Haptic Oblique Effect Research Articles

Haptic two-dimensional angle categorization and discrimination

This study examined the extent to which haptic perception of two-dimensional (2-D) shape is modified by the design of the perceptual task (single-interval categorization vs. two-interval discrimination), the orientation of the angles in space (oblique vs. horizontal), and the exploration strategy (one or two passes over the angle). Subjects (n = 12) explored 2-D angles using the index finger of the outstretched arm. In the categorization task, subjects scanned individual angles, categorizing each as "large" or "small" (2 angles presented in each block of trials; range 80° vs. 100° to 89° vs. 91°; implicit standard 90°). In the discrimination task, a pair of angles was scanned (standard 90°; comparison 91-103°) and subjects identified the larger angle. The threshold for 2-D angle categorization was significantly lower than for 2-D angle discrimination, 4° versus 7.2°. Performance in the categorization task did not vary with either the orientation of the angles (horizontal vs. oblique, 3.9° vs. 4°) or the number of passes over the angle (1 vs. 2 passes, 3.9° vs. 4°). We suggest that the lower threshold with angle categorization likely reflects the reduced cognitive demands of this task. We found no evidence for a haptic oblique effect (higher threshold with oblique angles), likely reflecting the presence of an explicit external frame of reference formed by the intersection of the two bars forming the 2-D angles. Although one-interval haptic categorization is a more sensitive method for assessing 2-D haptic angle perception, perceptual invariances for exploratory strategy and angle orientation were, nevertheless, task-independent.

Haptic orientation perception benefits from visual experience: Evidence from early-blind, late-blind, and sighted people

Early-blind, late-blind, and blindfolded sighted participants were presented with two haptic allocentric spatial tasks: a parallel-setting task, in an immediate and a 10-sec delay condition, and a task in which the orientation of a single bar was judged verbally. With respect to deviation size, the data suggest that mental visual processing filled a beneficial role in both tasks. In the parallel-setting task, the early blind performed more variably and showed no improvement with delay, whereas the late blind did improve, but less than the sighted did. In the verbal judgment task, both early- and late-blind participants displayed larger deviations than the sighted controls. Differences between the groups were absent or much weaker with respect to the haptic oblique effect, a finding that reinforces the view that this effect is not of visual origin. The role of visual processing mechanisms and visual experience in haptic spatial tasks is discussed.

The haptic perception of spatial orientations.

This review examines the isotropy of the perception of spatial orientations in the haptic system. It shows the existence of an oblique effect (i.e., a better perception of vertical and horizontal orientations than oblique orientations) in a spatial plane intrinsic to the haptic system, determined by the gravitational cues and the cognitive resources and defined in a subjective frame of reference. Similar results are observed from infancy to adulthood. In 3D space, the haptic processing of orientations is also anisotropic and seems to use both egocentric and allocentric cues. Taken together, these results revealed that the haptic oblique effect occurs when the sensory motor traces associated with exploratory movement are represented more abstractly at a cognitive level.

Reference frame and haptic discrimination of orientations in infants

Body-tilt effect on the haptic discrimination of orientations and the 'oblique effect' (better discrimination of the vertical orientation than of an oblique orientation) were examined in 5-month-old infants. Body tilt leads to a mismatch between egocentric and gravitational reference frames and indicates in which reference frame orientations and oblique effect are defined. A familiarization/reaction to novelty procedure was used in upright body and tilted body conditions. Results revealed the occurrence of a haptic oblique effect in the upright body position, which disappeared when the body was tilted. The results suggest that spatial orientations and the oblique effect depend on a mixed reference frame that integrates not only gravitational information but also egocentric information.

Effects of Hand Orientation and Delay on the Verbal Judgment of Haptically Perceived Orientation

We examined the haptic perception of orientations of a single bar throughout the horizontal plane using a verbal response: participants were to assign a number of minutes to the orientation of a bar defined with respect to the stimulus table. Performance was found to be systematically biased. Deviations were consistent with, yet much smaller than, those resulting from haptic motor matching tasks. The size and direction of the deviations were found to correlate with hand orientation, and not to depend on spatial location per se, suggesting a role for hand-centred reference frames in biasing performance. Delaying the response by 10 s led to a small improvement only of right-hand perceptions, indicating different hemispheric involvement in processes involved in retaining and/or recoding of haptic orientation information. Also the haptic oblique effect was found with the current verbal response. Importantly, it was affected neither by hand orientation nor by delay, suggesting that the oblique effect is independent of the aforementioned deviations in orientation 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 this 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.

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).

Role of memorization conditions in the haptic processing of orientations and the 'oblique effect'.

