Kinesthetic estimation of the main orientations from the upright and supine positions

The Effects of Posture on Mind Wandering

Using two executive tasks, we explored how body posture influences mind wandering, a universal internally self-generated activity. Specifically, participants were instructed to perform the Sustained Attention Response Task (SART) and the Flanker task under three postural conditions: lying supine, sitting, and standing upright. These tasks reflect the proactive and reactive modes of executive control, respectively. To measure the frequency of mind wandering, we employed the probe-caught technique, presenting prompts at irregular intervals. The results indicate that, compared to standing and sitting positions, lying supine significantly increased mind wandering, while posture had no effect on either measure of executive control. We suggest that changes in posture alter cognitive activity related to self-generated thoughts and external tasks, whereas the relationship between mind wandering and executive control requires further research.

Psychophysical Haptic Measurement of Vertical Perception: Elucidating a Hand Sensory Bias

The primary sensory modality for probing spatial perception can vary among psychophysical paradigms. In the subjective visual vertical (SVV) task, the brain must account for the position of the eye within the orbit to generate an estimate of a visual line orientation, whereas in the subjective haptic vertical (SHV) task, the position of the hand is used to sense the orientation of a haptic bar. Here we investigated whether a hand sensory bias can affect SHV measurement. We measured SHV in 12 subjects (6 left-handed and 6 right-handed) with a forced-choice paradigm using their left and right hands separately. The SHV measurement was less accurate than the SVV measurements (-0.6 ± 0.7) and it was biased in the direction of the hand used in the task but was not affected by handedness; SHV left hand -6.8 ± 2.1° (left-handed -7.9 ± 3.6°, right-handed -5.8 ± 2.5°) and right hand 9.8 ± 1.5° (left-handed 7.4 ± 2.2°, right-handed 12.3 ± 1.8°). SHV measurement with the same hand was also affected by the haptic bar placement on the left or right side versus midline, showing a side effect (left vs midline -2.0 ± 1.3°, right vs midline 3.8 ± 1.7°). Midline SHV measures using the left and right hands were different, confirming a laterality effect (left hand -4.5 ± 1.7°, right hand 6.4 ± 2.0°). These results demonstrate a sensory bias in SHV measurement related to the effects of both hand-in-body (i.e., right vs left hand) and hand-in-space positions. Such modality-specific bias may result in disparity between SHV and SVV measurements, and therefore cannot be generalized to vertical or spatial perception.

The effects of posture on mind wandering

Using a reading comprehension task, we explored whether body postures would influence mind wandering, a universal internally self-generated activity. Specifically, participants were instructed to perform a reading comprehension task under three postural conditions (lying supine, sitting, and standing upright). Probe-caught technique with prompts presented at irregular intervals was adapted to measure the frequency of mind wandering. Self-caught method was used to measure the meta-awareness of mind wandering by self-reports. Results indicated that the radio of mind wandering was significantly greater in lying than standing and sitting, but the meta-awareness of it was not different among three postures. Moreover, the reading performance, an indirect indicator of executive control, decreased in lying compared to standing and sitting. We suggested that the increase of mind wandering in lying posture may due to the dysfunction of executive control, which also results in the redistribution of cognitive resources. Suggestions for future research are proposed.

Perception of Upright: Multisensory Convergence and the Role of Temporo-Parietal Cortex

We inherently maintain a stable perception of the world despite frequent changes in the head, eye, and body positions. Such "orientation constancy" is a prerequisite for coherent spatial perception and sensorimotor planning. As a multimodal sensory reference, perception of upright represents neural processes that subserve orientation constancy through integration of sensory information encoding the eye, head, and body positions. Although perception of upright is distinct from perception of body orientation, they share similar neural substrates within the cerebral cortical networks involved in perception of spatial orientation. These cortical networks, mainly within the temporo-parietal junction, are crucial for multisensory processing and integration that generate sensory reference frames for coherent perception of self-position and extrapersonal space transformations. In this review, we focus on these neural mechanisms and discuss (i) neurobehavioral aspects of orientation constancy, (ii) sensory models that address the neurophysiology underlying perception of upright, and (iii) the current evidence for the role of cerebral cortex in perception of upright and orientation constancy, including findings from the neurological disorders that affect cortical function.

