Duangudom, Francis, and Herzog (
2007) showed that the global spatial layout of a mask has a profound effect on its masking strength. In their study, participants were asked to indicate the offset direction (left/right) of a vernier target that was followed by one flanking line on each side of the vernier, i.e., a typical metacontrast mask. B-type masking occurred. When the number of flanks on each side was increased to six flanks, strong A-type masking occurred (even though this mask is also a metacontrast mask; see
Figure 1a). The 2 × 6 flank mask contains the 2 × 1 mask. Hence, the shape of the masking function can be changed by global spatial manipulations. Interestingly, when the length of the 2 × 6 lines is increased, masking strength decreases—even though the LST-energy increased (Duangudom et al.,
2007; for a similar effect with pattern masks, see Hermens & Herzog,
2007; Herzog & Fahle,
2002; Herzog & Koch,
2001). Clearly, low-level explanations fail to explain these effects. We proposed an account in terms of perceptual grouping processes. The idea is that when target and mask are grouped into a single perceptual unit, the features of the target are no longer accessible even though the target itself is visible as a part of the group (Malania, Herzog, & Westheimer,
2007). For example for an SOA of 0 ms, the vernier target fits nicely between the two arrays of 6 flanking lines (
Figure 1a). Thus, the vernier plus the 2 × 6 metacontrast mask makes up a regular structure of 13 equally spaced, clearly visible, and identical lines except for the vernier offset which is hardly visible. When the SOA changes, temporal cues break this grouping and performance improves. Francis and Cho (
2008) likewise argued that the integrated target and mask percept blocks access to the individual properties of the target. Under these conditions, A-type masking occurs regardless of the mask energy.