The influence of stimulus tilt on haptic curvature matching and discrimination by dynamic touch

We investigated whether haptic comparison of the curvature of strips is influenced by the tilt (the average slope relative to the horizontal) of the curved strips. This particular stimulus manipulation was chosen to decide between two broad ways in which dynamic curvature comparison might be done: on the basis of the attitude (slope) differences over the surfaces or on the basis of the attitude (slope) differences between successively presented surfaces. For this purpose we conducted matching and discrimination experiments in which strips of constant curvature 9 cm long were touched dynamically and the tilts of the strips were varied. The results of the matching experiment showed some influence of tilt: the strips were judged to be slightly more curved if the tilt was nonzero than if it was zero. The results of the discrimination experiment did not show an effect of tilt. Because the attitude (slope) differences over the surfaces were independent of the tilt it is concluded that curvature comparison is based primarily on attitude differences over the curved surfaces.     

Haptic curvature comparison of convex and concave shapes

A sculpture and the mould in which it was formed are typical examples of objects with an identical, but opponent, surface shape: each convex (ie outward pointing) surface part of a sculpture has a concave counterpart in the mould. The question arises whether the object features of opponent shapes can be compared by touch. Therefore, we investigated whether human observers were able to discriminate the curvatures of convex and concave shapes, irrespective of whether the shape was convex or concave. Using a 2AFC procedure, subjects had to compare the curvature of a convex shape to the curvature of a concave shape. In addition, results were also obtained for congruent shapes, when the curvature of either only convex shapes or only concave shapes had to be compared. Psychometric curves were fitted to the data to obtain threshold and bias results. When subjects explored the stimuli with a single index finger, significantly higher thresholds were obtained for the opponent shapes than for the congruent shapes. However, when the stimuli were touched by two index fingers, one finger per surface, we found similar thresholds. Systematic biases were found when the curvature of opponent shapes was compared: the curvature of a more curved convex surface was judged equal to the curvature of a less curved concave surface. We conclude that human observers had the ability to compare the curvature of shapes with an opposite direction, but that their performance decreased when they sensed the opponent surfaces with the same finger. Moreover, they systematically underestimated the curvature of convex shapes compared to the curvature of concave shapes.     

Anisotropy in haptic curvature and shape perception

An investigation was undertaken into whether haptic comparison of curvature and of shape is influenced by the length/width ratio of the hand. For this purpose three experiments were conducted to test the curvature matching of curved strips (experiment 1), the curvature matching of cylindrically curved hand-sized surfaces (experiment 2), and the shape discrimination of elliptically curved hand-sized surfaces (experiment 3). The orientation of the stimuli with respect to the fingers was varied. The results of the two matching experiments showed that a given curvature is judged to be more curved when touched along the fingers than when touched across the fingers. The phenomenal flatness along and across the fingers was found to be different and subject dependent. The results of the shape-discrimination experiment showed that the orientation of ellipsoidal surfaces influences the judgments of the shapes of these surfaces. This influence could be predicted on the basis of results of the second matching experiment. It is concluded that similar mechanisms underlie the (anisotropic) perception of curvature and shape. For the major part the trends in the results can be explained by the length/width ratio of the hand and the phenomenal flatnesses.     

An after-effect in haptic space perception

After-effects in haptic perception have been little studied. But they seem to be prominent, and easy to elicit. An after-effect of the perception of the vergence of two surfaces is described, and is shown to be independent of the slant of the surfaces. It is not “figural.” Spatial perception with the hands, as well as with the eyes, is apparently based on a fluid and adaptable receptive system.     

Attenuating the haptic horizontal—vertical curvature illusion

In a number of experiments, blindfolded subjects traced convex curves whose verticals were equal to their horizontal extent at the base. Overestimation of verticals, as compared with horizontals, was found, indicating the presence of a horizontal-vertical illusion with haptic curves, as well as with visible curves. Experiment 1 showed that the illusion occurred with stimuli in the frontal plane and with stimuli that were flat on the table surface in vision and touch. In the second experiment, the stimuli were rotated, and differences between vision and touch were revealed, with a stronger illusion in touch. The haptic horizontal-vertical illusion was virtually eliminated when the stimuli were bimanually touched using free exploration at the body midline, but a strong illusion was obtained when curves were felt with two index fingers or with a single hand at the midline. Bimanual exploration eliminated the illusion for smaller 2.5- through 10.2-cm stimuli, but a weakened illusion remained for the largest 12.7-cm patterns. The illusion was present when the stimuli were bimanually explored in the left and right hemispace. Thus, the benefits of bimanual exploration derived from the use of the two hands at the body midline combined with free exploration, rather than from bimanual free exploration per se. The results indicate the importance of haptic exploration at the body midline, where the body can serve as a familiar reference metric for size judgments. Alternative interpretations of the results are discussed, including the impact of movement-based heuristics as a causal factor for the illusion. It was suggested that tracing the curve’s peak served to bisect the curve in haptics, because of the change in direction.     

