Judging beforehand the possibility of passing under obstacles without motion: the influence of egocentric and geocentric frames of reference

Previous studies have shown that the perception of the earth-based visual horizon, also named Gravity Referenced Eye Level (GREL), is modified by body tilt around a trans-ocular axis. Here, we investigated whether estimates of the elevation of a luminous horizontal line presented on a screen in otherwise darkness and estimates of the possibility of passing under are identically related to body tilt in absence of motion. Results showed that subjects overestimated the elevation of the projected line, whatever their body orientation. In the same way, subjects also overestimated their capacity of passing under the line. Both estimates appeared as a linear function of body tilt, that is, forward body tilt yielded increased overestimations, and backward body tilt yielded decreased overestimations. More strikingly, the linear effect of body tilt upon these estimates is comparable to that previously observed for direct GREL judgements. Overall, these data strongly suggest that the perception of the elevation of a visible obstacle and the perception of the ability of passing under in otherwise darkness shared common processes which are intimately linked to the GREL perception. The effect of body tilt upon these perceptions may illustrate an egocentric influence upon the semi-geocentric frame of reference required to perform the task. Possible interactions between egocentric and geocentric frames of reference are discussed.     

Pitch body orientation influences the perception of self-motion direction induced by optic flow

We studied the effect of static pitch body tilts on the perception of self-motion direction induced by a visual stimulus. Subjects were seated in front of a screen on which was projected a 3D cluster of moving dots visually simulating a forward motion of the observer with upward or downward directional biases (relative to a true earth horizontal direction). The subjects were tilted at various angles relative to gravity and were asked to estimate the direction of the perceived motion (nose-up, as during take-off or nose-down, as during landing). The data showed that body orientation proportionally affected the amount of error in the reported perceived direction (by 40% of body tilt magnitude in a range of ±20°) and these errors were systematically recorded in the direction of body tilt. As a consequence, a same visual stimulus was differently interpreted depending on body orientation. While the subjects were required to perform the task in a geocentric reference frame (i.e., relative to a gravity-related direction), they were obviously influenced by egocentric references. These results suggest that the perception of self-motion is not elaborated within an exclusive reference frame (either egocentric or geocentric) but rather results from the combined influence of both.     

Gaze influences finger movement-related and visual-related activation across the human brain

The brain uses gaze orientation to organize myriad spatial tasks including hand movements. However, the neural correlates of gaze signals and their interaction with brain systems for arm movement control remain unresolved. Many studies have shown that gaze orientation modifies neuronal spike discharge in monkeys and activation in humans related to reaching and finger movements in parietal and frontal areas. To continue earlier studies that addressed interaction of horizontal gaze and hand movements in humans (Baker et al. 1999), we assessed how horizontal and vertical gaze deviations modified finger-related activation, hypothesizing that areas throughout the brain would exhibit movement-related activation that depended on gaze angle. The results indicated finger movement-related activation related to combinations of horizontal, vertical, and diagonal gaze deviations. We extended our prior findings to observation of these gaze-dependent effects in visual cortex, parietal cortex, motor, supplementary motor area, putamen, and cerebellum. Most significantly, we found a modulation bias for increased activation toward rightward, upper-right and vertically upward gaze deviations. Our results indicate that gaze modulation of finger movement-related regions in the human brain is spatially organized and could subserve sensorimotor transformations.     

Role of gravity-based information on the orientation and localization of the perceived body midline

The present study focused on the influence of gravity-based information on the orientation and localization of the perceived body midline. The orientation was investigated by the rolling adjustment of a rod on the subjects’ Z-axis and the localization by the horizontal adjustment of a visual dot as being straight ahead. Experiment 1 investigated the effect of the dissociation between the Z-axis and the direction of gravity by placing subjects in roll tilt and supine postures. In roll tilt, the perception of the body midline orientation was deviated in the direction of body tilt and the perception of its localization was deviated in the opposite direction. In the supine body orientation, estimates of the Z-axis and straight-ahead remained veridical as when the body was upright. Experiment 2 highlighted the relative importance of the otolithic and tactile information using diffuse pressure stimulation. The estimation of body midline orientation was modified contrarily to the estimation of its localization. Thus, subjects had no absolute representation of their egocentric space. The main hypothesis regarding the dissociation between the orientation and localization of the body midline may be related to a difference in the integration of sensory information. It can be suggested that the horizontal component of the vestibulo-ocular reflex (VOR) contributed to the perceived localization of the body midline, whereas its orientation was mainly influenced by tactile information.     

