Postural responses to combinations of head and body displacements: vestibular-somatosensory interactions

Postural responses to head displacements are triggered by the vestibular system; responses to body displacements are triggered by the somatosensory system. We examined the interaction of responses to combinations of head and support surface perturbations. Head displacements were always in the opposite direction of body displacements. The time between head and support surface perturbations was varied. We measured amplitude and latency of gastrocnemius and tibialis anterior EMGs for various head backward/body forward and head forward/body backward displacement combinations. These responses were compared to head-only or body-only displacement trials, which served as controls. Relative to controls, the latency of somatosensory-evoked responses to body displacement was longer and vestibular-evoked responses were absent or of low amplitude for combinations where head and support surface perturbations were presented closely in time (10-50 ms apart). These results illustrate complex integration of vestibular and somatosensory information, suggesting that the vestibulospinal and somatosensory-spinal pathways are not two isolated systems independently driving motor neurons. Rather, these pathways may influence one another at premotoneuronal levels where common circuitry may be shared by both systems.     

Human eye movement response to z-axis linear acceleration: the effect of varying the phase relationships between visual and vestibular inputs

We investigated the effect of systematically varying the phase relationship between 0.5-Hz sinusoidal z-axis optokinetic (OKN) and linear acceleration stimuli upon the resulting vertical eye movement responses of five humans. Subjects lay supine on a linear sled which accelerated them sinusoidally along their z-axis at 0.4 g peak acceleration (peak velocity 1.25 m/s). A high-contrast, striped z-axis OKN stimulus moving sinusoidally at 0.5 Hz, 70°/s peak velocity was presented either concurrently or with the acceleration stimulus or alone. Subjects' vertical eye movements were recorded using scleral search coils. When stimuli were paired in the naturally occurring relationship (e.g., visual stripes moving upward paired with downward physical acceleration), the response was enhanced over the response to the visual stimulus presented alone. When the stimuli were opposed (e.g., visual stripes moving upward during upward physical acceleration, a combination that does not occur naturally), the response was not significantly different from the response to the visual stimulus presented alone. Enhancement was maximized when the velocities of the visual and motion stimuli were in their normal phase relationship, while the response took intermediate values for other phase relationships. The phase of the response depended upon the phase difference between the two inputs. We suggest that linear self-motion processing looks at agreement between the two stimuli — a sensory conflict model.     

Fast optical imaging of frontal cortex during active and passive oddball tasks

This study used the high spatial and temporal resolution of the event-related optical signal (EROS) to investigate the timing of neuronal activity in frontal cortex during auditory target detection and passive oddball tasks. Activation in right middle frontal gyrus (MFG) peaked approximately 350 ms following rare target tones. This corresponded closely to the latency of the simultaneously recorded electrical P3 component. In addition, we found activation in left lateral MFG peaking at approximately 130 ms following tone onset for conditions that may have required response inhibition. These results correspond with activation patterns observed in similar fMRI studies, but provide temporal tags for the activated locations. These data may help bridge the gap between electrophysiological and hemodynamic measures of target detection and contribute to our understanding of the spatiotemporal dynamics of brain activity during target processing.     

Initiation of evasive manoeuvres during self-motion: a test of three hypotheses

To understand performance of evasive and interceptive actions it is important to know how people decide when to initiate a movement—initiating at the right moment is often essential for successful performance. It has been proposed that initiation is triggered when a perceptually derived quantity reaches an invariant criterion value. Candidate quantities include time-to-collision (TTC), distance, and rate of image expansion (ROE), all of which have received empirical support. We studied initiation of an evasive manoeuvre in a computer-simulated steering task in which the observer was required to steer through a stationary visual environment and avoid colliding with an obstacle in their path. The results could not be explained by hypotheses which propose that evasive manoeuvre initiation is based on a fixed criterion value of TTC or distance. The overall pattern was, however, consistent with the use of a criterion ROE value. This was further tested by analyses designed to directly evaluate whether the ROE value used to initiate the response was the same across experimental conditions. Only two of the six participants showed evidence for using the ROE strategy.     

