The external noise plus perceptual learning paradigm and the perceptual template model (PTM) distinguish three mechanisms of perceptual learning: stimulus enhancement, template retuning, and multiplicative noise (or equivalently, contrast-gain control) reduction. Dosher & Lu (1) found a mixture of stimulus enhancement and template retuning in perceptual learning of orientation discrimination in periphery. A key test of the PTM framework is the separability of mechanisms. Pure template retuning was found in learning of orientation discrimination in fovea (2). Here, pre-training in high external noise is used to separate stimulus enhancement from template retuning in foveal motion direction discrimination. Two groups, either with or w/o pre-training in high external noise, performed first-order sine-wave (2.3 c/d, 8 Hz) motion direction discrimination. In the subsequent training, contrast thresholds were measured 70.7% and 79.3% correct using staircase procedures for each of eight levels of external noise added to the stimulus. Observers were trained in ten sessions of 1120 trials each. Without pre-training, perceptual learning reduced contrast thresholds by about 50 % across all noise levels, or equal contributions of stimulus enhancement and template retuning. Contrast thresholds were reduced by 37% during high external noise pre-training. Subsequent training reduced contrast threshold by 54% in low noise but only 24% in high noise. Improvements in low noise continued over days, while no significant improvement was observed in high noise after the first two training sessions. Pre-training in high noise nearly saturated high noise learning, leaving performance in low noise to improve as though no pre-training was administered. This suggests that learning in low noise is separable from learning in high noise. Stimulus enhancement and template retuning in learning first-order motion direction discrimination are mixed but separable.

Dosher & Lu, PNAS, 1998

Lu & Dosher, ARVO, 2001

Supported by NSF and NIMH.