Otolith signals contribute to inter-individual differences in the perception of gravity-centered space

The aim of the present study was to investigate (1) the relative contribution of the egocentric reference as well as body orientation perception to visual horizon percept during tilt or during increased gravito-inertial acceleration (GiA, hypergravity environment) conditions and (2) the role of vestibular signals in the inter-individual differences observed in these perceptual modalities. Perceptual estimates analysis showed that backward tilt induced (1) an elevation of the visual horizon, (2) an elevation of the egocentric estimation (visual straight ahead) and (3) an overestimation of body tilt. The increase in the magnitude of GiA induced (1) a lowering of the apparent horizon, (2) a lowering of the straight ahead and (3) a perception of backward tilt. Overall, visual horizon percept can be expressed as the combination of body orientation perception and egocentric estimation. When assessing otolith reactivity using off-vertical axis rotation (OVAR), only visual egocentric estimation was significantly correlated with horizontal OVAR performance. On the one hand, we found a correlation between a low modulation amplitude of the otolith responses and straight ahead accuracy when the head axis was tilted relative to gravity. On the other hand, the bias of otolith responses was significantly correlated with straight ahead accuracy when subjects were submitted to an increase in the GiA. Thus, straight ahead sense would be dependent to some extent to otolith function. These results are discussed in terms of the contribution of otolith inputs in the overall multimodal integration subtending spatial constancy.     

Gaze anticipation during human locomotion

During locomotion, a top-down organization has been previously demonstrated with the head as a stabilized platform and gaze anticipating the horizontal direction of the trajectory. However, the quantitative assessment of the anticipatory sequence from gaze to trajectory and body segments has not been documented. The present paper provides a detailed investigation into the spatial and temporal anticipatory relationships among the direction of gaze and body segments during locomotion. Participants had to walk along several mentally simulated complex trajectories, without any visual cues indicating the trajectory to follow. The trajectory shapes were presented to the participants on a sheet of paper. Our study includes an analysis of the relationships between horizontal gaze anticipatory behavior direction and the upcoming changes in the trajectory. Our findings confirm the following: 1) The hierarchical ordered organization of gaze and body segment orientations during complex trajectories and free locomotion. Gaze direction anticipates the head orientation, and head orientation anticipates reorientation of the other body segments. 2) The influence of the curvature of the trajectory and constraints of the tasks on the temporal and spatial relationships between gaze and the body segments: Increased curvature resulted in increased time and spatial anticipation. 3) A different sequence of gaze movements at inflection points where gaze plans a much later segment of the trajectory.     

Locomotor behaviour of children while navigating through apertures

During everyday locomotion, we encounter a range of obstacles requiring specific motor responses; a narrow aperture which forces us to rotate our shoulders in order to pass through is one example. In adults, the decision to rotate their shoulders is body scaled (Warren and Whang in J Exp Psychol Hum Percept Perform 13:371–383, 1987), and the movement through is temporally and spatially tailored to the aperture size (Higuchi et al. in Exp Brain Res 175:50–59, 2006; Wilmut and Barnett in Hum Mov Sci 29:289–298, 2010). The aim of the current study was to determine how 8-to 10-year-old children make action judgements and movement adaptations while passing through a series of five aperture sizes which were scaled to body size (0.9, 1.1, 1.3, 1.5 and 1.7 times shoulder width). Spatial and temporal characteristics of movement speed and shoulder rotation were collected over the initial approach phase and while crossing the doorway threshold. In terms of making action judgements, results suggest that the decision to rotate the shoulders is not scaled in the same way as adults, with children showing a critical ratio of 1.61. Shoulder angle at the door could be predicted, for larger aperture ratios, by both shoulder angle variability and lateral trunk variability. This finding supports the dynamical scaling model (Snapp-Childs and Bingham in Exp Brain Res 198:527–533, 2009). In terms of movement adaptations, we have shown that children, like adults, spatially and temporally tailor their movements to aperture size.     

