Abstract: PURPOSE: The inner-outer asymmetry, the outer flanker induces stronger crowding than the inner one, is a hallmark of visual crowding. It is unclear the contribution of inner-outer asymmetry to the pattern of crowding errors (biased predominantly toward the flanker identities) and the role of perceptual learning on crowding errors.
METHODS: In a typical radial crowding display, twenty observers were asked to report the orientation of a target Gabor (7.5°-eccentricity) flanked by an inner or outer Gabor along the horizontal meridian. Nine observers continued to train under the outer flanker condition for four sessions.
RESULTS: The outer flanker conditions induced stronger crowding, accompanied by assimilative errors to the outer flanker for similar target/flanker elements. While the inner flanker condition exhibited weaker crowding, with no significant crowding error patterns. A population coding model showed that the flanker weights in the outer flanker condition were significantly higher than those in the inner flanker condition. Training significantly reduced inner-outer crowding asymmetry and flanker weights to the outer flanker. Learning effects were retained over 4~6 months. Individual variabilities in the appearance of crowding errors, the strength of inner-outer asymmetry, and the training effects were evident.
CONCLUSIONS: Different crowding mechanisms may be responsible for the asymmetric crowding effects induced by inner and outer flankers, with the outer flankers dominating the appearance more than the inner ones. Training reduces inner-outer crowding asymmetry by reducing target-flanker confusion and learning is persistent over months, suggesting that perceptual learning has the potential to improve visual performance by promoting neural plasticity.

Key words: visual crowding, inner-outer asymmetry, perceptual learning, crowding errors, peripheral vision