Influence of visually induced self-motion on postural stability

CONCLUSION: Our results indicate that the illusion of self-motion is a significant factor leading to spatial disorientation. OBJECTIVE: Under normal circumstances, self-motion is perceived in response to motion of the head and body. However, under certain conditions, such as virtual reality environments, visually induced self-motion can be perceived even though the subject is not actually moving, a phenomenon known as "vection". The aim of this study was to examine the possible influence of illusory self-rotation (circular vection) on postural adjustments. MATERIAL AND METHODS: The subjects were 10 young females with no history of ocular or vestibular disease. Video-motion analysis was applied to measure postural movements during vertical optokinetic stimulation. RESULTS: For most subjects, movement of the visual surroundings induced head and body displacements in the same direction as that of the visual stimulus, regardless of the onset of self-motion perception. However, there was a significant increase in postural instability after the subjects began to perceive false self-motion in the opposite direction to that of the visual stimulus.     

A comparison of the latencies of visually induced postural change and self-motion perception.

This study compared the latencies of visually induced postural change and self-motion perception under identical visual conditions. The results showed that a visual roll stimulus elicits postural tilt in the direction of scene motion and an increase in postural instability several seconds before the subject begins to perceive illusory self-motion (vection) in the opposite direction. Postural and vection latencies correlate highly with one another, but bear little relationship with the magnitude of either sway or vection.     

Perceived self-motion in two visual contexts: dissociable mechanisms underlie perception

We evaluated the influence of moving visual scenes and knowledge of spatial and physical context on visually induced self-motion perception in an immersive virtual environment. A sinusoidal, vertically oscillating visual stimulus induced perceptions of self-motion that matched changes in visual acceleration. Subjects reported peaks of perceived self-motion in synchrony with peaks of visual acceleration and opposite in direction to visual scene motion. Spatial context was manipulated by testing subjects in the environment that matched the room in the visual scene or by testing them in a separate chamber. Physical context was manipulated by testing the subject while seated in a stable, earth-fixed desk chair or in an apparatus capable of large linear motions, however, in both conditions no actual motion occurred. The compellingness of perceived self-motion was increased significantly when the spatial context matched the visual input and actual body displacement was possible, however, the latency and amplitude of perceived self-motion were unaffected by the spatial or physical context. We propose that two dissociable processes are involved in self-motion perception: one process, primarily driven by visual input, affects vection latency and path integration, the other process, receiving cognitive input, drives the compellingness of perceived self-motion.     

Head and body sway in response to vertical visual stimulation

Conclusions. Postural responses differed according to the stimulus direction, i.e. vertical visual stimulation induced head rather than trunk displacements. Accordingly, it could be that center of foot pressure (COP) responses tended to underestimate the postural sway during visual stimulation. Objectives. To investigate head and body sway in response to vertical visual surround motion, and to examine the correlation between the displacements of head and body segments derived from video-motion analysis and COP measurements. Material and methods. Postural sway was assessed in 10 young female subjects by video-motion analysis of four different head and body segments, and by use of force-plate posturography. Head and body sway in the pitch plane was induced by rotating a random pattern of dots about the subject's inter-aural axis at a constant acceleration of 1°/s² or a constant velocity of 60°/s in darkness. Results. Generally, head displacement was greater than that of other body parts during vertical optokinetic stimulation (OKS). In most subjects, maximum head displacements were induced in the same direction as the visual motion. Downward OKS induced a forward head and body sway. The COP trajectory correlated well with the displacements of each head and body segment during downward OKS. In contrast, postural responses to upward OKS were complicated in terms of their time course. The correlation coefficient between each head and body segment and the COP varied among individuals for upward OKS.     

Motion sickness susceptibility associated with visually induced postural instability and cardiac autonomic responses in healthy subjects

Conclusions This study supports the hypothesis that postural sway and autonomic responses to moving visual stimuli may be associated with motion sickness susceptibility. Characteristics of the cardiac sympathovagal balance during exposure to provocative stimulation may be a marker of individual susceptibility to motion sickness.

Objective To assess the relationship between postural and autonomic responses to a simulated visual motion environment and reported susceptibility to motion sickness.

Material and methods Fifteen healthy subjects were exposed to sinusoidally oscillating visual motion in roll at frequencies of 0.1–0.4 Hz. Recordings were made of postural sway and respiratory frequency and electrocardiograms were obtained from which heart rate variability (HRV) was computed in order to probe cardiac sympathetic and parasympathetic activity.