Accuracy level of pointing movements performed during slow passive whole-body rotations

Seated observers requested to detect low-velocity passive rotations show a high motion-detection threshold. However, when standing on a slowly rotating platform, their equilibrium is preserved, suggesting that cognitive sensing and sensorimotor reactions do not share the same central processes. The present experiments investigated the ability of observers seated on a slowly rotating chair in total darkness to indicate with their hand the position of briefly flashed targets (Experiment 1) and to indicate the subjective horizon with an outstretched arm (Experiment 2) or with a target driven by a joystick (Experiment 3). The overall hypothesis stated that egocentric coding of the position of a target should not be affected by sensing or not-sensing body rotation (Experiment 1), while geocentric positioning may (Experiments 2 and 3). Our data partially supported the hypothesis. Subjects pointed accurately to the memorized targets (Experiment 1), whereas misperception of body orientation was a source of inaccuracy for actions referred to a geocentric frame (Experiments 2 and 3). More interestingly, subjects' perceptions changed as a single, smooth, and monotonic function of tilt, independent of whether the perception of body orientation was present or not.     

Guidance of visual direction by topographical vibrotactile cues on the torso

Vibration on localised areas of skin can be used to signal spatial orientation, multi-directional motion and also to guide arm and hand movements. This study investigated the possibility that vibration at loci on the skin might also be used to cue gaze direction. Eight subjects made eye or (head + eye) gaze saccades in the dark cued by vibration stimulation at discrete loci spaced on a horizontal contour across the chest. Saccade and gaze amplitudes, latencies, and directions were analysed. In the first experiment, performed without training, subjects could only use vibration cues to direct their gaze in cardinal directions and gross quadrature. There was a high variability in the relationship between locus on the trunk and gaze direction in space, both within and between subjects. Saccade latencies ranged from 377 to 433 ms and were related to the loci of vibration; the further from the body midline the quicker the response. Since the association of skin loci with gaze direction did not appear intuitive a sub-group of four subjects were retested after intensive training with feedback until they attained criterion on midline ≡ 0° and 15 cm (to right/left of midline) ≡ 45° gaze shifts right and left. Training gave a moderate improvement in directional specificity of gaze to a particular locus on the skin. Gaze direction was linearly rescaled with respect to skin loci but variability and saccade latencies remained high. The uncertainty in the relationship between vibration locus and gaze direction and the prolonged latencies of responses indicate circuitous neuronal processing. There appears to be no pre-existing stimulus-response compatibility mapping between loci on the skin and gaze direction. Vibrotactile cues on the skin of the trunk only serve a gross indication of visual direction in space.     

Adaptation of postural orientation to changes in surface inclination

We previously showed that standing on an inclined surface resulted in an after-effect of leaning in many healthy, blindfolded subjects when they returned to standing on a horizontal surface (Kluzik et al. in Exp Brain Res 162:474–489, 2005). The direction of leaning depended on the direction of prior surface inclination, always in a direction that preserved the relative alignment between the body and the support surface. For example, subjects leaned forward after they stood on a toes-up-inclined surface. In the present study, we investigated how the amplitude of surface inclination affected postural muscle activity, joint position, body segment orientation, and body center of mass (CoM) and foot center of pressure (CoP) locations before, during, and after subjects stood on an inclined surface. We asked whether the mechanism that underlies the lean after-effect involves regulation of local postural variables, such as the position of the ankle joint or the level of muscle activity, or whether instead, the mechanism involves regulation of global, whole-body postural variables that can only be determined by multisensory processing, such as orientation of the trunk or the body’s CoM. In one experiment, we found that varying the amplitude of a toes-up surface inclination between 2.5° and 10° had a systematic, linear, effect on the post-incline orientation of the trunk and head, but did not systematically affect the post-incline orientation of the legs, position of the ankle joint, the level of EMG activity, or the location of the CoP. In a second experiment, we found that preventing the legs from leaning in the post-incline period did not abolish leaning of the upper body. These findings suggest that (1) the body-to-support-surface relationship is an important reference for the CNS internal representation of postural orientation which is subject to adaptive modification and (2) the adaptive mechanism underlying the post-incline after-effect of leaning acts at the level of global, whole-body postural variables.     