Visual-vestibular interactions in postural control during the execution of a dynamic task

The purpose of this experiment was to determine the interaction between visual and vestibular information during the transition from quiet standing to the completion of a forward step. Six subjects were asked to take one step forward at the sound of an audio tone, with their eyes open or closed, and terminate the step in a standing position. During stimulation trials, galvanic vestibular stimulation (GVS) was delivered 1500 ms before the auditory cue. GVS was delivered at an intensity three-fold that of each subject's quiet stance threshold with either stimulus right, left or no stimulation. Force data were collected from three forceplates for the calculation of centre of pressure (CoP), and kinematic data were used to calculate centre of mass (CoM) and body trajectories. In quiet stance all subjects responded to the GVS perturbation by demonstrating upper body segment roll and whole body sway towards the anode electrode. Unexpectedly, in the presence of vision during quiet stance, the upper body roll response was not attenuated, even though the CoP sway patterns were reduced when vision was available. During the initiation phase of the step, despite ongoing GVS stimulation, there were no significant effects seen in CoM, CoP or upper body roll responses. During step execution, however, both CoM displacement and upper body roll demonstrated significant effects and both responses were significantly reduced when subjects' eyes were open. Analysis of the medio-lateral CoP integrals also indicated a strong stimulation effect between conditions late in the execution phase, which were largely attenuated with vision. The results suggest that the importance of visual and vestibular information varies depending on the phase of the task. In addition, the different integration between visual and vestibular input during quiet standing suggests a dual role for vestibular information. We propose that vestibular information in quiet standing has a role in maintaining whole body postural stability, as well as playing an integral role in the alignment of the body segments in preparation for proper movement execution. Vision was demonstrated to differentially attenuate these responses based on the phase of the task. Thus, visual and vestibular information appear to be integrated differently across the different phases of a forward-stepping task.     

The role of vision in maintaining heading direction: effects of changing gaze and optic flow on human gait

How is heading direction maintained in human gait? This question was investigated with respect to the role of optic flow and in the context of different movement strategies. While walking on a treadmill the deviation from the ideal straight path was measured in terms of lateral sway induced by a lateral gaze shift (by looking at a moving visual target). The role of the focus of expansion (FOE) within a radially expanding optic flow pattern was investigated by varying its relative velocity of expansion from 0- to 4-fold (the equivalent of walking speed), thus increasing the perceptibility of FOE. If FOE was a relevant cue for maintaining heading direction, a reduction of lateral sway amplitude was expected with increasing flow velocity. The presence of a radially expanding flow pattern did not reduce lateral sway. Lateral sway was least when the visual background remained stable without any flow pattern. Increasing the velocity of the flow pattern resulted in an increase in lateral sway. If the relative velocity of the flow pattern was raised beyond that corresponding to walking speed, lateral sway amplitude approached the maximal values observed in the dark. In all experiments, sway amplitude increased linearly with the increasing excursion of the visual target. Different strategies to perform the gaze shift (eye or head turns) only resulted in minor differences in lateral sway amplitude. The results show that gaze shifts during locomotion induce lateral sway, which depends upon the presence, and characteristics, of background optic flow. Under the present conditions, the FOE within the flow field seems not to be a dominant cue to control heading. However, the systematic increase in lateral sway induced by high flow velocities indicates that motion parallax has an effect on heading during locomotion.     

Influence of fitness and gender on blood pressure responses during active or passive stress

We examined hemodynamic and autonomic components of blood pressure responses during active and passive stressor tasks in a sample of young, normotensive men and women who were physically active but differed on fitness (i.e., VO2peak). During the hand cold pressor, increases in systolic blood pressure were inversely related to fitness among women but not men. Regardless of gender, fitter participants had a greater increase in cardiac pace during mental arithmetic, coherent with a decreased cardiac-vagal component of heart rate variability, and a greater compensatory reduction in stroke volume. Fitness was otherwise unrelated to changes in cardiac output and vascular resistance during the stressor tasks. Our findings suggest that cardiorespiratory fitness augments the cardiac-vagal withdrawal that is characteristic of mental arithmetic. The blunted systolic blood pressure response to the hand cold pressor among fitter women suggests that cardiorespiratory fitness should be considered as a covariate in studies that examine the hand cold pressor as a predictor of future hypertension among women.     

Postural responses to simulated moving environments are not invariant for the direction of gaze

Summary Postural responses were measured for observers instructed to maintain an erect position when viewing a visual scene, which simulated motion of the observer relative to a corridor with texture on the walls. Both the direction and amplitude of the postural responses appeared to depend on the fixation direction of the observer. This result indicates that postural responses are not invariant with respect to fixation direction. Our results suggest that not one of the frequently suggested features in the optical flow in particular is used to maintain posture but rather that a combination of several optical flow components, that have been suggested in the literature, is relevant.     

