Identification of contrast-defined letters benefits from perceptual learning in adults with amblyopia

Abstract

Amblyopes show specific deficits in processing second-order spatial information (e.g. Wong, Levi, & McGraw (2001). Is second-order spatial loss in amblyopia explained by the loss of first-order spatial input? Vision Research, 41, 2951-2960). Recent work suggests there is a significant degree of plasticity in the visual pathway that processes first-order spatial information in adults with amblyopia. In this study, we asked whether or not there is similar plasticity in the ability to process second-order spatial information in adults with amblyopia. Ten adult observers with amblyopia (five strabismic, four anisometropic and one mixed) were trained to identify contrast-defined (second-order) letters using their amblyopic eyes. Before and after training, we determined observers' contrast thresholds for identifying luminance-defined (first-order) and contrast-defined letters, separately for the non-amblyopic and amblyopic eyes. Following training, eight of the 10 observers showed a significant reduction in contrast thresholds for identifying contrast-defined letters with the amblyopic eye. Five of these observers also showed a partial transfer of improvement to their fellow untrained non-amblyopic eye for identifying contrast-defined letters. There was a small but statistically significant transfer to the untrained task of identifying luminance-defined letters in the same trained eye. Similar to first-order spatial tasks, adults with amblyopia benefit from perceptual learning for identifying contrast-defined letters in their amblyopic eyes, suggesting a sizeable degree of plasticity in the visual pathway for processing second-order spatial information.

Figures

Fig. 1

A schematic cartoon illustrating the basic experimental design of the study.

Fig. 2

Acuity, or letter size threshold (in deg), for identifying contrast-defined letters is plotted as a function of that for identifying luminance-defined letters, for the non-amblyopic (NAE: unfilled symbols) and the amblyopic (AE: filled symbols) eye of each amblyopic observer. Data from four normal observers obtained at the fovea (+ symbols) and 10° eccentricity (∗ symbols) are also included for comparison. The diagonal lines represent threshold ratios between contrast-defined and luminance-defined letters of 1:1 and 6:1. In this and subsequent figures, observers are color-coded according to the type of amblyopia they exhibited (strabismic, red; anisometropic, green; both, blue).

Fig. 3

Differential contrast threshold (ΔC) for identifying contrast-defined letters is plotted as a function of training block, for each individual observer. Filled symbols in each panel represent thresholds obtained at the pre- and post-tests. The solid line in each panel represents the best-fit regression line to each set of data. The slope of this line, if different from 0, represents significant amount of learning (p-value given in each panel). Acuity and letter size used are given in each panel.

Fig. 4

Threshold ratios (post-test/pre-test) are compared for the four conditions (non-amblyopic eye (NAE) vs. amblyopic eye (AE), luminance-defined vs. contrast-defined letters). The trained condition (AE cont) is indicated by an asterisk. The group-averaged ratio for each condition is given under the label for each condition. Observers are ordered according to the decreasing magnitude of their ratios (improvement = 1 − ratio) for the trained condition. Error bars represent ± 1 SEM.