INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The vestibular system includes two types of sensors: the semicircular canals and the otolith organs. The semicircular canals behave as integrating angular accelerometers measuring head angular velocity (Wilson and Melvill Jones 1979). The otolith organs behave as linear accelerometers measuring the specific gravito-inertial force (GIF), which is the sum of gravitational force and inertial force due to linear acceleration. Using information from these sensors, the human CNS develops perceptions of spatial orientation and generates VORs that help stabilize gaze in response to head rotation and translation.

A problem arises when the CNS must distinguish head tilt with respect to gravity from linear translation (acceleration) of the head. For example, a compensatory response for tilting the head toward the left shoulder is a torsional eye movement while acceleration toward the right induces a compensatory horizontal eye movement. In both cases, the interaural shear force measured by the otolith organs might be identical, but the compensatory eye responses are quite different. Ideally, having devices that independently measure gravity and linear acceleration could solve the tilt-translation dilemma. However, Einstein's equivalence principle states that no physical device can distinguish gravitational force from inertial force due to linear acceleration. This implies that the CNS is unable to solve the tilt-translation problem using only information from the otolith organs or any other combination of linear accelerometers. Indeed, a given GIF (f) measured by the otolith organs can be produced by an infinite number of combinations of gravity (g), and linear acceleration (a). For example, while in a supine orientation, a linear acceleration of the head to the left down a slight slope (Fig. 1A) can lead to the exact same GIF as a slight yaw tilt () to the right (Fig. 1B).

View larger version (15K): [in this window] [in a new window]   Fig. 1. Our definitions of gravity and linear acceleration follow standard definitions where g is gravitational force (per unit mass) and a is the linear acceleration of a head fixed reference frame measured by an observer in an inertial reference frame. In an accelerating head fixed reference frame, an inertial force (per unit mass) can be defined which is opposite to the linear acceleration, a. This inertial force can be represented by the vector a in the head fixed reference frame. Summing the gravitational force and the inertial force due to linear acceleration yields the total gravito-inertial force: f = g + (a). Since these representations are mathematically identical, we simply refer to gravity minus linear acceleration (f = g  a). Different combinations of g and a can yield exactly the same measured gravito-inertial force (GIF). A: with a subject in a supine position on a sled, acceleration down the inclined slope produces a GIF with a magnitude of 1 g toward the back of the head at a slight angle () to the subject's right. B: with no linear acceleration, the exact same GIF (magnitude and direction with respect to the subject) can be obtained by tilting the subject into a position  degrees to the right of a supine orientation. Since the measurement of GIF relative to the head is identical for these 2 situations, additional information is required for the nervous system to distinguish tilt from translation and to determine how much of the measured GIF is due to gravity and how much is due to linear acceleration.

In many situations, the eye movements generated in response to head tilts and translations are appropriately compensatory (Angelaki et al. 1999; Merfeld and Young 1995), implying that the CNS is able to somehow distinguish gravity from linear acceleration. At least three hypotheses have been proposed to explain how the CNS makes use of GIF information from the otolith organs to influence eye movements, peripheral processing, frequency segregation, and GIF resolution.

The peripheral processing hypothesis states that some of the separation of the GIF into estimates of gravity and linear acceleration occurs peripherally with the phasic irregular otolith afferent signal interpreted primarily as linear acceleration, while the tonic regular otolith afferent signal might represent gravity (Mayne 1974; Young and Meiry 1967). The frequency segregation hypothesis states that the CNS might resolve the GIF ambiguity using a form of central processing in which low-frequency cues elicit tilt responses and high-frequency cues elicit translation responses (Mayne 1974; Paige and Tomko 1991). This hypothesis is in accordance with low-frequency characteristics of tilt perception during centrifugation (Clark and Graybiel 1951; Glasauer 1992). It also matches VOR data during interaural translation, which has both horizontal translation-related eye movements with high-frequency characteristics and torsional tilt-related eye movements with low-frequency characteristics (Paige and Tomko 1991). However, as observed by Angelaki (1998), frequency segregation during interaural translation is affected by the way translation and tilt sensitivities are expressed (Paige and Tomko 1991).

