How robust are the deviations in haptic parallelity?

Several studies have shown that physically parallel bars do not feel parallel and vice versa. The most plausible cause of this deviation is the biasing influence of an egocentric reference frame. The aim of the present study was to assess the strength of this egocentric contribution. The deviations from veridicality were measured in six experiments where subjects were presented with either haptic or visual information about parallelity or their deviations. It was found that even direct error feedback (either haptically or visually) did not even nearly result in veridical performance. The improvements found were attributed to a shift in focus towards a more allocentric reference frame, possibly reflecting the same mechanisms as found in delay and noninformative vision studies. We conclude that the illusionary percept of haptic parallelity is rather robust and is indeed caused by a strong reliance on an egocentric reference frame.     

Effects of visual information regarding allocentric processing in haptic parallelity matching

Research has revealed that haptic perception of parallelity deviates from physical reality. Large and systematic deviations have been found in haptic parallelity matching most likely due to the influence of the hand-centered egocentric reference frame. Providing information that increases the influence of allocentric processing has been shown to improve performance on haptic matching. In this study allocentric processing was stimulated by providing informative vision in haptic matching tasks that were performed using hand- and arm-centered reference frames.     

Performance in haptic geometrical matching tasks depends on movement and position of the arms

Previous research on the properties of haptic space has shown systematic deviations from Euclidean parallelity in haptic parallelity tasks. The mainstream explanation for these deviations is that, in order to perform the task, participants generate a spatial representation with a frame of reference that integrates egocentric and allocentric components. Several studies have shown that the amount and type of deviations are affected by the configurations with regard to the arms and the rods to be matched. The present study reports 4 experiments that further address the effects of task configurations and body movements. Experiments 1 and 2 replicate and extend previous results concerning haptic matching task and acoustic pointing tasks. The third experiment includes acoustic cues aligned differentially to the reference and test bars. The fourth experiment concerns a geometrical matching task performed in the rear peripersonal space. Results show that haptic deviations from the Euclidean space are modulated by the available cues and by the body configurations. This indicates the need for further analysis on the role of body, arm and shoulder positions, and movement effects in haptic space perception.     

Large systematic deviations in a bimanual parallelity task: further analysis of contributing factors

Previous studies showed that what subjects haptically perceive as parallel deviates largely from what is actually physically parallel [Perception 28 (1999) 1001; Acta Psychol. 109 (2002) 25; Perception 28 (1999) 781]. It also turned out that the deviations were strongly subject-dependent. It was hypothesized that what is haptically parallel is decided in a frame of reference intermediate to an allocentric and an egocentric one. The purposes of the present study were to collect more evidence for this hypothesis and to investigate the factor(s) that determines the specific weighting between the two reference frames. We found a highly significant reversal of a haptic oblique effect (in context: larger systematic deviations for oblique orientations) for subjects with large deviations. This reversal provides convincing evidence that an intermediate frame of reference is used for the decision of haptic parallelity. Contrary to common expectation, several factors that might have been of influence on the weighting of the two frames of reference, such as arm length, arm span, shoulder width, turned out to be irrelevant. Surprisingly, the only factors that seem to be of influence are gender and job experience or education.     

Egocentric reference frame bias in the palmar haptic perception of surface orientation

The effect of egocentric reference frames on palmar haptic perception of orientation was investigated in vertically separated locations in a sagittal plane. Reference stimuli to be haptically matched were presented either haptically (to the contralateral hand) or visually. As in prior investigations of haptic orientation perception, a strong egocentric bias was found, such that haptic orientation matches made in the lower part of personal space were much lower (i.e., were perceived as being higher) than those made at eye level. The same haptic bias was observed both when the reference surface to be matched was observed visually and when bimanual matching was used. These findings support the conclusion that, despite the presence of an unambiguous allocentric (gravitational) reference frame in vertical planes, haptic orientation perception in the sagittal plane reflects an egocentric bias.     

Haptic space processing--allocentric and egocentric reference frames.

In this paper a haptic matching, task is used to analyze haptic spatial processing. In various conditions, blindfolded participants were asked to make a test bar parallel to a reference bar. This always resulted in large but systematic deviations. It will be shown that the results can be described with a model in which an egocentric reference frame biases the participants' settings: What a participant haptically perceives as parallel is a weighted average of parallel in allocentric space and parallel in egocentric space. The basis of the egocentric reference frame is uncertain. There is strong evidence that at least a hand-centred reference frame is involved, but possibly a body-centred reference frame also plays a role.     

Haptic perception of parallelity in the midsagittal plane.

