Haptic discrimination of bilateral symmetry in 2-dimensional and 3-dimensional unfamiliar displays

In five experiments, we tested the accuracy and sensitivity of the haptic system in detecting bilateral symmetry of raised-line shapes (Experiments 1 and 2) and unfamiliar 3-D objects (Experiments 3–5) under different time constraints and different modes of exploration. Touch was moderately accurate for detecting this property in raised displays. Experiment 1 showed that asymmetric judgments were systematically more accurate than were symmetric judgments with scanning by one finger. Experiment 2 confirmed the results of Experiment 1 but also showed that bimanual exploration facilitated processing of symmetric shapes without improving asymmetric detections. Bimanual exploration of 3-D objects was very accurate and significantly facilitated processing of symmetric objects under different time constraints (Experiment 3). Unimanual exploration did not differ from bimanual exploration (Experiment 4), but restricting hand movements to one enclosure reduced performance significantly (Experiment 5). Spatial reference information, signal detection measures, and hand movements in processing bilateral symmetry by touch are discussed.     

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.     

THREE-DIMENSIONAL BILATERAL SYMMETRY BIAS IN JUDGMENTS OF FIGURAL IDENTITY AND ORIENTATION

Abstract— The two experiments reported explored a bias toward symmetry in judging identity and orientation of indeterminate two-dimensional shapes. Subjects viewed symmetric and asymmetric filled, random polygons and described, "what each figure looks like" and its orientation. Viewers almost universally interpreted the shapes as silhouettes of bilaterally symmetric three-dimensional (3-D) objects. This assumption of 3-D symmetry tended to constrain perceived vantage of the identified objects such that symmetric shapes were interpreted as straight-on views, and asymmetric shapes as profile or oblique views. Because most salient objects in the world are bilaterally symmetric, these findings are consistent with the view that assuming 3-D symmetry can be a robust heuristic for constraining orientation when identifying objects from indeterminate patterns.     

Discrimination thresholds for haptic perception of volume, surface area, and weight

The present study investigated the human ability to discriminate the size of 3-D objects by touch. Experiment 1 measured the just noticeable differences (JNDs) for three tasks: (1) discrimination of volume without availability of weight information, (2) discrimination of volume with weight information available, and (3) discrimination of surface area. Stimuli consisted of spheres, cubes, and tetrahedrons. For all shapes, two reference sizes were used (3.5 and 12 cm(3)). No significant effect of task on the discriminability of objects was found, but the effects of shape and size were significant, as well as the interaction between these two factors. Post hoc analysis revealed that for the small reference, the Weber fractions for the tetrahedron were significantly larger than the fractions for the cube and the sphere. In Experiment 2, the JNDs for haptic perception of weight were measured for the same objects as those used in Experiment 1. The shape of objects had no significant effect on the Weber fractions for weight, but the Weber fractions for the small stimuli were larger than the fractions for the large stimuli. Surprisingly, a comparison between the two experiments showed that the Weber fractions for weight were significantly larger than the fractions for volume with availability of weight information. Taken together, the results reveal that volume and weight information are not effectively combined in discrimination tasks. This study provides detailed insight into the accuracy of the haptic system in discriminating objects' size. This substantial set of data satisfies the need for more fundamental knowledge on haptic size perception, necessary for a greater understanding of the perception of related properties, as well as of more general perceptual processes.     

The visual and haptic perception of natural object shape

In this study, we evaluated observers' ability to compare naturally shaped three-dimensional (3-D) objects, using their senses of vision and touch. In one experiment, the observers haptically manipulated 1 object and then indicated which of 12 visible objects possessed the same shape. In the second experiment, pairs of objects were presented, and the observers indicated whether their 3-D shape was the same or different. The 2 objects were presented either unimodally (vision-vision or haptic-haptic) or cross-modally (vision-haptic or haptic-vision). In both experiments, the observers were able to compare 3-D shape across modalities with reasonably high levels of accuracy. In Experiment 1, for example, the observers' matching performance rose to 72% correct (chance performance was 8.3%) after five experimental sessions. In Experiment 2, small (but significant) differences in performance were obtained between the unimodal vision-vision condition and the two cross-modal conditions. Taken together, the results suggest that vision and touch have functionally overlapping, but not necessarily equivalent, representations of 3-D shape.     

Partial perceptual equivalence between vision and touch for texture information

The present study examined the extent to which vision and touch are perceptually equivalent for texture information in adults. Using Garbin's method, we selected two sets of textures having high versus low cross-modal dissimilarity values between vision and touch (Experiment 1). The two sets of textures were then used as material in a cross-modal matching task (Experiment 2). Results showed that asymmetries occurred in the performances when the stimuli had high cross-modal dissimilarity values, but not when the stimuli had low cross-modal dissimilarity values. These results extend Garbin's findings on shape information to the texture domain and support the idea that partial perceptual equivalence exists between vision and touch.     

