Inner-outer asymmetry in the Eriksen flanker task

Interference from task irrelevant visual stimuli (“flankers”) has been studied through the Eriksen flanker task and visual crowding, among others. However, despite obvious similarities between the two approaches, they have been studied predominantly separately. In order to explore possible links between them, we examined whether a well-established diagnostic property of crowding, stronger crowding by the outer (more peripheral) compared to the inner (more foveal) flanker, is also observable in the Eriksen flanker task. Specifically, using an orientation categorization task, we manipulated flanker (1) congruency (congruent/incongruent), (2) location (in/out) and (3) spacing (close/mid/far), relative to the target. Analogous to findings in crowding, the congruency effect (better performance in congruent compared to incongruent trials) for categorization accuracy decreased with increasing target-flanker spacings and this was more pronounced for the outer flanker. In contrast, for reaction times, congruency effects were stronger for the inner compared to the outer flanker, and their strength was not modulated by spacing. This unexpected finding could be accounted for by recent reports that the inner flanker is more often confused with the target than the outer flanker in crowding tasks. Thus, there are clear qualitative differences between the pattern of results for the two measures (accuracy and reaction times) of inner and outer flanker interference. These findings suggest that inner and outer flankers differ in the extent of interference they produce at visual processing and decision making stages, with outer flankers interfering most at the visual processing stage and inner flankers interfering more with decision making.