Kinaesthetic and visual perceptions of orientations

Subjective visual vertical in vestibular disorders measured with the bucket test

CONCLUSION

The 'bucket test' may indicate that patients with known vestibular disorders have spatial orientation deficits but due to the low receiver operating characteristics (ROC) values it is not useful for screening people for vestibular impairments.

OBJECTIVES

1) to verify that patients with unilateral peripheral vestibular weakness (UW) differ from normal subjects on the bucket test, 2) to determine if patients with unilateral benign paroxysmal positional vertigo (BPPV) differ from normal subjects, 3) to determine the sensitivity and specificity of the test.

METHODS

Patients with UW (n = 25) and BPPV (n = 25) were compared to normals (n = 50). Subjects looked into a clean bucket with a vertical line on the bottom, which rested on a table. It was rotated, clockwise or counterclockwise, for three trials per direction until the subject indicated that the line was vertical. The dependent measure was the mean absolute value of the deviations from the true vertical.

RESULTS

Some, but not all, patients' responses differed from normal subjects but responses also differed by age and sex. ROC values were all weak, i.e. < 0.8. No good cut-off points differentiated controls from patients. Thus, although the bucket test is useful for describing spatial deficits in patients this test is not useful for screening people for possible vestibular impairments.

Gender and line size factors modulate the deviations of the subjective visual vertical induced by head tilt

Background

The subjective visual vertical (SVV, the visual estimation of gravitational direction) is commonly considered as an indicator of the sense of orientation. The present study examined the impact of two methodological factors (the angle size of the stimulus and the participant's gender) on deviations of the SVV caused by head tilt. Forty healthy participants (20 men and 20 women) were asked to make visual vertical adjustments of a light bar with their head held vertically or roll-tilted by 30° to the left or to the right. Line angle sizes of 0.95° and 18.92° were presented.

Results

The SVV tended to move in the direction of head tilt in women but away from the direction of head tilt in men. Moreover, the head-tilt effect was also modulated by the stimulus' angle size. The large angle size led to deviations in the direction of head-tilt, whereas the small angle size had the opposite effect.

Conclusions

Our results showed that gender and line angle size have an impact on the evaluation of the SVV. These findings must be taken into account in the growing body of research that uses the SVV paradigm in disease settings. Moreover, this methodological issue may explain (at least in part) the discrepancies found in the literature on the head-tilt effect.

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.

Precision and accuracy of the subjective haptic vertical in the roll plane

BACKGROUND

When roll-tilted, the subjective visual vertical (SVV) deviates up to 40 degrees from earth-vertical and trial-to-trial variability increases with head roll. Imperfections in the central processing of visual information were postulated to explain these roll-angle dependent errors. For experimental conditions devoid of visual input, e.g. adjustments of body posture or of an object along vertical in darkness, significantly smaller errors were noted. Whereas the accuracy of verticality adjustments seems to depend strongly on the paradigm, we hypothesize that the precision, i.e. the inverse of trial-to-trial variability, is less influenced by the experimental setup and mainly reflects properties of the otoliths. Here we measured the subjective haptic vertical (SHV) and compared findings with previously reported SVV data. Twelve healthy right-handed human subjects (handedness assessed based on subjects' verbal report) adjusted a rod with the right hand along perceived earth-vertical during static head roll-tilts (0-360 degrees , steps of 20 degrees ).

RESULTS

SHV adjustments showed a tendency for clockwise rod rotations to deviate counter-clockwise and for counter-clockwise rod rotations to deviate clockwise, indicating hysteresis. Clockwise rod rotations resulted in counter-clockwise shifts of perceived earth-vertical up to -11.7 degrees and an average counter-clockwise SHV shift over all roll angles of -3.3 degrees (+/- 11.0 degrees ; +/- 1 StdDev). Counter-clockwise rod rotations yielded peak SHV deviations in clockwise direction of 8.9 degrees and an average clockwise SHV shift over all roll angles of 1.8 degrees (+/- 11.1 degrees ). Trial-to-trial variability was minimal in upright position, increased with increasing roll (peaking around 120-140 degrees ) and decreased to intermediate values in upside-down orientation. Compared to SVV, SHV variability near upright and upside-down was non-significantly (p > 0.05) larger; both showed an m-shaped pattern of variability as a function of roll position.

CONCLUSIONS

The reduction of adjustment errors by eliminating visual input supports the notion that deviations between perceived and actual earth-vertical in roll-tilted positions arise from central processing of visual information. The shared roll-tilt dependent modulation of trial-to-trial variability for both SVV and SHV, on the other hand, indicates that the perception of earth-verticality is dominated by the same sensory signal, i.e. the otolith signal, independent of whether the line/rod setting is under visual or tactile control.