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.     

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.     

Size constancy in extended haptic space

When a stick is used to extend the boundaries of the world perceived by touch, haptic parallels to visual angle and size constancy appear. In a pilot study, Ss using a stick to feel an out-of-reach gap were more likely to underestimate its size than were Ss feeling a nearby gap with the index finger, but size constancy was relatively high with the stick. The main study, comparing judgments of gaps at intermediate and far positions, confirmed these findings. Errors were greater for the more distant gap, but constancy was high in both positions. Correlations between far and intermediate judgments indicated that Ss were consistent in the type of error made. Similar findings emerged from a partial replication with blind and partially sighted Ss.     

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.     

Symmetry in haptic and in visual shape perception

Four experiments tested the hypothesis that bilateral symmetry is an incidental encoding property in vision, but can also be elicited as an incidental effect in touch, provided that sufficient spatial reference information is available initially for haptic inputs to be organized spatially. Experiment 1 showed that symmetry facilitated processing in vision, even though the task required judgments of stimulus closure rather than the detection of symmetry. The same task and stimuli failed to show symmetry effects in tactual scanning by one finger (Experiment 2). Experiment 3 found facilitating effects for vertically symmetric open stimuli, although not for closed patterns, in two-forefinger exploration when the fore-fingers had previously been aligned to the body midaxis to provide body-centered spatial reference. The one-finger exploration condition again failed to show symmetry effects. Experiment 4 replicated the facilitating effects of symmetry for open symmetric shapes in tactual exploration by the two (previously aligned) forefingers. Closed shapes again showed no effect. Spatial-reference information, finger movements, and stimulus factors in shape perception by touch are discussed.     

Salient features in 3-D haptic shape perception

Shape is an important cue for recognizing an object by touch. Several features, such as edges, curvature, surface area, and aspect ratio, are associated with 3-D shape. To investigate the saliency of 3-D shape features, we developed a haptic search task. The target and distractor items consisted of shapes (cube, sphere, tetrahedron, cylinder, and ellipsoid) that differed in several of these features. Exploratory movements were left as unconstrained as possible. Our results show that this type of haptic search task can be performed very efficiently (25 msec/item) and that edges and vertices are the most salient features. Furthermore, very salient local features, such as edges, can also be perceived through enclosure, an exploratory procedure usually associated with global shape. Since the subjects had to answer as quickly as possible, this suggests that speed may be a factor in selecting the appropriate exploratory procedure.     

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.     

Infants' haptic perception of object unity in rotating displays

Four-month-old infants were allowed to manipulate, without vision, two rings attached to a bar that permitted each ring to undergo rotary motion against a fixed surface. In different conditions, the relative motions of the rings were rigid, independent, or opposite, and they circled either the same fixed point outside the zone of manipulation or spatially separated points. Infants' perception of the ring assemblies were affected by the nature of the rotary motion in two ways. First, infants perceived a unitary object when the felt ends of the object underwent a common, rigid rotary motion; perception of object unity was stronger in this condition than when the ends underwent either independent or opposite rotary motions. Second, infants perceived two distinct objects when the felt ends of the objects underwent independent rotary motions that centred on distinct fixed points. Perception of the distinctness of the objects was less clear when the ends underwent opposite or independent rotary motions that centred on a common fixed point. These findings provide the first evidence that infants are sensitive to rotary motion patterns and can extrapolate a global pattern of rigid motion from the distinct, local velocities that they produce and experience at their two hands.     

Metamers in the haptic perception of heaviness and moveableness

It is hypothesized that heaviness perception for a freely wielded nonvisible object can be mapped to a point in a three-dimensional heaviness space. The three dimensions are mass, the volume of the inertia ellipsoid, and the symmetry of the inertia ellipsoid. Within this space, particular combinations yield heaviness metamers (objects of different mass that feel equally heavy), whereas other combinations yield analogues to the size-weight illusion (objects of the same mass that feel unequally heavy). Evidence for the two types of combinations was provided by experiments in which participants wielded occluded hand-held objects and estimated the heaviness of the objects relative to a standard. Further experiments with similar procedures showed that metamers of heaviness were metamers of moveableness but not metamers of length. A promising conjecture is that the haptic perceptual system maps the combination of an object's inertia for translation and inertia for rotation to a perception of the object's maneuverability.     

