Contribution of somatosensory information to perception of the visual vertical with body tilt and rotating visual field

This study was designed to explore the role of somatosensory information from the trunk in the perception of the visual vertical. Twelve normal subjects and 1 subject with no somatosensory function below the neck attempted to set a line to the true vertical in the sitting and lying positions, first with a static visual background and then with rotation of the background about the line of sight. The absence of somatosensory information did not affect accuracy when the subjects were in the upright position. When lying horizontally, all control subjects experienced a substantial perceived tilt of the vertical in the direction of body tilt (the A effect), but, in contrast, the subject lacking somatosensory function exhibited a small but consistent apparent tilt of the vertical in the opposite direction (the E effect). This finding is discussed in relation to two competing hypotheses regarding the mechanisms subserving apparent displacement of the subjective vertical in tilted subjects.     

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 role of cognitive factors in the rod-and-frame effect

We compared the contribution and the effectiveness of modulating the orientation perception of two types of visual information: the visual frame and the visual polarity of objects. In experiment 1, we examined the effect of a square frame, a mouse, an elephant, and a map of France on the apparent vertical. In the upright position, despite the presence of tilted individual component features, the visual objects had no illusory visual tilt effects. When tilted, these objects had a substantial effect on the direction that appeared to be vertical. However, rod-setting errors were smaller in the inducing objects than when observed with the frame display. In the second experiment, the results of experiment 1 were replicated with a meaningful circular contour--a porthole and a clock. The presence of the external circular contour did not abolish the illusion on the apparent vertical. Moreover, in experiment 3, a clock whose numbers were displaced and not tilted--to avoid the possible tilt influence of visual cues--was also able to deflect the subjective visual vertical. This finding suggests that through top-down processing shapes can act as a framework which serves as a reference influencing the perceived orientation of the inner objects.     

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.     

Perception of self-tilt in a true and illusory vertical plane

A tilted furnished room can induce strong visual reorientation illusions in stationary subjects. Supine subjects may perceive themselves upright when the room is tilted 90 degrees so that the visual polarity axis is kept aligned with the subject. This 'upright illusion' was used to induce roll tilt in a truly horizontal, but perceptually vertical, plane. A semistatic tilt profile was applied, in which the tilt angle gradually changed from 0 degrees to 90 degrees, and vice versa. This method produced larger illusory self-tilt than usually found with static tilt of a visual scene. Ten subjects indicated self-tilt by setting a tactile rod to perceived vertical. Six of them experienced the upright illusion and indicated illusory self-tilt with an average gain of about 0.5. This value is smaller than with true self-tilt (0.8), but comparable to the gain of visually induced self-tilt in erect subjects. Apparently, the contribution of nonvisual cues to gravity was independent of the subject's orientation to gravity itself. It therefore seems that the gain of visually induced self-tilt is smaller because of lacking, rather than conflicting, nonvisual cues. A vector analysis is used to discuss the results in terms of relative sensory weightings.     

Tactile memory in sighted and blind observers: the influence of orientation and rate of presentation

Sighted, early blind, and late blind subjects attempted to identify numerals or number sequences printed on their palms. The numerals were either upright, or inverted, or rotated perpendicular to the arm axis. Stimulus rotation degraded recognition in the early blind subjects, suggesting the influence of experience with visual frames of reference. Slower rates of presentation with upright number sequences improved recall in both sighted and blind observers. An experiment on tactual-visual braille recognition in the sighted observers showed that tilt degraded pattern identification, but visual guidance of the fingertip and ballpoint minimized this loss. A further experiment was performed to distinguish between visual imagery and visual frame of reference explanations of the visual guidance effect on recognition of rotated braille. Subjects explored upright or tilted braille characters while viewing only a light emitting diode on the exploratory fingertip. Sight of scanning movements did not aid pattern recognition with tilt. The results indicate that the benefits of visual guidance on recognition of tilted patterns were probably due to frame of reference information. It is concluded that spatial reference information may aid tactile memory in the sighted and late blind, since the early blind performed at a lower level in the retention task. It is proposed that visual imagery may only explain the superiority of the sighted and late blind when familiar stimuli are studied.     

