Heaviness perception. II. Contributions of object weight, haptic size, and density to the accurate perception of heaviness or lightness.

The present study investigated the contributions of object weight, haptic size, and density to the accurate perception of heaviness or lightness in the process of discriminating differences in weight between pairs of cubes with cue conflicts such as that resulting from the size-weight illusion. Fifteen subjects, with visual input blocked and relying on the input gained by grasping the cubes with only their fingertips, attempted to accurately discriminate possible differences in weight factor between the two respective cubes in each step of the trials. Three sets - one set each of copper (CP), aluminum (AL), and plastic (PL) - of seven cubes of various weight (0.10-0.74 N) were used. All of the cubes were covered with smooth, thin vinyl to eliminate possible input concerning density or material per se. Screens were strategically placed to eliminate any visual cues. One hundred and ninety-six trials with 37 combinations were pseudorandomly presented to subjects in the following conditions: PL versus AL, AL versus CP, and CP versus PL. Trials included 2 x 3 combinations on the basis of density (98 trials for higher and 98 for lower conditions) and weight (84 ascending trials for heavier, 28 for identical, and 84 descending for lighter conditions). The response for each trial given by each subject was regarded as correct when it accurately identified the weight relationship between the first and second cube. It was found that the subjects fairly accurately identified the weight relationship when density and weight both increased for the second cube (95.6% for given trials), and when density and weight both decreased (94.6%). The current results were markedly greater than those in the constant-density conditions obtained previously, suggesting that changes in density may be as much of an aid in the perception of heaviness and lightness as is weight. Whenever two cues conflicted directionally with each other, however, accuracy fell dramatically to 33.6% for lower density/ascending weight, and to 22.7% for higher density/descending weight. These results indicate the possibility of two different cues contributing to the perception of heaviness and lightness. Cue conflict such as the size-weight illusion naturally occurs when discriminating weight between objects. The present results, however, suggest that a person may perceive heaviness on the basis of the well-regulated relations between changes of density, size, and weight. The way in which these two cues are related through the haptic size is discussed.