Visual crowding and binocular vision: the locus of crowding relative to binocular rivalry and fusion

Crowding refers to the detrimental effect of surrounding flankers on the recognition of a central target, which is otherwise easily recognized when presented in isolation (Bouma, 1970). There have been a lot of debate regarding the mechanism underlying crowding, from low-level considerations such as the excessive feature integration hypothesis, to high-level considerations involving attention (He, Cavanagh, &. Intriligator, 1996). Attributing crowding to different theories leads to different predictions regarding the cortical locus (or loci) of crowding. I aim to investigate whether crowding occurs before or after binocular
rivalry and fusion. When explicitly incompatible images are presented to the two eyes, the two images compete for perceptual dominance, creating a percept with the two images alternating a few seconds at a time in a spontaneous manner. This is binocular rivalry. Incompatible images (and yet similar enough) viewed under finely controlled viewing conditions could be integrated, yielding a third unitary percept that is different from the original images. This is binocular fusion. Kooi, Toet, Tripathy, & Levi (1994) found that the similarity between the target and flankers influenced the strength of crowding. Flankers that are similar to the target (in various dimensions such as size, luminance, colours) induce
stronger crowding than those that are not. This hallmark property of crowding was used to probe the relative locus of crowding to the two binocular phenomena.

Two orientation discrimination experiments were carried out. Crowding was indicated by an elevation in orientation discrimination threshold compared to the single condition. In Experiment 1, flankers of different colours (red and blue) were presented dichoptically to the two eyes and a red target to only eye. Prior to the presentation of the target and flankers, an adaptation phase in which observers viewed either blue or red adaptor patches was introduced in order to bias the perceptual dominance to either red or blue flankers in the subsequent stimulus phase. This allowed the target-flanker colour similarity to be manipulated independently at the monocular and perceptual (or binocular) levels. The monocular target-flanker similarity depended on the colour of the flankers presented together with the target to only one eye while the perceptual target-flanker similarity depended on the colour of the adaptor patches, which affected the final percept of rivalling flankers (flankers with adapted colour were totally suppressed, biasing the percept to flankers with non-adapted colour). In Experiment 2, the flankers with different colours between the two eyes (red and green) were allowed to fuse before the target (always red) was presented. The monocular target-flanker similarity was manipulated in the same way as in Experiment 1, while the perceptual target-flanker similarity depended on the fused percept of the dichoptic flankers.

Results of both experiments revealed that the strength of crowding relied on the monocular similarity between target and flankers. Data of the current study suggest that crowding occur before binocular rivalry and fusion and do not support a high-level explanation (such as attentional resolution) to crowding .