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Ninety-two subjects, schoolchildren and undergraduate and postgraduate students, took part in a series of experiments on the haptic perception of curvature. A graded series of surfaces was produced using plano-convex lenses masked off to produce curved strips that could be explored without using arm movements. Thresholds were measured using the constant method and a staircase procedure. Experiments 1 and 2 yielded data on the absolute and difference thresholds for curvature. Experiment 3 demonstrated that the effective stimulus for curvature is represented by the overall gradient of a curved surface. Using this measure, it was shown that the present absolute thresholds for curvature are lower than those previously reported. In Experiment 4, absolute thresholds were compared using spherical and cylindrical curves: the results showed that, at least with the narrow strips used, the type of curvature does not exert a significant influence on performance. In Experiment 5, the subjective response to curvature was assessed using a rating procedure. Power functions are reported, although the relationship between stimuli and responses had a strong linear component. This suggests that haptically perceived curvature may be a metathetic rather than a prothetic continuum.This report presents psychophysical data on the haptic perception of curved surfaces. The need for such data arose during an investigation of haptic detection of undulations in surfaces (Gordon & Cooper, 1975). The majority of studies of the perception of curvature have used quite long surfaces. One obvious reason for this is that such surfaces are easier to produce. We thought it would be interesting to see how well people can perceive curvature when the surfaces are short and can be explored without gross movements of the hand and arm.There are relatively few publications on the perception of curvature, and the most relevant of these can be described briefly. Crewdson and Zangwill (1940), Hunter (1954), and Rubin (1936) used similar procedures, and only one of these studies will be described. In Crewdson and Zangwill's investigation, a blind adult judged whether the edge of a 247-mm flexible steel strip was straight or curved. The strip was supported at each end and its curvature manipulated by displacing the center. Detecting that the strip was not straight required approximately 2-mm displacement of the center. Interestingly, some concave curvature was required for the strip to be judged straight.In a more recent study, Davidson (1972) examined the performance of blind and sighted subjects as they judged the curvatures of the edges of 200-mm strips. Four arc heights (the perpendicular distance from the midpoint of the arc's chord to the midpoint of the This research was supported by a grant from the British Medical Research Council. We thank S. E. G. Lea for valuable advice. Address reprint requests to either author, Department of Psychology, Washington Singer Laboratories, University of Exeter, Exeter EX4 4QG, England. arc) were presented in concave an...

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It was shown by magnitude estimation that the perception of causality first described by Michotte (1946/1963) also occurs consistently and strongly with stroboscopic, ie apparent or phi, movement. This is so when the 'causal' movement is stroboscopic and the 'caused' movement real, when these movements are reversed, and when both movements are stroboscopic. The effect is not due to prior experience with Michotte-type displays.