Recognizing familiar objects by hand and foot: Haptic shape perception generalizes to inputs from unusual locations and untrained body parts

The limits of generalization of our 3-D shape recognition system to identifying objects by touch was investigated by testing exploration at unusual locations and using untrained effectors. In Experiments 1 and 2, people found identification by hand of real objects, plastic 3-D models of objects, and raised line drawings placed in front of themselves no easier than when exploration was behind their back. Experiment 3 compared one-handed, two-handed, one-footed, and two-footed haptic object recognition of familiar objects. Recognition by foot was slower (7 vs. 13 s) and much less accurate (9 % vs. 47 % errors) than recognition by either one or both hands. Nevertheless, item difficulty was similar across hand and foot exploration, and there was a strong correlation between an individual’s hand and foot performance. Furthermore, foot recognition was better with the largest 20 of the 80 items (32 % errors), suggesting that physical limitations hampered exploration by foot. Thus, object recognition by hand generalized efficiently across the spatial location of stimuli, while object recognition by foot seemed surprisingly good given that no prior training was provided. Active touch (haptics) thus efficiently extracts 3-D shape information and accesses stored representations of familiar objects from novel modes of input.     

Learning to perceive differences in solid shape through vision and touch

A single experiment was designed to investigate perceptual learning and the discrimination of 3-D object shape. Ninety-six observers were presented with naturally shaped solid objects either visually, haptically, or across the modalities of vision and touch. The observers' task was to judge whether the two sequentially presented objects on any given trial possessed the same or different 3-D shapes. The results of the experiment revealed that significant perceptual learning occurred in all modality conditions, both unimodal and cross-modal. The amount of the observers' perceptual learning, as indexed by increases in hit rate and d', was similar for all of the modality conditions. The observers' hit rates were highest for the unimodal conditions and lowest in the cross-modal conditions. Lengthening the inter-stimulus interval from 3 to 15 s led to increases in hit rates and decreases in response bias. The results also revealed the existence of an asymmetry between two otherwise equivalent cross-modal conditions: in particular, the observers' perceptual sensitivity was higher for the vision-haptic condition and lower for the haptic-vision condition. In general, the results indicate that effective cross-modal shape comparisons can be made between the modalities of vision and active touch, but that complete information transfer does not occur.     

The dynamic-stimulus advantage of visual symmetry perception

It has been speculated that visual symmetry perception from dynamic stimuli involves mechanisms different from those for static stimuli. However, previous studies found no evidence that dynamic stimuli lead to active temporal processing and improve symmetry detection. In this study, four psychophysical experiments investigated temporal processing in symmetry perception using both dynamic and static stimulus presentations of dot patterns. In Experiment 1, rapid successive presentations of symmetric patterns (e.g., 16 patterns per 853 ms) produced more accurate discrimination of orientations of symmetry axes than static stimuli (single pattern presented through 853 ms). In Experiments 2-4, we confirmed that the dynamic-stimulus advantage depended upon presentation of a large number of unique patterns within a brief period (853 ms) in the dynamic conditions. Evidently, human vision takes advantage of temporal processing for symmetry perception from dynamic stimuli.     

Visual symmetry recognition by pigeons

Summary Pigeons learned to discriminate a large number of bilateral symmetric and asymmetric visual patterns successively projected on the pecking-key of an operant conditioning chamber. Responses to the positive stimuli were reinforced according to a variable interval schedule. Once acquisition was complete generalization trials, involving sets of new stimuli, were instituted under extinction. The birds classified these novel test stimuli with high accuracy throughout, according to their symmetry or asymmetry. Their performance was not disturbed by sets of test stimuli whose geometrical style differed considerably from the training stimuli. Pigeons were even able to discriminate when only allowed the use of one eye. The generalization series were partly designed to test some classical symmetry recognition theories. None was found to be adequate. Subsidiary experiments suggested that most pigeons have a slight spontaneous preference for asymmetric patterns and that symmetry/asymmetry differences can aid pattern discrimination learning at an early stage. It is concluded that pigeons, much like humans, can discriminate bilaterally symmetric from non-symmetric visual forms in a concept-like, generalizing way. The ontogenetic and phylogenetic development of this competence is considered. A novel symmetry recognition hypothesis based on spatial frequency analysis and neuronal feature-detector considerations is proposed.     

Symmetry and elongation of objects influence perceived direction of translational motion

Five experiments were conducted to examine how perceived direction of motion is influenced by aspects of shape of a moving object such as symmetry and elongation. Random polygons moving obliquely were presented on a computer screen and perceived direction of motion was measured. Experiments 1 and 2 showed that a symmetric object moving off the axis of symmetry caused motion to be perceived as more aligned with the axis than it actually was. However, Experiment 3 showed that motion did not influence perceived orientation of symmetry axis. Experiment 4 revealed that symmetric shapes resulted in faster judgments on direction of motion than asymmetric shapes only when the motion is along the axis. Experiment 5 showed that elongation causes a bias in perceived direction of motion similar to effects of symmetry. Existence of such biases is consistent with the hypothesis that in the course of evolution, the visual system has been adapted to regularities of motion in the animate world.     

