The decomposition analysis, described in
Method, was applied to the data from the training group to assess the contributions of acuity, crowding and mislocations. The results are shown in
Figure 5.
In the decomposition profiles, the area representing the crowding effect (yellow) is larger than the areas representing the acuity (red) and mislocation (blue) effects. This means that crowding contributes more than acuity loss or mislocation errors to the reduction from 100% in accuracy in the trigram letter-recognition task. We observed a notable shrinkage of the area representing crowding (yellow) from pretest to posttest. There was also slight shrinkage of the area representing mislocation errors (red). In contrast, there was no significant change in the very small area representing the impact of acuity (blue).
A 3 × 2 [influencing factors (acuity, crowding, mislocations) × test time (pre-/ posttest)] two-way repeated measures ANOVA was conducted to examine the training-related changes in the three factors. In the trained field, there was a significant main effect of the influencing factors, F(2, 30) = 7.43, p = 0.011, indicating a difference among the magnitudes (in bits) of the three factors. Pairwise comparison between the three factors indicated that in both pre- and posttests, the impact of crowding (pretest: 10.29 bits; posttest: 5.46 bits) was significantly larger than that of acuity (pretest: 0.23 bits; posttest: 0.31 bits) or mislocations (pretest: 1.3 bits; posttest: 0.62 bits), but the impact of acuity and mislocations did not differ. Also, there was a significant main effect of the test time, F(1, 30) = 53.41, p < 0.001, in the trained field, revealing a change in the size of the factors from pretest to posttest. For the untrained field, there were significant main effects of the influencing factors, F(2, 30) = 7.85, p = 0.0088, and test time, F(1, 30) = 97.05, p < 0.001, with similar results from the pairwise comparison between the size of the three factors (pretest: 11.65, 1.7 and 0.23 bits for the impact of crowding, mislocations and acuity; posttest: 7.39, 1.08 and 0.55 bits).
Results from paired-samples
t-tests showed that after training, in both the trained and untrained field, there was a significant reduction in the impact of crowding (trained: 4.83 bits; untrained: 4.26 bits) and mislocations (trained: 0.68 bits; untrained: 0.62 bits), but the impact of acuity remained the same (
Table 2). Further comparison revealed that in both visual fields, the reduction of crowding in bits was significantly larger than that of mislocations. Despite the fact that our training reduced the impact of both crowding and mislocations by approximately the same percentage, the larger magnitude of the reduction in crowding is more significant in guiding future designs of training protocols.
In sum, these results show that: 1) Crowding has a greater influence on the size of the visual span than letter acuity and mislocations; 2) After training, there was a substantial reduction in the magnitude of crowding and a smaller reduction for mislocations while the impact of letter acuity was unchanged; 3) The training effects largely transferred to an untrained retinal location.