Perceiving the vertical distances of surfaces by means of a hand-held probe.

Nine experiments were conducted on the haptic capacity of people to perceive the distances of horizontal surfaces solely on the basis of mechanical stimulation resulting from contacting the surfaces with a vertically held rod. Participants touched target surfaces with rods inside a wooden cabinet and reported the perceived surface location with an indicator outside the cabinet. The target surface, rod, and the participant's hand were occluded, and the sound produced in exploration was muffled. Properties of the probe (length, mass, moment of inertia, center of mass, and shape) were manipulated, along with surface distance and the method and angle of probing. Results suggest that for the most common method of probing, namely, tapping, perceived vertical distance is specific to a particular relation among the rotational inertia of the probe, the distance of the point of contact with the surface from the probe's center of percussion, and the inclination at contact of the probe to the surface. They also suggest that the probe length and the distance probed are independently perceivable. The results were discussed in terms of information specificity versus percept-percept coupling and parallels between selective attention in haptic and visual perception.     

Haptic curvature discrimination at several regions of the hand

Static haptic discrimination of the curvature of convex, concave, or straight 20-cm-long strips was investigated for nine placements on the hand. In one condition, the strips were touched with the palmar side of the hand, and in the other condition, with the dorsal side. The influence of the lengths of the strips, and thus of contact lengths, was also investigated. For all placements, discrimination was poorer in the dorsal than in the palmar condition, owing to poorer cutaneous resolution on the dorsal side of the hand (the kinesthetic stimulation was the same in both conditions). Thus cutaneous stimulation is important. In both experiments, performance appeared to depend primarily on contact length. Moreover, the discrimination thresholds for all different placements and contact lengths followed the same trend. We conclude that in these experiments the effective stimulus for the discrimination of curved strips is the total difference of local surface attitude—that is, the slope difference over the far ends of the stimulus.     

A "unity assumption" does not promote intersensory integration

An account of intersensory integration is premised on knowing that different sensory inputs arise from the same object. Could, however, the combination of the inputs be impaired although the "unity assumption" holds? Forty observers viewed a square through a minifying (50%) lens while they simultaneously touched the square. Half could see and half could not see their haptic explorations of the square. Both groups, however, had reason to believe that they were touching and viewing the same square. Subsequent matches of the inspected square were mutually biased by touch and vision when the exploratory movements were visible. However, the matches were biased in the direction of the square's haptic size when observers could not see their exploratory movements. This impaired integration without the visible haptic explorations suggests that the unity assumption alone is not enough to promote intersensory integration.     

Influence of shape on haptic curvature perception

Curvature discrimination of hand-sized doubly curved surfaces by means of static touch was investigated. Stimuli consisted of hyperbolical, cylindrical, elliptical and spherical surfaces of various curvatures. In the first experiment subjects had to discriminate the curvature along a specified orientation (the discrimination orientation) of a doubly curved surface from a flat surface. The curvature to be discriminated was oriented either along the middle finger or across the middle finger of the right hand. Independent of the shape of the surface, thresholds were found to be about 1.6 times smaller along the middle finger than across the middle finger. Discrimination biases were found to be strongly influenced by the shape of the surface; subjects judged a curvature to be more convex when the perpendicular curvature was convex than when this curvature was concave. With the results of the second experiment it could be ruled out that the influence of shape on curvature perception was simply due to a systematic error made by the subject regarding the discrimination orientation.     

