One problem of interpreting research on subconscious processing is the possibility that participants are weakly conscious of the stimuli. Here, we compared the fMRI BOLD response in healthy adults to clearly visible single letters with the response to letters presented in the absence of any behavioural evidence of visibility . No letter catch trials served as a control condition. Forced-choice responses did not differ from chance when letter-to-background contrast was low, whereas they were almost 100% correct when contrast was high. (...) A comparison of fMRI BOLD signals for supraliminal and subliminal letters with the control trials revealed a signal increase in left BA 37 . Comparison of supraliminal with subliminal letters showed a significant increase in the right inferior frontal gyrus . Finally, a comparison of subliminal with supraliminal letters showed increases in the left middle temporal gyrus and the right extrastriate cortex. (shrink)