Human perception of horizontal trunk and head rotation in space during vestibular and neck stimulation

Summary The vestibular signal of head motion in space must be complemented by a neck signal of the trunk-to-head excursion in order to provide the individual with information on trunk motion in space. This consideration led us to study psychophysically the role of vestibular-neck interaction for human self-motion perception. Subjects (Ss) were presented with passive horizontal rotations of their trunk and/or head (sinusoidal rotations, f=0.025 –0.4 Hz) in the dark for vestibular and neck stimulation, as well as for combinations of both. Ss' perception was evaluated in terms of gain (veridical perception of stimulus magnitude, G=1), phase, and detection threshold. (1) Perception of trunk rotation in space. During vestibular stimulation (whole-body rotation) and neck stimulation (trunk rotation with the head kept stationary) the frequency-transfer characteristics underlying this perception were very similar. The gain fell short; it was only about 0.7 at 0.4 and 0.2 Hz stimulus frequency and was further attenuated with decreasing frequency. In contrast, the phase was close to that of actual trunk position. The gain attenuation was found to be a function of the peak angular velocity of the stimulus, a fact, which we related to a velocity threshold of the order of 1 deg/s. During the various vestibular-neck combinations used, Ss' perception was again erroneous, reflecting essentially the sum of its two non-ideal constituents. However, there was one noticeable exception; during the combination head rotation on stationary trunk, Ss veridically perceived their trunk as stationary (compatible with the notion that the sum yielded zero). (2) Perception of head rotation in space. During vestibular stimulation, Ss' estimates showed the same non-ideal gain-vs.-frequency characteristics as described above for the trunk. Neck stimulation induced an illusion as if the head had been rotated in space. This neck contribution was such that, when it was combined with its vestibular counterpart during head rotation on stationary trunk, the perception became almost veridical. On closer inspection, however, this neck contribution was found to reflect the sum of two components; one was the non-ideal neck signal contributing to the perception of trunk in space, the other was an almost ideal neck signal of head-on-trunk rotation. (3) The results could be described by a simple model. In this model, the erroneous vestibular signal head in space is primarily used to create an internal representation of trunk in space. To this end, it is combined with the closely matching neck signal of trunk to head. The perception of head rotation in space is achieved by summing this trunk in space signal with the almost ideal head on trunk signal, again of nuchal origin. These seeming complex interactions have two implications: (i) the head is referred to trunk coordinates, whereas the trunk is referred to space coordinates; (ii) there is at least one condition in the dark where orientation is correct (despite an erroneous vestibular signal), i.e., during head rotation on stationary trunk.Supported by Deutsche Forschungsgemeinschaft, SFB 325     

Grasping at sticks: pseudoneglect for perception but not action

A current question in theories of visual cognition is whether distinct cognitive processes subserve perceptual judgments and perception for action. This paper examines bisection tasks which have previously been used to demonstrate a dissociation between perception and action in brain injured patients. Forty neurologically intact participants completed a standard line bisection task and a variant of this task—rod bisection. A typical leftwards bias was observed for line bisection but when asked to locate the centre of wooden rods using perceptual judgments, a distinct rightwards bias was shown. By contrast, when participants were asked to pick the rods up by the centre, their judgments showed no bias. The results are in line with theories suggesting that perception and action are independent; however, alternative explanations are also considered.     

Object motion perception is shaped by the motor control mechanism of ocular pursuit

