20 relating TMS to stimulus oVset, again two masking periods were identiWed rather than one (Silvanto et al. 2005; Koiv- isto et al. 2010). Most TMS-masking studies at any rate identiWed the ‘classical’ masking eVect around 100 ms. However, the precise latency at which masking was strongest, or even present, seems to diVer substantially between studies (Kammer 2007a, b). For instance, masking eVects have been reported at 60–75 ms (Romei et al. 2007), 80–100 ms (Amassian et al. 1989), and 120 ms (Kammer et al. 2003). Thus, since it seems impossible to pinpoint chronometri- cally when a TMS pulse has its eVect, a window spanning perhaps 60–120 ms is regarded to house a TMS-masking eVect. We will continue to refer to this as the ‘classical- masking time window’. Nearly all previous TMS-masking studies evaluated vision by an objective measure. If the research interest con- cerns visual awareness, one might argue that a subjective measure is appropriate. After all, a core aspect of visual consciousness is the subjective and reportable nature of it (Boyer et al. 2005; Lau and Passingham 2006; Koivisto et al. 2010). Moreover, the combination of both objective and subjective measures of vision may yield very interest- ing insights. Boyer et al. (2005) asked their subjects on every trial to discriminate the orientation of a bar and to moreover subjectively report whether they consciously per- ceived it or not. When evaluating objective discrimination performance on trials that were reported not to be con- sciously perceived, performance was still far above-chance. This has been dubbed ‘TMS-induced blindsight’. That par- ticular study applied TMS pulses only in the classical- masking time window around 100 ms. This Wnding raises the question of what happens in other time windows when both subjective and objective measures of vision are imple- mented. Are dissociations time speciWc? How does unim- paired objective performance square with previous reports of strong objective TMS masking at this late time window? More generally: does objective visual performance reXect the subjective experience of the visual stimuli? Could both measures be used alternately, or in unison, to investigate visual consciousness? There is to date no study that per- formed a rigorous high-resolution exploration of the chro- nometry of TMS masking of subjective and objective vision. This was one goal of the current work. The second goal was to evaluate what happens to the TMS-induced masking curve, over time, if the TMS-mask- ing paradigm is made to be as sensitive as possible. The purported advantage of single-pulse TMS is the high tem- poral resolution (Wassermann et al. 2008). However, even if the magnetic Weld change of a single TMS pulse lasts under a millisecond, it does not mean the neural eVects of that pulse are equally brief. In fact, TMS-induced diVeren- tial eVects on neuron behavior can last up to hundreds of 123 Exp Brain Res (2011) 209:19–27 milliseconds (Moliadze et al. 2003, 2005). The stronger the TMS pulse is, the longer these neural eVects can be expected to last. Traditional TMS-masking studies used circular coils (e.g., Amassian et al. 1989; Beckers and Homberg 1991), and indeed they are yet often used (Boyer et al. 2005). The spatial extent of disturbance may aVect how long the neural eVects last as well. Indeed, as pointed out, most TMS-masking studies reveal masking in broad time windows around 100 ms, rather than a single peak in one time window (say between 60 and 120 ms, see above). A Wgure-8 coil can at least provide focal stimulation, addressing this spatial resolution issue. It seems plausible that within a time window of tens of milliseconds early visual cortex is diVerentially involved in visual processing over this time, rather than occupied in the same way uni- formly (see e.g., Foxe and Simpson 2002). The question is whether TMS masking is sensitive enough to reveal puta- tive multiple stages of such involvement. One last factor that may aVect the masking curve is the stimulus itself. Larger and clearer stimuli may need higher TMS intensities to yield any masking eVect, since the information that needs to be suppressed is more salient. Higher intensities in turn may again decrease temporal resolution. Thus, a more sen- sitive paradigm might require non-salient stimuli. Altogether, these considerations lead us to hypothesize that, to obtain the highest eVective temporal resolution, one should (1) use diYcult and small visual target stimuli, (2) a spatially precise TMS coil, to allow masking with (3) the lowest TMS intensities possible (that still yield masking eVects), and (4) high temporal sampling rate (minimizing gaps between measured SOAs). In the current study, we thus implemented such a ‘sensitive TMS paradigm’ and explored its eVects on both objective and subjective visual processing. Methods Participants Fourteen participants (four men) volunteered for this study. None had a history of neuropsychiatric disorders, and all had normal or corrected-to-normal vision. The experiment was approved by the local medical-ethical committee, and written informed consent was obtained before participation. An independent medical supervisor screened all partici- pants for TMS experimentation safety prior to measure- ments, and participants were compensated with gift certiWcates. Since localization of TMS target site involved elicitation of phosphenes, the four participants in whom phosphenes could not be elicited reliably or in the proper visual Weld location were not tested in the experiment, leav- ing a total of ten included participants.