EXPRESS: How are grouping and interference related in crowding?

IIn visual crowding, perception of an element deteriorates in the presence of clutter. The mechanism(s) underlying crowding have been controversially discussed for decades. Whereas it is well established that grouping determines which elements are prone to interference, it is unclear what causes interference, e.g., feature pooling, lateral inhibition, or substitution. To address this question, we presented an array of lines of which the central line was offset (vernier offset). Observers were asked to report this offset. The flanking lines were straight or had the same (pro-vernier) or opposite (anti-vernier) offset direction compared to the target vernier. Participants' performance was hardly influenced by a single pro-vernier contrary to what is expected both from pooling and substitution mechanisms, which predict increased performance. When the number of pro- as well as anti-verniers increased, performance significantly increased or decreased, respectively. When the flankers were presented closer to the fovea, the influence of the flankers was larger than when presented further away, contrary to the well-known in-out anisotropy. We show that there are processes at work, which can hardly be captured by any existing model of crowding.