PERCEPTUAL-LEARNING REFLECTS EXTERNAL NOISE FILTERING AND INTERNAL NOISE-REDUCTION THROUGH CHANNEL REWEIGHTING

To investigate the nature of plasticity in the adult visual system, pe rceptual learning was measured in a peripheral orientation discriminat ion task with systematically varying amounts of external (environmenta l) noise. The signal contrasts required to achieve threshold were redu ced by a factor or two or more after training at all levels of externa l noise. The strong quantitative regularities revealed by this novel p aradigm ruled out changes in multiplicative internal noise, changes in transducer nonlinearites, and simple attentional tradeoffs. Instead, the regularities specify the mechanisms of perceptual learning at the behavioral level as a combination of external noise exclusion and stim ulus enhancement via additive internal noise reduction. The findings a lso constrain the neural architecture of perceptual learning. Plastici ty in the weights between basic visual channels and decision is suffic ient to account for perceptual learning without requiring the retuning of visual mechanisms.