Abstract
When a material is stretched along one spatial axis, the material is compressed along a perpendicular axis, so-called the Poisson effect. Poisson’s ratio is known as a measure of the Poisson effect, which corresponds to the ratio of transverse contraction strain to longitudinal extension strain in stretched direction. For common stretchy materials, the Poisson’s ratio is in range 0–0.5; more than 0.5 the volume of the material is reduced in comparison with the material’s original state. The present study investigated how human observers judged the causality of the orthogonal deformations in the Poisson effect. In stimuli, a bright rectangle against a neutral gray background was horizontally stretched while the upper and lower sides of the rectangle was vertically compressed with sinusoidal spatial modulation. We controlled the initial horizontal width of the rectangle and the amplitude of vertical compression because the Poisson’s ratios vary with those two parameters. The magnitude of horizontal stretch was constant among all conditions. The observers were asked to report whether the vertical compression of the rectangle was causally related with the horizontal stretch. The proportions of reports for non-causality increased with the amplitude of the vertical compression. Thresholds for non-causality judgments increased with the initial horizontal width of the rectangle. However, the pattern of the threshold variations was not consistently accounted for by the Poisson’s ratios. Rather, the results were well accounted for by the ratio of the area of the rectangle before the deformations to the area of the shrunk rectangle after the deformations. The results indicate that human observers judge the causality for orthogonal deformations due to material stretch not by estimating physical parameters but by utilizing simple visual features such as the change in areas between before and after the deformations.