Hysteresis of haptic vertical and straight ahead in healthy human subjects

Background

The subjective haptic vertical (SHV) task requires subjects to adjust the roll orientation of an object, mostly in the roll plane, in such a way that it is parallel to perceived direction of gravity. Previously we found a tendency for clockwise rod rotations to deviate counter-clockwise and vice versa, indicating hysteresis. However, the contributing factors remained unclear. To clarify this we characterized the SHV in terms of handedness, hand used, direction of hand rotation, type of grasping (wrap vs. precision grip) and gender, and compared findings with perceived straight-ahead (PSA). Healthy subjects repetitively performed adjustments along SHV (n = 21) and PSA (n = 10) in complete darkness.

Results

For both SHV and PSA significant effects of the hand used and the direction of rod/plate rotation were found. The latter effect was similar for SHV and PSA, leading to significantly larger counter-clockwise shifts (relative to true earth-vertical and objective straight-ahead) for clockwise rotations compared to counter-clockwise rotations irrespective of the handedness and the type of grip. The effect of hand used, however, was opposite in the two tasks: while the SHV showed a counter-clockwise bias when the right hand was used and no bias for the left hand, in the PSA a counter-clockwise bias was obtained for the left hand without a bias for the right hand. No effects of grip and handedness (studied for SHV only) on accuracy were observed, however, SHV precision was significantly (p < 0.005) better in right-handed subjects compared to left-handed subjects and in male subjects.

Conclusions

Unimanual haptic tasks require control for the hand used and the type of grip as these factors significantly affect task performance. Furthermore, aligning objects with the SHV and PSA resulted in systematic direction-dependent deviations that could not be attributed to handedness, the hand used, or the type of grip. These deviations are consistent with hysteresis and are likely not related to gravitational pull, as they were observed in both planes tested, i.e. parallel and perpendicular to gravity. Short-term adaptation that shifts attention towards previous adjustment positions may provide an explanation for such biases of spatial orientation in both the horizontal and frontal plane.

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.

Kinaesthetic and visual perceptions of orientations

In the present study we compare the kinaesthetic and visual perception of the vertical and horizontal orientations (subjective vertical and subjective horizontal) to determine whether the perception of cardinal orientations is amodal or modality-specific. The influence of methodological factors on the accuracy of perception is also investigated by varying the stimulus position as a function of its initial tilt (clockwise or counterclockwise) and its angle (22 degrees, 45 degrees, 67 degrees, and 90 degrees) in respect to its physical orientation. Ten participants estimated the vertical and horizontal orientations by repositioning a rod in the kinaesthetic condition or two luminous points, forming a 'virtual line' in the visual condition. Results within the visual modality replicated previous findings by showing that estimation of the physical orientations is very accurate regardless of the initial position of the virtual line. In contrast, the perception of orientation with the kinaesthetic modality was less accurate and systematically influenced by the angle between the initial position of the rod and the required orientation. The findings question the assumption that the subjective vertical is derived from an internal representation of gravity and highlight the necessity of taking into account methodological factors in studies on subjective orientations.

Interaction between reference frames during subjective vertical estimates in a tilted immersive virtual environment

Numerous studies highlighted the influence of a tilted visual frame on the perception of the visual vertical ('rod-and-frame effect' or RFE). Here, we investigated whether this influence can be modified in a virtual immersive environment (CAVE-like) by the structure of the visual scene and by the adjustment mode allowing visual or visuo-kinaesthetic control (V and VK mode, respectively). The way this influence might dynamically evolve throughout the adjustment was also investigated in two groups of subjects with the head unrestrained or restrained upright. RFE observed in the immersive environment was qualitatively comparable to that obtained in a real display (portable rod-and-frame test; Oltman 1968, Perceptual and Motor Skills 26 503-506). Moreover, RFE in the immersive environment appeared significantly influenced by the structure of the visual scene and by the adjustment mode: the more geometrical and meaningful 3-D features the visual scene contained, the greater the RFE. The RFE was also greater when the subjective vertical was assessed under visual control only, as compared to visuo-kinaesthetic control. Furthermore, the results showed a significant RFE increase throughout the adjustment, indicating that the influence of the visual scene upon subjective vertical might dynamically evolve over time. The latter effect was more pronounced for structured visual scenes and under visuo-kinaesthetic control. On the other hand, no difference was observed between the two groups of subjects having the head restrained or unrestrained. These results are discussed in terms of dynamic combination between coexisting reference frames for spatial orientation.