The role of neck afferents in subjective orientation in the visual and tactile sensory modalities

Using Neck Muscle Afferentation to Control an Ongoing Limb Movement? Individual Differences in the Influence of Brief Neck Vibration

When preparing and executing goal-directed actions, neck proprioceptive information is critical to determining the relative positions of the body and target in space. While the contribution of neck proprioception for upper-limb movements has been previously investigated, we could not find evidence discerning its impact on the planning vs. online control of upper-limb trajectories. To investigate these distinct sensorimotor processes, participants performed discrete reaches towards a virtual target. On some trials, neck vibration was randomly applied before and/or during the movement, or not at all. The main dependent variable was the medio-lateral/directional bias of the reaching finger. The neck vibration conditions induced early leftward trajectory biases in some participants and late rightward trajectory biases in others. These different patterns of trajectory biases were explained by individual differences in the use of body-centered and head-centered frames of reference. Importantly, the current study provides direct evidence that sensory cues from the neck muscles contribute to the online control of goal-directed arm movements, likely accompanied by significant individual differences.

Visuo-vestibular conflicts within the roll plane modulate multisensory verticality perception

The integration of visuo-vestibular information is crucial when interacting with the external environment. Under normal circumstances, vision and vestibular signals provide corroborating information, for example regarding the direction and speed of self-motion. However, conflicts in visuo-vestibular signalling, such as optic flow presented to a stationary observer, can change subsequent processing in either modality. While previous studies have demonstrated the impact of sensory conflict on unisensory visual or vestibular percepts, here we investigated whether visuo-vestibular conflicts impact sensitivity to multisensory percepts, specifically verticality. Participants were exposed to a visuo-vestibular conflicting or non-conflicting motion adaptor before completing a Vertical Detection Task. Sensitivity to vertical stimuli was reduced following visuo-vestibular conflict. No significant differences in criterion were found. Our findings suggest that visuo-vestibular conflicts not only modulate processing in unimodal channels, but also broader multisensory percepts, which may have implications for higher-level processing dependent on the integration of visual and vestibular signals.

Visuospatial orientation: Differential effects of head and body positions

To orientate in space, the brain must integrate sensory information that encodes the position of the body with the visual cues from the surrounding environment. In this process, the extent of reliance on visual information is known as the visual dependence. Here, we asked whether the relative positions of the head and body can modulate such visual dependence (VD). We used the effect of optokinetic stimulation (30°/s) on subjective visual vertical (SVV) to quantify VD as the average optokinetic-induced SVV bias in clockwise and counter-clockwise directions. The VD bias was measured in eight subjects with a head-on-body tilt (HBT) where only the head was tilted on the body, and also with a whole-body tilt (WBT) where the head and body were tilted together. The VD bias with HBT of 20° was in the same direction of the head tilt position (left tilt VD -1.35 ± 0.1.2°; right VD 1.60 ± 0.9°), whereas the VD bias with WBT of 20° was in a direction away from the body tilt position (left tilt VD 2.5 ± 1.1°; right tilt VD -2.1 ± 0.9°). These findings show differential effects of relative head and body positions on visual cue integration, a process which could facilitate optimal interaction with the surrounding environment for spatial orientation.

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.

Which way is down? Visual and tactile verticality perception in expert dancers and non-experts

Gravity provides an absolute verticality reference for all spatial perception, allowing us to move within and interact effectively with our world. Bayesian inference models explain verticality perception as a combination of online sensory cues with a prior prediction that the head is usually upright. Until now, these Bayesian models have been formulated for judgements of the perceived orientation of visual stimuli. Here, we investigated whether judgements of the verticality of tactile stimuli follow a similar pattern of Bayesian perceptual inference. We also explored whether verticality perception is affected by the postural and balance expertise of dancers. We tested both the subjective visual vertical (SVV) and the subjective tactile vertical (STV) in ballet dancers and non-dancers. A robotic arm traced downward-moving visual or tactile stimuli in separate blocks while participants held their head either upright or tilted 30° to their right. Participants reported whether these stimuli deviated to the left (clockwise) or right (anti-clockwise) of the gravitational vertical. Tilting the head biased the SVV away from the longitudinal head axis (the classical E-effect), consistent with a failure to compensate for the vestibulo-ocular counter-roll reflex. On the contrary, tilting the head biased the STV toward the longitudinal head axis (the classical A-effect), consistent with a strong upright head prior. Critically, tilting the head reduced the precision of verticality perception, particularly for ballet dancers' STV judgements. Head tilt is thought to increase vestibular noise, so ballet dancers seem to be surprisingly susceptible to degradation of vestibular inputs, giving them an inappropriately high weighting in verticality judgements.

