Fred W. Mast

Perceived body position and the visual horizontal

This contribution examines the relation between the subjective visual vertical, the subjective visual horizontal, and the perceived body position of human subjects. Firmly fixed on a tiltable chair with head and torso restrained, 11 healthy subjects were rolled sideways and indicated their subjective horizontal body position. In these positions the subjects were also asked to adjust a luminous line alternately to the vertical and to the horizontal. The adjustments of the subjective horizontal body position cluster around a mean of 96.3 degrees with a remarkably broad range (SD: 19.7 degrees). In the subjective horizontal body position, the luminous line does not appear horizontal when in line with ones own spinal axis. It is set further down by 27.4 degrees on average and, therefore, perpendicular to the subjective visual vertical. This finding supports the idea that the judgement of the own body position and the judgement of the orientation of a seen object respective to gravity are based on different references. Contradictory to other investigations [23,24], is the empirical fact that the individual subjects were not able to adjust the horizontal body position with the reported accuracy (range of mean adjustments 77.5 to 117.6 degrees).

Eye movements during visual mental imagery

It has long been debated whether eye movements play a functional role in visual mental imagery. A recent paper by Laeng and Teodorescu presents new evidence that eye movements are stored as a spatial index that is used to arrange the component parts correctly when mental images are generated.

Visual mental imagery during caloric vestibular stimulation

We investigated high-resolution mental imagery and mental rotation, while the participants received caloric vestibular stimulation. High-resolution visual mental imagery tasks have been shown to activate early visual cortex, which is deactivated by vestibular input. Thus, we predicted that vestibular stimulation would disrupt high-resolution mental imagery; this prediction was confirmed. In addition, mental rotation tasks have been shown to activate posterior parietal cortex, which is also engaged in the processing of vestibular stimulation. As predicted, we also found that mental rotation is impaired during vestibular stimulation. In contrast, such stimulation did not affect performance of a low-imagery control task. These data document previously unsuspected interactions between the vestibular system and the high-level visual system.

Visual mental images can be ambiguous: insights from individual differences in spatial transformation abilities

The debate about whether objects in mental images can be ambiguous has produced ambiguous results. In some studies, participants could not reinterpret objects in images, but even in the studies where participants could reinterpret visualized patterns, the results are not conclusive. The present study used a novel task to investigate the reinterpretation of ambiguous figures in imagery, which required the participants to mentally rotate a figure 180 degrees before attempting to see an alternate interpretation. In addition, the participants did not know the purpose of the study in advance, nor did they see alternate interpretations of the stimuli; moreover, we explicitly measured individual differences in key mental imagery abilities. Eight of the 44 participants discovered the alternate version while they were memorizing the figure; 16 reported it after mentally rotating an image; and 20 were not able to see the alternate version. The ability to rotate images, assessed with an independent task, was highly associated with reports of image reversals, whereas measures of other imagery abilities were not.

Four types of visual mental imagery processing in upright and tilted observers.

We investigated the role of body position on performance in four distinct types of mental imagery processing. Previous studies used the upright body position as standard procedure and therefore do not address the issue of whether mental imagery tasks are processed in accordance with ego-centered or gravitational coordinates. In the present study, the subjects were brought into one of three different body positions: upright, horizontal, or supine. In each of these body positions, we measured performance in four imagery tasks, which assessed (1) the ability to generate vivid, high-resolution mental images; (2) the ability to compose mental images from separate parts; (3) the ability to inspect patterns in mental images; and, (4) the ability to mentally rotate patterns in images. Not all processes were affected in the same way when subjects performed them in different body positions. Performance in the image composition and detection tasks depended on body position, whereas there was no such effect for the transformation and resolution tasks.

Mental Imagery of Visual Motion Modifies the Perception of Roll-Vection Stimulation

When viewing a wide-angle visual display, which rotates in the frontoparallel plane around the line of sight, observers experience an illusory shift of the direction of gravity; this shift leads to an apparent tilt of the body and displaces allocentric space coordinates. In this study, subjects adjusted an indicator to the apparent horizontal while viewing a rotating display. To determine whether top–down processes could affect the illusion, the subjects were asked to visualize a rotating configuration of dots onto a blank central portion of the moving visual field. Visualizing dots and actually viewing the dots deflected the spatial judgment in very similar ways. These results demonstrate that top–down processing can affect allocentric space coordinates.

