In the participant-based analysis, a main effect of phase scrambling was observed, F(2, 38) = 4.948, p = 0.012, η2p = 0.207, which indicated less positive ERPs in the 55% compared to the 65% and the 80% conditions: 80% vs. 55%, t(19) = 4.227, p < 0.001; 65% vs. 55%, t(19) = 3.427, p = 0.003. This effect was modulated by an interaction with categorization accuracy, F(2, 38) = 5.103, p = 0.011, η2p = 0.212. For correct trials, a main effect of phase scrambling indicated less positive ERPs as phase scrambling was reduced, F(2, 38) = 13.81, p < 0.001, η2p = 0.421, with significant differences between the 55% and all other degraded conditions: 80% vs. 55%, t(19) = 4.557, p < 0.001; 65% vs. 55%, t(19) = 3.95, p = 0.001. For incorrectly categorized trials, no effect of phase scrambling was observed, F(2, 38) = 0.252, p = 0.778 η2p = 0.013. Finally, after separate analysis of each phase-scrambling level, a significant effect of accuracy was observed only in the 55% phase-scrambling condition, t(19) = 3.326, p = 0.004, not for 80% and 65% phase scrambling, t(19) = −1.433, p = 0.168, and t(19) = −0.277, p = 0.785, respectively. Finally, a significant effect of SC rank was observed, F(1, 19) = 11.149, p = 0.003, η2p = 0.37, indicating less positive ERPs for pictures which were low compared to high in spatial coherence.
The image-based analysis investigated the effects of phase scrambling and categorization accuracy on early ERP modulation, while controlling for the spatial coherence of the original scenes. ERP waveforms in the three degraded conditions are reported in
Figure 4. Different from the participant-based analysis, the main effect of phase scrambling was not significant in the image-based analysis,
F(2, 348) = 0.688,
p = 0.494,
η2p = 0.004. Similar to the participant-based analysis, a significant interaction of phase scrambling and accuracy was observed,
F(2, 348) = 4.451,
p = 0.014,
η2p = 0.025. When participants correctly categorized pictures, a significant main effect of phase scrambling was observed,
F(2, 476) = 16.09,
p < 0.001,
η2p = 0.063, indicating less positive ERPs in the 55% compared to the other degraded conditions: 80% vs. 55%,
t(239) = 5.426,
p < 0.001; 65% vs. 55%,
t(239) = 4.489,
p < 0.001. When participants could not categorize stimuli correctly, no significant effect of phase scrambling was observed,
F(2, 476) = 1.444,
p = 0.238,
η2p = 0.008. On analyzing the Accuracy × Phase Scrambling interaction separately for each phase-scrambling level, significant effects of accuracy were observed for only the 55% phase-scrambling condition,
t(175) = 2.887,
p = 0.004 (see
Figure 3)—with less positive ERPs for correct compared to incorrect categorizations—not for 80% and 65% phase scrambling,
t(239) = 0.31,
p = 0.741, and
t(239) = 0.542,
p = 0.588, respectively.
Different from the participant-based analysis, in which a significant effect of spatial coherence was observed, the main effect of SC rank did not reach standard significance in the image-based analysis, F(1, 174) = 3.698, p = 0.056, η2p = 0.021, nor was any interaction involving SC rank observed. Finally, the image-based analysis differed from the participant-based analysis in that a main effect of accuracy was observed, F(1, 174) = 5.27, p = 0.02, η2p = 0.03, indicating a less positive ERP amplitude for correctly categorized trials compared to incorrect trials.
In summary, both the participant-based and the image-based analyses revealed a significant interaction of phase scrambling and categorization accuracy, with less positive ERPs for correct compared to incorrect trials in the least degraded level (55% phase scrambling). In the image-based analysis only, the main effect of accuracy reached significance. Finally, in the participant-based analysis, significant effects of spatial coherence and of phase scrambling were observed, but these effects were not observed in the image-based analysis, which used image as a random factor.