It is still a matter of debate whether the control of smooth pursuit eye movements involves an internal drive signal from object motion perception. We measured human target velocity and target position perceptions and compared them with the presumed pursuit control mechanism (model simulations). We presented normal subjects (Ns) and vestibular loss patients (Ps) with visual target motion in space. Concurrently, a visual background was presented, which was kept stationary or was moved with or against the target (five combinations). The motion stimuli consisted of smoothed ramp displacements with different dominant frequencies and peak velocities (0.05, 0.2, 0.8 Hz; 0.2–25.6°/s). Subjects always pursued the target with their eyes. In a first experiment they gave verbal magnitude estimates of perceived target velocity in space and of self-motion in space. The target velocity estimates of both Ns and Ps tended to saturate at 0.8 Hz and with peak velocities >3°/s. Below these ranges the velocity estimates showed a pronounced modulation in relation to the relative target-to-background motion ('background effect'; for example, 'background with'-motion decreased and 'against'-motion increased perceived target velocity). Pronounced only in Ps and not in Ns, there was an additional modulation in relation to the relative head-to-background motion, which co-varied with an illusion of self-motion in space (circular vection, CV) in Ps. In a second experiment, subjects performed retrospective reproduction of perceived target start and end positions with the same stimuli. Perceived end position was essentially veridical in both Ns and Ps (apart from a small constant offset). Reproduced start position showed an almost negligible background effect in Ns. In contrast, it showed a pronounced modulation in Ps, which again was related to CV. The results were compared with simulations of a model that we have recently presented for velocity control of eye pursuit. We found that the main features of target velocity perception (in terms of dynamics and modulation by background) closely correspond to those of the internal drive signal for target pursuit, compatible with the notion of a common source of both the perception and the drive signal. In contrast, the eye pursuit movement is almost free of the background effect. As an explanation, we postulate that the target-to-background component in the target pursuit drive signal largely neutralises the background-to-eye retinal slip signal (optokinetic reflex signal) that feeds into the eye premotor mechanism as a competitor of the target retinal slip signal. An extension of the model allowed us to simulate also the findings of the target position perception. It is assumed to be represented in a perceptual channel that is distinct from the velocity perception, building on an efference copy of the essentially accurate eye position. We hold that other visuomotor behaviour, such as target reaching with the hand, builds mainly on this target position percept and therefore is not contaminated by the background effect in the velocity percept. Generally, the coincidence of an erroneous velocity percept and an almost perfect eye pursuit movement during background motion is discussed as an instructive example of an action-perception dissociation. This dissociation cannot be taken to indicate that the two functions are internally represented in separate brain control systems, but rather reflects the intimate coupling between both functions. Electronic Publication     

Is there a preferred coordinate system for perception of hand orientation in three-dimensional space?

Summary The purpose of this experiment was to determine the preferred coordinate system for representation of hand orientation in 3-dimensional space. The ability of human subjects to perceive angles of the hand in 3-dimensional space (elevation, yaw, roll angles extrinsic coordinate system) was compared to their ability to perceive hand angles relative to the proximal upper limb segments (wrist joint angles, forearm pronation intrinsic coordinate system). With eyes closed, subjects performed a matching task in which the experimenter positioned the left arm, forearm and hand and the right arm and forearm. Subjects were then told to match an angle in one of the two coordinate systems by moving only the right hand at the wrist or the forearm as in pronation or roll matching. Absolute constant error (ACE), variable error (VE) and normalized variable error (NVE-normalized to tested range of motion) of matching were quantified for each subject for each of the six angles matched. It was hypothesized that matching angles in a preferred coordinate system would be associated with lower ACE, VE and NVE. Overall, ACE and VE were lower for matching hand angles in the intrinsic coordinate system. This suggests that the preferred coordinate system involved specification of hand angles relative to forearm and arm angles (joint angles) rather than the hand angles relative to axes external to the upper limb. However, matching of pronation angles was associated with larger VE and NVE than roll angle matching. There were no significant differences in ACE between pronation and roll matching. In a second experiment subjects with their forearms constrained at different elevations matched hand elevation and wrist flexion angles. Thus, errors in matching the angles in the non preferred coordinate system were predictable if the subjects were biased toward matching angles in the preferred coordinate system. Trends in the data suggested that subjects preferred matching hand elevation angles but these trends were not consistent within or between subjects. Thus a preferred intrinsic coordinate system for wrist flexion matching was not observed in this experiment. We suggest that matching angles when proximal limb segments are constrained is a simpler task for the subjects (VE lower than in the first experiment) and may bias the matching toward the extrinsic coordinate system. Thus, hand orientation in 3-dimensional space may be perceived as follows: wrist flexion and abduction angles together with forearm elevation and yaw are used to specify hand elevation and yaw; these together with hand roll angle, completely specify the hand angle in 3-dimensional space.     