Effects of varying the apparent tilt of the inspection figure on a kinaesthetic after-effect

Three experiments were conducted to determine whether kinaesthetic after-effects, measured by settings of a bar to the horizontal, are controlled by the apparent or the physical tilt of the bar during inspection. Two arrangements were used to induce a discrepancy between apparent and physical tilt. Data obtained with one arrangement did not permit a proper test of the issue. With the other arrangement either distorted or undistorted visual information about tilt was present during kinaesthetic inspection. It was found that post-inspection settings were displaced from pre-inspection settings in the direction of the tilt of the inspection figure when there was no discrepancy between information presented by the two modalities. When there was a discrepancy, the displacements attributable to apparent tilt were in the direction opposite to the apparent tilt of the inspection figure. The former effects dissipated rapidly after inspection; the latter did not. It is suggested that after-effects obtained following an inspection period during which there is no discrepancy between physical and apparent tilt are controlled by the physical tilt of the inspection figure and that apparent tilt, when it is an effective variable, operates by modifying the judgemental frame of reference.     

Illusory visual direction during and after backward head tilts

When the head is returned to upright after prolonged backward tilt, people who are asked to look straight ahead look higher than they did before the backward tilting. One explanation attributes the illusory visual direction to a change in muscle responsiveness which develops after the head is returned to upright. According to this explanation, the illusory shift should be absent, or at least reduced, if the subjects are not returned to upright before testing. In the present study, the illusion was the same whether subjects remained tilted for testing or were returned to upright.     

Involvement of neck proprioceptive system in visual after-effect from prolonged head tilt

Judgements of object orientation after prolonged head tilt differ relative to those made before tilt. This visual spatial after-effect was investigated with lateral head tilt relative to the upright and supine body. The data from two experiments indicate that the magnitude and direction of the visual after-effect does not differ under the two body postures. It is concluded that the proprioceptive system of the neck, as opposed to the otolith system of the utricle, is involved in the after-effect.     

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.     

Orientation Constancy in Neurons of Monkey Visual Cortex

We studied the effect of body tilt on the orientation selectivity of single neurons in the visual cortex of an alert monkey. The monkey performed a visual fixation task either in the upright position or with its whole body tilted about the naso-occipital (roll) axis by ±25° or ±30°. We determined the preferred stimulus orientation for 51 of 117 neurons in two or, if possible, three body positions (i.e. with the whole body upright, and tilted either left ear or right ear down). In striate cortex, most of the neurons were of a non-compensatory type, showing a change in the preferred orientation according to the body tilt and the estimated counterrolling of the eye. By contrast, about 40% of the neurons in prestriate cortex were of a compensatory type, preferring similar orientations in all body positions. This suggests that mechanisms which produce orientation constancy with respect to the direction of gravity are implemented at an early stage of cortical processing.     

In the footsteps of biological motion and multisensory perception: judgments of audiovisual temporal relations are enhanced for upright walkers

Observers judged whether a periodically moving visual display (point-light walker) had the same temporal frequency as a series of auditory beeps that in some cases coincided with the apparent footsteps of the walker. Performance in this multisensory judgment was consistently better for upright point-light walkers than for inverted point-light walkers or scrambled control stimuli, even though the temporal information was the same in the three types of stimuli. The advantage with upright walkers disappeared when the visual "footsteps" were not phase-locked with the auditory events (and instead offset by 50% of the gait cycle). This finding indicates there was some specificity to the naturally experienced multisensory relation, and that temporal perception was not simply better for upright walkers per se. These experiments indicate that the gestalt of visual stimuli can substantially affect multisensory judgments, even in the context of a temporal task (for which audition is often considered dominant). This effect appears to be constrained by the ecological validity of the particular pairings.     

