Crowding between first- and second-order letters in amblyopia

Abstract

To test whether first- and second-order stimuli are processed independently in amblyopic vision, we measured thresholds for identifying a target letter flanked by two letters for all combinations of first- and second-order targets and flankers. We found that (1) the magnitude of crowding is greater for second- than for first-order letters for target and flankers of the same order type; (2) substantial but asymmetric cross-over crowding occurs such that stronger crowding is found for a second-order letter flanked by first-order letters than for the converse; (3) the spatial extent of crowding is independent of the order type of the letters. Our findings are consistent with the hypothesis that crowding results from an abnormal integration of target and flankers beyond the stage of feature detection, which takes place over a large distance in amblyopic vision.

Figure 1

The trigram “skp” illustrated for the four testing conditions: (top-left) a first-order target (middle) letter flanked by two first-order letters (the “111” condition); (top-right) a second-order target letter flanked by second-order letters (“222”); (bottom-left) a first-order target letter flanked by second-order letters (“212”) and (bottom-right) a second-order target letter flanked by first-order letters (“121”).

Figure 2

Threshold elevation, ratio of contrast threshold for identifying flanked and unflanked (single) letters, is plotted as a function of letter separation in (A) degrees or (B) multiples of letter size, for the four trigram conditions. Data shown are obtained from observers AP and RH, but are representative of other five observers. Solid lines represent the best-fit curves (see text for details) through each set of data. Dashed lines represent the null effect (absence of threshold elevation). Error bars represent ± 1 S.E.M.

Figure 3

Critical distance in degrees (upper panels), representing the spatial extent of crowding and threshold elevation (lower panels) representing the peak magnitude of crowding are plotted for the four trigram conditions, for the non-amblyopic (left) and amblyopic eyes (right). Data for each observer are plotted as different colored symbols (red: strabismic observers; green: anisometropic observer; blue: strabismicanisometropic observer) and the group-averaged values (± 95% confidence intervals) are plotted as filled black circles. For comparison, data from the normal fovea (unfilled gray symbols) and periphery (10° eccentricity: filled gray symbols) are replotted from Chung et al (2007) as gray symbols, and the extent of crowding measured using stimuli close to size thresholds from Levi et al (2007) are replotted as small symbols. Dashed lines in the lower panels represent the null effect (no crowding).

Figure 4

Scatter plots showing the magnitude of crowding as a function of the spatial extent of crowding (deg; values replotted from the “critical distance” in Figure 3) for the non-amblyopic (NAE: unfilled symbols) and amblyopic (AE: filled symbols) eyes of each observer (different color). Panel A presents data for the same-order crowding (111 and 222 conditions) while panel B presents data for the cross-over conditions (212 and 121). Although the spatial extent of crowding seems to be larger for the amblyopic than the non-amblyopic eyes (note the horizontal displacement between filled and unfilled symbols), the ranges of the magnitude of crowding are similar between the two eyes.