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Report
Department of Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110
Submitted 29 October 2003; accepted in final form 30 November 2003
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ABSTRACT |
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INTRODUCTION |
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, 1998; Paige and Tomko 1991a,b; Schwarz and Miles 1991; Schwarz et al. 1989). Because a linear displacement of the head must be compensated by a rotation of gaze in space, to maintain focus on a point in space, TVOR amplitude scales inversely proportional to viewing distance (Angelaki and McHenry 1999; McHenry and Angelaki 2000; Paige and Tomko 1991b; Schwarz and Miles 1991; Schwarz et al. 1989; Telford et al. 1997). However, in both humans and monkeys, the dependency of the TVOR on viewing distance is less than ideal, such that TVOR responses are typically undercompensatory for near targets (Schwarz and Miles 1991; Telford et al. 1997).
Despite less-than-ideal TVOR gains during passive movements, studies with human subjects actively moving through their environment have reported nearly perfect image stabilization (i.e., retinal slip <4°/s; Crane and Demer 1997; Moore et al. 1999). In these studies, involving walking and running, translation of the head in one direction was always associated with a head rotation in the other direction (Bloomberg et al. 1992; Crane and Demer 1997; Demer and Viirre 1996; Hirasaki et al. 1999; Moore et al. 1999; Pozzo et al. 1990). This synergy was observed for both horizontal and vertical translations, which were coupled to yaw or pitch head rotations, respectively. Importantly, because the rotational vestibuloocular reflex (RVOR) gain (eye velocity divided by head rotational velocity) was often as low as 0.55 under these conditions, these compensatory head movements contributed to gaze stability. Interestingly, the ratio of head rotation to translation increased inversely proportional to target distance (Bloomberg et al. 1992; Moore et al. 1999), as would be expected from a functionally compensatory process.
In another recent human study involving actively generated translatory hip-and-head movements, gaze stabilization was partly achieved through an eye-in-head rotation and partly through a head-in-space horizontal rotation in the opposite direction to translation (Medendorp et al. 2002). Thus a head horizontal rotation appears to be a clear constituent of gaze stabilization in human subjects during actively generated lateral body movements. The origin of this compensatory head rotation remains unknown. There are at least two possible explanations for the observation that a head rotation contributes to gaze stability during active translations. First, it could represent an active strategy adopted by the subjects to circumvent a less-than-ideal TVOR performance. Alternatively, it could represent a vestibularly driven (vestibulocollic) reflex (TVCR) that causes the head to rotate in a direction opposite to translation (i.e., in a compensatory fashion; Hirasaki et al. 1999; Moore et al. 1999). In support of the latter hypothesis, Takahashi et al. (1990) reported that passive vertical translation induced compensatory pitch head movements whose amplitude increased with frequency in the range of 1–3 Hz. In addition, compensatory pitch head movements during locomotion were degraded in patients with bilateral vestibular deficits (Grossman and Leigh 1990; Pozzo et al. 1991) and after space flight (Bloomberg et al. 1997; Reschke et al. 1994).
The vestibulocollic reflex hypothesis—that is, the idea that a vestibularly driven horizontal rotation of the head opposite to the direction of translation contributes to gaze stability—is based on the conjecture that the RVOR is suppressed during the head rotation (as is the case for actively generated gaze shifts; Guitton and Volle 1987; Roy and Cullen 1998; Tabak et al. 1996; Tomlinson and Bahra 1986). In fact, the low RVOR gains during the oppositely directed translation associated with walking and running (Demer and Viirre 1996) would be consistent with such a hypothesis. During translation to the right, for example, gaze would be redirected to the left through a leftward rotation of both the eyes and the head (Fig. 1A). Of course, a head contribution to gaze stability would be appropriate only for reducing the conjugate component of the retinal slip. Thus a TVCR contribution should be most obvious during lateral and vertical translations, but not as large during fore–aft motion, when vergence eye movements dominate the response. Interestingly, only the conjugate eye movement component is consistently undercompensatory during passive translation in head-fixed animals; Vergence velocity gains are typically close to unity (Angelaki and Hess 2001; McHenry and Angelaki 2000).
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METHODS |
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Data presented here were obtained from 4 juvenile rhesus monkeys (Macacca mulatta). All animals were implanted with a lightweight delrin head ring and dual eye coils on each eye, as described in detail elsewhere (Angelaki 1998; Angelaki et al. 2000). Eye and head movements were recorded using a 3-magnetic field system (CNC Engineering). All animals were trained with juice rewards to fixate targets at different distances. All surgeries and experimental protocols were in accordance with institutional and National Institutes of Health guidelines.
During experiments, animals were seated in a primate chair with their head positioned such that the horizontal stereotaxic plane was earth-horizontal. Passive displacements were generated using a linear sled (Neurokinetics, Pittsburgh, PA). The linear acceleration stimuli consisted of 5-Hz sinusoidal oscillations (0.25 cm peak amplitude and 0.25 G peak acceleration). In 3 of the animals, lower-frequency translational motions were also delivered (4 Hz: ±0.4 cm, ±0.25 G; 2 Hz: ±1.6 cm, ±0.25 G; 1 Hz: ±5.0 cm, ±0.20 G; and 0.5 Hz: ±9.9 cm, ±0.10 G). These protocols were tested first with the head fixed, as well as under conditions where the head was free to rotate in the horizontal plane (but with all other degrees of freedom restrained). This was done by fixing the monkey's head ring to a 2.5-in. center-bore bearing that could be locked in place (head-fixed condition) or released to allow the head to freely rotate in the horizontal plane about a vertical axis that was centered on the interaural line (head-free condition). To measure the rotation of the head, a dual coil (Hess 1990) was mounted on the head ring, as close to the eye coils as possible. Both eye coils and the head coil were always kept within a radius of 3 cm from the center of the magnetic field coils and within the homogeneous part of the field (measured to have <5% error). We verified that animals with their heads partially restrained as described above could freely engage both the eyes and head to follow visual targets, as illustrated in Fig. 1B for horizontal gaze pursuit (0.1 Hz, ±30°).
