In previous studies, the effect on perception of individual features such as curvature and edges have been studied with specifically designed stimuli. However, the effect of local properties on the perception of the global object has so far received little attention. In this study, cylinders with an elliptical cross section and rectangular blocks were used to investigate the effect and relative importance of curvature, change in curvature and edges, as local properties, on the ability of subjects to determine the orientation of the stimuli, which is a global property. We found that when curvature was present the threshold to determine the orientation was 43 percent lower than when curvature was absent. When, in addition, the change in curvature could be felt, the threshold was 37 percent lower than when only curvature could be felt. Finally, when edges were felt during exploration, the threshold increased by 46 percent compared to when the subjects were instructed to avoid the edges in the blocks. We conclude that the perception of curvature and change in curvature improve the performance of humans in perception of the whole shape, whereas edges, when not directly contributing to the task, disrupt performance.
From previous studies, it is unclear how bimanual length discrimination differs from unimanual length discrimination. To investigate the difference, we designed an experiment with four conditions. In the first two conditions, unimanual and bimanual discrimination thresholds are determined. In the third and fourth conditions, length is explored with the two index fingers like in the bimanual condition, but the reference is either internal, by clasping the hands together, or external, by grasping handles connected to the table. We find that thresholds for the unimanual condition (7.0%) and the clasping condition (9.2%) are both lower than for the bimanual condition (16%) and the grasping handles condition (15%). We conclude that when discriminating length unimanually and bimanually while clasping the hands together, the internal reference within the hand can be used and that explains the lower discrimination thresholds.
For small stimuli, it has been shown that subjects are very accurate in distinguishing a cylinder with an elliptical cross section from one with a circular cross section. In such a task, both curvature and length are integrated effectively. Large cylinders are explored differently: either by one hand or by two hands sliding over the surface. However, the same cues are available. We investigated the integration of position and curvature in unimanual and bimanual explorations. In Experiment 1, curved surfaces were presented as part of a horizontal cylinder with a cross section that was either a horizontally or a vertically elongated ellipse. We found that discrimination thresholds for unimanual exploration were significantly larger than for bimanual exploration. In Experiment 2, we found that position discrimination thresholds were independent of the type of exploration (unimanual or bimanual) and surprisingly also independent of the reference length. In Experiment 3, we found that discrimination thresholds for the position of the midsagittal plane were on an average lower than the position discrimination thresholds found in Experiment 2. From these findings, we conclude that the lower thresholds in Experiment 1 for bimanual exploration compared to unimanual exploration are due to the integration of curvature, not position or uncertainty of the midsagittal plane in unimanual exploration.
Salient features help identify objects. However, they can also influence subsequent comparison between objects. In this study, we investigate the influence of edges on perception of the shape of 3D objects. We tested two conditions in which subjects were asked to indicate the orientation of a block by exploring it either statically without touching the edges or freely. When edges were excluded, the threshold to determine the orientation of a block was an aspect ratio of 1.030. When the edges were explored, the threshold was 1.045. We conclude that edges disrupt the precision at which 3D objects can be perceived.
We investigated if and how length and curvature information are integrated when an object is explored in one hand. Subjects were asked to explore four types of objects between thumb and index finger. Objects differed in either length, curvature, both length and curvature correlated as in a circle, or anti-correlated. We found that when both length and curvature are present, performance is significantly better than when only one of the two cues is available. Therefore, we conclude that there is integration of length and curvature. Moreover, if the two cues are correlated in a circular cross-section instead of in an anti-correlated way, performance is better than predicted by a combination of two independent cues. We conclude that integration of curvature and length is highly efficient when the cues in the object are combined as in a circle, which is the most common combination of curvature and length in daily life.
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