Spatial coding of eye movements relative to perceived earth and head orientations during static roll tilt

This purpose of this study was to examine the spatial coding of eye movements during static roll tilt (up to ±45°) relative to perceived earth and head orientations. Binocular videographic recordings obtained in darkness from eight subjects allowed us to quantify the mean deviations in gaze trajectories along both horizontal and vertical coordinates relative to the true earth and head orientations. We found that both variability and curvature of gaze trajectories increased with roll tilt. The trajectories of eye movements made along the perceived earth-horizontal (PEH) were more accurate than movements along the perceived head-horizontal (PHH). The trajectories of both PEH and PHH saccades tended to deviate in the same direction as the head tilt. The deviations in gaze trajectories along the perceived earth-vertical (PEV) and perceived head-vertical (PHV) were both similar to the PHH orientation, except that saccades along the PEV deviated in the opposite direction relative to the head tilt. The magnitude of deviations along the PEV, PHH, and PHV corresponded to perceptual overestimations of roll tilt obtained from verbal reports. Both PEV gaze trajectories and perceptual estimates of tilt orientation were different following clockwise rather than counterclockwise tilt rotation; however, the PEH gaze trajectories were less affected by the direction of tilt rotation. Our results suggest that errors in gaze trajectories along PEV and perceived head orientations increase during roll tilt in a similar way to perceptual errors of tilt orientation. Although PEH and PEV gaze trajectories became nonorthogonal during roll tilt, we conclude that the spatial coding of eye movements during roll tilt is overall more accurate for the perceived earth reference frame than for the perceived head reference frame. Received: 22 April 1997 / Accepted: 18 December 1997     

Effects of electrical coupling among layer 4 inhibitory interneurons on contrast-invariant orientation tuning

Simulations of orientation selectivity in visual cortex have shown that layer 4 complex cells lacking orientation tuning are ideal for providing global inhibition that scales with contrast in order to produce simple cells with contrast-invariant orientation tuning (Lauritzen and Miller in J Neurosci 23:10201–10213, 2003). Inhibitory cortical cells have been shown to be electrically coupled by gap junctions (Fukuda and Kosaka in J Neurosci 120:5–20, 2003). Such coupling promotes, among other effects, spike synchronization and coordination of postsynaptic IPSPs (Beierlein et al. in Nat Neurosci 3:904–910, 2000; Galarreta and Hestrin in Nat Rev Neurosci 2:425–433, 2001). Consequently, it was expected (Miller in Cereb Cortex 13:73–82, 2003) that electrical coupling would promote nonspecific functional responses consistent with the complex inhibitory cells seen in layer 4 which provide broad inhibition in response to stimuli of all orientations (Miller et al. in Curr Opin Neurobiol 11:488–497, 2001). This was tested using a mechanistic modeling approach. The orientation selectivity model of Lauritzen and Miller (J Neurosci 23:10201–10213, 2003) was reproduced with and without electrical coupling between complex inhibitory neurons. Although extensive coupling promotes uniform firing in complex cells, there were no detectable improvements in contrast-invariant orientation selectivity unless there were coincident changes in complex cell firing rates to offset the untuned excitatory component that grows with contrast. Thus, changes in firing rates alone (with or without coupling) could improve contrast-invariant orientation tuning of simple cells but not synchronization of complex inhibitory neurons alone.     

Transsaccadic integration of visual features in a line intersection task

Transsaccadic integration (TSI) refers to the perceptual integration of visual information collected across separate gaze fixations. Current theories of TSI disagree on whether it relies solely on visual algorithms or also uses extra-retinal signals. We designed a task in which subjects had to rely on internal oculomotor signals to synthesize remembered stimulus features presented within separate fixations. Using a mouse-controlled pointer, subjects estimated the intersection point of two successively presented bars, in the dark, under two conditions: Saccade task (bars viewed in separate fixations) and Fixation task (bars viewed in one fixation). Small, but systematic biases were observed in both intersection tasks, including position-dependent vertical undershoots and order-dependent horizontal biases. However, the magnitude of these errors was statistically indistinguishable in the Saccade and Fixation tasks. Moreover, part of the errors in the Saccade task were dependent on saccade metrics, showing that egocentric oculomotor signals were used to fuse remembered location and orientation features across saccades. We hypothesize that these extra-retinal signals are normally used to reduce the computational load of calculating visual correspondence between fixations. We further hypothesize that TSI may be implemented within dynamically updated recurrent feedback loops that interconnect a common eye-centered map in occipital cortex with both the “dorsal” and “ventral” streams of visual analysis.     