Results In subjects with a low susceptibility to motion sickness as rated using a standardized questionnaire, there was no significant effect of visual stimulus on postural sway or HRV at any frequency of motion. Subjects with a high susceptibility to motion sickness showed significant postural instability induced by visual stimuli (p<0.01). Visual stimuli presented at a frequency of 0.1 Hz significantly increased the low-frequency power (LF) of HRV, decreased the high-frequency power (HF) of HRV and increased the LF:HF ratio in these subjects (p<0.05).     

Head and body sway in response to vertical visual stimulation

CONCLUSIONS: Postural responses differed according to the stimulus direction, i.e. vertical visual stimulation induced head rather than trunk displacements. Accordingly, it could be that center of foot pressure (COP) responses tended to underestimate the postural sway during visual stimulation. OBJECTIVES: To investigate head and body sway in response to vertical visual surround motion, and to examine the correlation between the displacements of head and body segments derived from video-motion analysis and COP measurements. MATERIAL AND METHODS: Postural sway was assessed in 10 young female subjects by video-motion analysis of four different head and body segments, and by use of force-plate posturography. Head and body sway in the pitch plane was induced by rotating a random pattern of dots about the subject's inter-aural axis at a constant acceleration of 1 degree/s(2) or a constant velocity of 60 degrees/s in darkness. RESULTS: Generally, head displacement was greater than that of other body parts during vertical optokinetic stimulation (OKS). In most subjects, maximum head displacements were induced in the same direction as the visual motion. Downward OKS induced a forward head and body sway. The COP trajectory correlated well with the displacements of each head and body segment during downward OKS. In contrast, postural responses to upward OKS were complicated in terms of their time course. The correlation coefficient between each head and body segment and the COP varied among individuals for upward OKS.     

Directional preponderance in pitch circular vection

We used optokinetic stimulation (OKS) in eighteen normal adults aged 18-30 years to investigate vertical self-motion perception. In order to induce self-rotation, either a stripe pattern or a random dot pattern was projected onto the inner wall of a hemispherical dome with a diameter of 150 cm. The pattern was rotated either about the subject's vertical axis (yaw) or about the subject's interaural axis (pitch) for 80 s at a constant acceleration of 1 deg/s2. Stimuli were randomly repeated three to four times in each direction. The latency of onset as well as the perceived intensity of circular vection (CV) was measured for each stimulus presentation. CV latencies for upward rotational stimulation were significantly longer than those for downward rotational stimulation under both types of stimulus conditions. There was no significant difference in CV latency between rightward and leftward rotational stimulation. For most subjects, the magnitudes of the perceived CV for rightward rotational stimulation were equal to those for leftward rotational stimulation, whereas the magnitudes of the perceived CV for vertical stimulation showed large intersubject variability. These results provide additional evidence that fundamental differences exist between different types of self-motion. Possible explanations for the directional asymmetry in vertical perception of self-motion will also be discussed.     

Time course analysis of angular control of the body and head while rising from a chair

Objective To use time course information to improve understanding of the vestibular contribution to postural control as one rises from a chair.

Material and methods A total of 24 healthy controls and 42 patients with varying degrees of vestibular dysfunction were studied. The time course of the angular motion of the body and head when rising from a chair with eyes open and closed was evaluated. The delay between the onset of the motions of the body and head was compared between subject groups. We also investigated transition points from forward lean of the body to backward reversion and from backward tilt of the head to forward reversion. Results With regard to the onset of chair rise, we found a significant difference in the delay between head and body motion between healthy controls and subjects with bilaterally impaired vestibular deficiency only when the eyes were closed. The time between the transition points of the head and body was stable between these groups.

Conclusion The mechanisms controlling the onset of head and body movements differ between normal subjects and those with bilateral vestibular deficits. In the latter, the loss of a reference of gravity causes a decrease in feed-forward postural control, which is compensated for by a somato-sensory feedback mechanism. Visual input seems to provide an alternative reference of gravity.     

