Bimanual curvature discrimination of hand-sized surfaces placed at different positions

This study explores bimanual curvature discrimination of cylindrically curved, hand-sized surfaces. The setup was designed so that the postures of the observers' left and right arms and hands were the same as if the observers were holding a large object in their hands. We measured psychometric curves for observers who used active, dynamic touch; these curvatures ranged from 1.18 to 4.05/m. Bimanual discrimination thresholds were found to be between 0.26 and 0.38/m on average; they were in the same range as unimanual thresholds reported in previous studies. Variation of (1) the horizontal distance between the stimuli or (2) the position of the setup had no effect on thresholds. In addition, we found that a number of observers showed discrimination biases in which they judged two physically different curvatures to be equal. Biases were of the same order of magnitude as the thresholds and could be either positive or negative. These biases can possibly be explained by small differences in left and right arm movements, an explanation that is supported by the position dependence of biases for individual observers.