In experiment 2, we examined whether there was a mismatch in perceived contrast above threshold. This experiment was similar to previous matching experiments
24 –26 but different because observers matched the target and standard gratings under dichoptic viewing.
Figure 3 shows matching contrast as a function of standard contrast. The results fell into two categories. Four amblyopes (ADS, AM, MaS, MoS) were able to match the contrasts for all the conditions (
Fig. 3a, left). On the other hand, for three other amblyopes (AA, KS, MA), matching contrast was out of the range (higher than 100%) for the standard contrast of 50% when the fellow fixing eye saw a target (
Fig. 3a, middle, missing data point). We subjected the data to three-way ANOVA with factors of eye (amblyopic/nondominant eye vs. fellow fixing/dominant eye), standard contrast (10%, 25%, or 50%), and amblyopia (amblyopes vs. controls). Data for the standard contrast of 50% was excluded from this ANOVA because not all observers with amblyopia could perform a match at this standard contrast level as described. The main effect of eye was significant (
F (1,7) = 7.75,
P = 0.027), indicating that matching contrast was higher when the fellow fixing eye viewed the target stimulus (
Fig. 3a, open circle) than when the amblyopic eye did (
Fig. 3a, gray diamond). This suggests overcompensation for the amblyopic eye. In other words, perceived contrast was higher for the amblyopic eye than for the fellow fixing eye. Although the eye × amblyopia interaction was only marginal (
F (1,7) = 4.62,
P = 0.069), this was driven by the fact that the interocular mismatch was larger for amblyopes than for the control group (
Fig. 3a). Because the spatial frequency of gratings was low (0.5 cyc/deg), three amblyopes repeated the matching task using gratings with a higher spatial frequency (4 cyc/deg). As can be seen in
Figure 3b, there was little mismatch between the two eyes for these three participants. However, one observer (KS) was not able to perceive the gratings presented to the amblyopic eye at the lowest contrast (10%). As a whole, these results indicate that the amblyopic eye has no loss of perceived contrast for dichoptic stimuli above threshold, extending previous findings obtained under monocular viewing conditions.
24 –26