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).     

Body tilt effect on the reproduction of orientations: studies on the visual oblique effect and subjective orientations

Body tilt effects on the visual reproduction of orientations and the Class 2 oblique effect (E. A. Essock, 1980) were examined. Body tilts indicate whether the oblique effect (i.e., lower performance in oblique orientations than in vertical-horizontal orientations) is defined in an egocentric or a gravitational reference frame. Results showed that the oblique effect observed in upright posture disappeared in tilted conditions, mainly due to a decrease in the precision of the vertical and horizontal settings. In tilted conditions, the subjective visual vertical proved to be the orientation reproduced the most precisely. Thus, the oblique effect seemed to be not purely gravitationally or egocentrically defined but, rather, to depend on a subjective gravitational reference frame tilted in the same direction as body tilts.     

Mental rotation under head tilt: Factors influencing the location of the subjective reference frame

We report five experiments on the effect of head tilt on the mental rotation of patterns to the “upright.” In Experiment 1, subjects rotated alphanumeric characters, displayed within a circular surround. Experiment 2 was similar except that the character was an unfamiliar letter-like symbol. In Experiment 3, subjects again rotated alphanumeric characters, but they were displayed within a rectangular frame tilted 60° to the right. Experiment 4 was similar, except that the subjects were instructed to rotate the characters to the “upright” defined by the tilted frame. In all four experiments, the subjects performed the task with their heads either upright or tilted 60°. In Experiment 5, subjects had their heads and bodies tilted 90°, and rotated alphanumeric characters displayed within a circular surround. In all except Experiment 4, analysis of response latencies revealed that the subjective vertical lay closer to the gravitational than to the retinal vertical, although it was somewhat displaced in the direction of the head tilt—more so in Experiments 2 and 3 than in Experiment 1, and more so still in Experiment 5. In Experiment 4, instructions to adopt the axes of the frame land thus of the retina) succeeded in bringing the subjective vertical closer to the retinal than to the gravitational vertical, although the subjective vertical was still some 20° on average from the gravitational vertical. The results show that the subjective reference frame is distinct from both gravitational and the retinal frames, and that the gravitational frame exerts the stronger influence. They also argue against the primacy of a “retinal factor” in the perception of orientation.     

Haptic parallelity perception on the frontoparallel plane: the involvement of reference frames

It has been established that spatial representation in the haptic modality is subject to systematic distortions. In this study, the haptic perception of parallelity on the frontoparallel plane was investigated in a bimanual matching paradigm. Eight reference orientations and 23 combinations of stimulus locations were used. The current hypothesis from studies conducted on the horizontal and midsagittal planes presupposes that what is haptically perceived as parallel is a product of weighted contributions from both egocentric and allocentric reference frames. In our study, we assessed a correlation between deviations from the veridical and hand/arm postures and found support for the role of an intermediate frame of reference in modulating haptic parallelity on the frontoparallel plane as well. Moreover, a subject-dependent biasing influence of the egocentric reference frame determines both the reversal of the oblique effect and a scaling effect in deviations as a function of bar position.     

The visual perception coordinate system uses axes defined by the earth, trunk, and vision

Eight young adults adjusted a line located on one side of a computer display parallel to internally specified Earth-fixed vertical (display in frontal plane), to the horizontal trunk-fixed anterior-posterior axis (display in horizontal plane), and to an oblique line (display in horizontal and vertical planes). All tasks were completed in a dark room with the head and trunk in both a standard erect posture and varied postures. Errors were lowest when setting the line to internally specified vertical in the frontal plane and to an oblique line in the horizontal plane when head and trunk orientations were varied. Constant errors for setting one line parallel to a second line were in opposite directions when the second line was located on the left versus right side of the display, but did not differ in direction when setting the line parallel to internally specified axes. Also, the oblique effect was preserved when the head and trunk were tilted to various orientations, suggesting that it results from integration of an internally specified gravitational reference with visual input. We conclude that the visual perceptual coordinate system uses internally specified vertical and, when available, a visually specified horizontal reference axis to define object orientation.     

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.     

Role of gravitational cues in the haptic perception of orientation

The haptic perception of vertical, horizontal, +45°-oblique, and +135°-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.     

What’s up in mental rotation?