The Tilted Self: Visuo-Graviceptive Mismatch in the Full-Body Illusion

The bodily self is a fundamental part of human self-consciousness and relies on online multimodal information and prior beliefs about one's own body. While the contribution of the vestibular system in this process remains under-investigated, it has been theorized to be important. The present experiment investigates the influence of conflicting gravity-related visual and bodily information on the sense of a body and, vice versa, the influence of altered embodiment on verticality and own-body orientation perception. In a full-body illusion setup, participants saw in a head-mounted display a projection of their own body 2 m in front of them, on which they saw a tactile stimulation on their back displayed either synchronously or asynchronously. By tilting the seen body to one side, an additional visuo-graviceptive conflict about the body orientation was created. Self-identification with the seen body was measured explicitly with a questionnaire and implicitly with skin temperature. As measures of orientation with respect to gravity, we assessed subjective haptic vertical and the haptic body orientation. Finally, we measured the individual visual field dependence using the rod-and-frame test. The results show a decrease in self-identification during the additional visuo-graviceptive conflict, but no modulation of perceived verticality or subjective body orientation. Furthermore, explorative analyses suggest a stimulation-dependent modulation of the perceived body orientation in individuals with a strong visual field dependence only. The results suggest a mutual interaction of graviceptive and other sensory signals and the individual's weighting style in defining our sense of a bodily self.

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.

Sensorimotor and cognitive factors associated with the age-related increase of visual field dependence: a cross-sectional study

Reliance on the visual frame of reference for spatial orientation (or visual field dependence) has been reported to increase with age. This has implications on old adults' daily living tasks as it affects stability, attention, and adaptation capacities. However, the nature and underlying mechanisms of this increase are not well defined. We investigated sensorimotor and cognitive factors possibly associated with increased visual field dependence in old age, by considering functions that are both known to degrade with age and important for spatial orientation and sensorimotor control: reliance on the (somatosensory-based) egocentric frame of reference, visual fixation stability, and attentional processing of complex visual scenes (useful field of view, UFOV). Twenty young, 18 middle-aged, and 20 old adults completed a visual examination, three tests of visual field dependence (RFT, RDT, and GEFT), a test of egocentric dependence (subjective vertical estimation with the body erect and tilted at 70°), a visual fixation task, and a test of visual attentional processing (UFOV®). Increased visual field dependence with age was associated with reduced egocentric dependence, visual fixation stability, and visual attentional processing. In addition, visual fixation instability and reduced UFOV were correlated. Results of middle-aged adults fell between those of the young and old, revealing the progressive nature of the age effects we evaluated. We discuss results in terms of reference frame selection with respect to ageing as well as visual and non-visual information processing. Inter-individual differences amongst old adults are highlighted and discussed with respect to the functionality of increased visual field dependence.

Quantifying effects of stochasticity in reference frame transformations on posterior distributions

Reference frame transformations are usually considered to be deterministic. However, translations, scaling or rotation angles could be stochastic. Indeed, variability of these entities often originates from noisy estimation processes. The impact of transformation noise on the statistics of the transformed signals is unknown and a quantification of these effects is the goal of this study. We first quantify analytically and numerically how stochastic reference frame transformations (SRFT) alter the posterior distribution of the transformed signals. We then propose an new empirical measure to quantify deviations from a given distribution when only limited data is available. We apply this empirical measure to an example in sensory-motor neuroscience to quantify how different head roll angles change the distribution of reach endpoints away from the normal distribution.

