Effect of prolonged tilt on visual orientation

Visual orientation during lateral tilt is viewed in terms of orientation constancy. The postural systems involved in the maintenance of constancy are considered to be those of the otolith, neck and trunk. The relative contribution of these systems was investigated by obtaining visual verticality judgments immediately upon and several minutes after head, body, and trunk tilts. Due to the apparent non-adaptation of the otolith system any changes in visual orientation resulting from prolonged tilt would be attributed to adaptation of the proprioceptive system stimulated. For 30° head tilt visual orientation over-constancy was reduced by about 2°, reflecting the influence of the neck system. Prolonged body tilts of 30°, 60° and 90° reduced the constancy operating by approximately 1°, 3° and 8°, respectively. This was taken to indicate the contribution of the trunk system, which increased with increasing degrees of body tilt. The above interpretations received strong support from experiments involving trunk tilt, which stimulates only the neck and trunk systems.     

Visual orientation during lateral head tilt when upright and supine

Egocentric visual orientation was investigated with lateral head tilts relative to the upright and supine trunk. A significant difference was found in the trend of visual head axis judgments between 40° left and 40° right head tilt for the two conditions, such that they varied systematically with the degree of tilt for the upright but not for the supine condition.     

The interaction of postural systems in visual orientation

Visual orientation judgments were made during right head tilt, left body tilt, and left trunk tilt, each of 30 deg. In these postures the otolith, neck, and trunk systems are stimulated in pairs. The hypothesis that the visual vertical during trunk tilt is equal to the algebraic sum ofthose in the same direction ofbody tilt and the opposite head tilt was tested and accepted. This result demonstrates the interaction of these postural systems in visual orientation, and complements the confirmation of the same hypothesis for the visual aftereffect following prolonged tilt.     

THE EFFECT OF WATER IMMERSION ON PERCEPTION OF THE VISUAL VERTICAL

Judgements of the apparent vertieality of a single visible line of light were compared under normal support conditions and under water. During head, body and trunk tilts up to 40° the visual vertical was not influenced by water immersion. Greater degrees of lateral body tilt (up to 180°) resulted in slightly greater departures of the visual from the gravitational vertical during immersion relative to terrestrial performance. It was suggested that this was due to the reduction of information from surface pressure receptors in the trunk. The mean visual aftereffects following head, body and trunk tilts were not affected by water immersion in any consistent manner. It was concluded that visual orientation constancy is only marginally reduced by immersion in water.     

The role of gravitational cues in the judgment of visual orientation

It is argued that, contrary to the views of some theorists, the role of gravitational cues is essentially one of maintaining orientation constancy. In support of this claim, it is shown that the loss of relevant gravitational information when the body is supine results in a significant increase in the disorienting effects of both a tilted visual frame and tilt of the head relative to the trunk.     

Contrast and size variables and the tilt after-effect

The relationship between the contrast and bar width of adapting and test gratings in the determination of the magnitude of the tilt after-effect was explored. When the contrast of adapting and test gratings is varied concomitantly over four levels, the magnitude of the tilt after-effect does not change. When the contrast of the adapting grating is higher than the test grating, the magnitude of the after-effect is increased and when its contrast is lower than the test grating, the magnitude of the after-effect is decreased. When the bar width of the test grating is narrower or broader than that of the adapting grating, the magnitude of the after-effect does not change. The implications of these results for a neurophysiological explanation of the tilt after-effect are discussed.     

Postural configuration affects the perception of earth-based space during pitch tilt

This study investigates the relative contribution of body parts in the elaboration of a whole-body egocentric attraction phenomenon previously observed during earth-based judgments. This was addressed through a particular earth-based task requiring estimating the possibility of passing under a projected line, imagining a forward horizontal displacement. Different postural configurations were tested, involving whole-body tilt, trunk tilt alone or head tilt alone. Two legs positions relative to the trunk were manipulated. Results showed systematic deviations of the subjective “passability” toward the tilt, linearly related to the tilt magnitude. For each postural configuration, the egocentric influence appeared to be highly dependent on the position of trunk and head axes, whereas the legs position appeared not relevant. When compared to the whole-body tilt condition, tilting the trunk alone consistently reduced the amount of the deviation toward the tilt, whereas tilting the head alone consistently increased it. Our results suggest that several specific effects from multiple body parts can account for the global deviation of the estimates observed during whole-body tilt. Most importantly, we support that the relative contribution of the body segments could mainly depend on a reweighting process, probably based on the reliability of sensory information available for a particular postural set.     