Can a unilateral loss of otolithic function be clinically detected by assessment of the subjective visual vertical

Asymmetries in the settings of the subjective visual vertical after unilateral vestibular neurectomy during eccentric centrifugation [3] might provide a clinical test for unilateral otolithic function. This study investigates whether these asymmetries can also be revealed by a technically much easier practicable roll tilt of the subject relative to gravity instead of a roll tilt of the gravitational force on a human centrifuge. Twenty-seven normal subjects and 13 patients before surgery indicated verticality very accurately in the upright position. In 26 degrees roll positions (subjects seated on a slanted chair), they were only slightly more variable with no asymmetries larger than 5.3 and 7.8 degrees in normals and preoperative patients, respectively, between the roll positions toward the healthy and toward the affected ear. One week after surgical unilateral vestibular deafferentation, there was a consistent shift (mean 11.9 degrees) of the subjective vertical toward the affected ear in all patients and in all body positions. When the settings in the two roll tilt positions were referred to the setting in upright position, the group means of the patients were symmetrical although single subjects revealed asymmetries up to 22.4 degrees. Only one of four patients who were tested also during eccentric rotation revealed an important asymmetry with decreased sensitivity for tilts of the gravitational vector toward the affected ear. Measuring the subjective visual vertical assesses only asymmetrical tonic otolithic input, while a simple clinical test for unilateral otolithic sensitivity still has to be found.

Does the world rock when the eyes roll

When the head is inclined sideways, the eyes are counter-rotated with respect to the head (ocular-counterroll, OCR). In man, the gain of OCR in static body tilts is limited to about 10% of the angle of roll tilt, which suggests that its function is vestigial. However, it is still unclear how the residual OCR is related to the perceived orientation of visual stimuli. Wade and Curthoys (1997) claim that the brain does not “take into account” the OCR, so that the eye position directly interferes with perception of visual orientation. Alternately, it has been argued that OCR is partly compensated by an extraretinal eye-position information such as, e.g., an efference copy (Haustein, 1992; Haustein & Mittelstaedt, 1990). The two experiments reported in this study are targeted towards critically examining this inter-relation between OCR and perceived visual orientation. The latter was assessed via the subjective visual vertical, SVV, which is determined when a subject judges the orientation of an indicator (e.g...

A novel automatic procedure for measuring ocular counterrolling: A computeranalytical method to determine the eye’s roll angle while subjects work on perceptual tasks

In investigations of the perception of space, the consideration of ocular counterrolling-the movement of the eye around its visual axis in response to body movement-is crucially important. The angle of this movement must be known in order for one to determine the precise retinal coordinates of a distal object. Following transformation, this stimulus serves as a reliable cue for visual direction. The otolith organs provide information about body tilt and are responsible for ocular counterrolling. A novel, noninvasive method to measure ocular counterrolling, based on the cross-correlation of digitized video pictures of the eyes, is presented. The resolution attained was ≤ 0.1°. The computer analysis is fully automatic and fast, and it can be performed while subjects work on perceptual tasks. No direct access to the eyeballs is required. Data from 4 subjects showing the counterrolling profile in various body positions are presented.

Human perception of verticality: Psychophysical experiments on the centrifuge and their neuronal implications

The role of the otoliths in the perception of verticality is analyzed in two different gravitational environments, 1 g and 1 .5 g, and in different roll body positions between upright and upside down. The subjective visual vertical (SW) is determined when a subject judges the orrentation of an indicator as apparently vertical. An increase of g level hardly affects the SW in the subjects frontal plane (y-z plane). However, for the first time, a three-dimensionally adjustable indicator was used for the SW and this revealed a new phenomenon: An increase of g level induces a backward slant of the SW into subjects median plane (x-z plane). The data are discussed with regard to Mittelstaedts SW theory; particular emphasis is given to the otolith-head coordrnate transformatron and the normalization of afferent otolith components. The results of this study provrde evidence that the former is implemented at an earlier level and thus orecedes the latter.