Abstract: PURPOSE: Perceptual learning has been widely used to study the plasticity of the visual system in adults. However, the learning protocol is usually time-consuming, which requires subjects to practice a task for thousands of times over weeks. We explored the relationship between the training amount and the behavioral improvement.
METHODS: Four groups of subjects underwent motion discrimination training over 8 suc- cessive days for 40, 120, 360, or 1080 trials per day. Subjects were trained around one mo- tion direction, and were tested at 0°, 30°, 60°, and 90° away from the trained direction be- fore, one day after, and two weeks after training.
RESULTS: In both the tests after training, we observed a significant reduction in the threshold at the trained direction. Surprisingly, the magnitudes in the threshold reduction were similar across groups with different daily training amounts, ranging from 40 trials (2 minutes) to 1080 trials (54 minutes) per day. We further quantified the specificity of learn- ing as the difference between the threshold change at the trained direction and the aver- age threshold change at the untrained directions. We found that, immediately after train- ing, the group with the smallest training amount showed a weaker specificity compared to the other groups. However, the difference in the specificity among the groups disappeared two weeks after training.
CONCLUSIONS: These findings demonstrated that the behavioral improvement from train- ing was independent of the daily training amount. Also, the development of learning spe- cificity in the time course indicated that the neural mechanisms underlying perceptual learning could be dependent on the training amount and evolve with time.

Key words: Perceptual learning, Specificity, Plasticity