Steering behaviour can be modulated by different optic flows during walking

Optic flow is a typical pattern of visual motion that can be used to control locomotion. While the ability to discriminate translational or rotational optic flows have been extensively studied, how these flows control steering during locomotion is not known. The goal of this study was to compare the steering behaviour of subjects subjected to rotational, translational, or combined (rotational added to translational) optic flows with a focus of expansion (FOE) located to the right, left, or straight ahead. Ten healthy young subjects were instructed to walk straight in a virtual room viewed through a helmet mounted display while the location of the FOE was randomly offset. Horizontal trajectory of the body's centre of mass (CoM), as well as rotations of the head, trunk and foot were recorded in coordinates of both the physical and virtual worlds. Results show that subjects experienced a mediolateral shift in CoM opposite to the FOE location, with larger corrections being observed at more eccentric FOE locations. Head and body segment reorientations were only observed for optic flows containing a rotational component. CoM trajectory corrections in the physical world were also of small magnitude, leading to deviation errors in the virtual world. Altogether, these results suggest a profound influence of vision, especially due to the pattern of visual motion, on steering behaviours during locomotion.     

Psychophysiological characteristics of narcissism during active and passive coping

This study provides the first psychophysiological analysis of narcissism by measuring autonomic responses during active and passive anticipatory coping in 40 undergraduate men who scored high or low on the Narcissistic Personality Inventory (NPI). Compared to the low NPI group, the high NPI group showed greater preejection period (PEP) shortening, cardiac deceleration, and skin conductance response (SCR) habituation during anticipation of an aversive stimulus (p < .02). As expected, SCR and PEP reactivity were greater during active than passive coping. In the case of PEP, this effect emerged only in the low NPI group; the high NPI group showed the greatest PEP reactivity during the first task, regardless of coping demands. These data support hypothesized relationships among narcissism, psychopathy, and psychological predictors of cardiovascular disease, and suggest that a psychobiological dimension may underlie important features of narcissism.     

Sensory re-weighting in human postural control during moving-scene perturbations

The aim of the current study was to further investigate a recently proposed “sensory re-weighting” hypothesis, by evoking anterior–posterior (AP) body sway using visual stimuli during sway-referencing of the support surface. Twelve healthy adults participated in this study. Subjects stood on the platform while looking at a visual scene that encompassed the full horizontal field of view. A sequence of scene movements was presented to the subjects consisting of multiple visual push/pull perturbations; in between the first two push/pull sequences, the scene either moved randomly or was stationary. The peak-squared velocity of AP center-of-pressure (COP) was computed within a 6 s window following each push and pull. The peak-squared velocity was lowest for the push/pull sequence immediately following the random moving scene. These results are consistent with the sensory re-weighting hypothesis, wherein the sensory integration process reduced the contribution of visual sensory input during the random moving scene interval. We also found evidence of habituation to moving scene perturbations with repeated exposure.     

Estimation of self-turning in the dark: comparison between active and passive rotation

The present work compares passive and active rotations in darkness with the aim of characterizing the contribution of efferent and proprioceptive information to the perception of angular displacement. The perception of angular displacements was measured in 12 naive subjects (Ss), who either stood on a rotating platform (passive mode, P) or actively turned about their vertical axis by stepping around ”on the spot” on a stationary platform (active mode, A). Rotations consisted of short acceleration epochs followed by constant velocity periods of 18.5, 37, and 55°/s, with angular displacements ranging from 30° to 810° (presented in a randomized order); in the case of active turning, Ss had learned to approximately produce any of these three velocity levels on command. Ss indicated perceived displacement either verbally (verbal estimation mode, E), or by stopping their rotation when self-displacement appeared to match the magnitude specified by the experimenter (targeting, T). The resulting four conditions (PE, PT, AE, AT) were administered blockwise. In none of the four conditions was there a systematic dependence of perception on turning velocity. Therefore, the results were pooled across velocities, and the Ss’ performance was summarized in the form of estimation curves showing median estimates as a function of physical displacement. There were several differences between the passive and active modes: AE- and AT-estimation curves were linear, close to veracity, and fairly similar to each other. In contrast, the PE-curve was curved rightwardly (”saturation”), with small displacements being overestimated and large ones underestimated, whereas the PT-curve was linear and indicated a pronounced overestimation of large displacements. Moreover, both the random and the systematic errors (measures of individual consistency and correctness of individual calibration, respectively) were significantly smaller in the active than in the passive modes. The observed independence of Ss’ perception from turning velocity also during passive rotation suggests that the perceptual time constant was significantly longer than 16 s (a value cited as typical for vestibular perception), being possibly ”enhanced” by contextual implications and by expectations of the Ss. The clear improvement of perceptual performance in the active mode testifies to the importance of the efferent and proprioceptive signals arising during active motion. On the assumption that these signals are about as ”noisy” as the vestibular ones, the smaller errors during active turning could result from their combination with the vestibular signal. Alternatively, they could also be intrinsically less noisy than the vestibular signal and simply replace the latter during active motion. In the context of these alternatives (which are not exhaustive), the general problem of sensory fusion is discussed, that is, by which mechanisms are signals from different sensory sources combined to obtain a unified representation of the self’s orientation. Received: 4 January 1999 / Accepted: 6 May 1999     