Unconscious perception of a flash can trigger line motion illusion

When a flash of light precedes a static line segment, an illusory motion sensation is observed with the line propagating away from the flash’s location towards the opposite side (Hikosaka et al. in Vision Res 33:1219–1240, 1993). Here we report that a similar illusory motion percept can be triggered by a non-consciously perceived flash. Observers reported illusory line motion (ILM) arising from the flash’s location when a stationary line was presented and the flash was not detected. The results imply that the line motion illusion does not depend on conscious awareness of the flash and suggest that processing of unconscious information can modulate the responses of the neural mechanisms involved in motion perception.     

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.     

Interaction of vestibular and proprioceptive inputs for human self-motion perception.

Human perception of horizontal self(body)-motion in space was studied during various combinations of vestibular and leg-proprioceptive stimuli in the dark. During sinusoidal rotations of the trunk relative to the stationary feet (functionally synergistic combination) the perception was almost veridical over the frequency range tested (0.025-0.4 Hz). This finding suggested a dominance of the proprioceptive over the vestibular input, since the quantitative aspects of the perception (gain, phase, and detection threshold): (a) closely resembled those of the proprioceptive foot-to-trunk perception, and (b) clearly differed from those of the vestibular self-motion perception. However, when using other combinations, the self-motion perception changed in a monotonous way as a function of the two inputs, indicating that the two inputs do interact in a linear way. In a model of these findings the interaction occurs in two stages: (1) summation of a vestibular trunk-in-space signal and a (dynamically matched) proprioceptive foot-to-trunk signal yields an internal representation of foot support motion in space; (2) superposition of the latter by an almost ideal proprioceptive trunk-to-foot signal results in a representation of trunk-in-space motion (essentially proprioception-dependent and ideal when the feet are stationary).     

Disynaptic vestibulospinal and reticulospinal excitation in cat lumbosacral motoneurons: modulation during fictive locomotion

This study assessed the ability of mildly affected Parkinson's disease (PD) subjects (n=16) to perform attentional cognitive tasks within a three-dimensional object. A hollow cube was displayed on a computer screen and the subject was required to respond as quickly as possible to the highlighting of one of the cube angles by pressing the spacebar of the keyboard. Prior to the appearance of this imperative stimulus, the same (valid trials) or an alternative (invalid trials) angle was highlighted. For the invalid trials this meant that the subject oriented attention to the cued angle but, on imperative stimulus appearance, was unexpectedly required to redirect attention to another angle, which could be on a different cube face to that which had been cued. For one experimental session the cube was stationary, that is, object-centred and viewer-centred coordinates of a cube angle corresponded. For another session, the cube rotated such that the viewer-centred coordinates of an angle changed between appearance of the cue and appearance of the stimulus, but the angle's object-centred coordinates remained constant. The finding of lower reaction times for the valid than for the invalid trials, even when the cube was rotating, indicated that PD subjects could operate attention using an object-centred coordinate system. However, PD subjects showed exaggerated reaction times when the stimulus appeared in a cube face that was opposite to, rather than the same as, that of the invalidly cued angle. It is suggested that this reflects a dysfunction in the grouping of the structural components of the whole object at an attentional level.     

Treadmill locomotion captures visual perception of apparent motion

A visual illusion of perceived motion direction induced by treadmill locomotion is reported. Directionally ambiguous motions of shifting frames of sinusoidal horizontal gratings are perceived moving downward more frequently when the stimuli are shown in front of the observers’ feet while walking on a treadmill. To confirm this effect quantitatively, we asked naive observers to answer whether the direction of the motion of the grating pattern was perceived as upward or downward while they were walking or standing on a treadmill. The frequency of the “downward” response was significantly higher under the walking condition. This effect reveals that treadmill locomotion captures perceived direction of ambiguous motion in accordance with the direction of the optic flow during natural walking. This finding suggests that the effect reflects a perceptual mechanism to compensate for the absence of inputs in the action–perception cycle during locomotion.     

