Subliminal presentation can also be achieved with threshold stimuli, where the contrast or energy of a stimulus is progressively reduced until its presence is unnoticeable. Binocular rivalry is another common paradigm whereby the image in one eye becomes subliminal by competition with a rivaling image presented in the other

eye. Participants typically report temporal alternations in the image that is consciously perceived. However, a variant of binocular rivalry, the continuous flash suppression paradigm allows an image to be made permanently invisible by presenting continuously flashing shapes in the other eye ( Tsuchiya and Koch, 2005). An equally large range of techniques allows for preconscious presentation. In inattentional blindness, a potentially visible but ABT737 unexpected stimulus remains unreported when the participants’ attention is focused on another task ( Mack and Rock, 1998 and Simons and Ambinder, 2005). The attentional

blink (AB) is a short-term variant of this effect where a brief distraction by a first stimulus T1 prevents the conscious perception of a second stimulus T2 briefly presented within a few hundreds of milliseconds of T1 ( Raymond et al., 1992). In the related psychological refractory period (PRP) effect ( Pashler, 1994 and Welford, 1952), T2 is unmasked and is therefore eventually perceived and processed, PLX-4720 cell line but only after a delay during which it remains nonconscious ( Corallo et al., 2008 and Marti et al., 2010). The “distracting” event T1 can be a surprise event that merely captures attention ( Asplund et al., 2010). The minimum requirement, in order to induce AB, appears to be that T1 is consciously perceived ( Nieuwenstein et al., 2009). Thus, PRP and AB are closely related phenomena that point to a serial limit or “bottleneck” in conscous access ( Jolicoeur, 1999, Marti et al., 2010 and Wong, PLEKHG4 2002) and can be used to contrast

the neural fate of two identical stimuli, only one of which is consciously perceived ( Sergent et al., 2005). How can an experimenter decide whether his experimental subject was or was not conscious of a stimulus? According to a long psychophysical tradition, grounded in signal-detection theory, a stimulus should be accepted as nonconscious only if subjects are unable to perform above chance on some direct task of stimulus detection or classification. This strict objective criterion raises problems, however ( Persaud et al., 2007 and Schurger and Sher, 2008). First, it tends to overestimate conscious perception: there are many conditions in which subjects perform better than chance, yet still deny perceiving the stimulus. Second, performance can be at chance level for some tasks, but not others, raising the issue of which tasks count as evidence of conscious perception or merely of subliminal processing.