Signal but not noise changes with perceptual learning

Perceptual discrimination improves with practice. This 'perceptual learning ' is often specific to the stimuli presented during training(1-5): indicati ng that practice may alter the response characteristics of cortical sensory neurons(6,7). Although much is known about how learning modifies cortical circuits(8), it remains unclear how these changes relate to behaviour. Diff erent theories assume that practice improves discrimination by enhancing th e signal(1,9,10) diminishing internal noise(11,12) or both(13). Here, to di stinguish among these alternatives, we fashioned sets of faces and textures whose signal strength could be varied, and we trained observers to identif y these patterns embedded in noise. Performance increased by up to 400% acr oss several sessions over several days. Comparisons of human performance to that of an ideal discriminator showed that learning increased the efficien cy with which observers encoded task-relevant information. Observer respons e consistency, measured by a double-pass technique in which identical stimu li are shown twice in each experimental session(14,15), did not change duri ng training, showing that learning had no effect on internal noise. These r esults indicate that perceptual learning may enhance signal strength, and p rovide important constraints for theories of learning.