Mechanisms of generalization in perceptual learning

Learning in many visual perceptual tasks has been shown to be specific to p racticed stimuli, while new stimuli have to be learned from scratch. Here w e demonstrate generalization using a novel paradigm in motion discriminatio n where learning has been previously shown to be specific. We trained subje cts to discriminate directions of moving dots, and verified the previous re sults that learning does not transfer from a trained direction to a new one . However, by tracking the subjects' performance across time in the new dir ection, we found that their speed of learning doubled. Therefore, we found generalization in a task previously considered too difficult to generalize. We also replicated, in a second experiment, transfer following training wi th 'easy' stimuli, when the difference between motion directions as enlarge d. In a third experiment we found a new mode of generalization: after maste ring the task with an easy stimulus, subjects who have practiced briefly to discriminate the easy stimulus in a new direction generalize to a difficul t stimulus in that direction. This generalization depends on both the maste ring and the brief practice. The specificity of perceptual learning and the dichotomy between learning of 'easy' versus 'difficult' tasks have been as sumed to involve different learning processes at different cortical areas. Here we show how to interpret these results in terms of signal detection th eory. With the assumption of limited computational capacity, we obtain the observed phenomena - direct transfer and acceleration of learning - for inc reasing levels of task difficulty. Human perceptual learning and generaliza tion, therefore, concur with a generic discrimination system. (C) 1999 Else vier Science Ltd. All rights reserved.