Frame of reference for visual perception in young infants during change of body position

The visual and vestibular systems begin functioning early in life. However, it is unclear whether young infants perceive the dynamic world based on the retinal coordinate (egocentric reference frame) or the environmental coordinate (allocentric reference frame) when they encounter incongruence between frames of reference due to changes in body position. In this study, we performed the habituation–dishabituation procedure to assess novelty detection in a visual display, and a change in body position was included between the habituation and dishabituation phases in order to test whether infants dishabituate to the change in stimulus on the retinal or environmental coordinate. Twenty infants aged 3–4 months were placed in the right-side-down position (RSDp) and habituated to an animated human-like character that walked horizontally in the environmental frame of reference. Subsequently, their body position was changed in the roll plane. Ten infants were repositioned to the upright position (UPp) and the rest, to the RSDp after rotation. In the test phase, the displays that were spatially identical to those shown in the habituation phase and 90° rotated displays were alternately presented, and visual preference was examined. The results revealed that infants looked longer at changes in the display on the retinal coordinate than at changes in the display on the environmental coordinate. This suggests that changes in body position from lying to upright produced incongruence of the egocentric and allocentric reference frames for perception of dynamic visual displays and that infants may rely more on the egocentric reference frame.     

Interhemispheric transfer of visual motion information after a posterior callosal lesion: a neuropsychological and fMRI study

Interhemispheric transfer of visual information was investigated behaviourally and with functional magnetic resonance imaging (fMRI) 6 months after a lesion of the posterior two-thirds of the corpus callosum. On tachistoscopical left hemifield presentation, the patient was severely impaired in reading letters, words and geographical names and moderately impaired in naming pictures and colours. In contrast, interhemispheric transfer of visual motion information, tested by verbal report of the direction of short sequences of coherent dot motion presented within the left hemifield, was preserved. The pattern of cerebral activation elicited by apparent motion stimuli was studied with fMRI and compared to that of normal subjects. In normal subjects, apparent motion stimuli, as compared to darkness, activated strongly striate and extrastriate cortex. When presented to one hemifield only, the contralateral calcarine region was activated while regions on the occipital convexity, including putative area V5, were activated bilaterally. A similar activation pattern was found in the patient with a posterior callosal lesion; unilateral left or right hemifield stimulation was accompanied by activation in the contralateral and ipsilateral occipital convexity. Ipsilateral hemifield representation in the extrastriate visual cortex is believed to depend on callosal input. Our observation suggests that this is not the case for visual motion representation and that other, probably parallel, pathways may mediate visual motion transfer after posterior callosotomy.     

Perception of forearm angles in 3-dimensional space

Summary The purpose of this study was to determine a preferred coordinate system for representation of forearm orientation in 3-dimensional space. In one experiment, the ability of human subjects to perceive angles of the forearm in 3-dimensional space (forearm elevation and yaw — extrinsic coordinate system) was compared to their ability to perceive elbow joint angle (intrinsic coordinate system). While blindfolded, subjects performed an angle reproduction task in which the experimenter first positioned the upper limb in a reference trial. This was followed, after movement of the subject's entire upper limb to a different position, by an attempt to reproduce or match a criterior angle of the reference trial by motion of the forearm in elbow flexion or extension only. Note that matching of the criterion forearm angle in the new upper limb position could not be accomplished by reproducing the entire reference upper limb position, but only by angular motion at the elbow. Matching of all 3 criterion angles was accomplished with about equal accuracy in terms of absolute constant errors and variable errors. Correlation analysis of the perceptual errors showed that forearm elevation and elbow angle perception errors were not biased but that forearm yaw angle matching showed a bias toward elbow angle matching in 7 of 9 subjects. That is, errors in forearm yaw perception were attributed to a tendency toward a preferred intrinsic coordinate system for perception of forearm orientation. These results show that subjects can accurately perceive angles in both extrinsic and intrinsic coordinate systems in 3-dimensional space. Thus, these data conflict with previous reports of highly inaccurate perception of elbow joint angles in comparison to perception of forearm elevation. In an attempt to resolve this conflict with previous results, a second experiment was carried out in which perception of forearm elevation and elbow joint angles with the forearm motion constrained to a vertical plane. Results of this experiment showed that during a two-limb elbow angle matching task, four of five subjects exhibited a clear bias toward forearm elevation angle. During a one-limb angle reproduction task only two of five subjects exhibited such a bias. Perception of elevation angles show little bias toward elbow angle matching. These results indicate that use of tasks in which the limb is supported against gravity and motion is constrained to a vertical plane cause subjects to make perceptual errors during elbow angle matching such that the slopes of the forearms in a vertical plane (elevation angles) are more easily matched. It is concluded that human subjects can use both extrinsic and intrinsic coordinate systems in planning movements. Kinematic aspects may be planned in terms of an extrinsic coordinate system because of the use of vision in specifying location of external targets, but kinetic aspects of movement planning probably requires use of both forearm elevation angles and elbow joint angles to accurately specify forces and torques for muscles spanning the elbow.     