Previous studies [Perception 28 (1999) 1001; Perception 28 (1999) 781] on the haptic perception of parallelity on a horizontal plane showed that what subjects haptically perceive as being parallel deviates considerably from what is physically parallel. The deviations could be described with a subject-dependent orientation gradient in the left-right direction. The gradients found in the bimanual conditions were significantly larger (about 70%) than those in the unimanual conditions. The questions to be answered in the present study are the following: (1) Does the haptic perception of parallelity in the midsagittal plane also show systematic deviations from veridicality? (2) Are the unimanual and bimanual performances again quantitatively but not qualitatively different? The set-up consisted of a plate positioned in the midsagittal plane of the subject. The subject touched the right side of the plate with his/her right hand and the left side with the left hand. The results show again large systematic deviations. The major part of the deviations can be described by means of a subject-dependent orientation gradient in the vertical direction. The quantitative (but not qualitative) difference between the unimanual and the bimanual conditions is much larger in the midsagittal plane than in the horizontal plane.     

Haptic experiences influence visually acquired memories: Reference frames during multimodal spatial learning

In two experiments, we investigated whether reference frames acquired through touch could influence memories for locations learned through vision. Participants learned two objects through touch, and haptic egocentric (Experiment 1) and environmental (Experiment 2) cues encouraged selection of a specific reference frame. Participants later learned eight new objects through vision. Haptic cues were manipulated, whereas visual learning was held constant in order to observe any potential influence of the haptically experienced reference frame on memories for visually learned locations. When the haptically experienced reference frame was defined primarily by egocentric cues, cue manipulation had no effect on memories for objects learned through vision. Instead, visually learned locations were remembered using a reference frame selected from the visual study perspective. When the haptically experienced reference frame was defined by both egocentric and environmental cues, visually learned objects were remembered in the context of the haptically experienced reference frame. These findings support the common reference frame hypothesis, which proposes that locations learned through different sensory modalities are represented within a common reference frame.     

Reference frames and haptic perception of orientation: Body and head tilt effects on the oblique effect

The aim of this study was to examine the effect of body and head tilts on the haptic oblique effect. This effect reflects the more accurate processing of vertical and horizontal orientations, relative to oblique orientations. Body or head tilts lead to a mismatch between egocentric and gravitational axes and indicate whether the haptic oblique effect is defined in an egocentric or a gravitational reference frame. The ability to reproduce principal (vertical and horizontal) and oblique orientations was studied in upright and tilted postures. Moreover, by controlling the deviation of the haptic subjective vertical provoked by postural tilt, the possible role of a subjective gravitational reference frame was tested. Results showed that the haptic reproduction of orientations was strongly affected by both the position of the body (Experiment 1) and the position of the head (Experiment 2). In particular, the classical haptic oblique effect observed in the upright posture disappeared in tilted conditions, mainly because of a decrease in the accuracy of the vertical and horizontal settings. The subjective vertical appeared to be the orientation reproduced the most accurately. These results suggest that the haptic oblique effect is not purely gravitationally or egocentrically defined but, rather, depends on a subjective gravitational reference frame that is tilted in a direction opposite to that of the head in tilted postures (Experiment 3).     

Reference frames and haptic perception of orientation: body and head tilt effects on the oblique effect.

The aim of this study was to examine the effect of body and head tilts on the haptic oblique effect. This effect reflects the more accurate processing of vertical and horizontal orientations, relative to oblique orientations. Body or head tilts lead to a mismatch between egocentric and gravitational axes and indicate whether the haptic oblique effect is defined in an egocentric or a gravitational reference frame. The ability to reproduce principal (vertical and horizontal) and oblique orientations was studied in upright and tilted postures. Moreover, by controlling the deviation of the haptic subjective vertical provoked by postural tilt, the possible role of a subjective gravitational reference frame was tested. Results showed that the haptic reproduction of orientations was strongly affected by both the position of the body (Experiment 1) and the position of the head (Experiment 2). In particular, the classical haptic oblique effect observed in the upright posture disappeared in tilted conditions, mainly because of a decrease in the accuracy of the vertical and horizontal settings. The subjective vertical appeared to be the orientation reproduced the most accurately. These results suggest that the haptic oblique effect is not purely gravitationally or egocentrically defined but, rather, depends on a subjective gravitational reference frame that is tilted in a direction opposite to that of the head in tilted postures (Experiment 3).     

The contributions of egocentric and allocentric reference frames in haptic spatial tasks

The influence of egocentric and allocentric reference frames on performance in haptic spatial tasks, was tested in three conditions. Blindfolded subjects had to make two bars haptically parallel, perpendicular or mirrored in the midsagittal plane. The hypothesis is that the contributions of egocentric and allocentric reference frames are combined, resulting in settings that lie in between the allo-representation and the ego-representation. This leads to different predictions for the outcome of different conditions. All findings were consistent with the hypothesis. In addition, for subjects with large deviations a reversal of the oblique effect was found once again, which provides extra support for the hypothesis.     

Haptically straight lines

In this research, we set out to investigate haptically perceived space. Large deviations with respect to physical space have already been shown to exist. Here, research on haptic space is continued by investigating straight lines constructed by touch. In four experiments, subjects were asked to produce straight lines between two reference markers that were in the horizontal plane at a fixed distance from each other. Each experiment corresponded to a different task: two different interpolation tasks, an intersection task, and a pointing task. Straight lines had an orientation that was approximately frontoparallel. Subjects used both hands; manipulation was unrestricted. Although we found considerable differences between observers, the overall pattern of results showed that haptically straight lines were generally curved away from the observer. However, in one of the interpolation tasks they corresponded to physically straight lines. In addition, the pointing task generally produced larger deviations than the other three tasks. Taken together, the results show that there is no unique definition of the straight line, a conclusion that questions the viability of the concept of haptic space.     

