Learning to identify near-threshold luminance-defined and contrast-defined letters in observers with amblyopia

Abstract

We assessed whether or not the sensitivity for identifying luminance-defined and contrast-defined letters improved with training in a group of amblyopic observers who have passed the critical period of development. In Experiment 1, we tracked the contrast threshold for identifying luminance-defined letters with training in a group of 11 amblyopic observers. Following training, six observers showed a reduction in thresholds, averaging 20%, for identifying luminance-defined letters. This improvement transferred extremely well to the untrained task of identifying contrast-defined letters (average improvement=38%) but did not transfer to an acuity measurement. Seven of the 11 observers were subsequently trained on identifying contrast-defined letters in Experiment 2. Following training, five of these seven observers demonstrated a further improvement, averaging 17%, for identifying contrast-defined letters. This improvement did not transfer to the untrained task of identifying luminance-defined letters. Our findings are consistent with predictions based on the locus of learning for first- and second-order stimuli according to the filter-rectifier-filter model for second-order visual processing.

Figures

Figure 1

A schematic illustration of the basic experimental design of the study. Experiment 1 consisted of learning to identify luminance-defined letters. Experiment 2 immediately succeeded Experiment 1 and consisted of learning to identify contrast-defined letters. In each experiment, a pre-test preceded the training, which was in turn, followed by a post-test. Contrast thresholds for identifying luminance-defined and contrast-defined letters were measured separately for the non-amblyopic (NAE) and amblyopic (AE) eyes during pre- and post-tests.

Figure 2

Contrast threshold for identifying luminance-defined letters (Experiment 1) is plotted as a function of training block, for each individual observer. Each unfilled symbol represents threshold obtained for a training block (100 trials). Observers were trained for 10 blocks per day for a total of eight days. Filled symbols in each panel represent thresholds obtained at the pre- and post-test. 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 zero, represents significant amount of learning (p-value given in each panel). Observers were divided into two groups: those who showed learning (top two rows, data represented by circular symbols) and those who did not show learning (bottom two rows, data represented by triangular symbols). Acuity and letter size used are given in each panel. In this and subsequent figures, observers are color-coded according to the type of amblyopia they exhibited (strabismic, red; anisometropic, green; mixed, blue).

Figure 3

Post-test thresholds are plotted against pre-test thresholds for identifying luminance-defined (trained task: left panel) and contrast-defined letters (untrained task: right panel), for non-amblyopic (NAE: unfilled symbols) and amblyopic eyes (AE: filled symbols), for the six observers who showed significant learning (see text for criterion of “significant” learning). Dashed lines represent the 1:1 lines, indicating no change in thresholds before and after learning.

Figure 4

Acuities obtained at post-tests are plotted as a function of those obtained at pre-tests (deg) for the six observers who showed learning in Experiment 1, for luminance-defined and contrast-defined letters. In each panel, data obtained from the non-amblyopic and amblyopic eyes are represented by unfilled and filled symbols, respectively. Dashed lines represent the 1:1 line, indicating no change in acuity before and after learning.

Figure 5

Post/pre-test threshold ratios for the untrained task of identifying contrast-defined letters (Cont) were plotted as a function of the post/pre-test threshold ratios for the trained task of identifying luminance-defined letters (Lum), for the non-amblyopic eyes (NAE: left panel) and amblyopic eyes (AE: right panel). Each symbol represents the data for one observer. Dashed lines drawn at threshold ratios of 1 divide each panel into four quadrants, with the lower left quadrant representing improvements for both luminance-defined (trained task) and contrast-defined (untrained task) letters following training in Experiment 1. The solid line in each panel represents the best-fit regression line to each set of data, with the correlation coefficient (r) given in the panel.

Figure 6

Differential contrast threshold (ΔC) for identifying contrast-defined letters is plotted as a function of training block, for the seven observers who took part in Experiment 2. Two observers (SCF and CF) were trained for 100 blocks instead of 80 due to experimenter’s errors (see text for details). Details of the plotting are the same as those in Figure 2.

Figure 7

Post-test thresholds are plotted against pre-test thresholds for identifying luminance-defined (untrained task: left panel) and contrast-defined letters (trained task: right panel), for non-amblyopic (NAE: unfilled symbols) and amblyopic eyes (AE: filled symbols), for the five observers who showed significant learning. Dashed lines represent the 1:1 lines, indicating no change in thresholds before and after learning.