Exp Brain Res (2011) 209:19–27 21 Fig. 1 Stimulus and design. a The stimulus was a horizontal or vertical bar. The stimulus was deliberately small and diYcult to distinguish, in line with a ‘sensitive TMS-masking paradigm’ (see main text). b Stimuli were presented for 16.7 ms only, after which a TMS pulse at one SOA between 0 and 150 ms (except on no-TMS trials) was admin- istered. Participants were asked to Wrst make a forced-choice judgment about stimulus orientation. Second, they were asked to indicate on a scale of 1–4 how clearly they perceived the orientation of the stimulus Stimuli and tasks We used stimuli in the form of small rectangular bars (»0.025 £ »0.035 visual degrees), that could be either horizontal or vertical in orientation, and either red or blue in color (for an example, see Fig. 1). Each stimulus was always presented for only 16.7 ms, one frame on a monitor set to 60-Hz refresh rate. Average luminance of the bars was 65 cd/m2, of background 180 cd/m2. The experiment involved a forced-choice discrimination task, and a subjec- tive visibility rating task. In the objective task, subjects were required to indicate, using button presses, whether the orientation of the visual target stimulus was vertical (right index Wnger) or horizontal (right middle Wnger). These forced-choice discriminations were made Wrst and consti- tuted the objective measure of visual awareness. After this judgment, participants were asked, in the same trial, to indi- cate subjectively the visibility of the stimulus. This was done using a 4-point scale, in which value ‘1’ indicated “I didn’t see the orientation at all”; ‘2’ indicated “I don’t think I saw the orientation”; ‘3’ indicated “I think I did see the orientation”; ‘4’ indicated “I saw the orientation clearly”. We indicated that these ratings were a subjective rating of how clearly the stimulus was perceived, not a conWdence rating. Moreover, we emphasized that the descriptions were a guideline, but that consistency of the rating throughout the experiment was most important. Subjects were given ample time to practice with both discrimination tasks and subjective ratings. Stimuli were presented on a standard TFT computer monitor (Samsung Syncmaster 931BF), using Presentation software (Neurobehavioral Systems, San Francisco, CA, USA). Viewing distance was 60 cm. The location of the stimuli was 4 degrees visual angle to the lower left or right of Wxation—depending on where participants perceived phosphenes more easily in the required eccentricity and polar angle (on the left side in three subjects). Subjects Wxated throughout the experiment. Design and TMS parameters The experiment consisted of one session per subject. A total of 17 conditions (16 time windows + no-TMS) with 15 trials per time window resulted in 255 orientation task trials per session.1 Each trial took a minimum of 6 s, with subjects otherwise determining the pace by means of button presses—manually indicating readiness to proceed to the next trial. The diVerent TMS SOAs (0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, and 150 ms) and no-TMS trials were randomly interleaved throughout the whole experiment. The diVerent conditions were grouped in 15 task blocks of 17 trials. The order of these blocks was pseudo-randomized. Subjects could take a break whenever and however long they wanted between task blocks, but were asked to at least take a break halfway through the ses- sion to rest. In 4 subjects, a separate ‘NoTMS’ run was added, con- taining 20 trials. Because of the absence of a TMS pulse in the NoTMS condition during the actual experiment, partici- pants could respond diVerently due to the expectance of a pulse. Indeed, the average no-TMS performance over all four participants was 0.90 in the separate no-TMS block, while it was 0.78 for these four participants in the no-TMS trials interleaved in the actual experiment. Considering this large diVerence, we decided not to use no-TMS as baseline, 1 In the same session an equal number of trials, using the same stimuli, were obtained using a secondary task for a diVerent experiment involv- ing color judgment: this experiment will not be reported here or else- where. The original intentions behind this secondary experiment caused our stimuli to be red on half the orientation trials, and blue on the other half of trials. These color diVerences were balanced between time windows and of no further inXuence or import. 123