The haptic processing of vertical, horizontal, 45 degrees and 135 degrees oblique orientations was studied in blindfolded sighted adults in an exploration-reproduction task. The purpose was to determine whether the variations of the memorization conditions between the exploration and reproduction phases would influence the global performance and the oblique effect (lower performance in oblique orientations than in vertical-horizontal orientations). If orientation coding depended on attentional resources, the increase in memory constraints would affect the haptic processing of orientations and the oblique effect. Memory constraints were therefore varied by changing the length and the nature of the delay in two tasks in which previous research has shown that the availability of gravitational cues affected orientation coding. Blindfolded adults were asked to explore haptically a rod with minimal (Expt 1) or natural (Expt 2) gravitational cues and then to reproduce the orientation of this rod ipsilaterally after one of four memorization conditions: with 5 s or 30 s unfilled delays, and 30 s delays filled with verbal or haptic interpolated tasks. When the delay was unfilled, whatever its length (5 s or 30 s), the performance depended on the conditions of manual exploration: the oblique effect was absent when the gravitational cues were minimal (Expt 1) and was present when these cues were natural (Expt 2). By contrast, when the delay was filled with interpolated tasks, the haptic oblique effect was present whatever the conditions of manual exploration. Taken together, these results showed that memorization conditions played a role in the haptic processing of orientations and in the oblique effect when the gravitational cues were minimal during manual exploration.

Large systematic deviations in the haptic perception of parallelity.

Recently, Kappers and Koenderink (1999 Perception 28 781-795) showed that what subjects haptically perceive as parallel often deviates greatly from what is actually physically parallel. In their experiment, subjects had to rotate a test bar in such a way that it felt as though it was parallel to a reference bar. Their data were obtained with the right hand on a table plane to the right side of the median plane of the subject. The present study extends that work in a number of ways: (1) the locations of the stimuli cover the total reachable table plane; (2) distances between stimuli can also be large (more than 1 m); (3) experiments are done both unimanually (with the right and left hand) and bimanually. Like in the previous study, the results show large systematic deviations that correlate significantly with horizontal (left-right) distance between the two bars but not with vertical (forward-backward) distance. Thus we have established that a description of the results in terms of a horizontal gradient in the deviations is valid over a large part of haptic space, over large distances, and in both unimanual and bimanual conditions. The subject-dependent horizontal gradients ranged from -12 degrees m-1 to -27 degrees m-1 in the present experiment. In all conditions a significant haptic oblique effect can be demonstrated.

Is haptic perception continuous with cognition?

A further step in Pylyshyn's discontinuity thesis is to examine the penetrability of haptic (tactual-kinesthetic) perception. The study of the perception of orientation and the “oblique effect” (lower performance in oblique orientations than in vertical–horizontal orientations) in the visual and haptic modalities allows this question to be discussed. We suggest that part of the visual process generating the visual oblique effect is cognitively impenetrable, whereas all haptic processes generating the haptic oblique effect are cognitively penetrable.

Haptic perception of spatial relations.

There are some indications that haptic space like visual space is not Euclidean (e.g. Blumenfeld, 1937 Acta Psychologica 2 125-174). In a series of experiments, we investigated the haptic perception of spatial relations in a systematic way. We restricted ourselves to a horizontal plane at waist height. Blindfolded subjects were asked to perform three tasks with their right hand: (i) a reference bar was presented under four different orientations and subjects were asked to rotate a test bar such that it felt to be parallel to the reference bar; (ii) subjects had to rotate two test bars in such a way that they felt collinear; (iii) subjects had to point a test bar in the direction of a marker. Bars and marker could appear at nine different locations. In all experiments large systematic deviations (up to 40 degrees) were made. The deviations strongly correlated with horizontal (right-left) but not with vertical (forward-backward) distance. Subjects showed qualitatively identical trends but the size of the deviations was strongly subject-dependent. In addition, a significant haptic oblique effect was found. These results provide strong evidence that haptic space in non-Euclidean.

The haptic oblique effect in the perception of rod orientation by blind adults.

The haptic perception of vertical, horizontal, +45 degrees oblique, and +135 degrees oblique orientations was studied in completely blind adults. The purpose was to determine whether the variations of the gravitational cues provided by the arm-hand system during scanning would affect the manifestation of the oblique effect (lower performance in oblique orientations than in vertical-horizontal ones) as they did in blindfolded sighted people (Gentaz & Hatwell, 1996). In blindfolded sighted adults, the oblique effect was reduced or absent when the magnitude of gravitational cues was decreased. If visual experience participated in the haptic oblique effect, we should observe no oblique effect in early blind subjects in the conditions of manual exploration where late blind and blindfolded sighted manifest this effect. The magnitude of gravitational cues was therefore varied by changing gravity constraints, whereas the variability of these cues was varied by changing the plane in which the task was performed: horizontal (low variability) and frontal (high variability). Early and late blind adults were asked to explore haptically a rod and then to reproduce its orientation ipsilaterally in one of two exploratory conditions in each plane. In the horizontal plane, the oblique effect was absent, whatever the gravity constraints, in both groups (early and late blind subjects). In the frontal plane, the oblique effect was present, whatever the gravity constraints, in both groups. Taken together, these results showed that, in blind people, the variability of gravitational cues played a role in the haptic oblique effect; no effect of previous visual experience was observed.

