Academic journal article Perception and Psychophysics

Poor Shape Perception Is the Reason Reaches-to-Grasp Are Visually Guided Online

Academic journal article Perception and Psychophysics

Poor Shape Perception Is the Reason Reaches-to-Grasp Are Visually Guided Online

Article excerpt

Both judgment studies and studies of feedforward reaching have shown that the visual perception of object distance, size, and shape are inaccurate. However, feedback has been shown to calibrate feedfoward reaches-to-grasp to make them accurate with respect to object distance and size. We now investigate whether shape perception (in particular, the aspect ratio of object depth to width) can be calibrated in the context of reaches-to-grasp. We used cylindrical objects with elliptical cross-sections of varying eccentricity. Our participants reached to grasp the width or the depth of these objects with the index finger and thumb. The maximum grasp aperture and the terminal grasp aperture were used to evaluate perception. Both occur before the hand has contacted an object. In Experiments 1 and 2, we investigated whether perceived shape is recalibrated by distorted haptic feedback. Although somewhat equivocal, the results suggest that it is not. In Experiment 3, we tested the accuracy of feedforward grasping with respect to shape with haptic feedback to allow calibration. Grasping was inaccurate in ways comparable to findings in shape perception judgment studies. In Experiment 4, we hypothesized that online guidance is needed for accurate grasping. Participants reached to grasp either with or without vision of the hand. The result was that the former was accurate, whereas the latter was not. We conclude that shape perception is not calibrated by feedback from reaches-to-grasp and that online visual guidance is required for accurate grasping because shape perception is poor.

There is currently a puzzle in perception/action research on visually guided reaching and grasping. Many psychophysical studies have demonstrated that human observers cannot accurately perceive metric 3-D shape, but we ordinarily have no difficulty hi reaching for and grasping solid objects. The puzzle is that accurate reaches-to-grasp would appear to require accurate shape perception, because grasping typically involves contact of the fingers with the back of an object. Grasps are known to be accurately sized relative to the size of an object as a hand approaches an object. When the grasp involves contact of thumb and fingers on the front and back of an object, respectively, then the specification of the relevant extent of the object (in depth) requires combined information about object size and shape. The shape can be characterized by the aspect ratio of object depth to width or by curvedness (Koenderink, 1990, pp. 319-324; Perotti, Todd, Lappin, & Phillips, 1998). Specification of the metric object size in the frontoparallel plane together with the aspect ratio (or the curvedness) would determine the metric object depth. How can information about 3-D shape be determined accurately in order to permit accurate reaches-to-grasp? A solution is suggested by the collected results from studies on shape perception. One possible solution involves using feedback information from grasping to calibrate information about shape.

A large number of shape perception studies have investigated the relation between perceived shape and actual physical shape. Early studies began with single cues, such as binocular disparity or motion parallax. In monocular structure-from-motion studies (e.g., Norman & Lappin, 1992; Norman & Todd, 1993; Perotti et al., 1998; Tittle, Todd, Perotti, & Norman, 1995; Todd & Bressan, 1990; Todd & Norman, 1991) and studies of stereopsis (e.g., Johnston, 1991; Tittle et al., 1995), the relative depth of objects has not been perceived accurately. Johnston examined perceived 3-D structure from binocular disparity and showed that shapes in depth tend to be systematically compressed at larger distances and systematically expanded at shorter distances. In other words, truly circular cylinders appeared flattened at a far distance and elongated at a near distance. Similar evidence for distortions in the perception of 3-D structure has been obtained in structure-from-motion studies. …

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