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Perceptual discrimination improves with practice. This ‘perceptual learning’ is often specific to the stimuli presented during training^,,,,, indicating that practice may alter the response characteristics of cortical sensory neurons^,. Although much is known about how learning modifies cortical circuits, it remains unclear how these changes relate to behaviour. Different theories assume that practice improves discrimination by enhancing the signal^,,, diminishing internal noise^, or both. Here, to distinguish among these alternatives, we fashioned sets of faces and textures whose signal strength could be varied, and we trained observers to identify these patterns embedded in noise. Performance increased by up to 400% across several sessions over several days. Comparisons of human performance to that of an ideal discriminator showed that learning increased the efficiency with which observers encoded task-relevant …