Eye and neck proprioceptive messages contribute to the spatial coding of retinal input in visually oriented activities

Summary The egocentric localization of objects in extrapersonal space requires that the retinal and extraretinal signals specifying the gaze direction be simultaneously processed. The question as to whether the extraretinal signal is of central or peripheral origin is still a matter of controversy, however. Three experiments were carried out to investigate the following hypotheses: 1) that the proprioceptive feedback originating in eye and neck muscles might provide the CNS with some indication about the gaze direction; and 2) that the retinal and proprioceptive extraretinal inputs might be jointly processed depending on whether they are of monocular or binocular origin. Application of low amplitude mechanical vibrations to either the extraocular or neck muscles (or both) of a subject looking monocularly at a small luminous target in darkness resulted in an illusory movement of the target, the direction of which depended on which muscle was stimulated. A slow upward target displacement occurred on vibrating the eye inferior rectus or the neck sterno-cleido-mastoidus muscles, whereas a downward shift was induced when the dorsal neck muscles (trapezius and splenius) were vibrated. The extent of the perceptual effects reported by subjects was measured in an open-loop pointing task in which they were asked to point at the perceived position of the target. These results extend to visually-oriented behavior the role of extraocular and neck proprioceptive inputs previously described in the case of postural regulation, since they clearly show that these messages contribute to specifying the gaze direction. This suggests that the extraretinal signal might include a proprioceptive component. The proposition that a directional body reference frame may be based on the common processing of various proprioceptive feedbacks is discussed.     

Automatic control of postural sway by visual motion parallax

The purpose of this study was to establish whether visual motion parallax participates in the control of postural sway. Body sway was measured in ten normal subjects by photoelectric recordings of head movements and by force-plate posturography. Subjects viewed a visual display (“background”), which briefly moved (2 s) along the y (horizontal) axis, under three different conditions: (1) direct fixation of the background, (2) fixation of a stationary window frame in the foreground, and (3) fixation of the background in the presence of the window in the foreground (“through the window”). In response to background fixation, subjects swayed in the same direction as stimulus motion, but during foreground (window) fixation they swayed in the opposite direction. The earlier forces observed on the force platform occurred at circa 250 ms in both conditions. The results show that motion parallax generates postural responses. The direction of these parallax-evoked postural responses — opposite to other visually evoked postural responses reported so far — is appropriate for stabilizating posture in natural circumstances. The findings show that motion parallax is an important source of self-motion information and that this information participates in the process of automatic postural control. In the “fixating through the window” condition, which does not mimic visual conditions induced by body sway, no consistent postural responses were elicited. This implies that postural reactions elicited by visual motion are not rigid responses to optokinetic stimulation but responses to visual stimuli signalling self-motion.     

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.     

Interactions between vestibular and proprioceptive inputs triggering and modulating human balance-correcting responses differ across muscles

