Distinct spatial patterns of flanker interference differentiate visual crowding from flanker compatibility effects in the Eriksen task

Recognising and responding to task-relevant stimuli may be hindered by nearby task-irrelevant flanker stimuli. Such effects occur both in visual crowding and in Flanker Compatibility Effects (FCE). Whereas crowding is a visual phenomenon that reflects a breakdown of object recognition in clutter, the FCE in the Eriksen flanker task is thought to be due to conflict during decision-making. In two experiments we investigated if and how these two seemingly independent phenomena are related. We employed an orientation categorisation task that allowed us to concurrently quantify crowding and FCEs. Specifically, we examined whether the spatial arrangement of stimuli affects the FCE in a similar way as in crowding. Interestingly, even when flankers were outside the crowding range, larger FCEs were observed for radially placed flankers compared to tangentially placed ones and for two compared to one flanker, corresponding to established patterns in crowding. However, inner flankers produced larger FCEs than outer flankers - the opposite of what is observed in crowding. In Experiment 2, we further investigated this reversed inner-outer asymmetry while manipulating the magnitude of crowding through varying target-flanker spacings. As expected, the outer flankers produced stronger crowding than inner ones. Crucially, the inner flankers produced a larger reaction time FCE and this inner-flanker interference was highest at the largest spacing. These findings demonstrate that the spatial layout of visual stimuli modulates conflict at both visual and decision-making stages, but the opposite patterns of the inner-outer asymmetry in the two phenomena provide a clear demarcation between the processes underlying them.