The first learning effect for the multiple stimulus conditions was calculated as the percent threshold improvement from the first to the fifth (last) training session (
Figures 1b and
1c). For the random group (
n = 10), the stimulus location and orientation were randomized from trial to trial. The orientation thresholds with multiple stimulus conditions were about three times as high as the pre-/post-training condition that contained a single stimulus, and training reduced the thresholds with multiple stimulus conditions by 37.0 ± 6.2% (
t9 = 5.93;
p < 0.001; 95% confidence interval [CI], 22.9–51.1; Cohen's
d = 1.87; two-tailed paired
t-test here and in later analyses unless otherwise specified) (
Figures 1b and
1g). We suspected that the higher thresholds might have resulted from increased stimulus uncertainty due to stimulus randomization. Therefore, we had the rotating group (
n = 8) practice the same stimuli while the stimulus location and orientation were rotated. Such orderly stimulus presentations would reduce stimulus uncertainty and facilitate learning (
Kuai, Zhang, Klein, Levi, & Yu, 2005;
Zhang et al., 2008). However, the orientation thresholds with rotating conditions were also more than twice as high as those with the pre-/post-training condition, indicating that the high thresholds were not much related to stimulus uncertainty. Training reduced orientation thresholds with rotating conditions by 46.0 ± 3.3% (
t7 = 13.74;
p < 0.0001; 95% CI, 38.1–53.9; Cohen's
d = 4.86) (
Figures 1c and
1g). There was no significant difference for learning between the random and rotating groups (
t16 = 1.17;
p = 0.259; 95% CI, –7.2 to 25.1; Cohen's
d = 0.58; two-tailed independent
t-test). These results indicate that our observers were able to learn orientation discrimination with changing stimulus locations and orientations, regardless of whether the stimuli were presented in random or rotating orders.