Direction of Galvanically-induced Vestibulo-postural Responses during Active and Passive Neck Torsion

The direction of a postural response induced by galvanic vestibular stimulation depends on the head and trunk position. The relative importance of afferent information (proprioception) and efferent motor command/corollary discharge is unknown. We studied the direction of body sway evoked by galvanic vestibular stimulation in 9 healthy subjects during active and passive head positioning at 0° frontal position, 35° to the left, and 75° to the right, using a custom-built collar. At 0° and 75° there were no significant differences in sway direction between active and passive head positioning. The galvanic stimulation invoked sway toward the anode, mainly in the inter-aural direction. The sway direction differed significantly between active and passive positioning at 35° to the side (p&lt;0.05). When the head was actively kept in this position, the body sway was mainly in an inter-aural direction. The sway shifted to a naso-occipital direction when the head was passively positioned at 35°. Our results indicate that the afferent proprioceptive information has the largest influence on the direction of the galvanically-induced postural response, although some dependence on efferent motor commands and non-linear cervical proprioception cannot be ruled out entirely.     

Short-term Adaptation in the Peripheral Auditory System is Related to the AMPA Receptor

A pattern of sound-induced paroxysms of the eye and head and other spinal motor neuron synkinesis (Tullio's phenomenon) in human subjects always implies either a pathological contiguity of the tympano-ossicular chain and membranous labyrinth or a dehiscence of the bone overlying the superior semicircular canal. However, it has become clear in the last decade that sound-evoked vestibular stimulation is not only a sign of disease but also a physiological phenomenon. The examination of such physiologically sound-induced vestibular (saccular) responses contributes today to the clinical testing of the vestibular organ, mainly in the form of vestibular-evoked myogenic potentials. In this study it was observed that, in a group of 20 normal subjects, a 500 Hz tonal stimulus of high intensity (105 dB HL=118.5 dB SPL), applied monoaurally, elicited postural responses. Each subject was studied under 4 different conditions: (i) head facing forwards, eyes open; (ii) head facing forwards, eyes closed; (iii) head rotated &;#44 90° to the right, eyes closed; and (iv) head rotated 90° to the left, eyes closed. Body sway, measured using a force platform, was recorded in all subjects, with eyes either open or closed. Postural responses, which were also elicited with a 250 Hz tonal stimulus, were not observed with a tone of 2000 Hz, with legs slightly flexed or with binaural stimulation. The postural sway (head facing forwards, eyes open or closed) was in a lateral direction towards the stimulated ear: with the stimulus applied to the right ear the subject had postural sway towards the right, with the stimulus applied to the left ear towards the left. When the head was rotated &;#44 90° sideways and the stimulus was given facing forwards (i.e. head rotated contralaterally to stimulated ear) the postural sway was in a forward direction; when the head was rotated &;#44 90° sideways and the stimulus was given facing backwards (i.e. head rotated ipsilaterally to stimulated ear) the postural sway was in a backward direction. The mean values (mm) of body sway obtained with the head facing forwards and the eyes closed were higher than those with the eyes open (21.7 and 22.8 vs 15.7 and 14.7 for the right and left ears, respectively); higher mean values were obtained with the head turned to the side contralateral to the ear stimulated and the eyes closed (29.3 and 24.8 for the right and left ears, respectively). Under this condition the body sway was mainly in a forward direction. The sound-evoked vestibulopostural reflex seems to be a useful test for exploring the saccular function and, as a click-evoked vestibulocollic reflex, can be considered a physiological Tullio phenomenon.     

Dynamic visual acuity in benign paroxysmal positional vertigo

Abstract

Background: Head motions cause transient vertigo in patients with benign paroxysmal positional vertigo (BPPV) and may reduce visual ability.

Objectives: The aim is to investigate the clinical utility of dynamic visual acuity (DVA) test in those patients.

Material and methods: Thirty patients, 11 with lateral canal BPPV (6 geotropic and 5 ageotropic), and 19 with posterior canal BPPV, were evaluated with DVA test during rapid horizontal (left and right) and vertical (up and down) head movements. Patients were asked to identify the direction of the letter C, ranging from 1.0 to 0.0 logMAR, while moving their heads. The orientation of the optotype was randomly changed by a computer-generated program. Ten subjects were served as control. Data were analyzed with a one-way ANOVA. p Value <.01 was considered significant.

Results: Patients with LC and PC BPPV had significant DVA loss as compared with control subjects (p?<?.01). However, no significant difference was found in vDVA between patients with LC and PC BPPV, or in hDVA between ipsilesional and contralesional head rotation in patients with LC or PC BPPV (LC: p?=?.755, PC: p?=?.765).