We report two experiments on the influence of head tilt on mental rotation. In Experiment I, subjects decided whether dot patterns were or were not repeated about a line. Their reaction times (RTs) were consistent with the interpretation that they mentally rotated the patterns so that the line was subjectively vertical before making their decisions. When the subjects tilted their heads, the RT functions shifted in the direction of the tilt, indicating that the subjective vertical lay closer to the retinal than to the gravitational vertical. In Experiment II, subjects decided whether singly presented alphanumeric characters in various orientations were standard or backward (mirror-reversed). Again, analysis of their RTs suggested mental rotation to the standard upright, but the function was unaffected by head tilt; in this case, the subjects operated in subjective gravitational rather than retinal coordinates. The choice of retinal or gravitational coordinates may depend on whether the stimuli are interpreted egocentrically or as part of the external world.     

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.     

The perception of verticality and the frame of reference of the visual tilt aftereffect

Previous research has suggested that the visual tilt aftereffect operates according to a gravitational frame of reference. Three experiments were conducted to test this conclusion further. In each experiment, observers (with head upright) adjusted an illuminated bar to apparent vertical following various adaptation conditions. In Experiment 1, observers were given clear visual cues for objective vertical while adjusting the bar. In Experiment 2, they were not given visual cues for vertical. The adaptation conditions in Experiments 1 and 2 consisted of various combinations of head and stimulus tilt. Experiment 3 investigated the effects of head tilt alone. The results indicated that the tilt aftereffect follows a retinal frame of reference under some conditions (Experiment 1) and appears to follow a gravitational frame under others (Experiment 2). These results can be predicted by a simple model involving two factors, a purely visual aftereffect that follows a retinal frame and an extravisual aftereffect that appears to follow a gravitational frame.     

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

The haptic perception of vertical, horizontal, +45° oblique, and +135° 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 ipsilateraUy 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.     

Visual orientation during and after lateral head,body, and trunk tilt

Visual judgments of orientation were investigated during (effect) and after (aftereffect) different body postures. In Experiment 1 four trained Ss made apparent verticality (AV) judgments before and after 2 min in each of seven orientations: head tilt left and right, body tilt left and right, trunk tilt left and right and a control condition with head ’and body upright. The aftereffect was significant for all postures excepting trunk tilt left and the control. The aftereffect from head tilt was greater than that from the same degree of body tilt, and that in the trunk tilt condition was in the same direction as’ predicted from neck stimulation. In Experiment 2, 30 Ss made AV judgments during tilt in the same seven postures. The E-phenomenon resulted from both head and body tilts, and an effect was found for trunk tilt in the direction predicted from neck stimulation. The results are discussed in terms of the otolith, neck, and trunk receptor systems.     

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.     

The importance of being upright: Use of environmental and viewer-centered reference frames in shape discriminations of novel three-dimensional objects

We investigated the frame of reference that people use to make shape discriminations when their heads are either upright or tilted. Observers madesame-different judgments of pairs of novel threedimensional objects that were aligned along their length within the frontal-parallel plane and rotated in depth around an axis parallel to their own axes of elongation. The aligned objects were displayed vertically, tilted 45°, or horizontally with respect to the environmental upright, but the distance of each pair from the upright was irrelevant to resolving the angular disparity between the stimuli for thesame-different judgment. Nevertheless, when observers’ heads were upright, the time to encode the stimuli was a linear function of the distance of the stimuli from the environmental upright, whereas when observers’ heads were tilted 45°, encoding times for tilted and vertical stimuli did not differ and were faster than the times to encode horizontal stimuli. We interpreted these data to mean that observers either rotate or reference the top of an object to the environmental upright, and they can use either a gravitational or retinal reference frame to do so when either they or the objects are not upright.     

Tachistoscopic perception under head tilt

Subjects attempted to identify singly presented letters, flashed briefly at six different locations equidistant from fixation. The locations were at 1, 3, 5, 7, 9, and 11 o’clock on an imaginary circle. The subjects performed both with their heads upright or tilted 60° (i.e., through 2 and 8 o’clock or through 10 and 4 o’clock). In Experiment 1, the letters were projected in a gravitationally fixed rectangular surround, and accuracy of report depended largely on the locations of the letters in gravitational coordinates, although there was clear evidence of a retinal influence as well. In Experiment 2, the surround was circular and accuracy depended principally on retinal location, although there was also a gravitational influence. Analysis of individual data suggested that, when their heads were tilted, subjects generally adopted a reference frame which lay between the retinal and gravitational frames, closer to one than the other but typically coinciding with neither. We conclude that the subjective reference frame does not depend on a poststimulus correction, but rather that stimulus information is incorporated directly into the subjective frame. The data also suggested that lateral asymmetries depend on a perceptual rather than a retinal interpretation of left and right visual fields.     