Orientation contrast effects in the rod-and-frame test

Previous research indicates that a tilted visual display is capable of inducing eye torsion and an illusion of self-tilt in objectively upright observers. These effects may contribute to performance errors on the rod-and-frame test by rotating the perceived axes of visual space toward the tilted frame. The kinesthetic-matching method was used in the present study to see whether an effect of the visual orientation contrast between red and frame sides might also contribute to rod-and-frame test performance. Observers aligned invisible hand-held rods with the visual rod at various tilts under a control condition when the frame was absent, and under experimental conditions with the frame upright or set at 45°. The frame induced matching errors in the direction away from the frame sides which were most nearly parallel to the rod. Since no rotation of apparent visual axes should occur under these conditions, the data suggest that an orientation contrast effect is involved in the rod-and-frame test.     

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.     

The role of balance dynamics in the active perception of orientation.

The Stoffregen and Riccio (1988) hypothesis that perceived orientation is determined primarily by balance dynamics was tested. Perception of orientation was evaluated in the context of a task that required Ss to control the roll orientation of a device in which they were seated. The device's direction of balance was manipulated across trials and thus was independent of gravity. Eighteen Ss participated in the investigation. After each trial, Ss estimated their mean tilt with respect to upright. Correlations of perceived tilt with tilt from balance were consistently higher than the correlations with gravity tilt. The dominance of balance over gravity depended on the magnitude of tilt from balance.     

The effect of orientation on visual and tactual braille recognition

Five experiments are reported in which subjects matched tangible or visible braille characters against either visual or tangible arrays. In both modalities recognition was impaired when the characters were tilted, but visual performance was superior to that for touch. Touch may be more sensitive than vision to tilt, since very small deviations from the upright decreased recognition accuracy. Orientation influenced pattern recognition with and without prior information about orientation. Tilting patterns slowed down recognition for tactual-visual matching, but only when orientation was studied with repeated measures. The results are consistent with the hypothesis that it is difficult to code braille patterns tactually as global outline shapes.     

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.     

Decay of visual adaptation to tilt and displacement

Persistence in the dark following 48 min of visual adaptation to tilt and displacement was compared in two experiments to determine if same or different processes are involved in the two kinds of adaptation. Decay of tilt adaptation occurred rapidly, all within about 16 min. However, it was not complete and some residual tilt adaptation persisted for at least as long as 56 min. Decay of displacement adaptation occurred more slowly but was clearly complete after at most 56 min in the dark. Displacement adaptation appears to be entirely subject to decay, while tilt adaptation involves an additional, more long-term component. Results are interpreted in terms of independent systems for the perception of location and orientation.     

The rod and frame effect and induced head tilt as a function of observation distance

Several experiments investigated the influence of a tilted luminous frame on the rod and frame effect (RFE) and on adjustments of the head to apparent vertical, while observation distance to the frame was varied between 1 and 5 m. Comparisons of repeated measures from the same subjects as well as between independent groups showed that distance was a highly effective variable. Both the RFE and the apparent head tilt varied inversely with distance to the frame, diminishing Lo zero at about 5 m. Since head settings were in the direction of the tilted frame, the latter may be inferred to have caused an induced head tilt (IHT) opposite in direction to the tilted frame, when the head was objectively upright. Issues discussed were the role of IHT in the RFE and factors underlying the role of distance in both phenomena.     

Central visual learning and illusory visual direction after backward head tilts

When the head is returned to upright after prolonged backward tilt, people who are asked to look straight ahead look higher than they did before the backward tilting. This has been interpreted in terms of hypotheses about central visual learning or by hypotheses about peripheral muscle physiology. According to the learning hypotheses, the illusion of visual direction that occurs after head tilts depends upon the presence of discordant cues about direction. In the present study, the illusion was the same with or without discordant information.