The haptic perception of curvature

Abstract

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...

Similar mechanisms underlie curvature comparison by static and dynamic touch

“…Pont et aI. (1996) and Gordon and Morison (1982) found curvature discrimination thresholds in terms of local attitude in the order of magnitude of 1 0 • This is in the same order of magnitude as that which we found in the present investigation. The results of the first experiment show that the local attitudes on a curved surface can be determined per finger.…”

“…It is the first-order geometrical structure (the attitude difference) that seems to represent the effective stimulus for curvature discrimination in this experiment. Gordon and Morison (1982) also came to this conclusion when they studied dynamic discrimination ofcurved surfaces of I X maximally 4 em with the index finger. They found a gradient of .009 at 75% correct, which corresponds to an attitude difference of2.8°over the surface at 84% correct.…”

“…The curvatures of these stimuli cannot be discriminated statically from flat with a single fingerpad. Gordon and Morison (1982) investigated discrimination and rating of curved stimuli with a width of 1 em and a length of2-4 em that were dynamically touched with the index finger. They found that the threshold was constant if expressed in the base-to-peak height divided by halfthe length of the stimulus.…”

Transfer of the curvature aftereffect in dynamic touch

“…In a number of psychophysical studies, the curvature of the stimuli was below the threshold for static touch with a single fingertip (Davidson, 1972;Gordon & Morison, 1982;Pont et al, 1999). Hence, movement was required to perceive the shape of the surface.…”

Haptic Discrimination of Doubly Curved Surfaces

“…Quantitative studies have been mainly focussed on curvature (eg Davidson 1972;Gordon and Morison 1982) and have often been restricted to small stimuli applied passively to the skin (eg Goodwin et al 1991;Goodwin and Wheat 1992).…”