Selective transfer of perceptual learning in parafoveal second-order motion: generalization to first-order motion but not to static second-order stimuli.

Perceptual learning of motion provides a powerful tool for investigating the hierarchical organization of the visual system. Previous studies have shown that training on second-order (contrast-defined) motion transfers asymmetrically to first-order (luminance-defined) motion, but the locus of this learning effect remains unclear. Here we tested whether second-order motion training transfers to static second-order perception, a condition that shares early texture-defined features but lacks motion components. Twenty-four subjects were assigned to a training group (n = 16) or a control group (n = 8). The training group underwent eight sessions of second-order motion direction discrimination training in the parafovea, while the control group received no training. Contrast sensitivity for second-order motion, first-order motion, and static first- and second-order stimuli was measured before and after training. The training group showed substantial improvements in contrast sensitivity for both second-order and first-order motion, with gains of approximately 1.8 dB and 2.7 dB, respectively. These improvements were significantly greater than those in the control group for motion stimuli. In contrast, improvements in static second-order perception were comparable between the training and control groups, indicating no specific training effect. Correlation analyses revealed a significant relationship between improvements in first- and second-order motion, but no correlation between second-order motion and static second-order perception. These findings suggest that second-order motion training primarily strengthens processing at a stage dedicated to motion extraction, rather than at early texture-feature extraction stages, providing behavioral evidence for the locus of plasticity within the dual-pathway model.