Interactions between proprioceptive and vestibular inputs contributing to the generation of balance corrections may vary across muscles depending on the availability of sensory information at centres initiating and modulating muscle synergies, and the efficacy with which the muscle action can prevent a fall. Information which is not available from one sensory system may be obtained by switching to another. Alternatively, interactions between sensory systems and the muscle to which this interaction is targeted may be fixed during neural development and not switchable. To investigate these different concepts, balance corrections with three different sets of proprioceptive trigger signals were examined under eyes-open and eyes-closed conditions in the muscles of normal subjects and compared with those of subjects with bilateral peripheral vestibular loss. The different sets of early proprioceptive inputs were obtained by employing three combinations of support surface rotation and translation, for which ankle inputs were nulled, normal or enhanced, the knees were either locked or in flexion, and the trunk was either in flexion or extension. Three types of proprioceptive and vestibulospinal interactions were identified in muscles responses. These interactions were typified by the responses of triceps surae, quadriceps, and paraspinal muscles. The amplitudes of stretch responses at 50 ms after the onset of ankle flexion in triceps surae muscles were related to the velocity of ankle stretch. The amplitude of balance-correcting responses at 100 ms corresponded more with stretch of the biarticular gastrocnemius when the knee was re-extended at 60 ms. Absent stretch reflexes at 50 ms in triceps surae with nulled ankle inputs caused a minor, 12-ms delay in the onset of balance-correcting responses in triceps surae muscles. Vestibular loss caused no change in the amplitude of balance-correcting responses, but a negligible decrease in onset latency in triceps surae even with nulled ankle inputs. Stretch responses in quadriceps at 80 ms increased with the velocity of knee flexion but were overall lower in amplitude in vestibular loss subjects. Balance-correcting responses in quadriceps had amplitudes which were related to the directions of initial trunk movements, were still present when knee inputs were negligible and were also altered after vestibular loss. Stretch and unloading responses in paraspinals at 80 ms were consistent with the direction of initial trunk flexion and extension. Subsequent balance-correcting responses in paraspinals were delayed 20 ms in onset and altered in amplitude by vestibular loss. The changes in the amplitudes of ankle (tibialis anterior), knee (quadriceps) and trunk (paraspinal) muscle responses with vestibular loss affected the amplitudes and timing of trunk angular velocities, requiring increased stabilizing tibialis anterior, paraspinal and trapezius responses post 240 ms as these subjects attempted to remain upright. The results suggest that trunk inputs provide an ideal candidate for triggering balance corrections as these would still be present when vestibular, ankle and knee inputs are absent. The disparity between the amplitudes of stretch reflex and automatic balance-correcting responses in triceps surae and the insignificant alteration in the timing of balance-correcting responses in these muscles with nulled ankle inputs indicates that ankle inputs do not trigger balance corrections. Furthermore, modulation of balance corrections normally performed by vestibular inputs in some but not all muscles is not achieved by switching to another sensory system on vestibular loss. We postulate that a confluence of trunk and upper-leg proprioceptive input establishes the basic timing of automatic, triggered balance corrections which is then preferentially weighted by vestibular modulation in muscles that prevent falling. The organisation of balance corrections around trunk inputs portrayed here would have considerable advantage for the infant learning balance control, but forces balance control centres to rely on limited sensory information related to this most unstable body segment, the trunk, when triggering balance corrections. Received: 13 October 1997 / Accepted: 30 March 1998     

Linear acceleration perception in the roll plane before and after unilateral vestibular neurectomy

Summary The ability of 33 patients to perceive the direction, relative to the body long axis, of a linear acceleration vector acting in the coronal plane, rolltilt perception, was studied at various times, before and from 1 week to 6 months after unilateral, selective vestibular neurectomy for Meniere's disease, acoustic neuroma or intractable paroxysmal vertigo. The results of these patients were compared with the results of 31 normal subjects and two control patients who had both vestibular nerves surgically sectioned. Rotating on a fixed-chair centrifuge in an otherwise darkened room, each observer was required to indicate his perception of the direction of the resultant gravito-inertial vector by setting a small, motor-driven, illuminated bar, attached to the chair but rotatable in the frontoparallel plane, to the perceived gravitational horizontal. Normal subjects accurately align the bar with respect to the gravito-inertial resultant vector which, in the dark, they assume to be the gravitational vertical. This percept has been called the oculogravic illusion. Accurate roll-tilt perception is due to vestibular (probably mainly otolithic) sensory information since patients with bilateral vestibular neurectomies do not perceive the resultant vector accurately. Whereas normal subjects perceive resultant vectors directed to the right and to the left equally accurately, roll-tilt perception was invariably asymmetrical one week after unilateral vestibular neurectomy. Even at rest there was an asymmetry in the baseline settings, so that patients set the bar down on the side of the operated ear, in order for it to appear gravitationally horizontal: if a patient had a right vestibular nerve section then he set the bar clockwise (from the patient's view) below the true gravitational horizontal. With increasing gravitoinertial resultant angles there was an increasing asymmetry of roll-tilt perception due both to decreased sensitivity to roll-tilt stimuli directed towards the operated ear and to transiently increased sensitivity to roll-tilt stimuli directed towards the intact ear. A progressive decrease in both perceptual asymmetries followed, rapidly in the first 3 weeks, more slowly in the next 6 months. Based on these results, which are consistent with what is known about the responses of primary and secondary otolithic neurons to linear acceleration, we propose: (1) that the asymmetric roll-tilt perceptual response following unilateral vestibular neurectomy is an otolithic analogue of Ewald's second law; (2) that the perceptual asymmetries may be due to decreased spontaneous activity in the deafferented lateral vestibular nucleus; (3) that the progressive recovery of roll-tilt perceptual symmetry after vestibular neurectomy may be part of the otolithic component of the total recovery phenomenon known as vestibular compensation; (4) that ocular torsion caused by the unilateral vestibular neurectomy is a major factor contributing to the systematic errors in baseline settings to the gravitational horizontal one week after operation.     

