Suppression of task-irrelevant learning achieved through withdrawal of excitation in early sensory areas

Little is known about neural mechanisms that suppress learning, although such mechanisms are crucial for adaptive learning, which occurs selectively for items relevant to an observer's task but not for task-irrelevant items. Here, we examined neural mechanisms involved in suppressing visual perceptual learning (VPL), a type of visual skill learning, for a repeatedly exposed task-irrelevant coherent motion direction. The direction was either perceptually salient (suprathreshold for coherent motion detection) or weak (near detection threshold). Excitatory and inhibitory processing of the task-irrelevant direction during exposure was measured in early visual areas (V1 and V2) in the occipital lobes with functional magnetic resonance spectroscopy (fMRS). The results showed that, compared to near threshold exposure, VPL for the coherent motion direction was suppressed in suprathreshold exposure. Excitatory processing in early visual areas, as reflected in the concentration of glutamate, was reduced during suprathreshold compared to near threshold exposure. No modulation of inhibitory processing, as reflected in the concentration of γ-aminobutyric acid (GABA), was found. These results suggest that a gating mechanism operates within early visual areas that acts to suppress VPL for salient task-irrelevant signals by reducing excitatory processing.