Conclusions: Patients with BPPV may have impaired visual acuity, particularly during acute onset condition. Present study indicated that DVA test was not helpful for differentiation of the pathologic and normal ears.     

Postural Stability Using Different Neck Positions in Normal Subjects and Patients with Neck Trauma

Subjects with neck problems, such as whiplash injuries, often complain of disturbed equilibrium and, in some instances, provocation of the neck position can elicit such problems. The importance of neck proprioceptors for maintaining balance is gaining increased interest, moreover the function or malfunction of the otoliths may disturb equilibrium in certain head positions. The aim of the study was to create a reference material for postural control and its dependence on head position in healthy subjects and to compare this with a set of patients with known neck problems and associated vertiginous problems. A total of 32 healthy subjects (16 men, 16 women; age range 21-58 years) as well as 10 patients age range 27-62 years (mean 44 years) with neck problems and associated balance problems since a whiplash injury were tested for postural control using the EquiTest dynamic posturographic model. The normal subjects were initially split into four age groups in order to estimate the effects of age on performance. The postural stability was evaluated for dependence of support surface conditions (stable or sway-referenced), visual input (eyes open or closed) and head position (neutral, left rotated, right rotated, extended backwards or flexed forward) using analysis of variance (ANOVA) with Tukey's post hoc test in case of a significant factor effect. As expected, visual cues as well as stable support surface improve postural stability (p&lt;0.001). Postural stability is statistically different in the head extended backwards condition compared with the other four head positions (p&lt;0.001 in all cases) in both patients and controls. Eliminating this test condition from the analysis, only a slight (p&lt;0.05) difference between head forwards and head turned left remained. This pattern of results remained if the normal subjects were only split into two age groups instead of four. Finally, the patient group exhibited significantly lower postural performance than all the groups of normal subjects (p&lt;0.01), but none of the normal groups differed significantly from each other. It is concluded that the postural control system is significantly challenged in the head extended backwards condition in both normal subjects and patients with previous whiplash injury and persistent neck problems. The patient group differed statistically from all groups of normal subjects. This suggests that neck problems impair postural control, and that the head extended position is a more challenging task for the postural system to adapt to. Whether this is due to utricular malpositioning, central integrative functions or cervical proprioceptive afferents is not within the scope of this study to answer.     

Sound-evoked postural responses in normal subjects

A pattern of sound-induced paroxysms of the eye and head and other spinal motor neuron synkinesis (Tullio's phenomenon) in human subjects always implies either a pathological contiguity of the tympano-ossicular chain and membranous labyrinth or a dehiscence of the bone overlying the superior semicircular canal. However, it has become clear in the last decade that sound-evoked vestibular stimulation is not only a sign of disease but also a physiological phenomenon, The examination of such physiologically sound-induced vestibular (saccular) responses contributes today to the clinical testing of the vestibular organ, mainly in the form of vestibular-evoked myogenic potentials. In this study it was observed that, in a group of 20 normal subjects, a 500 Hz tonal stimulus of high intensity (105 dB HL = 118.5 dB SPL), applied monoaurally, elicited postural responses. Each subject was studied under 4 different conditions: (i) head facing forwards, eyes open; (ii) head facing forwards, eyes closed; (iii) head rotated approximately 90 degrees to the right, eyes closed: and (iv) head rotated approximately 90 degrees to the left, eyes closed. Body sway, measured using a force platform, was recorded in all subjects, with eyes either open or closed. Postural responses, which were also elicited with a 250 Hz tonal stimulus, were not observed with a tone of 2000 Hz, with legs slightly flexed or with binaural stimulation. The postural sway (head facing forwards, eyes open or closed) was in a lateral direction towards the stimulated ear: with the stimulus applied to the right ear the subject had postural sway towards the right, with the stimulus applied to the left ear towards the left. When the head was rotated approximately 90 degrees sideways and the stimulus was given facing forwards (i.e. head rotated contralaterally to stimulated ear) the postural sway was in a forward direction; when the head was rotated approximately 90 degrees sideways and the stimulus was given facing backwards (i.e. head rotated ipsilaterally to stimulated ear) the postural sway was in a backward direction. The mean values (mm) of body sway obtained with the head facing forwards and the eyes closed were higher than those with the eyes open (21.7 and 22.8 vs 15.7 and 14.7 for the right and left ears, respectively); higher mean values were obtained with the head turned to the side contralateral to the ear stimulated and the eyes closed (29.3 and 24.8 for the right and left ears, respectively). Under this condition the body sway was mainly in a forward direction. The sound-evoked vestibulopostural reflex seems to be a useful test for exploring the saccular function and, as a click-evoked vestibulocollic reflex, can be considered a physiological Tullio phenomenon.     

