Walking without vision to previously viewed targets was compared with visual perception of allocentric distance in two experiments. Experimental evidence had shown that physically equal distances in a sagittal plane on the ground were perceptually underestimated as compared with those in a frontoparallel plane, even under full-cue conditions. In spite of this perceptual anisotropy of space, Loomis et al (1992 Journal of Experimental Psychology. Human Perception and Performance 18 906-921) found that subjects could match both types of distances in a blind-walking task. In experiment 1 of the present study, subjects were required to reproduce the extent of allocentric distance between two targets by either walking towards the targets, or by walking in a direction incompatible with the locations of the targets. The latter condition required subjects to derive an accurate allocentric distance from information based on the perceived locations of the two targets. The walked distance in the two conditions was almost identical whether the two targets were presented in depth (depth-presentation condition) or in the frontoparallel plane (width-presentation condition). The results of a perceptual-matching task showed that the depth distances had to be much greater than the width distances in order to be judged to be equal in length (depth compression). In experiment 2, subjects were required to reproduce the extent of allocentric distance from the viewing point by blindly walking in a direction other than toward the targets. The walked distance in the depth-presentation condition was shorter than that in the width-presentation condition. This anisotropy in motor responses, however, was mainly caused by apparent overestimation of length oriented in width, not by depth compression. In addition, the walked distances were much better scaled than those in experiment 1. These results suggest that the perceptual and motor systems share a common representation of the location of targets, whereas a dissociation in allocentric distance exists between the two systems in full-cue conditions.

Human prehension movements have been studied with regard to the parallel processing of motor control and sensorimotor coordination. Temporal aspects of the movement (e.g., onset time and duration) have been studied extensively, while spatial aspects have not been studied systematically. Thus, the purpose of this study was to examine spatiotemporal variability of the transport (wrist trajectory) and grasp (grip aperture between the index finger and the thumb) components. In this experiment, the extrinsic (e.g., distance) and intrinsic object properties (e.g., object size) were manipulated. Subjects were required to pick up an aluminum cylinder as quickly and accurately as possible using the index finger and the thumb. It was found that object size significantly affected both transport and grasp components. Distance mainly affected the transport component. These kinematic results were consistent with the findings of earlier studies. Furthermore, the distribution of mean within-subject variability across normalized movement time for the transport component was not the same as that of the grasp component, suggesting that the different motor control processes exist. The peak amplitudes in variability of the wrist trajectory and the grip aperture were obtained at similar points throughout movement time. Furthermore, the peak of wrist variability depended on distance not object size, while that of aperture variability depended on both distance and object size. These results strongly support the hypothesis that the grasp component is adjusted using dynamic information provided from the transport component as the wrist moves toward the object. We also found that wrist variability converged to the target point, while aperture variability was biphasic: it converged, at least, around the point of maximum aperture in the first phase and then remained constant in the second phase. This result suggests that the two components are under different control processes. We hypothesize that the transport component can be modeled as a single feedforward system, while the grasp component can be divided into two separate mechanisms.

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