The GIF resolution hypothesis (Merfeld and Young 1995; Merfeld et al. 1993a), an explicit refinement of the "multisensory integration" hypothesis (Guedry 1974; Mayne 1974; Oman 1982; Young 1984), states that additional sensory information is required for the CNS to resolve the GIF ambiguity. Specifically, the CNS systematically separates the otolith GIF measurement (f) into estimates of gravity () and linear acceleration (â) using multi-sensory convergence, so that the difference between these estimates approximately matches the measured GIF (f =   â). [The specific gravito-inertial force measured by the otolith organs (f) is defined as the sum of gravitational force (g) plus an inertial force per unit mass (a) acting on the otolith organs and exactly opposing the direction of linear acceleration (a). We choose to adopt the notation of Young (1984) for representing the effect that gravity and linear acceleration have on the otolith organs. Physical variables are mathematically represented by three-dimensional vectors, f, g, and a.] The GIF resolution hypothesis appears indistinguishable (other than notational differences) from another recently formulated approach used to explain how the CNS uses semicircular canal cues to distinguish sinusoidal tilt from sinusoidal translation (Angelaki et al. 1999).

When visual cues are absent or cannot be used, the semicircular canal and otolith cues may provide the CNS with conflicting sensory information. Consider the sensory situation experienced by a subject statically positioned in a nose-up orientation (Fig. 2A) following a sustained counterclockwise rotation (toward the subject's left) in darkness about an earth-horizontal axis. First, for this experimental protocol, the otolith organs measure a constant GIF due to gravity alone (f = g). In the nose-up orientation, this force is aligned with the subject's nasooccipital axis. At the same time, the semicircular canals, because of their dynamics (Wilson and Melvill Jones 1979), have a post-rotatory response indicating an on-going rotation () in a clockwise (toward the subject's right) direction even though the subject is actually at rest. Rotational cues are known to influence the perceived orientation of gravity (Stockwell and Guedry 1970), often leading to illusory tilt (Dichgans et al. 1972; Merfeld et al. 1999; von Holst and Grisebach 1951). This evidence suggests that the yaw rotational cue following post-rotatory tilt () influences the estimated orientation of gravity () such that the estimate of gravity () rotates in the same direction as if there was an actual rotation. We hypothesize that the CNS computes estimates of gravity () and linear acceleration (â) such that their difference matches the measured GIF, which equals gravity in this example (f = g =   â). Therefore a nonzero estimate of linear acceleration (â =   g) would be generated whenever the estimate of gravity () does not match true gravity (g), as demonstrated in Fig. 2A. The estimate of interaural linear acceleration (â_y) may induce a horizontal VOR component (Fig. 2B) that should be similar to a horizontal VOR response compensatory to an actual interaural linear acceleration; we refer to this VOR component as an induced VOR. This induced VOR component should combine more or less linearly with the horizontal angular VOR component as previously demonstrated in squirrel monkeys (Sargent and Paige 1991). Since the tilt direction of the estimate of gravity () depends on rotation direction, the induced VOR component must depend on rotation direction. Similarly, since the estimate of interaural linear acceleration (â_y) depends on subject orientation (, Fig. 2C), the induced VOR component must depend on subject orientation.

View larger version (13K): [in this window] [in a new window]   Fig. 2. If the subject is oriented "nose-up" (NU) a clockwise (CW) yaw rotational cue () rotates the estimated gravity () toward the subject's right. A: when the estimated gravity () is tilted relative to the actual orientation of gravity (g), the GIF resolution hypothesis suggests that a neural internal model, representing the physical relationship between gravity and linear acceleration, will develop an estimated linear acceleration (â) that equals the estimated gravity () minus measured GIF (f). That is, f =   â, and therefore â =   f. In this case, the measured GIF (f) equals gravity (f = g). B: the estimate of linear acceleration (tilted by an angle from the earth-horizontal) has a nonzero projection on the interaural axis (ây). C: the projection of the estimated linear acceleration on the interaural axis ây () varies sinusoidally with subject orientation ().  = 0° is the nose-up orientation,  = 90° is the left-down (LD) orientation,  = 180° is the nose-down (ND) orientation, and  = 270° is the right-down (RD) orientation.

To gain insight into how human subjects distinguish tilt from translation, we designed two experiments that created a conflicting sensory situation between semicircular canal and otolith cues, and we measured reflexive slow phase eye movements. In one experiment, subjects were tilted 90° from an upright orientation after they were rotated for 150 s at a constant velocity about an earth-vertical axis ("dumping" protocol). In another experiment, subjects were rotated for 150 s at a constant velocity about an earth-horizontal axis and then decelerated to a stop ("barbecue" protocol). For both protocols, we focused on the reflexive eye movements following deceleration to a stop, referred to as the post-rotatory responses. Both of these protocols are described in detail in the following section.

