Stimulus Dependence of Peripheral Flanker Effects During a Central Task: An EEG Study

Background/Objectives: Surrounding stimuli affect the visual processing of a target; however, whether this effect varies across stimulus types when peripheral targets do not need to be actively discriminated remains unclear. Methods: We recorded EEG activity from 20 participants during a central color judgment task while they viewed peripheral Gabor patterns, vertical stripes, letters, and numbers that were irrelevant to the task. Flankers appeared 1000 ms before targets in the flanked condition, providing an existing visual background. In the unflanked condition, only the fixation point was present before target onset. We compared event-related potentials (ERPs) and beta power between these conditions. Results: A spatiotemporal cluster permutation test revealed a significant interaction between stimulus type and flanker condition in the ERP results, with a cluster spanning 146–252 ms after target onset. Further comparisons revealed that flanker effects on ERP amplitude differed between Gabor patterns and vertical stripes. Within each stimulus condition, ERP changes were observed over different time ranges and scalp locations. Beta power was lower in the flanked condition for Gabor patterns, vertical stripes, and numbers, but no significant interaction was detected for beta power. Central color judgment responses were faster in the flanked condition, whereas accuracy did not differ significantly between conditions. Conclusions: These findings suggest that an existing flanker background can affect EEG responses after target onset without active discrimination of peripheral targets. Differences in flanker effects across stimulus types were supported by the ERP results, while beta power provided additional evidence of changes within stimulus conditions.