Why does such a seemingly minor stimulus difference, filled-in versus outlined elements, determine whether or not pure level-priming occurs? One might argue that perceptual grouping differs when elements are filled-in versus outlined circles. However, we find no direct evidence for this claim. First, overall RTs did not differ between stimulus depictions; although RTs appear to be somewhat longer for outlined than filled-in stimuli, the difference was statistically unreliable,
F(1, 60) = 1.6,
n.s. (
Figures 3 and
4; note that the within-subjects error bars shown are not appropriate for making this between-subjects comparison). Thus, performance was roughly equated across the filled-in and outlined stimulus sets. Second, where we might have seen a difference based on a direct grouping manipulation, for connected versus unconnected stimuli (connected parts are treated differently than segmented ones, e.g., Franconeri, Bemis, & Alvarez,
2009; Palmer & Rock,
1994; Saiki & Hummel,
1998; Xu,
2006), we found no local target RT difference,
t(62) = 1.07,
n.s. This suggests that the difficulty for identifying the local patterns was equivalent whether the constituent circles were connected or unconnected. Third, if we examine local and global RTs, assuming that these levels may be differentially affected by grouping processes that might differ for filled-in and outlined stimuli, we failed to find any main effects of level,
F(1, 60) = 2.3,
n.s., or depiction,
F(1, 60) = 1.6,
n.s., or an interaction between the two,
F(1, 60) = 1.19,
n.s. Furthermore, in a direct comparison of filled-in and outlined stimuli, RTs for local or global targets were equivalent, |
t|s < 1.5,
n.s. Thus, although null effects limit our ability to conclusively argue against grouping differences across levels, the difficulty of grouping circles to identify a local pattern and that of grouping local patterns to identify a global pattern were statistically undifferentiated for our filled-in and outlined stimuli. Lastly, we failed to find any evidence of pure level-priming varying as a function of level (outlined: local 23 ms vs. global 23 ms, |
t| < 1, and filled-in: local 1 ms vs. global 7 ms, |
t| < 1). Thus, it is unlikely that differences in grouping for filled-in and outlined stimuli could explain why attentional persistence to scale occurred for outlined but not filled-in stimuli.