When the picture is complete, crowding disappears, and grouping rules

In crowding, the perception of a target strongly deteriorates when flanked by neighboring elements. Crowding is often explained in terms of pooling, i.e., averaging target and flanker signals. The pooling hypothesis predicts stronger crowding when the number of flankers increases- in stark contrast to recent findings. Crowding of a vernier decreased when the number of flankers, longer than the vernier, increased. However, crowding remained virtually unchanged when the number of equal-length flankers increased. We proposed that crowding reduces when more long flankers are presented because this increases ungrouping of the vernier from the flankers, whereas grouping of same-length flankers is invariant with the number of flankers. Here, we show that the vernier ungroups from equal-length flankers when we "complete the flanker array". A vernier was presented at 4 deg eccentricity. The vernier was offset to the left or right. Observers indicated this offset direction. As with previous results, Vernier discrimination thresholds raised by a factor of about 15 when a single vernier was flanked by arrays of eight same-length flankers on each side. We then presented an additional same-length flanker superimposed at the position of the target vernier. Surprisingly, crowding did not increase but was halved. We propose that adding the flanker "completed" the two separate arrays of flankers into one coherent grating of flankers. In this constellation, the vernier ungroups from the "full" grating even though the vernier and additional flanker are at the same position (except for the vernier offset). We found similar effects for peripherally presented Gabors and letters.