Effects of the spatio-temporal structure of optical flow on postural readjustments in man

How does the spatio-temporal structure of an oscillating radial optical flow affect postural stability? In order to investigate this problem, two different types of stimulus pattern were presented to human subjects. These stimuli were generated either with a constant spatial frequency or with a spatial frequency gradient providing monocular depth cues. When the stimulation was set in motion, the gain response of the antero-posterior postural changes depended upon the oscillation frequency of the visual scene. The amplitude of the postural response did not change with the amplitude of the visual scene motion. The spatial orientation of the postural sway (major axis of sway) depended strictly and solely on the structure of the visual scene. In static conditions, depth information resulting from the presence of a spatial frequency gradient enhanced postural stability. When set in motion, a visual scene with a spatial frequency gradient induced an organization of postural sway in the direction of the visual motion. Considering visual dynamic cues, postural instability depended linearly both on the logarithm of the velocity and on the logarithm of the temporal frequency. A nonlinear relationship existed between the amplitude of the fore-aft postural sway at the driving frequency and the temporal frequency, with a peak around 2–4 Hz. These results are discussed in terms of their implications for the separation of visual and biomechanical factors influencing visuo-postural control.     

Mental effort during active and passive coping: A dual-task analysis

This study was designed to examine the invested mental effort during active and passive coping by means of performance data. Dual-task performance was measured while subjects coped with a situation, either actively or passively. Thirty-six male students worked on a primary mental arithmetic task (MAT) and a secondary choice reaction time task simultaneously. Half of the sample could avoid an aversive tone by performing well on the MAT. For the remaining half, their performance had no influence on the tone. The aversive tone stimulation of these subjects was yoked to a “partner,” in the first group. Subjects with control showed elevated cardiovascular responses and inferior secondary task performance than subjects without control. No differences were found in the performance on the MAT. These results were in line with the assumption that subjects with control spent more effort on the primary task. Subjects under the active coping condition probably payed more attention to the tone, which consumed more cognitive resources.     

Sound localisation during illusory self-rotation

Auditory elevation localisation was investigated under conditions of illusory self-rotation (i.e., vection) induced by movement of wide-field visual stimuli around participants’ z-axes. Contrary to previous findings which suggest that auditory cues to sound-source elevation are discounted during vection, we found little evidence that vection affects judgements of source elevation. Our results indicate that the percept of auditory space during vection is generally consistent with the available head-centered auditory cues to source elevation. Auditory information about the head-centered location of a source appears to be integrated, without modification, with visual information about head motion to determine the perceived exocentric location of the source.     

Updating visual space during passive and voluntary head-in-space movements

The accuracy of our spatially oriented behaviors largely depends on the precision of monitoring the change in body position with respect to space during self-motion. We investigated observers’ capacity to determine, before and after head rotations about the yaw axis, the position of a memorized earth-fixed visual target positioned 21° laterally. The subjects (n=6) showed small errors (mean=–0.6°) and little variability (mean=0.9°) in determining the position of an extinguished visual-target position when the head (and gaze) remained in a straight-ahead position. This accuracy was preserved when subjects voluntary rotated the head by various magnitudes in the direction of the memorized visual target (head rotations ranged between 5° and 60°). However, when the chair on which the subjects were seated was unexpectedly rotated about the yaw axis in the direction of the target (chair rotations ranged between 6° and 36°) during the head-on-trunk rotations, the performance was markedly decreased, both in terms of spatial precision (mean error=5.6°) and variability (mean=5.7°). A control experiment showed that the prior knowledge of chair rotation occurrence had no effect on the perceived target position after head-trunk movements. Updating an earth-fixed target position during head-on-trunk rotations could be achieved through both cervical and vestibular signals processing, but, in the present experiment, the vestibular output was the only signal that had the potentiality to contribute to accurate coding of the target position after simultaneous head and trunk movements. Our results therefore suggest that the vestibular output is a noisy signal for the central nervous signal to update the visual space during head-in-space motion. Received: 2 June 1997 / Accepted: 16 March 1998     