Mach's square-or-diamond phenomenon in microgravity during parabolic flight

In a classic demonstration, Ernst Mach showed that the same figure could be perceived as a square or as a diamond depending on the orientation of the subject relative to gravity. Such phenomenon is based on the use of a geocentric reference frame for object perception. If the central nervous system perceives an object with respect to the gravitationally defined vertical, what will happen if this reference frame is removed? We investigated the Mach phenomenon in subjects placed in short-term microgravity during parabolic flight. Subjects were presented with a square with a corner pointing upwards, and asked whether they perceived it as a diamond or square with the head upright or tilted 45 degrees in roll both in normal gravity and when free-floating in microgravity during parabolic flight. The addition of a rectangular frame around the figure was also investigated. In contrast to the normal gravity condition, with the head tilted the subjects still perceived a diamond figure in microgravity, indicating that they had switched from a geocentric to an egocentric reference frame. Also in contrast to the normal gravity condition, adding a rectangular frame around the figure did not significantly change the perception of the object in microgravity, suggesting that an intrinsic reference determined by the axis of elongation or symmetry of the object does not easily override an egocentric reference frame like it does for a geocentric reference frame.     

Age-related differences during a gaze reorientation task while standing or walking on a treadmill

Falls among adults over the age of 65 years have become a growing concern. Two factors related to high incidence of falls in this group of adults are decreased head stability and impaired balance. Older adults’ level of control of head stability or balance is unknown when they must reorient their gaze. In the current study, ten older adults (69 ± 3.27 years) performed a gaze reorienting task while standing or walking on a treadmill. The task was the same as that used on young adults by Cinelli et al. (2007). The results show that older adults use a different strategy than young adults when reorienting gaze. Shoulder and hip rotations occurred synchronously when standing and were more variable when walking on a treadmill. As well, there was a larger difference between the onset of eye movements and body segment movement in the older adults. These differences can be accounted for by decreases in physiological subsystems. The visual presence of a visual target helped the older adults stabilize their heads-in-space by incorporating information from more than one sensory system.     

Effects of neck muscles vibration on the perception of the head and trunk midline position

The present study focused on the influence of neck vibration on the perception of the head and trunk midline position (orientation and localization). The orientation of the head and trunk was investigated by the rolling adjustment of a rod on their midline while their localization was investigated by the adjustment of the position of a visual dot as being straight-ahead the eyes or the sternum. The first experiment investigated whether a head–trunk dissociation was induced by the unilateral vibration of neck muscles in upright and restrained subjects. Results showed that the subjective orientation and localization of whole-body midline were shifted toward the vibrated side. The second experiment determined the effect of the neck muscles vibration when the subjects were lying on their side. The effect of vibration disappeared when the side of vibration was opposed to the side of postural inclination and it was stronger than in the upright position when the side of vibration and the side of postural inclination were congruent. Whereas, results suggested that the input from neck muscle proprioceptors participates directly to the elaboration of the egocentric space, the question may be raised as to how the sensory cues interacted in their contribution to the neural generation of an egocentric, body centred coordinate system.     