Identification of the visual motion area (area V5) in the human brain by dipole source analysis

The retinal periphery of nine healthy subjects was stimulated with computer-generated random-dot kinematograms. These stimuli provided almost isolated visual motion information and minimal position cues. Pattern-reversal stimuli at the same location in the visual field were used for control. Stimulus-related electrical brain activity was recorded from 29 scalp electrodes. Total mean and individual data were analyzed with a spatiotemporal multiple dipole model. The scalp potentials showed a different spatial distribution for motion and pattern stimulation in the time range of 160–200 ms. In this epoch, the predominant motion-related source activity was localized in the region of the contralateral occipital-temporal-parietal border. A significant ipsilateral source activity was not found. The predominant source activity related to the pattern stimulus occurred in the same epoch. The corresponding equivalent dipole was localized more medially and deeper in the brain. The orientation of these major dipole activities was markedly different. These dipoles appeared to represent activity of distinct extrastriate areas, in contrast to earlier activity which was modelled by more posterior dipoles in the occipital lobe. The latter dipoles were at comparable contralateral locations and had similar peak activities around 100 ms, suggesting an origin in the striate cortex.     

Convergence and interaction of neck and macular vestibular inputs on reticulospinal neurons

Extracellular recordings were obtained in decerebrate cats from neurons located in the inhibitory area of the medullary reticular formation, namely in the medial aspects of the nucleus reticularis gigantocellularis, magnocellularis and ventralis. Of 127 medullary reticular units examined, 77 were reticulospinal neurons antidromically identified following stimulation of the spinal cord at T12-L1; the remaining 50 neurons were not activated antidromically. Unit firing rate was analyzed under separate stimulation of macular vestibular, neck, or combined receptors by using sinusoidal rotations about the longitudinal axis at 0.026 Hz, 10 peak amplitude. Among the 127 reticular units, 84 (66.1%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 93 (73.2%) responded to neck rotation. Convergence of macular and neck inputs was found in 71/127 (55.9%) reticular neurons; in these units, the gain as well as the sensitivity of the first harmonic of response corresponded on the average to 0.49 +/- 0.41, SD imp/s/deg and 5.10 +/- 4.27, SD %/deg for the neck responses and to 0.40 +/- 0.39, SD imp/s/deg and 3.90 +/- 3.80, SD %/deg for the macular responses, respectively. Most of the convergent reticular units were maximally excited by the direction of stimulus orientation, the first hormonic or responses showing an average phase lead of about +42.7 with respect to neck position and +24.9 with respect to animal position. Two populations of convergent neurons were observed. The first group of units (58/71, i.e. 81.7%) showed reciprocal ("out of phase") responses to the two inputs in that they were mainly excited during side-down neck rotation, but inhibited during side-down animal tilt. The remaining group of units (13/71, i.e. 18.3%) showed parallel ("in phase") responses to the two inputs and they were mainly excited by side-down neck rotation and animal tilt. The response characteristics of medullary reticular neurons to the combined neck and macular inputs, elicited during head rotation, closely corresponded to those predicted by a vectorial summation of the individual neck and macular responses. In particular, "out of phase" units displayed small amplitudes and large phase leads of the responses with respect to head position, when both types of receptors were costimulated. In contrast, "in phase" units displayed large amplitude and small phase leads during head rotation.(ABSTRACT TRUNCATED AT 400 WORDS)