Preference for symmetry is experience dependent in newborn chicks (Gallus gallus)

Spontaneous pecking preferences toward symmetric or asymmetric stimuli were tested in newborn chicks (Gallus gallus). A preference for asymmetric patterns was found in na?ve chicks (either 24 or 48 hours old), although a preference for symmetry appeared at retest after chicks had experienced standard rearing conditions (Experiments 1 and 2). Only food-experienced chicks preferred symmetric patterns; food-deprived and hand-fed chicks did not show any preference (Experiment 3). A key factor that allowed for the emergence of a preference for symmetry may relate to the improving of pecking sensorimotor skills occurring during active food manipulation. Possible explanations are discussed for the late emergence of the preference for symmetry and for the preference for asymmetry found in na?ve chicks.     

Visual and tactile memory for 2-D patterns: Effects of changes in size and left-right orientation

Visual identification of 3-D objects depends on representations that are invariant across changes in size and left-right orientation. We examined whether this finding reflects the unique demands of processing 3-D objects, or whether it generalizes to 2-D patterns and to the tactile modality. Our findings suggest that object representation for identification is influenced greatly by the processing demands of stimulus materials (e.g., 2-D vs. 3-D objects) and stimulus modality (touch vs. vision). Identification of 2-D patterns in vision is adversely affected by left-right orientation changes, but not size changes. Identification of the same patterns in touch is adversely affected by both changes. Together, the results suggest that the unique processing demands of stimulus materials and modality shape the representation of objects in memory.     

Symmetry brings an impression of familiarity but does not improve recognition memory

Studies have shown that symmetric stimuli are recognized better than asymmetric stimuli but evidence suggests that this advantage may result from a familiarity bias induced by symmetry. We used a classic episodic memory paradigm to test this bias and see if it truly accounts for the symmetry advantage. Subjects first encoded symmetric and asymmetric figures. During a subsequent recognition phase, they discriminated the encoded (old) figures from new intermixed figures. The recognition rate of old figures was higher with symmetric figures than asymmetric figures. However, the tendency to falsely recognize new figures was also higher when they were symmetric, meaning that the higher recognition rate for symmetric figures was artificially inflated by a response bias. Three other experiments further tested this finding and examined the influence of some variables (rotation in virtual 3D space, stimulus meaningfulness, and redundancy of information) on the bias. A fifth experiment with photo stimuli confirmed that the response bias also applies to objects that we regularly encounter in everyday life. In conclusion, our results show that symmetry does not enhance mnemonic processes but instead induces a response bias leading individuals to judge such stimuli as having been seen.     

Effects of surface pre-presentation on symmetry detection on a 3-D bumpy surface

Abstract:  The aim of this study was to investigate the visual detection mechanism of mirror symmetrical dot patterns drawn on a 3-D bumpy surface (i.e., 3-D noisy curved surface). The focus of this investigation was whether symmetry detection on the bumpy surface is associated with structure recovery of the surface. In Experiment 1, we confirmed that the human visual system can distinguish the bumpy surface from a distorted dot pattern on a transparent bumpy surface. In Experiment 2, we required participants to discriminate between a symmetrical pattern on a transparent (non-visible) bumpy surface and an opaque (visible) bumpy surface for the same stimuli as in Experiment 1. Finally, in Experiment 3, we examined the effect of pre-presentation of the opaque bumpy surface on pattern discrimination. The results showed that pre-presentation of the opaque surface facilitated the detection of a diagonal symmetry, but not in the case of the detection of a cardinal symmetry. These results suggested that diagonal symmetry involves the process of surface recovery, whereas cardinal symmetry does not.     

Peak Shift on an Artificial Dimension

Blough (1975) proposed an elemental model of generalization and discrimination phenomena in which stimuli from physical dimensions, such as wavelength, are conceptualized as overlapping sets of elements. In order to test predictions generated by this model, we constructed an artificial 'dimension' of stimuli as a series of overlapping sets of arbitrary 'icons' (small, unrelated shapes). In Experiment 1, we trained pigeons to discriminate two neighbouring stimuli from this artificial dimension and then assessed their responses to other stimuli from the dimension. The results of these tests typified those obtained with stimuli from genuine physical dimensions-that is, we obtained positive and negative peak shifts. In Experiment 2, human subjects were given a similar discrimination task and produced an analogous set of results to those of the pigeons. These results support Blough's elemental theory and are not readily accommodated by Pearce's (1987) configural theory. In Experiment 3, both humans and pigeons were trained and tested with stimuli from a real physical dimension (luminance). The pigeons' results were again consistent with Blough's analysis, but those of the humans suggested the use of more sophisticated strategies that are unavailable to pigeons and that lie outside the scope of elemental models of discrimination learning.     

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.