Roughness perception across the hands

It has previously been shown that the perceived roughness of a surface touched by one digit is influenced by the roughness of a different surface touched simultaneously by another digit on the same hand. The present study was designed to examine whether this is the case when surfaces of varying roughness are touched using digits on separate hands. Participants touched pairs of sandpaper surfaces, in sequence, using the same digit, and identified which of the two was rougher. Roughness discrimination was measured in the presence of distractor surfaces touched simultaneously with the target surface, but using a different digit either on the same or on the other hand. The overall perception of roughness of the attended surfaces was better on the left than on the right hand. Perceived roughness also varied systematically with the roughness of the distractor surfaces. Attended surfaces were more likely to be perceived as smoother when they were paired with smooth rather than rough distractors. Likewise, attended surfaces tended to be perceived as rougher with rough distractors. This pattern of results occurred whether the attended and distractor digits were on the same hand or different hands. These data confirm that it is difficult to restrict tactile attention for roughness to a single digit and show that this difficulty extends to restricting attention to a single hand. Furthermore, the effect of a stimulus at an unattended body location was not simply to impair perception in general, but to bias it in the roughness direction of the distractor surface.     

Haptic identification of curved surfaces

In two experiments, the active haptic identification of three-dimensional mathematically welldefined objects is investigated. The objects, quadric surfaces, are defined in terms of the shape index, a quantity describing the shape, and curvedness, a quantity describing overall curvature. Both shape index and curvedness are found to have a significant influence on haptic shape identification . Concave surfaces lead to a larger spread in responses than convex ones. Hyperbolic surfaces show a slight tendency to be identified with more difficulty than elliptic ones. Surfaces with a high curvedness are identified more easily than those with a low curvedness. Results from experiments with constant and with random curvedness are indistinguishable . It is concluded that shape index and curvedness are psychophysically not confounded.     

Touching matters: Embodiments of intimacy

Touch is, I propose, a foundational, “intercorporeal” form of intimacy. Such intercorporeal intimacy precedes developmentally and undergirds permanently the “intersubjective” intimacy that is possible between adult subjects. For, it is in the affective intimacy of touching and being touched that we first realize (i.e., make real, actualize) both a coexistence or participation with other bodies, and an organization and differentiation of ourselves as embodied beings. Section 1 lays out phenomena of interpersonal (and interanimal) relations that require thinking touch as much more than either the exploration of a physical surface by an embodied subject or a conventional form of communication: I note the powerful existential effects of being or not being touched. In Section 2, I recall philosopher Maurice Merleau-Ponty’s account of embodiment, focusing on features that provide resources for understanding touch. I argue that touching must be understood as potentially transformative of the toucher, that “being touched” can equally be transformative, and that touching and being touched are inherently intertwined. This intertwining and transformative power is what makes touch an intercorporeal form of intimacy and accounts for its ability to inaugurate and enliven, at the affective level, our sense of self as differentiated from and in relation to others.     

The visual perception of length along intrinsically curved surfaces

The ability of observers to perceive three-dimensional (3-D) distances or lengths along intrinsically curved surfaces was investigated in three experiments. Three physically curved surfaces were used: convex and/or concave hemispheres (Experiments 1 and 3) and a hyperbolic paraboloid (Experiment 2). The first two experiments employed a visual length-matching task, but in the final experiment the observers estimated the surface lengths motorically by varying the separation between their two index fingers. In general, the observers' judgments of surface length in both tasks (perceptual vs. motoric matching) were very precise but were not necessarily accurate. Large individual differences (overestimation, underestimation, etc.) in the perception of length occurred. There were also significant effects of viewing distance, type of surface, and orientation of the spatial intervals on the observers' judgments of surface length. The individual differences and failures of perceptual constancy that were obtained indicate that there is no single relationship between physical and perceived distances on 3-D surfaces that is consistent across observers.     

Sites of excess: The spatial politics of touch for drag queens in Aotearoa New Zealand

This paper explores the role of touch for drag queens. I examine the ways in which touch – being touched, touching others, and feelings associated with touch – is an important component of sexualised subjectivities and places. The first part of the paper reviews the recent literature called ‘haptic geographies’. I highlight why sexualised touch has, for the most part, been absent from this literature before bringing together the limited references made to touch, feelings, sexuality and place. In the second part of the paper I draw on various media, my involvement as a member of the queer community group Hamilton Pride Incorporated and in-depth interviews to examine the role of touch and feelings associated with touch for drag queens in Aotearoa New Zealand. I pay attention to the complex politics and performances of drag queens in order to highlight the co-construction of haptic geographies and sexualised subjectivities. I argue that drag queens’ bodies and spaces may be understood as sites of excess where the pleasures and pains of touch may form and break bodily and spatial boundaries associated with hetero/homo and masculine/feminine subjectivities. A focus on drag queens and touch queers our understandings of embodiment and haptic geographies further.     

