Repeated practice of perceptual tasks often produces marked improvements in behavioral performance. While such perceptual learning is found across a variety of visual tasks, physiological evidence of corresponding neural changes in humans has remained elusive. We hypothesized that practice of a task thought to be mediated by early visual mechanisms (i.e., simple contrast-detection) would produce measurable changes in human primary visual cortex (V1). Here were used fMRI to measure the effects of perceptual learning on activity in V1. Subjects spent more than twenty days (>10,000 trials) practicing detection of sinusoidal gratings. In order to quantify the specificity of learning, fMRI measurements and behavioral thresholds for contrast-detection and orientation-discrimination were collected before and after training for several eccentricities and grating orientations. We found that V1 responses for practiced gratings were reliably greater than those responses collected prior to training. Critically, the specificity of changes in V1 responses mirrored the specificity of changes in perceptual thresholds; both the changes in detection thresholds and V1 responses were orientation, and spatial-location specific. While these results are consistent with several models of the neural mechanisms of perceptual learning, the simplest explanation of our data may be a bottom-up gain change produced by a re-weighting of synaptic connections.