Anisotropy in the extended haptic perception of longitudinal distances

Using horizontal rotational motions of a hand-held rod, participants in four experiments probed the aperture between two blocks separated in depth. The farther block could be either on the outer or on the inner side of the hand-rod system, defining apertures of opposite sense. The participants reported the perceived size of the in-depth intervals by adjusting, with the nonprobing hand, the lateral separation between two blocks or the egocentric distances of two blocks. Over variations in hand and geometric arrangement of stimulus and report blocks, perceived aperture size depended on the aperture's sense: Apertures with the farther edge on the outer side of the hand-rod were reported as being larger than their mirror image counterparts. The effect was not observed in judgments of a single edge distance (Experiment 5); it was configuration dependent. The sense effect would not be expected from the physical quantity accommodating perceived frontal apertures. Possible expansions of the physical model are discussed.     

The role of pictorial convention in haptic picture perception

An investigation of tactile picture perception is reported. Blindfolded sighted subjects explored either 'line drawings' or 'textured' tactile pictures produced on Zytex swell paper. All pictures were 'two-dimensional', that is they depicted only one object face and so did not represent a third dimension. Both picture sets represented the same objects. Results revealed that the textured pictures, in which solid surfaces of depicted objects were uniformly textured, were recognised more often than tactile line drawings, in which surfaces of objects were simply bounded by lines. There were no significant correlations between imagery ability (visual, cutaneous, or kinaesthetic) and picture recognition success. Texture may be a form of 'uniform connectedness' (Palmer and Rock 1994 Psychonomic Bulletin & Review 1 29-55) or 'common region' (Palmer 1992 Cognitive Psychology 24 436-447), highlighting the global characteristics of stimuli. We argue that textured pictures may encourage the haptic system to take a more globally oriented approach to tactile picture perception, benefiting recognition.     

Application of the sensory-tonic theory of perception to figural after-effect

Current theoretical positions concerned with figural after-effects were criticized for their inadequacy in accounting for figural after-effects data and for their failure to link these phenomena with other perceptual phenomena in any systematic fashion. On the basis of the relative broadness of Gibson's early formulation, it was decided to adapt a similar adaptation theory, the sensory-tonic field theory of perception, to figural after-effect displacements. Appropriate hypotheses were stated and predictions made. These were tested and confirmed. It was concluded that although the success of the theory is limited, there appears to be hope for its extension.     

Visual short-term memory is influenced by haptic perception

The present study investigated whether and how visual memory and haptic perception are related. Participants were required to compare a visual reference velocity with a visual test velocity separated by a 4-s interval. During the retention interval, a fast or slow hand movement was performed. Although the hand movement was not visible, effects of the speed of the distracting body movement occurred. Slow movements resulted in a lowering of the represented visual velocity, whereas fast movements heightened the represented velocity. Subsequent experiments extended the effect to body movements that differed from the visual motion and ruled out the possibility that the effect was due to changes in visual perception or interference from semantic, verbal, and acoustic memory codes. Perhaps haptic velocity information and visual velocity information stored in short-term memory are blended.     

Visual size perception and haptic calibration during development

It is still unclear how the visual system perceives accurately the size of objects at different distances. One suggestion, dating back to Berkeley’s famous essay, is that vision is calibrated by touch. If so, we may expect different mechanisms involved for near, reachable distances and far, unreachable distances. To study how the haptic system calibrates vision we measured size constancy in children (from 6 to 16 years of age) and adults, at various distances. At all ages, accuracy of the visual size perception changes with distance, and is almost veridical inside the haptic workspace, in agreement with the idea that the haptic system acts to calibrate visual size perception. Outside this space, systematic errors occurred, which varied with age. Adults tended to overestimate visual size of distant objects (over‐compensation for distance), while children younger than 14 underestimated their size (under‐compensation). At 16 years of age there seemed to be a transition point, with veridical perception of distant objects. When young subjects were allowed to touch the object inside the haptic workspace, the visual biases disappeared, while older subjects showed multisensory integration. All results are consistent with the idea that the haptic system can be used to calibrate visual size perception during development, more effectively within than outside the haptic workspace, and that the calibration mechanisms are different in children than in adults.     

Visual and haptic perception of the horizontal-vertical illusion

The horizontal-vertical illusion consists of two lines of the same length (one horizontal and the other vertical) at a 90 degree angle from one another forming either an inverted-T or an L-shape. The illusion occurs when the length of a vertical line is perceived as longer than the horizontal line even though they are the same physical length. The illusion has been shown both visually and haptically. The present purpose was to assess differences between the visual or haptic perception of the illusions and also whether differences occur between the inverted-T and the L-shape illusions. The current study showed a greater effect in the haptic perception of the horizontal-vertical illusion than in visual perception. There is also greater illusory susceptibility of the inverted-T than the L-shape.