Core temperature thresholds for hyperpnea during passive hyperthermia in humans

Humans have higher ventilation when they are hyperthermic but it is not known whether core temperature thresholds for ventilation exist, nor has a physiological rationale been presented for this response. To examine this question, ventilation was studied in relation to core temperatures in humans rendered hyperthermic in a warm bath. Seven subjects [mean (SE), 23.3 (1.4) years] wearing only shorts and a thick felt hat with ear flaps were immersed to the neck in a bath at 41 (0.5)°C for 25 min. Tympanic (T _ty), esophageal (T _es), thigh skin and forehead skin temperatures, heart rate, inspired minute ventilation (V _I at body temperature and pressure, saturated), ventilation frequency and oxygen consumption (VO₂ at standard temperature and pressure, dry) were recorded at 30-s intervals. At immersion V _I briefly increased to 18.6 (3.0)l·min^–1 returned to about the pre-immersion value,, and significantly increased to 19.3 (3.0) l·min^–1 by the end of immersion. VO₂ increased significantly from the pre-immersion value of 0.27 l·min^–1 to 0.67 l·min^–1 by the first 0.5 min of immersion, but then returned to its pre-immersion value. T _ty increased to 38.7 (0.2)°C and T _es increased to 39.0 (0.2)°C by the end of immersion. Core temperature thresholds for increases in V _I were evident at 38.1°C when expressed against T _ty and at 38.5°C when expressed against T _es. The results indicated that during body warming core temperature thresholds for V _I are reached and subsequently a hyperpnea was evident, despite VO₂ remaining at a resting value. This hyperpnea is seen as a thermoregulatory response likely to participate in selective brain cooling.     

Perception of vibrotactile stimuli during motor activity in human subjects

Previous studies have shown that voluntary motor activity decreases the ability to detect nearthreshold electrical stimuli applied to the skin, but has no effect on the perception of either suprathreshold electrical stimuli or natural thermal stimuli (warmth, heat pain). The present study was undertaken to determine if the perception of natural tactile stimuli (vibrotactile) is diminished by motor activity (rhythmical isometric flexions and extensions about the elbow). The stimuli were applied at three different sites on the operant arm — ventral forearm, thenar eminence and distal digit — to examine also the influence of the proximity of the stimulated site to the active muscles on perception. The ability to detect near-threshold stimuli at the two more proximal stimulation sites was significantly reduced during the motor task, and these effects were more pronounced and widespread with higher levels of target force (20 N vs 50 N). Discrimination of small differences in the intensity of suprathreshold stimuli, at all three sites, was unchanged during the motor task. Finally, the subjective intensity of suprathreshold vibrotactile stimuli was reduced, in a nonlinear fashion, during the motor task; proximity again influenced the degree of modulation. In contrast a previous study showed no change in magnitude estimates of suprathreshold electrical stimuli during isotonic flexion and extension. Some possible reasons for the discrepancy are discussed. In addition, our previous suggestion that movement produces a simple reduction in the signal-to-noise ratio (i.e. the gating signal modelled as a masking stimulus) cannot explain the present results, so more complex models are required.     

Dynamics model for analyzing reaching movements during active and passive torso rotation

We have developed an inverse dynamics model of unrestrained natural reaching movements. Such movements are usually not planar and often involve complex deformation of the shoulder girdle as well as rotary and linear torso motion. Our model takes as its input kinematic data about the positions of the finger, wrist, elbow, left and right acromion processes, and the sternum and produces the torques and forces developed at the shoulder, elbow, and wrist joints. The model can also be used to simulate the consequences of introducing passive torso rotation or linear acceleration on arm movements and to simulate the consequences of applying mechanical perturbations to the reaching limb. It separately quantifies the contributions of inertial forces resulting from torso rotation and translation. In experimental paradigms involving arm movements, different dynamic components can be present such as active or passive torso rotation and translation, external forces and Coriolis forces. Our model provides a means of evaluating the different sources of force and the total muscle force needed to control the trajectory of the arm in their presence.

Postural responses and spatial orientation to neck proprioceptive and vestibular inputs during locomotion in young and older adults

Both vestibular and neck proprioceptive inputs contribute towards maintaining a walking trajectory. We investigated how aging alters neck proprioceptive and vestibular interaction for preserving equilibrium and spatial orientation during locomotion. Young and healthy elderly were exposed to two sensory manipulations as they walked, eyes closed, to a target located straight ahead: (1) right side dorsal neck muscle vibration (Vib), and (2) Vib + transmastoidal galvanic vestibular stimulation (Vib + GVS). The maximum path deviation, average frontal centre of mass velocity and average trunk roll were evaluated. Trunk yaw rotation was computed at every metre of the path. We observed that directional responses to neck muscle stimulation were very sensitive to the reference frame generated by vestibular information. The attenuation of path deviation in older adults can be attributed to a reduced sensitivity of the neck proprioceptive system rather than the vestibular system.