The role of contextual cues in the haptic perception of orientations and the oblique effect

Blindfolded right-handed participants were asked to position, with the right hand, a frontoparallel rod to one of three orientations: vertical (0°) and left 45° and right 45° obliques. Simultaneously, three different backgrounds were explored with the left hand: smooth, congruent stripes (parallel to the orientation to be produced), or incongruent stripes (tilted relative to the orientation to be produced). The analysis of variable errors showed that the oblique effect (higher precision for the vertical orientation than for the oblique orientations) was weakened in the presence of contextual cues, because of an improvement in oblique precision. Moreover, the analysis of constant errors revealed that the perception of orientations erred in the direction of the stripes, similar to the effect that has been found with vision, where visual contextual cues (tilted frame or lines) divert the perception of the vertical. These results are discussed in relation to a patterncentric frame of reference hypothesis or as a congruency effect.     

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.     

Tactile orientation constancy: Do proprioception and attention affect the tactile vertical?1

Abstract: Tactile vertical, defined as the edge orientation that participants perceive to be vertical, was examined in four experiments. In Experiment 1, we touched the participants’ cheek, lips, or hand with an edge and asked them to judge its orientation with regard to gravitational vertical, both when the stimulated body part was upright (or, in the case of the lips, aligned), and when it was tilted (lips, distorted). We found that when the head or hand was tilted forward 30°, or when the lower lip was distorted approximately 38° to the left or right, the tactile vertical shifted in the same direction by only a fraction (8.7, 8.6, and 36.3% for the cheek, lips, and hand, respectively) of the change in orientation of the stimulated region. The results indicated considerable, but usually incomplete, orientation constancy. In Experiment 2, we measured tactile vertical on the hand for forward tilts from 0° to 45°. We found that as the hand was tilted, the tactile vertical increasingly shifted in the same direction as the hand (i.e., a tactile Aubert effect). In Experiment 3, the effect of attentional focus on tactile vertical was examined by comparing the tactile vertical of participants who attended to body‐centered coordinates, and others who attended to gravitation‐centered coordinates. We found that focusing on body‐centered coordinates caused a decrease in orientation constancy. We sought to examine the role of attention further in Experiment 4, measuring tactile vertical on the cheek of persons with temporomandibular disorders. Compared with normal participants, these participants displayed significantly lower constancy. The results were accounted for by a narrowing of attention to painful signals, so that proprioceptive information was attended to less. In conclusion, the degree of tactile orientation constancy that participants demonstrate varies as a function of body site and attentional focus.     

The role of head-centric spatial reference with a static and kinetic visual disturbance

A static or kinetic visual disturbance affects subjects’ ability to estimate the direction of the gravitational vertical. This kind of error is increased by a head roll inclination. In two experiments, we combined head orientation with a static (Experiment 1: tilted frame) versus kinetic (Experiment 2: rotating disk) visual disturbance. The results showed that with a static visual disturbance, the increase of errors in the inclined head condition was mainly the consequence of a postural head effect like an Aubert effect. On the contrary, with a kinetic visual disturbance, it appears that the disk effect increases with head inclination. However, individual errors observed with the head inclined in front of a stationary disk were systematically correlated with the errors triggered by the same head inclination in front of a rotating disk. These observations confirm that the head axis spatial reference plays an important role in orientation perception, whatever the head position and the kind of visual display.     

The relationship between allocentric and egocentric frames of reference and categorical and coordinate spatial information processing

We report two experiments on the relationship between allocentric/egocentric frames of reference and categorical/coordinate spatial relations. Jager and Postma (2003) suggest two theoretical possibilities about their relationship: categorical judgements are better when combined with an allocentric reference frame and coordinate judgements with an egocentric reference frame (interaction hypothesis); allocentric/egocentric and categorical/coordinate form independent dimensions (independence hypothesis). Participants saw stimuli comprising two vertical bars (targets), one above and the other below a horizontal bar. They had to judge whether the targets appeared on the same side (categorical) or at the same distance (coordinate) with respect either to their body-midline (egocentric) or to the centre of the horizontal bar (allocentric). The results from Experiment 1 showed a facilitation in the allocentric and categorical conditions. In line with the independence hypothesis, no interaction effect emerged. To see whether the results were affected by the visual salience of the stimuli, in Experiment 2 the luminance of the horizontal bar was reduced. As a consequence, a significant interaction effect emerged indicating that categorical judgements were more accurate than coordinate ones, and especially so in the allocentric condition. Furthermore, egocentric judgements were as accurate as allocentric ones with a specific improvement when combined with coordinate spatial relations. The data from Experiment 2 showed that the visual salience of stimuli affected the relationship between allocentric/egocentric and categorical/coordinate dimensions. This suggests that the emergence of a selective interaction between the two dimensions may be modulated by the characteristics of the task.