Horizontal responses alone have been presented in a previous report for 200°/s dumping protocols (Merfeld et al. 1999). The present report extends this previous study by including VOR responses for 50 and 100°/s dumping protocols to investigate the influence of the magnitude of the rotational stimulus on the GIF resolution. In addition, vertical VOR responses are investigated to identify properties of the axis-shift. Moreover responses following barbecue-spit rotation are presented to evaluate the dynamic influence of post-rotational tilt on the measured responses.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Experimental setup and eye movement recording

Informed consent was obtained in accordance with institutional procedures, and subjects were instructed about potential risks, including motion sickness, prior to each testing session.

All protocols were conducted on a two-axis rotation device. An inner gimbal powered by a 160-N-m DC motor (velocity servo control) provided full-circle rotations about the subject's yaw axis. An outer pitch/roll gimbal powered by a 2,300-N-m hydraulic actuator (position servo control) provided rotations about an earth-horizontal axis. Each subject was seated in a kneeling position on a car-race-type seat mounted in the device's inner yaw gimbal. The subject's body was restrained using a 5-point seat-belt system, lateral shoulder supports, a wide waist belt, and knee restraints. Foam pads were added as needed to ensure maximum stability. During testing the subject gripped a pair of handlebars that provided additional stabilization. The subject's head was secured in an adjustable foam-lined head restraint. The subject's interaural axis was aligned with the outer gimbal earth-horizontal axis and the interaural axis midpoint was aligned with the inner gimbal yaw axis.

Binocular eye movements were recorded (Panasonic AG-DS850 SVHS VCR) using small video cameras (Machine Vision Hyper CCD Cameras CV-36SH) mounted on a bite-bar assembly. Infrared light-emitting diodes (LEDs) provided lighting for the video cameras. A mold of each subject's mouth was formed on the bite-bar using a dental-impression compound (3 M Express, 3 M Dental Products, St. Paul, MN). The weight of the camera assembly was supported by elastic bands attached to the yaw gimbal.

An off-line application of a custom image-analysis program provided measurements of the horizontal and vertical coordinates of the pupil center for each field of the video image (59.94 video fields/s). The video system was calibrated by having the subject sequentially direct gaze at 9 horizontal, 9 vertical, and 12 oblique LED target lights. Target viewing evoked horizontal and/or vertical eye movements up to approximately ±20°. Measurements of the three-dimensional (3-D) locations in space of both the eyes and target LEDs were used to determine the horizontal and vertical angular orientations (Fick coordinates) of the eyes with respect to the head for each gaze orientation. The calibration procedure used these known gaze orientations to determine polynomial equations that related the horizontal and vertical pupil center coordinates to the horizontal and vertical eye orientation angles. These polynomial equations were applied to video data obtained during subsequent test runs to determine horizontal and vertical eye orientation. The overall accuracy of the video measurements is provided by the root mean square error between the absolute eye position prediction from the polynomial fit and the presumed actual eye position. The rms error was less than 0.5° for both horizontal and vertical eye orientations. The repeatability of the video measurements is provided by results of redigitizing and reanalyzing a single video image. This procedure produced a standard deviation of 0.05° for both horizontal and vertical eye position measures. All reported eye movement responses are at least one order of magnitude greater than the video-system sensitivity. Horizontal and vertical eye position data were digitally filtered and differentiated to yield horizontal and vertical eye velocity, respectively. Torsional eye position was not calculated. Fast phases were automatically removed using a computer algorithm based on peak acceleration detection, with manual editing by experienced personnel, leaving the slow phase eye velocity (SPV).

Experimental protocols

dumping protocol. Eight healthy subjects age 26-47 (6 males and 2 females) with no history of peripheral or central vestibular disorders volunteered for this study. Clinical testing (including but not limited to rotating chair test battery, Hallpike maneuvers, computerized dynamic posturography, and caloric testing) was performed on seven of the eight subjects and indicated no abnormalities. (The data from the subject not clinically examined were similar to the data from the other subjects.)

barbecue spit protocol. Four healthy male subjects, ages 29-49, with no history of peripheral and central vestibular disorders participated in this portion of the study (2 of the 4 also participated in the dumping protocol). The number of subjects performing this protocol was limited because of severe motion sickness that prevented most subjects from completing the test protocols.

