Practice improves the performance in visual tasks, but mechanisms underlying this adult brain plasticity are unclear. Single-cell studies reported no , weak , or moderate [3, 4] perceptual learning-related changes in macaque visual areas V1 and V4, whereas none were found in middle temporal (MT) . These conflicting results and modeling of human (e.g., [6, 7]) and monkey data  suggested that changes in the readout of visual cortical signals underlie perceptual learning, rather than changes in these signals. In the V4 learning studies, monkeys discriminated small differences in orientation, whereas in the MT study, the animals discriminated opponent motion directions. Analogous to the latter study, we trained monkeys to discriminate static orthogonal orientations masked by noise. V4 neurons showed robust increases in their capacity to discriminate the trained orientations during the course of the training. This effect was observed during discrimination and passive fixation but specifically for the trained orientations. The improvement in neural discrimination was due to decreased response variability and an increase of the difference between the mean responses for the two trained orientations. These findings demonstrate that perceptual learning in a coarse discrimination task indeed can change the response properties of a cortical sensory area.