Human perceptual overestimation of whole body roll tilt in hypergravity

Hypergravity provides a unique environment to study human perception of orientation. We utilized a long-radius centrifuge to study perception of both static and dynamic whole body roll tilt in hypergravity, across a range of angles, frequencies, and net gravito-inertial levels (referred to as G levels). While studies of static tilt perception in hypergravity have been published, this is the first to measure dynamic tilt perception (i.e., with time-varying canal stimulation) in hypergravity using a continuous matching task. In complete darkness, subjects reported their orientation perception using a haptic task, whereby they attempted to align a hand-held bar with their perceived horizontal. Static roll tilt was overestimated in hypergravity, with more overestimation at larger angles and higher G levels, across the conditions tested (overestimated by ∼35% per additional G level, P < 0.001). As our primary contribution, we show that dynamic roll tilt was also consistently overestimated in hypergravity (P < 0.001) at all angles and frequencies tested, again with more overestimation at higher G levels. The overestimation was similar to that for static tilts at low angular velocities but decreased at higher angular velocities (P = 0.006), consistent with semicircular canal sensory integration. To match our findings, we propose a modification to a previous Observer-type canal-otolith interaction model. Specifically, our data were better modeled by including the hypothesis that the central nervous system treats otolith stimulation in the utricular plane differently than stimulation out of the utricular plane. This modified model was able to simulate quantitatively both the static and the dynamic roll tilt overestimation in hypergravity measured experimentally.

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.

Does the integration of haptic and visual cues reduce the effect of a biased visual reference frame on the subjective head orientation?

Background

The selection of appropriate frames of reference (FOR) is a key factor in the elaboration of spatial perception and the production of robust interaction with our environment. The extent to which we perceive the head axis orientation (subjective head orientation, SHO) with both accuracy and precision likely contributes to the efficiency of these spatial interactions. A first goal of this study was to investigate the relative contribution of both the visual and egocentric FOR (centre-of-mass) in the SHO processing. A second goal was to investigate humans' ability to process SHO in various sensory response modalities (visual, haptic and visuo-haptic), and the way they modify the reliance to either the visual or egocentric FORs. A third goal was to question whether subjects combined visual and haptic cues optimally to increase SHO certainty and to decrease the FORs disruption effect.

Methodology/Principal Findings

Thirteen subjects were asked to indicate their SHO while the visual and/or egocentric FORs were deviated. Four results emerged from our study. First, visual rod settings to SHO were altered by the tilted visual frame but not by the egocentric FOR alteration, whereas no haptic settings alteration was observed whether due to the egocentric FOR alteration or the tilted visual frame. These results are modulated by individual analysis. Second, visual and egocentric FOR dependency appear to be negatively correlated. Third, the response modality enrichment appears to improve SHO. Fourth, several combination rules of the visuo-haptic cues such as the Maximum Likelihood Estimation (MLE), Winner-Take-All (WTA) or Unweighted Mean (UWM) rule seem to account for SHO improvements. However, the UWM rule seems to best account for the improvement of visuo-haptic estimates, especially in situations with high FOR incongruence. Finally, the data also indicated that FOR reliance resulted from the application of UWM rule. This was observed more particularly, in the visual dependent subject. Conclusions: Taken together, these findings emphasize the importance of identifying individual spatial FOR preferences to assess the efficiency of our interaction with the environment whilst performing spatial tasks.

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.

Influence of head orientation on visually and memory-guided arm movements

In the absence of visual supervision, tilting the head sideways gives rise to deviations in spatially defined arm movements. The purpose of this study was to determine whether these deviations are restricted to situations with impoverished visual information. Two experiments were conducted in which participants were positioned supine and reproduced with their unseen index finger a 2 dimensional figure either under visual supervision or from memory (eyes closed). In the former condition, the figure remained visible (using a mirror). In the latter condition, the figure was first observed and then reproduced from memory. Participants' head was either aligned with the trunk or tilted 30° towards the left or right shoulder. In experiment 1, participants observed first the figure with the head straight and then reproduced it with the head either aligned or tilted sideways. In Experiment 2, participants observed the figure with the head in the position in which the figure was later reproduced. Results of Experiment 1 and 2 showed deviations of the motor reproduction in the direction opposite to the head in both the memory and visually-guided conditions. However, the deviations decreased significantly under visual supervision when the head was tilted left. In Experiment 1, the perceptual visual bias induced by head tilt was evaluated. Participants were required to align the figure parallel to their median trunk axis. Results revealed that the figure was perceived as parallel with the trunk when it was actually tilted in the direction of the head. Perceptual and motor responses did not correlate. Therefore, as long as visual feedback of the arm is prevented, an internal bias, likely originating from head/trunk representation, alters hand-motor production irrespectively of whether visual feedback of the figure is available or not.

