PURPOSE: Our recent study showed that crowding effect can be completely eliminated by perceptual learning (Zhu, Fan, and Fang, JOV 2016). Here, we present data to further characterize this process.
METHODS: Subjects were trained on a crowded orientation discrimination task with a target centered at 10° eccentricity together with two abutting flankers positioned radially. The target and flankers were a circular patch of a sine-wave grating. Before and after training, we measured orientation discrimination thresholds with the crowded and isolated targets. In Experiment 1, the diameter of the target and flankers could be 1.5°, 2°, 2.5° or 3°.
RESULTS: We found that the extent of alleviation of the crowding effect by training depended on the center-to-center distance between the target and flankers. The greater the distance, the less crowding effect after training. When the distance was larger than 3°, the crowding effect can be completely eliminated. In Experiment 2, we first replicated our previous finding that there was little transfer of the learning effect between the left and right visual fields. A new finding is that the learning effect to eliminate crowding could completely transfer from the upper to the lower visual filed, but not vice versa. In Experiment 3, we examined whether the learned ability to eliminate the orientation crowding could generalize to eliminate letter crowding. Before and after training, we also measured the contrast thresholds for identifying crowded and isolated target letters, which had the same size as and were placed at the same location as the target grating. We found that the learning effect could completely transfer and eliminate the letter crowding effect.
CONCLUSIONS: Taken together, these results suggest that, with a relative large target, crowding effect is dominated by some high-level cognitive components, though constrained by visual hemifield properties. The cognitive components can be modified by perceptual training.