Performance on clinical tests of visual acuity can be influenced by the presence of nearby targets. This study compared the influence of neighboring flanking bars and letters on foveal and peripheral letter identification.
Contour interaction and crowding refer to an impairment of visual resolution or discrimination produced by different types of flanking stimuli. This study compared the impairment of percent correct letter identification that is produced in normal observers when a target letter is surrounded by an array of four flanking bars (contour interaction) or four flanking letters (crowding).
Performance was measured at the fovea and at eccentricities of 1.25, 2.5, and 5° for photopic (200 cd/m2) and mesopic stimuli (0.5 cd/m2) and a range of target-to-flanker separations.
Consistent with previous reports, foveal contour interaction and crowding were more pronounced for photopic than mesopic targets. However, no statistically significant difference existed between foveal contour-interaction and crowding functions at either luminance level. On the other hand, flanking bars produced much less impairment of letter identification than letter flankers at all three peripheral locations, indicating that crowding is more severe than contour interaction in peripheral vision. In contrast to the fovea, peripheral crowding and contour-interaction functions did not differ systematically for targets of photopic and mesopic luminance.
The similarity between foveal contour interaction and crowding and the dissimilarity between peripheral contour interaction and crowding suggest the involvement of different mechanisms at different retinal locations.