Curvature affects haptic length perception

One possible way of haptically perceiving length is to trace a path with one’s index finger and estimate the distance traversed. Here, we present an experiment in which observers judge the lengths of paths across cylindrically curved surfaces. We found that convex and concave surfaces had qualitatively different effects: convex lengths were overestimated, whereas concave lengths were underestimated. In addition, we observed that the index finger moved more slowly across the convex surface than across the concave one. As a result, movement times for convex lengths were longer. The considerable correlation between movement times and length estimates suggests that observers take the duration of movement as their primary measure of perceived length, but disregard movement speeds. Several mechanisms that could underlie observers’ failure to account for speed differences are considered.     

The haptic perception of curvature

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 piano-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.     

The role of haptic exploration of ground surface information in perception of overhead reachability

The authors performed an experiment in which participants (N = 24) made judgments about maximum jump and reachability on ground surfaces with different elastic properties: sand and a trampoline. Participants performed judgments in two conditions: (a) while standing and after having recently jumped on the surface in question and (b) while standing on a third control surface, eliminating haptic exploration of the surface in question. There was a high correlation between perceived maximum reachable height and actual maximum reachable height in all conditions. Judging performance on the basis of visual and haptic exploration of ground surface information was slightly overestimated, whereas performance on the basis of visual information alone was underestimated and variable for the different surfaces. The authors discuss possible causes for the observed errors. They emphasize that there is a considerable nonvisual aspect to the nature of the information specifying affordances for overhead reach and jumping and that perceptual performance is degraded when spontaneous exploratory movement is restricted.     

The Bourdon illusion in haptic space

A strong Bourdon illusion-the apparent bentness of a straight edge-in the active haptic mode was established in two experiments. Blindfolded subjects clasped the opposite surfaces of an object with the same frontal profile as the visual figure between thumb and forefinger and moved the latter together from end to end across the object. When the two triangular components of the objects were rotated through 180 degrees so that their bases instead of their apexes were contiguous, the illusion was reversed in direction and reduced by about half, as with the visual illusion. It was concluded that the active haptic Bourdon illusion is basically the same as the visual illusion and can be accounted for by a compromise in perception between the orientation of the test surface and that of the object of which it is an integral part.     

Ordinal structure in the visual perception and cognition of smoothly curved surfaces

In theoretical analyses of visual form perception, it is often assumed that the 3-dimensional structures of smoothly curved surfaces are perceptually represented as point-by-point mappings of metric depth and/or orientation relative to the observer. This article describes an alternative theory in which it is argued that our visual knowledge of smoothly curved surfaces can also be defined in terms of local, nonmetric order relations. A fundamental prediction of this analysis is that relative depth judgments between any two surface regions should be dramatically influenced by monotonicity of depth change (or lack of it) along the intervening portions of the surface through which they are separated. This prediction is confirmed in a series of experiments using surfaces depicted with either shading or texture. Additional experiments are reported, moreover, that demonstrate that smooth occlusion contours are a primary source of information about the ordinal structure of a surface and that the depth extrema in between contours can be optically specified by differences in luminance at the points of occlusion.     

Power implications of touch in male—Female relationships

Nancy Henley argues that nonreciprocal touch in male—female relations is used by men as a status reminder to keep women in their place. This study examines Henley's argument by exposing 60 observers to photographs of male—female interactions and asking them to rate the pictured actors on the degree to which each dominates the interaction. The interactions differ across two dimensions: status differences evident in the age and dress of the participants (female higher vs. equal vs. male higher) and who is touching whom (female toucher vs. no toucher vs. male toucher). Results of the study support but qualify the status reminder argument. Nonreciprocal touch reduces the perceived power of the person being touched whether the high-status or the low-status person is doing the touching and whether the man or the woman is being touched. Thus, nonreciprocal touch can be used by high-status men to remind lower-status women of their subordinate positions. But it can also be used by lower status women to undermine the status claims of higher status men. In the equal status interactions, nonreciprocal touch does not alter power perceptions as systematically. This finding suggests that without other status cues evident in the relationship, touch alone is insufficient to establish a power advantage for either party.