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.

Perceived self-orientation in allocentric and egocentric space: effects of visual and physical tilt on saccadic and tactile measures

Do physical tilt and tilt of the visual environment affect perception of allocentric and egocentric space? We addressed this question using two perceptual-motor tasks: alignment of a tactile rod (ROD) and saccadic eye movements (EM). Nine participants indicated the vertical axis of their heads (egocentric task), as well as the direction of gravity (allocentric task). Head orientation (+/-60 degrees and 0 degrees) and visual environment orientation (+/-120 degrees, +/-60 degrees and 0 degrees) were independently manipulated in the fronto-planar roll plane. ROD and EM estimates of both allocentric and egocentric reference directions varied with head and room orientation. Physical tilt dominated allocentric estimates in the dark where overestimates of physical tilt were noted up to 11 degrees using both measures. Allocentric ROD and EM estimates were significantly correlated across all head orientations (r=.70, p<.01) but only when upright for egocentric estimates (r=.38, p<.01). The relative contributions of the visual environment, gravity's direction and long-body axis to the estimation of allocentric and egocentric directions were determined by vector modeling. This modeling found that vision determined about 14% of the allocentric ROD and EM estimates, that the long-axis body reference played no discernible role, and that the largest factor was gravity, the effective direction of which was non-veridical. For egocentric estimates, vision contributed about 3% with the largest factor being the body reference. We conclude that perception of allocentric and egocentric space is likely influenced by multiple senses that define common egocentric and allocentric frames of reference accessible for saccadic and tactile estimates of perceived self-orientation.

Head orientation biases tactile localization

Does the perceived location of tactile stimuli presented on the torso depend on the orientation of our heads with respect to our bodies? An experiment is reported that was designed to assess whether the subjective perception of tactile stimuli on the torso changes as people turn their heads in different directions. Our participants used a scale presented on a computer monitor to indicate the perceived position of vibrotactile stimuli presented to one of eight different positions around the frontal side of their waist while they either looked straight ahead, turned their head to the left, or else turned their head to the right. The results showed that the perceived location of tactile stimuli was systematically influenced by head orientation. In particular, the perceived location of the tactile stimuli shifted away from their actual position in the direction opposite to the direction of the participant's head turn. Our results also revealed a systematic decline in the accuracy of tactile localization as a function of the physical distance of the tactile stimuli from the participant's navel. These results echo related findings in the auditory domain where it has been shown that changes in eye position affect auditory lateralization. Our results also have important implications for the design of tactile displays for presenting directional information in a variety of real-world applications.

Perceived versus actual head-on-trunk orientation during arm movement control

Static roll head tilt induces bias in the trajectory of upper limb voluntary movements. The aim of the experiment was to investigate whether this bias is dependant on the perception of body configuration rather than on its actual configuration. We used the 'return' phenomenon as a method to produce dissociation between perceived and actual head tilt. Static roll head tilt in supine subjects was sustained for 15 min during which subjects were periodically required to estimate verbally the tilt of their head respective to their trunk and draw, with their right index finger, straight lines aligned with their trunk. After 15 min, subjects' head were realigned with the trunk, and subjects continued to give verbal estimate of head position and perform the motor task. Results showed that the initial angular deviation of the lines in the direction opposite to head tilt gradually diminished. The adaptation was noticeable within the first 3-5 min of tilt and subsequently diminished. Verbal estimates confirmed the return phenomenon, i.e. subjects perceived their head as slowly returning towards its neutral position after a few minutes of sustained tilt. When realigned with the trunk, subjects experienced the illusion that their head was tilted in the opposite direction to the initial head tilt and a line deviation in the opposite direction to those made on initial exposure was observed (after-effect). These results indicate that the angular deviation in motor production observed in condition of static head tilt were largely related to the perceived body configuration and therefore favour the hypothesis that the conscious perception of body configuration plays a key role in organising sensorimotor tasks.