Comparing limb proprioception and oculomotor signals during hand-guided saccades

We previously showed that saccades tend to overshoot briefly flashed targets that were manually displaced in the dark (Ren et al. 2006). However it was not clear if the overshoot originated from a sensory error in measuring hand displacement or from a premotor error in saccade programming, because gaze and hand position started at the same central position. Here, we tested between these hypotheses by dissociating the initial eye and hand position. Five hand/target positions (center, far, near, right, left) on a frontally-placed horizontal surface were used in four paradigms: Center or Peripheral Eye-hand Association (CA or PA, both gaze and right hand started from the center or a same peripheral location) and Hand or Eye Dissociation (HD or ED, hand or gaze started from one of three non-target peripheral locations). Subjects never received any visual feedback about the final target location and the subjects’ hand displacement. In the CA paradigm, subjects showed the same overshoot that we showed previously. However, changing both initial eye and hand positions relative to the final target (PA) affected the pattern, significantly altering the directions of overshoots. Changing only the initial position of hand (HD) did not have this effect, whereas changing only initial eye position (ED) had the same effect as the PA condition (CA ≈ HD, PA ≈ ED). Furthermore, multiple regression analysis showed that the direction of the ideal saccade contributed significantly to the endpoint direction error, not the direction of the hand path. These results suggest that these errors do not primarily arise from misestimates of the hand trajectory, but rather from a process of comparing the initial eye position and the limb proprioceptive signal during saccade programming.     

Task relevance predicts gaze in videos of real moving scenes

Low-level stimulus salience and task relevance together determine the human fixation priority assigned to scene locations (Fecteau and Munoz in Trends Cogn Sci 10(8):382–390, 2006). However, surprisingly little is known about the contribution of task relevance to eye movements during real-world visual search where stimuli are in constant motion and where the ‘target’ for the visual search is abstract and semantic in nature. Here, we investigate this issue when participants continuously search an array of four closed-circuit television (CCTV) screens for suspicious events. We recorded eye movements whilst participants watched real CCTV footage and moved a joystick to continuously indicate perceived suspiciousness. We find that when multiple areas of a display compete for attention, gaze is allocated according to relative levels of reported suspiciousness. Furthermore, this measure of task relevance accounted for twice the amount of variance in gaze likelihood as the amount of low-level visual changes over time in the video stimuli.     

The anti-orienting phenomenon revisited: effects of gaze cues on antisaccade performance

When the eye gaze of a face is congruent with the direction of an upcoming target, saccadic eye movements of the observer towards that target are generated more quickly, in comparison with eye gaze incongruent with the direction of the target. This work examined the conflict in an antisaccade task, when eye gaze points towards the target, but the saccadic eye movement should be triggered in the opposite direction. In a gaze cueing paradigm, a central face provided an attentional gaze cue towards the target or away from the target. Participants (N = 38) generated pro- and antisaccades to peripheral targets that were congruent or incongruent with the previous gaze cue. Paradoxically, facilitatory effects of a gaze cue towards the target were observed for both the pro- and antisaccade tasks. The results are consistent with the idea that eye gaze cues are processed in the task set that is compatible with the saccade programme. Thus, in an antisaccade paradigm, participants may anti-orient with respect to the gaze cue, resulting in faster saccades on trials when the gaze cue is towards the target. The results resemble a previous observation by Fischer and Weber (Exp Brain Res 109:507–512, 1996) using low-level peripheral cues. The current study extends this finding to include central socially communicative cues.     

A body-centred frame of reference drives spatial priming in visual search

Spatial priming in visual search is a well-documented phenomenon. If the target of a visual search is presented at the same location in subsequent trials, the time taken to find the target at this repeated target location is significantly reduced. Previous studies did not determine which spatial reference frame is used to code the location. At least two reference frames can be distinguished: an observer-related frame of reference (egocentric) or a scene-based frame of reference (allocentric). While past studies suggest that an allocentric reference frame is more effective, we found that an egocentric reference frame is at least as effective as an allocentric one (Ball et al. Neuropsychologia 47(6):1585–1591, 2009). Our previous study did not identify which specific egocentric reference frame was used for the priming: participants could have used a retinotopic or a body-centred frame of reference. Here, we disentangled the retinotopic and body-centred reference frames. In the retinotopic condition, the position of the target stimulus, when repeated, changed with the fixation position, whereas in the body-centred condition, the position of the target stimulus remained the same relative to the display, and thus to the body-midline, but was different relative to the fixation position. We used a conjunction search task to assess the generality of our previous findings. We found that participants relied on body-centred information and not retinotopic cues. Thus, we provide further evidence that egocentric information, and specifically body-centred information, can persist for several seconds, and that these effects are not specific to either a feature or a conjunction search paradigm.