Data analysis

head-fixed reference frame. All vector coordinates (physical variables and internal estimates) were expressed in an orthogonal head-fixed frame of reference with x, y, and z axes corresponding to the subject's nasooccipital, interaural, and rostrocaudal axes respectively. The positive axes were directed nasally (x), toward the left ear (y), and toward the top of the skull (z).

fick angles. To describe eye position and eye angular velocity, we used Fick angles and angular rates defined by the right-hand rule with positive x, y, and z coordinates corresponding to CW torsional, downward vertical, and leftward horizontal movements, respectively.

sinusoidal fit. To characterize the variations of the horizontal induced VOR component, horizontal VOR time constant, and vertical VOR as a function of head orientation, we fit spatial sinusoids to these different data sets for each direction of rotation. For the horizontal-induced VOR component, a sinusoidal variation with head orientation is expected based on the following considerations. The induced VOR component should be proportional to the interaural projection of the estimate of linear acceleration (â_y, Fig. 2B). If the estimate of linear acceleration is constant in a space-fixed frame of reference, its interaural projection and the associated compensatory induced VOR component should vary sinusoidally with head orientation in a head-fixed frame of reference (Fig. 6). This sinusoidal variation was quantified by measuring the mean horizontal induced VOR component between 3 and 4 s after the tilt and fitting a sine function to these data (Figs. 6 and 8).

(1)

time constants. To determine time constants, a constrained nonlinear optimization algorithm was used (function const in Matlab 5.2 for Macintosh, The Mathworks). The slow phase velocity was fit with three different analytical functions D(t)

(2)

(3)

(4)

where A is the amplitude and T the time constant of the exponential fit. The decay time constant was also determined by measuring the time interval in which the post-rotatory SPV amplitude reached 37% of its peak amplitude (1/e  0.37). For data presented in this paper, we used the parameters obtained from Eq. 2 since they showed the lowest variability. (Except for higher variability, the parameters obtained using the other methods were similar to those from Eq. 2.)

statistics. Analysis of variance (ANOVA) was used to determine the statistical significance of velocities and time constants of VOR, angular VOR, and induced VOR components as a function of speed of rotation, direction of rotation, and head final orientation. All statistical analyses were performed with Systat 7.0 (SPSS).

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

This report focuses primarily on the responses following deceleration, i.e., slow phase velocity of the post-rotatory nystagmus. However, the per-rotatory responses to yaw rotations about an earth-vertical axis during dumping protocols had average amplitudes and average decay time constants within the normal range (Table 1) for all speeds (Hess et al. 1985; Honrubia et al. 1984). The per-rotatory horizontal VOR time constants decreased with increasing velocity as previously reported (Baloh et al. 1979; Paige 1989). For barbecue spit trials (Table 2), the per-rotatory responses showed the usual peak amplitudes and sinusoidal modulations (Wall and Furman 1989).

Post-rotatory horizontal eye movements following earth-vertical axis rotations ("dumping")

HORIZONTAL VOR. The horizontal post-rotatory VOR slow-phase velocity was determined in eight subjects for three speeds (200, 100, 50°/s), two directions (CW and CCW) of rotation, and eight final tilt orientations. During the first 20 s following the post-rotatory tilt, the post-rotatory VOR exhibited four very different patterns dependent on rotation direction and head orientations. First, for each of the three speeds tested, the VOR magnitude following a tilt in the NU orientation was greater than the VOR magnitude following a tilt in the ND orientation after identical CCW and CW rotations (Fig. 3, left). We are focusing here on the 20 s following the post-rotatory tilt period that begins 3.5 s after the VOR peaked (see graphs to the right of the vertical dash-dotted line on each plot in Fig. 3). Second, the VOR magnitude was greater for LD trials than for RD trials following identical CCW rotations, but the VOR magnitude was smaller for LD trials than for RD trials following identical CW rotations for each of the three speeds tested (Fig. 3, 2nd column). Third, for NU-RD and ND-LD orientations, little difference was observed following CCW rotation, but the VOR magnitude was greater for NU-RD trials than for ND-LD trials following CW rotation (Fig. 3, 3rd column). Fourth, for NU-LD and ND-RD trials, little difference was observed following CW rotation, but the VOR magnitude was greater for NU-LD trials than for ND-RD trials following CCW rotation (Fig. 3, 4th column). These response patterns were one of our principal findings and were predicted by the GIF resolution hypothesis as discussed later.

