Two separate mechanisms in motion speed learning revealed by task difficulty manipulation

The difficulty of a task is believed to be an important factor in determining the specificity of perceptual learning: difficult tasks usually yield more specific learning to the practiced stimulus. We reported motion sensitivity generalized across directions after practice in speed discrimination in a single direction (Tseng et al, 2007 Perception 36 Supplement, 53), in contrast to the literature revealing specific/limited motion learning. This may be due to our task being too easy, or speed learning may involve different learning mechanisms. We investigated whether we can induce similar specific learning by increasing the difficulty of our speed discrimination task by method A, replacing 50% of the single-direction signal dots with random-moving dots, or method B, by lessening speed difference between the standard and the comparison moving displays. Our results show that method A or method B alone did not generate direction-specific learning, and that easier conditions produce larger sensitivity enhancement (30%) than the more difficult conditions (19%). However, method A and B together did generate a notable M-shaped tuning curve centered at the practiced direction. Our results suggest at least two mechanisms are involved in speed discrimination learning: one increases the motion detector sensitivity and the other narrows the filter tuning by excluding external noise during speed learning.link_to_subscribed_fulltex