Perceptual learning (PL) of visual features depends on their relevance to a given task. Previous results showed that PL occurs for task-irrelevant features that are near detection threshold but not for task-irrelevant features that are above detection threshold (suprathreshold). Neuronal mechanisms modulating the occurrence of task-irrelevant PL remain largely unknown and are focus of the current research. We employed a design in which participants (n=24) trained on a rapid-serial-visual-presentation task at screen center while being exposed to coherent motion in the visual periphery as a task-irrelevant feature over the course of twelve daily sessions. Before the first, after the sixth, and after the final session, participants performed the task while brain activation was measured in an MRI scanner. Participants were assigned to one of two exposure conditions, which differed only in the salience of the exposed task-irrelevant feature (coherent motion near detection threshold or suprathreshold). The behavioral results showed that task-irrelevant PL occurred in the near threshold but not in the suprathreshold exposure condition. Task-irrelevant PL in the near-threshold condition was associated with increased activation in early visual cortex representing the task-irrelevant coherent motion in the visual periphery. In contrast, activation in early visual cortex during suprathreshold exposure was suppressed. These results indicate that neuronal mechanisms at the earliest stage of cortical visual processing are involved in filtering perceptually salient (suprathreshold) task-irrelevant features. However, such filtering mechanisms fail for perceptually weak (near threshold) features, leading to the occurrence of task-irrelevant learning.