Haptic shape discrimination in humans: insight into haptic frames of reference

This study investigates how a change in the physical relation between objects (two-dimensional, 2-D, angles) and a subject, as well as scanning conditions, modify the ability to discriminate small changes in 2-D shape. Subjects scanned pairs of angles (90 masculine standard; 91 masculine-103 masculine comparison angles) with the right index finger of the out-stretched arm, identifying the larger of each pair. When joint rotation was restricted to the shoulder, the discrimination threshold significantly increased when the angles were explored with the shoulder in a more eccentric position rather than closer to the midline (60 masculine versus 30 masculine to the right). This result was attributed to changes in proprioceptive sensitivity, since explorations restricted to distal joints (wrist/second metacarpophalangeal joint) showed no change with shoulder position. The results showed, moreover, that discrimination threshold was similar for distal and proximal joints when the delay between scanning the pairs of angles was long (15 s). This observation suggests that regional variations in proprioceptive acuity (proximal>distal) may reflect an adaptation to generate an invariant central representation of haptic shape. Using a shorter interscan delay (5 s), a position-dependent increase in discrimination threshold was revealed for distal explorations, an effect that disappeared when the head was turned in the direction of the unseen angle (vision occluded). We suggest that these results can be explained by the existence of two competing egocentric frames of reference with different time courses, one of short duration that is centred on the arm/hand, and a second of longer duration centred on the head. At the short delay, the reference frames interacted to distort the haptic representation when they were misaligned. This distortion was resolved at the long delay, possibly through suppression of the arm/hand-centred reference frame.

The visual perception coordinate system uses axes defined by the earth, trunk, and vision

Eight young adults adjusted a line located on one side of a computer display parallel to internally specified Earth-fixed vertical (display in frontal plane), to the horizontal trunk-fixed anterior-posterior axis (display in horizontal plane), and to an oblique line (display in horizontal and vertical planes). All tasks were completed in a dark room with the head and trunk in both a standard erect posture and varied postures. Errors were lowest when setting the line to internally specified vertical in the frontal plane and to an oblique line in the horizontal plane when head and trunk orientations were varied. Constant errors for setting one line parallel to a second line were in opposite directions when the second line was located on the left versus right side of the display, but did not differ in direction when setting the line parallel to internally specified axes. Also, the oblique effect was preserved when the head and trunk were tilted to various orientations, suggesting that it results from integration of an internally specified gravitational reference with visual input. We conclude that the visual perceptual coordinate system uses internally specified vertical and, when available, a visually specified horizontal reference axis to define object orientation.

Contribution of neck proprioception to subjective vertical perception among experts in physical activities and untrained women

The purpose of this study was to investigate the influence of physical training on subjective vertical perception with the different head positions in order to explore the involving of the neck proprioception. Visual field dependence was assessed using a rod and frame test on women practising judo or dance (international level) or no specific physical activity. Tests were performed with head upright or tilted head to disturb the Z-axis egocentric reference frame. A cluster analysis determined the distribution of visual field independent (VFI) and visual field dependent (VFD) participants. The first result showed no head position effect for the group of judoists (6 degrees +/-5 degrees ; 7 degrees +/-5 degrees ) and dancers (4 degrees +/-2 degrees ; 5 degrees +/-3 degrees ) but a significant effect for untrained participants (5 degrees +/-2 degrees ; 7 degrees +/-3 degrees ): their visual vertical perception was more disturbed with tilted head than with head upright. A variability exists among experts and would necessitate further analysis. The second result showed no influence of the head position for all VFD participants, whereas for VFI participants significant difference between upright and tilted head appeared both for experts (3 degrees +/-1 degrees ; 4 degrees +/-2 degrees ) and untrained participants (3 degrees +/-1 degrees ; 5 degrees +/-2 degrees ). In this research, whatever physical activity level, the VFI participants would mainly use the Z-axis reference frame and rely on proprioceptive information. VFD among experts and VFI among untrained participants suggest that proprioceptive reference frame of neck may not provide alone according the groups an appropriate postural control.

