PURPOSE: Our double training studies revealed that perceptual learning can generalize to not only new locations and orientations (Xiao et al., 2008; Zhang et al., 2010), but also new physical stimuli encoded at different cortical processing stages (Wang et al., 2016). These results suggest that perceptual learning involves concept learning that improves the abstract concept of a trained feature (e.g., the concept of orientation). Here we demonstrate that such a concept can be represented and improved across the sensory modalities.
METHODS & RESULTS: In Experiment I, participants first practiced tactile orientation discrimination (Tac_ori) using their index fingers that were stimulated by 3D-printed gratings at 40o or 130o. Although training improved Tac_ori thresholds (44.90%, p = 0.036 < 0.05), it had no effect on visual orientation discrimination (Vis_ori) at the trained orientation (p = 0.40) and untrained orthogonal orientation (p = 0.26). The participants then practiced an irrelevant visual contrast discrimination task (Vis_ctrst) at the trained orientation following a sequential double training procedure. After that, Vis_ori was improved at both trained orientation (27.7%, p = 0.012 < 0.05) and untrained orientation (22.5%, p = 0.025 < 0.05). Further training of Vis_ori failed to produce additional gain significantly (p = 0.093). In Experiment II, Tac_ori and Vis_ctrst were trained in the same sessions in alternating blocks of trials, which improved Vis_ori at both trained (26.20%, p = 0.017 < 0.05) and untrained orientation (21.34%, p = 0.025 < 0.05). Further training of Vis_ori did not improve Vis_ori thresholds significantly either (8.33%, p = 0.069). A control condition found that mere Vis_ctrst training had no impact on Vis_ori thresholds (p = 0.16). Therefore, tactile orientation discrimination learning can transfer completely to visual orientation discrimination with double training.
CONCLUSIONS: These results suggest that stimulus orientation as a concept can be represented and improved across tactile and visual modalities. Tactile training of orientation discrimination thus can teach visual orientation discrimination.