First trial response to sudden support surface displacement: The effect of vestibular compensation

Conclusion: The effect of visual condition is more intense in the first trial response in normal subjects and patients and in last trial response only in patients. The first trial effect is more evident in compensated patients in the eyes open condition with any type of perturbation, and in non-compensated patients with the angular displacements in either visual condition. Objective: The study of body reaction to FTR can help to understand the complex mechanisms involved in the postural response and to develop new therapies to improve stability and prevent falls in unilateral vestibular deficit (UVD). This work describes the adaptation effect and the visual influence on the postural response to repetitive balance perturbation stimulus in normal subjects, compensated, and uncompensated UVD patients. Methods: The magnitude of displacement has been measured when the support surface is linearly or angularly displaced. The differences between results in the first and late trial, and the differences between the eyes open and eyes closed situation have been compared. Results: Compensated patients recover the adaptation ability to unexpected changes on the support surface through visual preference mechanism. Not compensated patients present hypermetric postural response with greater instability in the eyes open and eyes closed situations.     

Contribution of the Vestibular Apparatus to Postural Control when Rising from a Chair

Objective—The everyday act of rising from a chair is known to require the combined angular control of a number of the body's joints, especially those within the pitch plane. Precisely how this control is exerted, however, remains controversial. The aim of this study was to obtain a better understanding of the contribution made by the vestibular apparatus to postural control of the body and head when an individual rises from a chair.

Material and Methods—A total of 24 healthy controls and 38 patients with varying degrees of vestibular dysfunction were examined. Electromagnetic motion sensors were used to analyze the angular control of the head and body as subjects rose from a chair with their eyes open or closed.

Results—We found that unilateral vestibular dysfunction caused fixation of the head with respect to the body, resulting in a loss of spatial stability of the head which was not compensated for by visual input. Visual input did appear to compensate for bilateral vestibular loss, enabling patients with bilateral vestibular apparatus impairment or central disorders to fix the position of their head in space.

Conclusion—The act of rising from a chair is normally controlled by vestibular and proprioceptive input; the head is aligned according to the gravitational reference so as to obtain stable visual information. In patients with unilateral vestibular hypofunction, posture is still controlled by these two inputs, although the ability to align the head is diminished. In patients with bilateral vestibular hypofunction or a central disorder, head alignment is maintained using visual input, although it may not be the sole or predominant stabilizing force.     

Dynamic visual acuity test findings of migraine patients: Observational case-control study

ObjectiveTo evaluate computerized dynamic visual acuity (DVA) test findings among patients with migraine and to determine whether self-motion sensitivity and visually induced migraine symptoms that are seen in migraine patients can be explained by DVA results.MethodsTwenty migraine patients and twenty control subjects were evaluated with DVA test during rapid horizontal (left and right) and vertical (up and down) head movements. The subjects were asked to quickly recognize the direction of the Landolt-C optotype on the monitor while moving their heads. Test began with the largest optotype and progressively decreased in size by 0.1 logMAR until the subject could no answer correctly state the orientation of the optotype. DVA test scores of migraine group and control group were compared.ResultsPatient with migraine had significant DVA loss as compared with control subjects in four positions (left DVA, right DVA, up DVA, down DVA respectively p < 0.001, <0.001 = 0.001 and <0.001).ConclusionMigraine patients were found to have abnormal DVA scores mediated vestibular-ocular reflex. These abnormal DVA findings can explain the pathophysiology of head motion hypersensitivity and visual motion sensitivity that encountered by migraine patients.     