Testing the tilt-constancy theory of visual illusions

Prinzmetal and Beck (2001) Journal of Experimental Psychology: Human Perception and Performance 27 206 - 217) argued that a subset of visual illusions is caused by the same mechanisms that are responsible for the perception of vertical and horizontal a theory they referred to as the tilt-constancy theory of visual illusions. They argued that these illusions should increase if the observer's head or head and body are tilted because extra reliance would then be placed on the illusion-inducing local visual context. Exactly that result had previously been reported in the case of the tilted-room and the rod-and-frame illusions. Prinzmetal and Beck reported similar increases in the tilt illusion (TI), as well as the Z?llner, Poggendorff, and Ponzo illusions. In two experiments, we re-examined the effect of head tilt on the TI. In experiment 1, we used more conventional TI stimuli, more standard experimental methods, and a more complete experimental design than Prinzmetal and Beck, and additionally extended the investigation to attraction as well as repulsion effects. Experiment 2 more closely replicated the Prinzmetal and Beck stimuli. Although we found that head tilt did increase TIs in both experiments, the increases were of the order of 1 degrees -2 degrees, more modest than the 7 degrees reported by Prinzmetal and Beck. Significantly, the TI increase was larger when inducing tilts and head tilts were in the same direction than when they were in opposite directions, suggesting that the tilt-constancy theory may be oversimplified. In addition, because previous evidence renders unlikely the claim that the Poggendorff illusion can be explained simply in terms of misperceived orientation of the transversals, the question arises whether there might be some other explanation for the increase in the Z?llner, Poggendorff, and Ponzo illusions with body tilt that Prinzmetal and Beck reported.     

Apparent head position as a basis for a visual aftereffect of prolonged head tilt

A possible explanation of the visual spatial aftereffect following head tilt with eyes closed is that it is an outcome of a proprioceptive aftereffect of head position. If the upright head is apparently tilted then it might be expected that a vertical line in a dark room would also be apparently tilted. This explanation predicts that the direction and magnitude of the visual and proprioceptive aftereffects would correspond. The second of two experiments showed that the trends of the two aftereffects as a function of head tilt angle were different. It was concluded that the visual aftereffect cannot be explained in terms of a proprioceptive aftereffect.     

Tilt from ahead-inverted position produces displacement of visual subjective vertical in the opposite direction

Observers who lie supine with their heads inverted report large (up to 60°) tilt of a light line in an otherwise dark room when their heads and/or bodies are tilted. Most observers report that visual subjective vertical is tilted in the direction opposite to the head/body tilt. The results can be interpreted by employing a model developed by Mittelstaedt (1983), which suggests that visual subjective vertical is derived from a gravity vector transduced by vestibular and somesthetic receptors combined with “idiotropic vectors” that represent the orientation of the observer’s own head and body axes.     

How do visual and postural cues combine for self-tilt perception during slow pitch rotations?

Self-orientation perception relies on the integration of multiple sensory inputs which convey spatially-related visual and postural cues. In the present study, an experimental set-up was used to tilt the body and/or the visual scene to investigate how these postural and visual cues are integrated for self-tilt perception (the subjective sensation of being tilted). Participants were required to repeatedly rate a confidence level for self-tilt perception during slow (0.05°·s^− 1) body and/or visual scene pitch tilts up to 19° relative to vertical. Concurrently, subjects also had to perform arm reaching movements toward a body-fixed target at certain specific angles of tilt. While performance of a concurrent motor task did not influence the main perceptual task, self-tilt detection did vary according to the visuo-postural stimuli. Slow forward or backward tilts of the visual scene alone did not induce a marked sensation of self-tilt contrary to actual body tilt. However, combined body and visual scene tilt influenced self-tilt perception more strongly, although this effect was dependent on the direction of visual scene tilt: only a forward visual scene tilt combined with a forward body tilt facilitated self-tilt detection. In such a case, visual scene tilt did not seem to induce vection but rather may have produced a deviation of the perceived orientation of the longitudinal body axis in the forward direction, which may have lowered the self-tilt detection threshold during actual forward body tilt.     

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.     

Influence of body roll on visually induced sensations of self-tilt and rotation

Illusory self-tilt and illusory self-motion (vection) produced by rotation of a 360 degrees visual scene about the subject's roll axis was measured as a function of the presence or absence of actual rotation of the subject during acceleration of the visual scene. Rotation of the subject to a tilt of 15 degrees was at two levels of acceleration (onset) and with or without a delay between initial rotation and subsequent return (washout) to the vertical position. In one set of conditions, visual motion and subject motion were in opposite directions (concordant) and in another set they were in the same direction (discordant). In two control conditions, the subject was rotated while the visual scene remained stationary. For concordant motion the main effect of body rotation was to reduce the time taken by the subject to indicate self-tilt as compared with the response time to visual motion alone. The magnitude of estimated self-tilt was increased by actual body tilt as could be expected from addition of the perceived actual body tilt and the illusory body tilt induced by visual rotation. This effect of augmented body tilt did not persist after the body was returned to the vertical. The magnitude of vection was not markedly influenced by body rotation and washout. For discordant motion of body and the visual scene, subjects were confused and their responses were very variable, suggesting a nonlinear visual--vestibular interaction.     