View larger version (44K): [in this window] [in a new window]   Fig. 3. Post-rotational horizontal vestibuloocular reflex (VOR) following 50, 100, and 200°/s yaw rotations about an earth-vertical axis followed by a 90° tilt ("dumping protocol"). Post-rotational responses are labeled using the actual direction of rotation preceding the tilt [clockwise (CW) or counterclockwise (CCW)], but the oppositely directed post-rotational canal response () is also indicated in the first row. Sketches illustrate subject orientation following the post-rotatory tilt. Due to the on-going rotation indicated by the semicircular canals (), the estimate of gravity () is no longer aligned with gravity (g), resulting in a nonzero estimate of linear acceleration (â) that may have a nonzero interaural projection (ây). Initial VOR responses following CCW rotation are always positive (slow phase to the left); responses following CW rotation are always negative (slow phase to the right). Time 0, which is signaled by a vertical dash-dotted line, indicates completion of tilt. The mean post-rotatory responses for 8 subjects are shown for the following orientations: NU and ND (1st column), LD and RD (2nd column), NU-RD and ND-LD (3rd column), and NU-LD and ND-RD (4th column).

View larger version (39K): [in this window] [in a new window]   Fig. 4. Sinusoidal fits to post-rotatory horizontal VOR mean time constants for dumping protocols. The post-rotatory horizontal VOR mean time constants for 8 subjects are plotted vs. subject orientation following both CW () and CCW () yaw rotations at each of 3 speeds (50, 100, and 200°/s). Responses are modulated sinusoidally; least-square-error sinusoidal fits to the data are shown (means ± 1 SE).

ANGULAR- AND ORIENTATION-DEPENDENT COMPONENTS OF THE VOR. We hypothesized that subject orientations separated by 180° (e.g., NU and ND) result in an opposite estimate of interaural linear acceleration (â_y), leading to oppositely directed induced VOR components as well (see DISCUSSION for more details). At the same time, rotational cues from the semicircular canals are identical, consistent with experimental psychophysical reports showing that the duration of rotation sensation following 90° tilts does not depend on subject orientation (Benson and Bodin 1966). Therefore if the VOR slow phase velocities for two head orientations separated by 180° are noted VOR₁ and VOR₂, we calculated, as a simple approximation, an estimate of the angular VOR (AVOR) component as

(5)

To highlight the orientation effects on the VOR response, we also calculated the difference between these VOR responses separated by 180° (e.g., NU minus ND or RD minus LD), and designated this orientation dependent component of the VOR as an "induced VOR" component

(6)

There are no assumptions underlying these calculations because (at this point) they are just a phenomenological tool to investigate similarities and differences in the eye movement responses.

View larger version (21K): [in this window] [in a new window]   Fig. 5. Post-rotational horizontal induced VOR following 50, 100, and 200°/s yaw rotations about an earth-vertical axis followed by a 90° tilt (dumping protocol). Post-rotational responses are labeled using the actual direction of rotation preceding the tilt (CW or CCW). The induced VOR is calculated as the NU response minus the ND response (divided by 2; 1st column); the LD response minus the RD response (divided by 2; 2nd column); the ND-LD response minus the NU-RD response (divided by 2; 3rd column); and the NU-LD response minus the ND-RD response (divided by 2; 4th column).

1

1

View larger version (28K): [in this window] [in a new window]   Fig. 6. Peak induced VOR component following 50, 100, and 200°/s yaw rotations vs. subject orientation. Left: the peak amplitude of the induced VOR component for each subject and each test condition was defined as the mean of the induced VOR component between 3 and 4 s after the completion of the tilt. The mean peak amplitudes (±1 SE) for 8 subjects are plotted vs. subject orientation following both CW () and CCW () rotation for all subject orientations. Responses are modulated sinusoidally; least-square-error sinusoidal fits to the data are used to estimate an amplitude (M) and a phase (). Right: the interaural component of linear acceleration (i.e., the projection of the estimate of linear acceleration onto the interaural axis) varies sinusoidally with head orientation for a space-fixed estimate of linear acceleration (â). For 50, 100, and 200°/s yaw rotations, the negative of the predicted magnitude of the interaural component of linear acceleration is plotted vs. subject orientation for angles of the estimate of linear acceleration with the earth-horizontal axis (, Fig. 2B) of 22.5° (1st row), 30° (2nd row), and 45° (3rd row), respectively. The negative is plotted to account for the 180° phase shift required of a compensatory response, making comparisons to data in the first column straightforward.

