Electrophysiological correlates of suppression and facilitation in crowding

In crowding, neighboring elements deteriorate performance on a target. The neural mechanisms of crowding are largely unknown. We have recently shown that the N1 component of the EEG is suppressed during crowding. It is difficult to disentangle the processing of the target and the flankers because they are presented synchronously. Here, we used a frequency-tagging technique to analyze EEG responses separately for the flankers and target. Subjects discriminated the offset direction of a vernier that was slowly increasing in size either to the left or right. Flanking lines were either longer than the vernier or of the same length. Flankers of the same length crowded more strongly than the longer flankers because the former grouped with the vernier. The vernier and the flankers flickered at two different frequencies. EEG responses to the vernier were suppressed and the responses to the flankers were enhanced during crowding (same length flankers) compared to uncrowding (longer flankers). Our results are consistent with the attentional hypothesis of crowding, where attention cannot be focused on the target and spreads to the flankers.