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.     

A visually induced illusion of body tilt in a horizontal plane

Unexpected spontaneous reports of an illusion of body orientation have been observed in previous studies on the effects of a tilted overhead frame on supine observers. The present studies were designed in an attempt to describe and measure this illusion. The findings suggest that when objectively aligned with invisible side walls of the room, some subjects may experience an illusion that they are tilted in a horizontal plane with respect to the room walls in the direction opposite to frame tilt from the longitudinal body axes. The results are discussed in terms of a model of visual-vestibular interactions.     

The influence of the acoustic context on vertical sound localization in the median plane

The influence of background sounds (frames) on vertical localization of single sound sources (targets) was examined in four experiments. Loudspeakers (five targets and four frames) were positioned in the median plane, ranging from +30 degrees to -30 degrees above and below the subject's ear level. The subjects determined the vertical position of the targets by either verbal judgments or manual pointing. Frame and target sounds were presented concurrently or successively with a 1-sec interval; both consisted of (1) 300-Hz square waves, (2) noise, or (3) targets of noise and frames of 300-Hz square waves. Particularly in the second condition, the subjects consistently shifted the apparent target positions away from the frame locations. This contrast effect persisted even 1 sec after the offset of the frames. No effect was found with different waveforms for the frame and the target. Results are related to recent findings indicating a similar effect in the azimuthal dimension. Possibly the effect is based on a mechanism in which the auditory system adapts to recently heard sound source positions.     

Infants' localization of sounds in the median vertical plane: estimates of minimum audible angle

Infants 6, 9, 12, 15, and 18 months of age were seated in a dark room directly facing an array of nine loudspeakers positioned along the median vertical plane. One loudspeaker was positioned at ear level, 0 degree, and four others each were positioned above and below 0 degree. To examine infants' resolution of auditory space in the median vertical plane we sought to determine the smallest angular shift in the vertical location of a sound that infants could reliably detect (i.e., minimum audible angle). A two-alternative forced-choice procedure was used in which a sequence of white noise bursts was presented initially at 0 degree, and then shifted vertically (i.e., above or below 0 degree) and continued to be presented until the infant made a directional response; correct responses were visually reinforced. The smallest angular shift in vertical location that was reliably detected systematically decreased with increasing age between 6 months (15 degrees) and 18 months (4 degrees), suggesting a finer partitioning of auditory space along the vertical axis over this age range. By 18 months infants' performance matched that of a group of adults tested under the same circumstances.     

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.     

Auditory saltation in the vertical midsagittal plane

Auditory saltation is an illusion in which a train of clicks, the first half of which is presented at one location and the other half of which is presented from a second location, is perceived as originating not only from the anchor points, but also from locations between them. That is, intermediate members of the series of clicks have their spatial locations systematically misperceived. In the present study, auditory saltation was examined for the first time in the vertical midsagittal plane. Subjects rated the perceived continuity of motion for 8-click trains systematically varied in inter-click interval (ICI), direction of motion (up, down), and trial type ('saltation' versus 'real' motion). In all listeners, saltation stimuli supported robust saltation, but only for trials with ICIs less than about 120 ms. Real motion was rated as continuous for all ICIs. These data indicate that the auditory-saltation illusion can exploit monaural stimulus cues for source location in the generation of the illusory motion percept.     

Gender differences in spatial perception of body tilt

The tendency for observers to overestimate slant is not simply a visual illusion but can also occur with another sense, such as proprioception, as in the case of overestimation of self-body tilt. In the present study, distortion in the perception of body tilt was examined as a function of gender and multisensory spatial information. We used a full-body-tilt apparatus to test when participants experienced being tilted by 45 degrees, with visual and auditory cues present or absent. Body tilt was overestimated in all conditions, with the largest bias occurring when there were no visual or auditory cues. Both visual and auditory information independently improved performance. We also found a gender difference, with women exhibiting more bias in the absence of auditory information and more improvement when auditory information was added. The findings support the view that perception of body tilt is multisensory and that women more strongly utilize auditory information in such multisensory spatial judgments.     

Visual and somesthetic influences on postural orientation in the median plane

We investigated optic and somesthetic contributions to perceived body orientation in the pitch dimension. In a within-subject factorial design, each of 12 subjects attempted to set his/her body erect or 45° back from erect while restrained in a movable bed surrounded by an adjustable box The box provided a visual environment consisting of either a grid pattern, two luminous lines, or complete darkness. Both the grid pattern and the luminous lines were effective at-biasing settings of body position when the box was pitched; the pitched grid was more effective than the pitched lines. Although the pitch of the box influenced orientation to both goals, the effect was greater for the diagonal goal than for the erect goal. We present a model of postural orientation in the median plane that involves vestibular, somatosensory, and visual inputs.     