View larger version (24K): [in this window] [in a new window]   Fig. 7. Parametric comparison between experimental trajectories of the induced VOR and theoretical trajectories of the estimate of linear acceleration for 50, 100, and 200°/s yaw rotations. Left: experimental trajectories of the induced VOR following CW and CCW rotations in a space-fixed reference frame. The experimental trajectory of the induced VOR is defined as the parametric plot X(t) = M(t) cos (t) and Y(t) = M(t) sin (t), where M(t) and (t) are the amplitude and phase of sinusoidal fits to the induced VOR data at times between 3 and 50 s after completion of the tilt. Right: in a space-fixed frame of reference where gravity g is vertical and oriented downward, the theoretical trajectory of the estimate of linear acceleration â = [;] is defined by the parametric equations (t) = â(t) cos (t) and (t) = â(t) sin (t). The theoretical trajectories of the estimate of linear acceleration are plotted for an estimate of linear acceleration (â) maximally tilted  = 22.5° (1st row),  = 30° (2nd row), and  = 45° (3rd row) from an earth-horizontal axis to the left and to the right (see Fig. 2B for details).

Post-rotatory horizontal eye movements following an earth-horizontal axis rotation ("barbecue")

HORIZONTAL EYE MOVEMENTS. Four subjects completed barbecue rotations with constant yaw angular velocity of 200°/s about an earth-horizontal axis followed by a stop in either the NU or ND orientations. The barbecue post-rotatory VOR (Fig. 8A) and induced VOR (Fig. 8B) were very similar to the results for 200°/s dumping trials (Figs. 3 and 5). During the 35 s following the tilt, for both CW and CCW rotations, the VOR magnitude following a stop in the NU orientation was greater than following a stop in the ND orientation. For both CW and CCW rotations, the VOR time constant was greater for the NU orientation (12.2 and 12.3 s, respectively) than for the ND orientation (8.7 and 8.9 s, respectively), but due to the small number (n = 4) of subjects tested, the difference was not significant (P = 0.07). These results confirm that the dynamic tilt rotation measured by the semicircular canals in the dumping protocol did not substantially affect or contaminate the horizontal VOR eye movements recorded in the post-rotatory period.

View larger version (30K): [in this window] [in a new window]   Fig. 8. Barbecue protocol experimental data. A: post-rotational horizontal VOR following 200°/s yaw rotations about an earth-horizontal axis for 4 subjects. As before, post-rotational responses are labeled using the actual direction of rotation preceding the tilt (CW or CCW). Time 0 indicates the end of the horizontal rotation plus a 2-s delay corresponding to the tilt duration used on dumping protocols. The mean post-rotatory responses for 4 subjects are shown for NU and ND orientations. B: the induced VOR component is calculated as the NU response minus the ND response (divided by 2). C: peak induced VOR following 200°/s yaw rotations about an earth-horizontal axis (barbecue protocol) for 2 subjects. The peak amplitude of the induced VOR component for each subject and each test condition was defined as the mean of the induced VOR component during a 1-s interval that corresponded to the same interval used for the dumping protocol. The mean peak amplitudes (±1 SE) for 2 subjects are plotted vs. subject orientation following both CW () and CCW () rotations. Responses are modulated sinusoidally; least-square-error sinusoidal fits to the data are shown. D: for comparison, peak induced VOR following 200°/s yaw rotations about an earth-vertical axis followed by a 90° tilt (dumping protocol) for the same 2 subjects.

Post-rotatory vertical eye movements following earth-vertical axis rotations (dumping)

In addition to the postulated induced VOR component discussed previously, previous results in humans (Fetter et al. 1996; Harris and Barnes 1987), rhesus monkeys (Angelaki and Hess 1994), and squirrel monkeys (Merfeld et al. 1993b) demonstrate that a shift in the axis of eye rotation is also observed following post-rotatory tilt. This axis-shift, which is not specifically predicted (nor precluded) by the GIF resolution hypothesis, indicates the tendency of the VOR rotation axis to align with gravity, as described by any one of several spatial orientation hypotheses (Angelaki and Hess 1994; Cohen et al. 1999; Merfeld et al. 1993b). While both AVOR and induced VOR components should be horizontal eye movements, any axis-shift of the VOR rotation axis toward alignment with gravity should induce either vertical or torsional eye movements, depending on the subject's orientation. The axis-shift of the VOR rotation axis is not directly related to the induced VOR component, although the induced VOR component would influence the magnitude of the axis-shift, since the induced VOR component modifies the horizontal VOR.

