Mechanisms of learning a visual motion discrimination task

Training can improve our ability to detect, discriminate or identify sensory stimuli. Despite the prevalence of this phenomenon, called perceptual learning, our understanding of the underlying neural plasticity is incomplete. Changes in early sensory areas of cortex have been inferred from psychophysical studies and identified in monkeys trained on auditory and somatosensory tasks. However, monkeys trained on visual tasks show only moderate or no change in early visual cortex. Changes in higher stages of processing, including those that contribute to decision-making and attention, have also been inferred from psychophysical studies but have not been identified directly in the brain. Here we exploit recent advances relating perception and single-unit activity in two areas of primate cortex, the middle temporal (MT) and lateral intraparietal (LIP) areas, and show that improved performance on a visual discrimination task corresponds to changes in LIP but not MT. We trained monkeys to discriminate the direction of random-dot motion and indicate their decision with an eye movement. The monkeys first learned the association between motion direction and saccadic response and then gradually learned to make accurate direction decisions using weaker motion stimuli. Neurons in area MT, which in trained monkeys represent visual motion information used to solve the task, maintained a consistently high level of sensitivity to the motion stimulus throughout training. In contrast, neurons in area LIP, which in trained monkeys represent formation of the categorical direction decision that converts visual input into oculomotor output, underwent dramatic changes that reflected improvements in the decision process as performance improved. These behavioural and neural changes can be accounted for using a reinforcement learning rule to shape functional connectivity between the sensory and decision neurons. Thus, for this task, perceptual learning appears to involve changes not in how sensory information is represented in the brain but rather how the sensory representation is interpreted to form the decision that guides behaviour