Role of gravitational cues in the haptic perception of orientation

The haptic perception of vertical, horizontal, +45 degrees-oblique, and +135 degrees-oblique orientations was studied in adults. The purpose was to establish whether the gravitational cues provided by the scanning arm-hand system were involved in the haptic oblique effect (lower performances in oblique orientations than in vertical-horizontal ones) and more generally in the haptic coding of orientation. The magnitude of these cues was manipulated by changing gravity constraints, and their variability was manipulated by changing the planes in which the task was performed (horizontal, frontal, and sagittal). In Experiment 1, only the horizontal plane was tested, either with the forearm resting on the disk supporting the rod ("supported forearm" condition) or with the forearm unsupported in the air. In the latter case, antigravitational forces were elicited during scanning. The oblique effect was present in the "unsupported" condition and was absent in the "supported" condition. In Experiment 2, the three planes were tested, either in a "natural" or in a "lightened forearm" condition in which the gravitational cues were reduced by lightening the subject's forearm. The magnitude of the oblique effect was lower in the "lightened" condition than in the "natural" one, and there was no plane effect. In Experiment 3, the subject's forearm was loaded with either a 500- or a 1,000-g bracelet, or it was not loaded. The oblique effect was the same in the three conditions, and the plane effect (lower performances in the horizontal plane than in the frontal and sagittal ones) was present only when the forearm was loaded. Taken together, these results suggested that gravitational cues may play a role in haptic coding of orientation, although the effects of decreasing or increasing these cues are not symmetrical.

The haptic 'oblique effect' in children's and adults' perception of orientation.

The haptic perception of vertical, horizontal, and diagonal orientations was studied in children (aged 7 and 9 years) and in adults. The purpose was to test the hypothesis that the haptic oblique effect results from the different scanning movements at work when one hand explores an oblique standard and the other hand sets the response rod. In experiment 1, blindfolded subjects reproduced the orientation of a standard rod presented in either the frontal, the horizontal, or the sagittal plane, and this task was achieved either ipsilaterally (the same hand explored the standard and set the response rod) or contralaterally (one hand explored the standard and the other hand set the response rod). Since, in the sagittal plane, scanning movements are analogous when the left and right hands explore oblique orientations, no oblique effect should be observed in this condition if the hypothesis is valid. Moreover, a development effect should be observed, since young children generally rely more on movement coding than do older children and adults. Results did not support these predictions: the same oblique effect appeared in the frontal and the sagittal planes both in the ipsilateral and in the contralateral condition, and the effect of age was not in the direction predicted by the hypothesis. The results were consistent with the hypothesis in the horizontal plane only. Experiments 2 and 3 provided further tests of this hypothesis but both failed to support it. Taken together, the results of these three experiments did not support the assumption and it is suggested that the haptic oblique effect may be linked to the gravitational cues provided by the arm-hand system when it acts in the three spatial planes.

Eliminating the haptic oblique effect: influence of scanning incongruity and prior knowledge of the standards.

Although the oblique effect has been conceptualized as a purely visual phenomenon, recent studies report its occurrence in a haptic matching task and present the hypothesis that differences in haptic orientational sensitivity might be responsible for the results. The possibility that procedural variables could be responsible was investigated. Specifically, the effect of prior knowledge of the stimulus orientation standards and of use of bilateral haptic exploration of standard and comparison orientations was examined. The results indicate that the reported oblique effect is reduced either when subjects are not informed which orientations will be tested, or when a unilateral matching procedure instead of a bilateral one is used. When both conditions are combined, the haptic oblique effect is eliminated. It is concluded that this particular manifestation of the oblique effect is not related to haptic sensitivity, but stems from the use of well-established imagery as referent for a match (imagery for oblique stimulus orientations is inferior) and the inherently different scanning patterns required in bilateral exploration of obliques (percepts of standard and comparison obliques will be necessarily different).

Effect of modality-specific experience on visual and haptic judgment of orientation.

Although the 'oblique effect' (poorer performance on oblique orientations as compared to performance on vertical and horizontal orientations) is generally understood as a strictly visual phenomenon, a haptic oblique effect occurs for blindfolded subjects required to set a stimulus rod by hand. Because oblique effects are often attributed to the observer's experience with a predominantly horizontal and vertical environment, we assessed the effect of visual and haptic experience by providing subjects with modality-specific inspection periods to familiarize them with the more poorly judged obliques. Oblique error was significantly reduced in magnitude for judgments made by the modality of experience, and for judgments made across modalities. Rate of improvement, consistency of transfer, and the subjective reports of subjects indicate that this haptic oblique effect is more strongly influenced by visual experience and imagery than by haptic experience. It need not be interpreted as an effect based on factors intrinsic to the haptic modality.