We evaluated the spatial reorientation of the post-rotatory VOR induced by head movements in the roll plane by plotting post-rotatory vertical VOR versus horizontal VOR (Fig. 9) for both LD or RD orientations, for all speeds, and for both directions of rotation. The upward vertical components were on average larger than the downward vertical components. The axis-shift was initially small (<10° for all speeds) at the time when the induced VOR component peaked (about 3 s after the post-rotatory tilt, Fig. 5), and then increased and peaked at an amplitude of roughly 15-30° between 10 and 20 s after the tilt, depending on the rotation angular velocity. A slow buildup in the axis-shift has been previously observed following post-rotatory tilt (Fetter et al. 1996) and following off-vertical axis rotation in humans (Furman and Koizuka 1994; Harris and Barnes 1987). Figure 10 shows plots of the mean vertical VOR between 10 and 11 s after completion of the tilt as a function of head orientation.

View larger version (32K): [in this window] [in a new window]   Fig. 9. Spatial reorientation of the average post-rotatory VOR (8 subjects) in the roll plane. The average post-rotatory vertical VOR for 8 subjects is plotted vs. the average post-rotatory horizontal VOR following both CW and CCW 50, 100, and 200°/s yaw rotations for left-down and right-down orientations. Dashed lines indicate reference axis-shift angles (0, ±15, ±30°) of the slow phase velocity axis of rotation toward gravity following the post-rotatory tilt. Data where the horizontal VOR was less than 5°/s are not plotted to avoid inaccurate computations due to experimental noise.

View larger version (35K): [in this window] [in a new window]   Fig. 10. Peak vertical VOR following 50, 100, and 200°/s yaw rotations. The peak amplitude of the vertical VOR component for each subject and each test condition was defined as the mean of the vertical VOR component between 10 and 11 s after the completion of the tilt, allowing time for the vertical eye movements to build up. The mean peak amplitudes (±1 SE) are plotted vs. subject orientation following both CW () and CCW () 50, 100, and 200°/s yaw rotations for all subject orientations. Responses are modulated sinusoidally; least-square-error sinusoidal fits to the data are shown.

For each rotation direction and speed, the peak amplitude of the vertical VOR calculated between 10 and 11 s after completion of the tilt varied sinusoidally as a function of head orientation for each individual subject (not shown). As described in METHODS, each individual sinusoidal fit can be represented by a pair of real number [X = M cos , Y = M sin ] where M is the amplitude and  the phase of the sinusoidal fit. For two speeds (100 and 200°/s), the group of real pairs [X₁,Y₁] for CW rotations were significantly different (P = 0.02 and P = 0.03, respectively, for Hotelling T²-test) from the group of real pairs [X₂,Y₂] for the CCW rotations. Since response amplitudes were not significantly different, we concluded that there was a significant phase difference between the results from CW and CCW rotations. The mean phase shift for vertical VOR following CW rotations (63 and 66°, respectively) was significantly different from following CCW rotations (58 and 64°, respectively).

The mean peak amplitude across subjects of the vertical VOR calculated between 10 and 11 s after completion of the tilt also varied as a function of head orientation (Fig. 10). The CW and CCW responses were well fit by sinusoids shifted with respect to one another. The amplitude, the bias, and the phase shift absolute value of the sinusoidal fits to the mean vertical VOR data decreased with decreasing speed for both CW and CCW rotations.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

For both dumping and barbecue protocols, the post-rotatory horizontal VOR demonstrated a clear dependence on both head orientation and direction of the preceding rotation, indicating that interactions between otolith and semicircular canal cues influenced the resulting eye movements. In addition, we have shown that the dynamic tilt of the dumping protocol cannot explain the various asymmetries observed in horizontal eye movements since eye responses following yaw rotation about an earth-horizontal axis demonstrated the same asymmetries (Fig. 8). We first compare our results to previous experimental findings and then consider whether any of the existing hypotheses (VOR spatial orientation, frequency segregation, and GIF resolution) that characterize interactions between otolith and semicircular canal cues are consistent with experimentally determined features of dumping- and barbecue-induced post-rotatory VOR.

