Abstract
The ability to make judgments about a peripheral target stimulus can be impaired when the target is surrounded by flanking stimuli. This effect is called crowding. Crowding is often related to relatively simple low-level mechanisms that pool visual information over spatial location. We tested this hypothesis by determining the effect of stimulus configuration on crowding. We manipulated only the ‘global’ configuration, keeping constant the orientation, spatial frequency, and spatial phase of the elements making up the stimuli.
We measured thresholds for contrast and orientation discrimination using a vertical Gabor patch as a target. The target was presented at an eccentricity of 5.7 degrees and was flanked on both sides by seven vertical Gabor patches. The spatial frequency of all stimuli was 3.3 cycles per degree.
Contrast and orientation discrimination were both strongly impaired when the target was flanked on both sides by Gabors of equal length. However, when the flanks were either shorter or longer than the target, the impairment was largely reduced or even abolished, indicating that crowding was strongly modulated by stimulus configuration. Control experiments confirmed that this modulation was not due to a change in contrast energy with changing flank size or to reduced uncertainty about the target location. Further, crowding was reduced when the flanks were composed of small collinear Gabors that had the same orientation, spatial frequency, and spatial phase as the target.
These results are not readily explained by a simple pooling account of crowding. Crowding seems to be weak whenever the target does not make up a coherent texture with the flanks, even when the ‘low-level’ properties of the stimuli are kept constant.
Supported by the Swiss National Science Foundation (SNF).