The effect of stimulus similarity in the Eriksen Flanker Task

Irrelevant and potentially conflicting information (‘flankers’) interferes with the processing of relevant information. Separate approaches to studying such flanker interference have evolved in visual perception and cognitive psychology. Two approaches that share experimental and conceptual similarities but have been investigated predominantly in isolation are crowding and the Eriksen flanker task, respectively. Whereas crowding is thought to arise from interference in the visual cortex, flanker compatibility effects as studied in the Eriksen flanker task have been ascribed to more downstream interference such as at the stage of response selection or cognitive control. In three experiments we examined whether interference at the visual processing stage also modulates the flanker compatibility effect by examining if the latter exhibits properties of crowding. It has been well-established that target similarity (e.g., in luminance polarity) strongly modulates crowding. Here, we assessed whether the same is the case in the Eriksen Flanker task. If visual processing does not modulate the flanker compatibility effect, we would expect that manipulating target-flanker similarity would not change the magnitude of the compatibility effect. We measured accuracy and reaction times to peripherally presented target stimuli while manipulating target-flanker congruency, spacing, and similarity in polarity. In all three experiments, we observed both a drop in accuracy as flankers approached the target (i.e., crowding) and a flanker congruency effect: reaction times were slower when flankers were incongruent. There was an interaction between spacing and similarity: the closer the flankers were to the target, the stronger the effect of similarity was on congruency. These effects were further enhanced when target polarity and similarity were blocked. Similarity seems to influence the congruency effect in a pattern comparable to that reported in the crowding literature. These findings suggest that interference at the visual processing stage also contributes to the flanker compatibility effect.