Direction of galvanically-induced vestibulo-postural responses during active and passive neck torsion

The direction of a postural response induced by galvanic vestibular stimulation depends on the head and trunk position. The relative importance of afferent information (proprioception) and efferent motor command/corollary discharge is unknown. We studied the direction of body sway evoked by galvanic vestibular stimulation in 9 healthy subjects during active and passive head positioning at 0 degrees frontal position, 35 degrees to the left, and 75 degrees to the right, using a custom-built collar. At 0 degrees and 75 degrees there were no significant differences in sway direction between active and passive head positioning. The galvanic stimulation invoked sway toward the anode, mainly in the inter-aural direction. The sway direction differed significantly between active and passive positioning at 35 degrees to the side (p < 0.05). When the head was actively kept in this position, the body sway was mainly in an inter-aural direction. The sway shifted to a naso-occipital direction when the head was passively positioned at 35 degrees. Our results indicate that the afferent proprioceptive information has the largest influence on the direction of the galvanically-induced postural response, although some dependence on efferent motor commands and non-linear cervical proprioception cannot be ruled out entirely.     

Postural control and sensory perception in patients with Parkinson's disease

Conclusions. This study suggests that patients with Parkinson's disease (PD), even in the early stages, have decreased body limits of stability (LOS) and changes in the visual input impair their postural control. Objective. To assess the LOS and the postural responses after changes in visual input in a group of PD patients in stage 1 of the Hoehn and Yahr classification. Subjects and methods. Twenty PD patients in stage 1 and a group of 24 normal subjects as control were assessed in two tests: (1) the LOS and (2) measurement of the body center of pressure area (COP) 10 s before and after sudden change in visual flow velocity. We also investigated labeling of the COP trajectory in these two periods. The stimulation paradigm was a horizontal optokinetic stimulation (60°/s and suddenly stopped) using a virtual reality system. Results. LOS showed significant decrease in PD patients as compared with the control group (p<0.001, Kruskal-Wallis and Wilcoxon ranked test). The COP values increased significantly (p<0.001, Wilcoxon signed rank test) after sudden changes in the visual flow velocity in relation to the control group. After the visual stop the PD patient showed a spatial ‘roaming’ approaching the limits of stability and therefore impairing the postural control.     

Motion sickness susceptibility associated with visually induced postural instability and cardiac autonomic responses in healthy subjects

CONCLUSIONS: This study supports the hypothesis that postural sway and autonomic responses to moving visual stimuli may be associated with motion sickness susceptibility. Characteristics of the cardiac sympathovagal balance during exposure to provocative stimulation may be a marker of individual susceptibility to motion sickness. OBJECTIVE: To assess the relationship between postural and autonomic responses to a simulated visual motion environment and reported susceptibility to motion sickness. MATERIAL AND METHODS: Fifteen healthy subjects were exposed to sinusoidally oscillating visual motion in roll at frequencies of 0.1-0.4 Hz. Recordings were made of postural sway and respiratory frequency and electrocardiograms were obtained from which heart rate variability (HRV) was computed in order to probe cardiac sympathetic and parasympathetic activity. RESULTS: In subjects with a low susceptibility to motion sickness as rated using a standardized questionnaire, there was no significant effect of visual stimulus on postural sway or HRV at any frequency of motion. Subjects with a high susceptibility to motion sickness showed significant postural instability induced by visual stimuli (p < 0.01). Visual stimuli presented at a frequency of 0.1 Hz significantly increased the low-frequency power (LF) of HRV, decreased the high-frequency power (HF) of HRV and increased the LF:HF ratio in these subjects (p < 0.05).     

Cross-coupling in a body-translating reaction: interaural optokinetic stimulation reflects a gravitational cue

Conclusion. The velocity storage integrator does not play a dominant role in the postural response to vertical visual cues; more likely, retinal slip provides the main driving force. By contrast, sideways eye movement can drive the velocity storage integrator and preserve a gravitational cue, which would be observed as a cross-coupling effect on the postural response. Objectives. To investigate the mechanism by which optokinetic stimulation causes the body to translate and to determine whether the optokinetic information is accompanied by a gravitational cue, which would appear as a cross-coupling effect. Materials and methods. Directionally diverse optokinetic stimuli were presented to seven healthy subjects, with and without a fixation target, and the body-translation of the subjects was recorded. Results. Horizontal optokinetic stimulation with a fixation target caused the body to translate in the same direction as the optic flow. Upward or downward vertical optokinetic stimulation caused the body to translate backward or forward, respectively, only when a fixation target was present. When the subject's interaural axis was parallel to the optokinetic flow, diagonal optokinetic stimulation in the absence of a fixation target elicited responses in the pitch plane similar to those elicited by vertical stimulation in the presence of a fixation target.