Two dimensions of visibility revealed by multidimensional scaling of metacontrast.

An increasing number of studies use subjective reports of visibility, so as to delineate the domain of perceptual awareness. It is generally assumed that degrees of visibility can be ordered on a single unidimensional scale. Here, I put this assumption to test with metacontrast, one of the most studied visual masking paradigms. By means of multidimensional scaling, I show that even though metacontrast stimuli only differ along the dimension of time, the perceptual space they generate unfolds in three dimensions: time and two kinds of visibilities, that are confounded when projected onto a unitary visibility scale. I argue that metacontrast creates multidimensional complex percepts, a property that may run counter to its use as a simple modulator of visibility. More broadly the results cast doubt on the use of visibility scales that ignore the qualities of the percepts.