Foveal Crowding Resolved

Abstract

Crowding is the substantial interference of neighboring items on target identification. Crowding with letter stimuli has been studied primarily in the visual periphery, with conflicting results for foveal stimuli. While a cortical locus for peripheral crowding is well established (with a large spatial extent up to half of the target eccentricity), disentangling the contributing factors in the fovea is more challenging due to optical limitations. Here, we used adaptive optics (AO) to overcome ocular aberrations and employed high-resolution stimuli to precisely characterize foveal lateral interactions with high-contrast letters flanked by letters. Crowding was present, with a maximal edge-to-edge interference zone of 0.75-1.3 minutes at typical unflanked performance levels. In agreement with earlier foveal contour interaction studies, performance was non-monotonic, revealing a recovery effect with proximal flankers. Modeling revealed that the deleterious effects of flankers can be described by a single function across stimulus sizes when the degradation is expressed as a reduction in sensitivity (expressed in Z-score units). The recovery, however, did not follow this pattern, likely reflecting a separate mechanism. Additional analysis reconciles multiple results from the literature, including the observed scale invariance of center-to-center spacing, as well as the size independence of edge-to-edge spacing.

Processing speed in perceptual visual crowding

“…A first step in introducing detection speed to test central crowding could be to flank the central target in combination with unflanked peripheral targets. Most challenging would still be to assess the limits of crowding on an arcminute scale due to optical limitation, such as irregularities in the cornea and lens blurring foveal vision (Coates et al, 2018). Such an approach would require a far more accurate eyetracking system because the resolution of the presently used remote infrared eye-tracker system would be too low.…”

“…shown for opposite-contrast stimuli (Hess, Dakin, & Kapoor, 2000). Others have reported that foveal crowding occurs over very small distances but that sophisticated techniques are required to overcome the optical limitations (Siderov, Waugh, & Bedell, 2013;Coates, Levi, Touch, & Sabesan, 2018).…”

Seven myths on crowding

“…Now does critical target-flanker gap size, with increasing target eccentricity, increase linearly from there (as would be expected from Bouma's law) or does it first behave differently for a few minutes of arc, and then increase ( Figure 6c)? The hockey stick 21 Coates et al (2018) also isolate a separate recovery mechanism, first observed by Flom et al, 1963, at even smaller distances -0.5-0.75 arcminutes -that can be left aside for the present discussion. 22 The kink in the hockey stick is at s=5'.…”

“…There is agreement that the interaction effect of foveal acuity targets, measured with conventional techniques, occurs "within a fixed angular zone of a few min arc" (3'-6') (Siderov, Waugh, & Bedell, 2013;Siderov et al, 2014, p. 147). However, a new study using adaptive optics (Coates et al, 2018) shows critical spacings are indeed even much smaller and only about a quarter of that range, 0.75 to 1.3 arcminutes edge-to-edge. 19 H. Wässle, personal communication 8/2019; there is no precise border so estimates vary widely.…”

“…for indirect vision) but is also present in the very centre. That has been controversial for a time but appears now well established (Flom, Weymouth, & Kahneman, 1963;Levi et al, 1985;Levi, Hariharan, & Klein, 2002a;Coates & Levi, 2014;Siderov, Waugh, & Bedell, 2014;Coates, Levi, Touch, & Sabesan, 2018;see Coates & Levi, 2014, for review up to 2014. There is agreement that the interaction effect of foveal acuity targets, measured with conventional techniques, occurs "within a fixed angular zone of a few min arc" (3'-6') (Siderov, Waugh, & Bedell, 2013;Siderov et al, 2014, p. 147).…”