Effect of head position and visual condition on balance control in inverted stance

This study analyzed the effect of head position and visual condition on the control of balance in handstand, a gymnastics posture that necessitates adaptation of sensory information processing. Five expert gymnasts participated. Centre of pressure trajectories and kinematics of different body segments were recorded. The gymnasts were instructed to maintain three handstands as long as possible in four head positions, with and without vision. Performances and postural stability was much better in the standard and dorsiflexion positions than in the aligned and ventroflexion positions under the two conditions of vision. Performances were lower without vision in the standard and dorsiflexion position. If vision clearly plays an important role, yet the tonic neck reflexes also seem to contribute greatly to control body sways during inverted posture.

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

This work investigated the accuracy of the perception of the main orientations (i.e., vertical and horizontal orientations) with the kinesthetic modality--a modality not previously used in this field of research. To further dissociate the influence of the postural and physical verticals, two body positions were explored (supine and upright). Twenty-two blindfolded participants were asked to set, as accurately as possible, a rod to both physical orientations while assuming one of the two body positions. The horizontal was perceived more accurately than the vertical orientation in the upright position but not in the supine position. Essentially, there were no differences in the supine position because the adjustments to the physical vertical were much more accurate than they were in the upright position. The lower accuracy in the estimation of the vertical orientation observed in the upright position might be linked to the dynamics associated with the maintenance of posture.

From head orientation to hand control: evidence of both neck and vestibular involvement in hand drawing

This research investigated the effect of head to trunk relation in a sensorimotor drawing task. In the first experiment, seated participants were asked to reproduce with eyes closed geometric shapes (square or diamond) with the tip of their right index finger in the frontoparallel plane. Their head was either aligned with the trunk or tilted 25 degrees towards the left or right shoulder. Results showed that drawings were subjected to an overall rotation of a few degrees in the opposite direction to the tilt. In two subsequent experiments, the respective contribution of both otoliths and neck receptors to this head tilt effect was investigated. In Experiment 2, seated participants kept their head straight but were subjected to 2.5 mA vestibular galvanic stimulation (GVS). Results indicated that GVS induced a small but significant deviation of the drawings towards the anode. Finally, in Experiment 3, subjects performed the drawing task either seated upright (seated condition) or lying on their back (supine condition). Unlike in the seated condition, tilting the head towards the shoulders in a supine posture does not modulate afferents from the otolith stimulation and therefore mainly stimulates neck receptors. Head tilt induced rotations of hand-drawn reproductions in both seated and supine conditions, suggesting a significant contribution of neck afferents in the control of hand motion in space in the absence of vision. Overall the data provided evidence for a strong head-hand linkage during kinaesthetically guided drawing movements.

Visual, proprioceptive and tactile performance in left neglect patients

Patients with unilateral spatial neglect due to right-hemisphere lesions typically fail to attend to and explore left-sided stimulus objects. It has been postulated that in right-brain damaged (RBD) patients an ipsilesional displacement of the egocentric frame of reference (ER), whether visual or tactile, may be responsible for a contralesional supramodal spatial bias causing their left neglect behavior. However, this hypothesis had been proposed without testing, in the same patients, the position of the ER or their performance in the visual and tactile modalities. Thus, the aim of the present study was to test the hypothesis that an ipsilateral shift of the ER is responsible for a supramodal spatial bias in neglect. For this purpose, a within-subject design is required. Consequently, 12 left neglect patients and 12 control subjects were asked to perform a proprioceptive straight-ahead pointing task while blindfolded, as well as visual and tactile bisection tasks. In the left neglect patients, we found:no systematic deviation of the ER on the ipsilesional right side;a significant rightward bias in visual bisection, and normal performance in tactile bisection;no correlation among the three tasks;that only visual bisection correlated with the severity of neglect. These results are discussed with regard to the egocentric and attentional hypothesis of neglect.