The Müller-Lyer illusion in touch and vision: implications for multisensory processes

In six experiments, we used the Müller-Lyer illusion to investigate factors in the integration of touch, movement, and spatial cues in haptic shape perception, and in the similarity with the visual illusion. Latencies provided evidence against the hypothesis that scanning times explain the haptic illusion. Distinctive fin effects supported the hypothesis that cue distinctiveness contributes to the illusion, but showed also that it depends on modality-specific conditions, and is not the main factor. Allocentric cues from scanning an external frame (EF) did not reduce the haptic illusion. Scanning elicited downward movements and more negative errors for horizontal convergent figures and more positive errors for vertical divergent figures, suggesting a modality-specific movement effect. But the Müller-Lyer illusion was highly significant for both vertical and horizontal figures. By contrast, instructions to use body-centered reference and to ignore the fins reduced the haptic illusion for vertical figures in touch from 12.60% to 1.7%. In vision, without explicit egocentric reference, instructions to ignore fins did not reduce the illusion to near floor level, though external cues were present. But the visual illusion was reduced to the same level as in touch with instructions that included the use of body-centered cues. The new evidence shows that the same instructions reduced the Müller-Lyer illusion almost to zero in both vision and touch. It suggests that the similarity of the illusions is not fortuitous. The results on touch supported the hypothesis that body-centered spatial reference is involved in integrating inputs from touch and movement for accurate haptic shape perception. The finding that explicit egocentric reference had the same effect on vision suggests that it may be a common factor in the integration of disparate inputs from multisensory sources.     

Effects of context on a visual 3-D pointing task

We examined the effects of egocentric and contextual references on a 3-D exocentric pointing task. Large systematic deviations were found for the slant (angle in the horizontal plane). For most observers, the deviations were smaller when the veridical pointing direction was parallel to a wall. For some observers the size of the deviations was also dependent on whether the veridical pointing direction was frontoparallel or not. For the tilt (angle in the vertical plane), the deviations were smaller and less systematic. Hence, although observers show comparable systematic deviations, the way in which the presence of structure in an environment is used for judging positions of objects is observer-dependent.     

Reference frame preferences in haptics differ for the blind and sighted in the horizontal but not in the vertical plane

We investigated which reference frames are preferred when matching spatial language to the haptic domain. Sighted, low-vision, and blind participants were tested on a haptic-sentence-verification task where participants had to haptically explore different configurations of a ball and a shoe and judge the relation between them. Results from the spatial relation "above", in the vertical plane, showed that various reference frames are available after haptic inspection of a configuration. Moreover, the pattern of results was similar for all three groups and resembled patterns found for the sighted on visual sentence-verification tasks. In contrast, when judging the spatial relation "in front", in the horizontal plane, the blind showed a markedly different response pattern. The sighted and low-vision participants did not show a clear preference for either the absolute/relative or the intrinsic reference frame when these frames were dissociated. The blind, on the other hand, showed a clear preference for the intrinsic reference frame. In the absence of a dominant cue, such as gravity in the vertical plane, the blind might emphasise the functional relationship between the objects owing to enhanced experience with haptic exploration of objects.     

Two reference frames for visual perception in two gravity conditions

The processing and storage of visual information concerning the orientation of objects in space is carried out in anisotropic reference frames in which all orientations are not treated equally. The perceptual anisotropies, and the implicit reference frames that they define, are evidenced by the observation of 'oblique effects' in which performance on a given perceptual task is better for horizontally and vertically oriented stimuli. The question remains how the preferred horizontal and vertical reference frames are defined. In these experiments cosmonaut subjects reproduced the remembered orientation of a visual stimulus in 1g (on the ground) and in 0g, both attached to a chair and while free-floating within the International Space Station. Results show that while the remembered orientation of a visual stimulus may be stored in a multimodal reference frame that includes gravity, an egocentric reference is sufficient to elicit the oblique effect when all gravitational and haptic cues are absent.     

The structure of frontoparallel haptic space is task dependent

In three experiments, we investigated the structure of frontoparallel haptic space. In the first experiment, we asked blindfolded participants to rotate a matching bar so that it felt parallel to the reference bar, the bars could be at various positions in the frontoparallel plane. Large systematic errors were observed, in which orientations that were perceived to be parallel were not physically parallel. In two subsequent experiments, we investigated the origin of these errors. In Experiment 2, we asked participants to verbally report the orientation of haptically presented bars. In this task, participants made errors that were considerably smaller than those made in Experiment 1. In Experiment 3, we asked participants to set bars in a verbally instructed orientation, and they also made errors significantly smaller than those observed in Experiment 1. The data suggest that the errors in the matching task originate from the transfer of the reference orientation to the matching-bar position.