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.     

To pass or not to pass: More a question of body orientation than visual cues

This study investigated the influence of pitch body tilt on judging the possibility of passing under high obstacles in the presence of an illusory horizontal self-motion. Seated subjects tilted at various body orientations were asked to estimate the possibility of passing under a projected bar (i.e., a parking barrier), while imagining a forward whole-body displacement normal to gravity. This task was performed under two visual conditions, providing either no visual surroundings or a translational horizontal optic flow that stopped just before the barrier appeared. The results showed a main overestimation of the possibility of passing under the bar in both cases and most importantly revealed a strong influence of body orientation despite the visual specification of horizontal self-motion by optic flow (i.e., both visual conditions yielded a comparable body tilt effect). Specifically, the subjective passability was proportionally deviated towards the body tilt by 46% of its magnitude when facing a horizontal optic flow and 43% without visual surroundings. This suggests that the egocentric attraction exerted by body tilt when referring the subjective passability to horizontal self-motion still persists even when anchoring horizontally related visual cues are displayed. These findings are discussed in terms of interaction between spatial references. The link between the reliability of available sensory inputs and the weight attributed to each reference is also addressed.     

Kinesthetic orientation judgments during later al head,body and trunk tilt

The effects of body, neck, and trunk tilt on judgments of kinesthetic verticality were compared using 104 Ss. The results showed that head tilt and body tilt produced equal and significant E effects and that trunk tilt produced no significant E effect. The data were interpreted as showing that otolith information is an important determinant of the kinesthetic E effect.     

Spatial representation in body coordinates: evidence from errors in remembering positions of visual and auditory targets after active eye, head, and body movements.

Eight participants were presented with auditory or visual targets and then indicated the target's remembered positions relative to their head eight seconds after actively moving their eyes, head or body to pull apart head, retinal, body, and external space reference frames. Remembered target position was indicated by repositioning sounds or lights. Localization errors were found related to head-on-body position but not of eye-in-head or body-in-space for both auditory (0.023 dB/deg in the direction of head displacement) and visual targets (0.068 deg/deg in the direction opposite to head displacement). The results indicate that both auditory and visual localization use head-on-body information, suggesting a common coding into body coordinates--the only conversion that requires this information.     

Is the figural after-effect accompanied by changes in discrimination?

In the figural after-effect one part of the visual field is apparently expanded relative to the rest, and it is suggested that the expansion might correspond to an improvement in discrimination. An experiment, in which subjects were required to judge the direction of curvature of lines after fixation of a straight line, showed no such improvement.     

Imagined body orientation and perception of the visual vertical

The existence of body orientation mental imagery was tested by examining whether self roll tilt imagery affects the subjective visual vertical (SVV). Twenty healthy subjects judged the orientation of a dim luminous bar with respect to gravitational vertical, while normally seated in complete darkness with their head firmly restrained earth vertically. SVV was measured in three conditions: a reference condition with no imagery, and a left and a right imagery condition, during which the bar orientation was to be judged while the subjects imagine themselves roll-tilted towards left or right, respectively. The imagined roll tilts were of the same magnitude as roll tilts which generally induce an E- effect, i.e., an SVV lean toward the side opposite to those of body tilt. If imagery and perception of self roll tilt share common processes, self roll tilt imagery should induce an E-like effect. Results show an imagery- induced E-like effect, which strongly supports the idea that humans can perform mental imagery of body orientation about gravity. Received: 4 April 2000 / Accepted: 1 September 2000     

The mysteries of the diagonal: gender-related perceptual asymmetries

In this article, we report a perceptual asymmetry for the two diagonals that is related to gender in that females prefer the diagonal spanning from top right to bottom left (/) whereas males prefer the opposite (\). This relationship is observed in a variety of tasks, including aesthetic judgment of paintings, spotting differences between two paintings, and visual search for a tilted line among similarly tilted distractors. This article does not provide an explanation of the relationship between this asymmetry and gender but rules out several potential mediating factors, such as eye dominance, head tilt, handedness, and hemispheric differences. At the same time, the scope of the phenomenon is outlined: The asymmetry is found for both meaningful and meaningless stimuli and in both brief and extended presentations. Moreover, the asymmetry is found to be related to the tilt of the visual elements that require processing, not to their location in the visual field.