Horizontal VOR decay following post-rotatory tilts

Previous human studies reported that an eye movement response attenuation (dumping) occurs when graviceptor cues are inconsistent with rotation cues, e.g., after yaw (Benson and Bodin 1966) and roll (Udo de Haes and Schöne 1970) rotation about an earth-horizontal axis and after active (Schrader et al. 1985) and passive (Benson and Bodin 1966) post-rotational tilts following yaw rotation about an earth-vertical axis. Our data confirm these findings, with the decay time constant of post-rotatory VOR significantly smaller for trials with post-rotatory tilt than for trials without post-rotatory tilt. Previous studies also demonstrated asymmetries in VOR responses for orientations separated by 180° after both passive (Benson and Bodin 1966) and active (Schrader et al. 1985) post-rotatory tilts. Other paradigms also appear to induce related orientation-dependent asymmetries. For example, human horizontal optokinetic nystagmus and afternystagmus induced by yaw rotation of an optokinetic surround, with subjects and optokinetic surround aligned with earth-horizontal, have been shown to be greater for NU than for ND orientations (Wall et al. 1999). A similar asymmetry has been observed during caloric stimulation of canal plugged squirrel monkeys (Minor and Goldberg 1990; Paige 1985). These extensive sets of data allow for detailed comparisons of predictions of different hypotheses regarding the interactions of otolith and semicircular canal cues on the generation of VOR eye movements.

VOR spatial orientation hypothesis

The VOR spatial orientation hypothesis has been proposed to explain orientation dependent asymmetries in the VOR (Cohen et al. 1999). First, this hypothesis states that the angular VOR time constant (10-20 s) is prolonged when compared with primary semicircular canal afferent (5-6 s) by a velocity-storage mechanism (Raphan et al. 1979). Second, the velocity storage mechanism is dependent on head orientation with respect to the direction of the GIF as demonstrated in rhesus and cynomolgus monkeys (Dai et al. 1991) and humans (Gizzi et al. 1994). Third, a "cross-coupling" in the velocity-storage mechanism occurs from the primary horizontal nystagmus to a secondary vertical nystagmus after a roll tilt or to a secondary torsional component after a pitch tilt (Raphan and Cohen 1988). Because of this cross-coupling, the rotation axis of reflexive eye movements demonstrates a tendency to shift toward alignment with the GIF direction. In several species of monkeys (rhesus, cynomolgus, and squirrel monkeys), this so-called axis-shift has been observed for optokinetic nystagmus (OKN) and afternystagmus (OKAN) (Dai et al. 1991), for VOR during centrifugation (Merfeld and Young 1995; Wearne et al. 1999), for VOR following off-vertical axis rotation (Raphan et al. 1992), and for VOR following a post-rotatory tilt (Angelaki and Hess 1994; Merfeld et al. 1993b). For similar protocols in humans (Fetter et al. 1996; Gizzi et al. 1994; Harris and Barnes 1987; Merfeld et al. 1998), the rotation axis of reflexive eye responses also tends to align with the GIF direction but to a much smaller extent.

According to the VOR spatial orientation hypothesis, a directional gain asymmetry in the secondary nystagmus induced by "cross-coupling" might lead to a time constant asymmetry in the horizontal primary nystagmus (Raphan and Sturm 1991). For example, upward OKN and OKAN are larger than downward OKN and OKAN in rhesus and cynomolgus monkeys (Matsuo et al. 1979). Therefore horizontal OKN and OKAN responses following a roll tilt in either LD or RD orientations demonstrate asymmetries that reverse with the direction of the rotational cues in rhesus and cynomolgus monkeys (Raphan and Cohen 1988). Assuming that a similar upward/downward response asymmetry exists for human VOR responses, the VOR spatial orientation hypothesis might be consistent with our observed LD/RD asymmetries in the horizontal VOR and their dependence on rotation direction.

While upward/downward response asymmetries are consistently observed in rhesus and cynomolgus monkeys, the presence of an upward/downward response asymmetry in humans is rather controversial and is certainly not consistent across subjects. An OKN/OKAN upward/downward asymmetry was reported in humans in some studies (Murasugi and Howard 1989; Wei et al. 1994) but not in other studies where asymmetries were reported for only a minority of human subjects (Baloh et al. 1986; Stiefel 1962). No consistent vertical VOR asymmetry has been reported in humans during pitch rotation about an earth-vertical axis (Allum et al. 1988; Baloh et al. 1983) or about an earth-horizontal axis (Baloh and Demer 1991).

It is certainly safe to conclude upward/downward response asymmetries are inconsistent and at best very weak in humans. Furthermore no asymmetries in torsional responses have ever been reported in humans for either optokinetic or vestibular stimulation (Morrow and Sharpe 1993; Peterka 1992; Seidman and Leigh 1989