X100). Since our results show diVerences between time windows that were 10 ms apart, we were prompted to reconWrm whether our experimental setup was accurate in this range. To this end, we performed calibration measurements. A pho- todiode was applied to the TFT monitor, measuring lumi- nance changes on an oscilloscope. Simultaneously, the TTL pulse leaving the experimental computer’s parallel port (which otherwise triggers the TMS machine with negligible delay) was measured. Using this setup, we tested latencies of visual stimulus and TTL pulses presented by the stimulation program also used in the experiment, in several neighboring visual stimulus-TMS triggering pulse SOAs (i.e., time win- dow conditions). By these measurements, we could not con- Wrm if all time windows were not shifted by milliseconds, although a stable monitor response delay should have been oVset by a programmed TMS discharge delay. These cali- bration measurements were rather performed to check whether requested increases in TMS SOA were reliable. Indeed, the measurements conWrmed that requested 10-ms increases in SOA between stimulus and TMS pulse were perfectly (accurate to the sub-millisecond range) presented by the experimental setup. These calibration measurements thus showed conclusively that whichever temporal pattern of Wndings we obtained (see “Results”) was not due to techni- cal decisions/limitations. Analysis Since this study was exploratory in nature, we included all ten measured participants in the analyses. This conserva- tively included also participants where individual masking curves showed limited eVects (see Fig. 3). In fact, two of the three subjects, where no multiple-dip pattern of results could be observed (see “Results”), had performance across conditions consistently above no-TMS. Thus, masking did not work at all in these participants. Nonetheless, they were 123 Exp Brain Res (2011) 209:19–27 included in all analyses—in acknowledgment of the explor- atory nature. Note that this conservative procedure across the board makes any and all eVects in the Wnal group results probably weaker than necessary. Since our no-TMS base- line was compromised, probably due to an ‘oddball-like’ disruptive eVect of the unexpected absence of the TMS pulse in randomly interleaved no-TMS trials, we also took a most conservative baseline to base our statistical tests on. We took the average (performance or visibility rating) of all trials in the experiment to compare Xuctuations around this mean to. This we refer to as the ‘all-round mean’. Note that potential TMS eVects would already bias this all-round mean downward, making it harder to detect these TMS eVects. With 17 conditions, it is not feasible to do statistical correction for multiple comparisons, given TMS eVect sizes in general but probably sensitive TMS-masking eVect sizes in particular. Thus, we reasoned that taking this conserva- tive all-round mean as reference for each individual time window, combined with several conservative analysis deci- sions mentioned above, would moderately compensate for the multiple comparisons problem. To thus explore TMS- masking eVects, we performed pairwise t test analyses (one-tailed) on each time window separately. The same analysis was performed for both objective orientation dis- crimination performance and subjective visibility ratings. As presented in the Results section, our data required a follow-up analysis to investigate whether objective mask- ing precedes subjective masking. For this analysis, we looked at individual masking curves and extracted peak masking latencies for these two measures. Peak masking was deWned as the lowest performance (or visibility rating) over all SOAs. If peak masking occurred at two SOAs with equal masking, the average of these SOAs was taken as peak masking latency value. Results In this study, we applied single-pulse TMS over all SOAs between 0 and 150 ms in steps of 10 ms. We measured the eVects of TMS pulses on objective visual task performance (an orientation discrimination task) and on subjective orien- tation visibility rating (on a scale of one to four). Figure 2 shows the resulting data for both measures separately. Objective discrimination performance The upper graph shows the group masking curve of all ten participants, of the objective measure (orientation discrimi- nation task). Whereas classic TMS-masking curves gener- ally display a single broad dip within the classical-masking time window (between 60 and 120 ms, see above), or sec- ondary dips in remote time windows, our data may suggest a