Localization of sound in the vertical plane with and without high-frequency spectral cues

Binaural localization of 3.0-kHz high- and lowpass noise presented in the median vertical plane (MVP) and lateral vertical plane (LVP) was investigated. We anticipated superior performance when localizing the highpass noise hy virtue of the availability of pinna cues. The viability of this supposition was strengthened by monaural localization tests in which performance proficiency for the highpass noise exceeded that for the lowpass noise (p < .01). The main result showed that binaural localization of proficiency for highpass noise surpassed that for lowpass noise for all listening conditions (p < .01). However, the importance of binaural temporal and level differences in vertical-plane localization was demonstrated by the highly respectable performances when the lowpass noise was presented in the LVP. Data from binaural localization in the MVP and monaural localization in the LVP suggested that the influence of pinna cues diminishes for source elevations above 45°.     

Effects of age and asymmetrical stimulation on the subjective median plane

Changes in subjective median plane (SMP) as a result of asymmetrical visual and bodily stimulation were measured for a total of 260 Ss, age 5–17. Asymmetrical visual stimulation resulted in a readjustment of the SMP in the direction of stimulation, with a decline in magnitude of response as a function of age. Asymmetrical bodily stimulation was assimilated to by the younger children (SMP adjusted in the direction of stimulation), . and counteracted by the older children (age 8 and older). In the combination of the two, the effects of visual stimulation dominated those of bodily stimulation throughout the age range, possibly due to insufficient exposure to the asymmetrical bodily stimulation. Correlational analysis showed that CA, rather than MA, was the major variable associated with the results. The results are in general agreement with sensory-tonic theory, and are discussed in this light.     

Localization of sound in the vertical plane with and without high-frequency spectral cues.

Binaural localization of 3.0-kHz high- and lowpass noise presented in the median vertical plane (MVP) and lateral vertical plane (LVP) was investigated. We anticipated superior performance when localizing the highpass noise by virtue of the availability of pinna cues. The viability of this supposition was strengthened by monaural localization tests in which performance proficiency for the highpass noise exceeded that for the lowpass noise (p less than .01). The main result showed that binaural localization of proficiency for highpass noise surpassed that for lowpass noise for all listening conditions (p less than .01). However, the importance of binaural temporal and level differences in vertical-plane localization was demonstrated by the highly respectable performances when the lowpass noise was presented in the LVP. Data from binaural localization in the MVP and monaural localization in the LVP suggested that the influence of pinna cues diminishes for source elevations above 45 degrees.     

Size and distance judgements in the vertical plane under water

Summary Experiments with subjects of varied diving experience in clear water of Malta showed that divers tend to underestimate the distance of the surface more than that of the seabed. The underestimation is most marked in clear empty water, but disappears with practice. Objects viewed vertically downwards tend to be correctly estimated in size; but this is an underestimate in comparison with the overestimation which normally occurs with horizontal viewing. These effects are similar to the moon illusion, but are probably due to visual rather than proprioceptive changes in the vertical plane.

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Résumé Des expériences faites avec des sujets possédant divers niveaux d'expérience de plonger en eau claire devant Malte ont montré ques les plongeurs ont tendance à sous-estimer léloignement de la surface plus que du fond de la mer. Cette sous-estimation se révèle la plus marquée en eau claire et vide, mais disparaît avec l'habitude. Des objets regardés verticalement en bas ont tendance à être estimés justement en grandeur; mais c'est là une sous-estimation en comparaison avec la surestimation qui se produit quand ils sont regardés horizontalement. Ces effets sont semblables à la lune-illusion, mais ont probablement pour cause des changements plutôt visuels que proprioceptifs dans le plan vertical.

     

Perception of the visual vertical: Utricular and somatosensory contributions

Summary The subjective visual vertical (SV) has been investigated in centrifugation experiments in order to test the hypothesis, that changes in the SV are proportional to changes in the shear force acting on the utricles. The results deviate from those predicted from the shear hypothesis. This deviation can be accounted for by assuming that the SV is determined by the average of utricular and somaesthetic inputs.Thanks are due to A. Stein for performing some of the experiments of series d and e. N.J. Wade was supported by a Forschungsstipendium from the Alexander von Humboldt-Stiftung. Renate Alton skillfully assisted with the drawings.     

Perception of objects oriented downward from a vertical position

In the present work we investigated people’s perceptions of orientation for surfaces that are conceived of as being sloped downward from vertical against a vertical reference frame. In the three conditions of Experiment 1, participants either (1) placed a ladder against a wall at what they thought was the most stable position, and then estimated its orientation; (2) gave a verbal (conceptual) estimate of what the most stable position of a ladder leaned against a wall would be; or (3) drew a line representing the most stable position of a ladder to be placed against a wall, and then gave a verbal estimate of the ladder’s orientation. Ladder placement was shallower than the most stable position, as were the verbal estimations of both the positioned and drawn orientations and the verbal (conceptual) estimates of the most stable position for a ladder to be leaned against a wall, relative to the actual orientations. In Experiment 2, participants verbally estimated various ladder orientations. The estimates were again shallower than the actual orientations. For orientations between 60° and 90°, the estimates showed a scale compression effect from horizontal. This perceived exaggeration of the orientation of an object typically oriented down from vertical is similar to the perceived exaggeration of the orientation of hills and ramps, typically thought of as oriented up from horizontal. This may point to a generic perceived exaggeration of slant whose direction depends on the conceptual or actual reference frame being used.