Dynamics of visual masking revealed by second-order metacontrast

Metacontrast is a powerful visual illusion by which the visibility of a brief stimulus is drastically reduced when it is followed by a snugly fitting second, masking stimulus. There have been longstanding debates about the levels at which metacontrast mechanisms operate and about the temporal unfolding of the masking effect. Here, we use second-order features (texture and movement) in order to set a lower bound to the level at which metacontrast may be found. First, we show that interactions of two second-order stimuli readily produce typical metacontrast masking. We then create second-order singletransient stimuli that induce visual percepts when a random uniform texture is locally replaced by a similar random uniform texture. We show that these ultra-brief stimuli can be used both as target and mask. Using these single-transient stimuli, we seek to disentangle the relative contributions of mask onset and offset. Results suggest that, at least in the context of second-order masking, nearly all of the mask's effectiveness is due to the very first visual event that follows the target.