Variance predicts visual-haptic adaptation in shape perception

When people are exposed repeatedly to a conflict in visually and haptically specified shapes, they adapt and the apparent conflict is eventually eliminated. The inter-modal adaptation literature suggests that the conflict is resolved by adapting the haptic shape estimator. Another possibility is that both estimators adapt by amounts that depend on their relative variances. Thus, the visual estimator could adapt if its variance were high enough. Is relative reliability the better predictor of visual-haptic adaptation? We examined this by manipulating the variance of the visual signal during inter-modal adaptation and then measuring changes in the within-modal (vision-alone and haptics-alone) shape percepts. The stimulus was a 3D object with a rectangular front surface. It was specified visually by random-dot stereograms and haptically by PHANToM force-feedback devices. In pre- and post-tests, observers judged whether its front surface was taller or shorter than it was wide. For each modality, we found the aspect ratio that was perceptually a square. During adaptation, a conflict was created between the visually and haptically specified shapes by independently altering the visual and haptic aspect ratios of the front surface. The variance of the visual estimator (determined by dot number) was either low or high. We assessed the amount of visual and haptic adaptation by comparing pre- and post-test shape estimates. When the visual estimators variance was low, essentially all of the adaptation occurred in the haptic estimator. When the visual estimators variance was high, we observed visual and haptic adaptation. These results suggest that the relative reliability of visual and haptic estimators determines the relative amounts of visual and haptic adaptation.