The viewing distance dependency of the TVOR was tested by requiring the animals to fixate horizontal targets, along the midsagittal plane, at distances of 12, 18, 32, or 102 cm in a softly illuminated room. All target spots were created by back-projecting a laser that was controlled by a mirror galvanometer system (General Scanning) onto a screen placed at different viewing distances. Binocular eye-in-space positions were computed on-line to monitor fixation based on geometrical target position windows (Angelaki et al. 2000). During motion, periods of target presentation (1.5–2.5 s) were alternated with periods in total darkness (1–1.5 s). Stimulus presentation and data acquisition were controlled with custom-written scripts within the Spike2 software environment using the Cambridge Electronics Device (CED, model power 1401) data-acquisition system. Data were antialias filtered (200 Hz, 6-pole Bessel) and digitized by the CED at a rate of 833.33 Hz (16-bit resolution). Positive eye and head movement directions were leftward and downward.
Data analyses
Data analysis was performed off-line using custom-written scripts in Matlab (Mathworks, Natick, MA). Horizontal and vertical eye movements were calibrated using a daily fixation task, then differentiated using a polynomial filter (Savitsky and Golay 1964). Calibration of the dual head coil inside the 3-field magnetic system was according to the procedure described by Tweed et al. (1990). Briefly, each coil's sensitivity vector was computed from the direction cosines recorded from the demodulated signals, with those for straight-ahead corresponding to the zero head position. Fast phases were identified and removed based on time and amplitude windows set for the second derivative of the eye velocity vector amplitude. The identified fast phases were visually displayed on a plot of the eye position components to interactively correct potential misidentification. Sinusoidal modulations in eye and head velocity were quantified by fitting a sum-of-sinusoids (1st and 2nd harmonics) to the data using a nonlinear, least-squares algorithm based on the Levenberg–Marquardt method. A similar fitting procedure was also applied to the stimulus (output of a linear accelerometer mounted on the primate chair).
The magnitude of the compensatory gaze movement during translation, which we will refer to as the TVOR response, was computed by taking the ratio of peak eye velocity (in °/s) to peak linear head velocity (in cm/s) for each individual movement cycle. Thus the TVOR response has units of °/cm. Phase values have been expressed as eye velocity (positive to the left) relative to linear head velocity (positive direction is to the right). Based on this, the phase of the compensatory horizontal eye velocity during lateral motion should be approximately 0°. Head movement amplitude and phase were expressed similarly relative to peak linear head velocity. For each cycle, a vergence angle was also computed as the difference between right and left eye positions. A "vergence-defined viewing distance" was estimated on a cycle-by-cycle basis from the mean vergence angle and interocular distance and expressed as meter-angles in units of m–1 (Paige and Tomko 1991a). One meter-angle corresponds to a vergence state where the 2 gaze directions intersect 1 m away from the subject. Data were analyzed separately for motion in the presence of visual stimuli and in complete darkness. During motion in darkness, analyses focused on movement cycles during which the vergence angle deviated by <10% from that with the target on for a particular viewing distance. This was done because changes in TVOR gain precede changes in vergence by several tens of milliseconds (Snyder et al. 1992). Because the goal was to quantify the relationship between TVOR and vergence angle under steady-state conditions, cycles when vergence angle was varying (decaying) were excluded.
During translation with the head free, any active rotation of the head will further translate the eyes relative to the target. The amount of head rotation–induced right eye translation (in the same direction as the passive translation) can be calculated to be (Fig. 1C)
 | (1) |
is the head rotation amplitude, 
is the initial head position angle relative to the lateral motion axis, and d is half the interocular distance. Equation 1 was used to compute the active translation of the right eye for each cycle of the sinusoidal displacement of the sled. As illustrated by Eq.1 and for the range of head movements observed here, the larger the head rotation, the larger the active translation of the eye with respect to the target that was stationary in space. Thus to compute the actual TVOR response (TVOR-corrected) relative to the total (passive and active) displacement of the eyes, we calculated the ratio of peak eye velocity (in °/s) to the total peak linear head velocity (in cm/s) for each individual movement cycle (the total peak linear velocity includes both the passive translational stimulus and the actively generated translation of the eye relative to the target). Statistical comparisons were based on linear regression and ANCOVA.
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RESULTS |
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DISCUSSION |
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; Crane and Demer 1997; Demer and Viirre 1996; Hirasaki et al. 1999; Medendorp et al. 2002; Moore et al. 1999; Pozzo et al. 1990). This observation, coupled with the presence of nearly perfect image stabilization with near-target viewing during natural activities, has raised the possibility that the compensatory head rotation represents a vestibularly driven head reflex (e.g., translational vestibulocollic reflex; Medendorp et al. 2002; Moore et al. 1999). As will be summarized below, although there exists substantial neurophysiological evidence to support such a hypothesis, we found no evidence for a compensatory head horizontal rotation under the present experimental conditions.
The main evidence in support of the TVCR hypothesis is the existence of an extensive and strong utriculospinal connectivity, in sharp contrast to sparse short-latency utriculoocular projections. For example, electrical stimulation of the utricular nerve evoked synaptic potentials in essentially all neck motoneurons (Bolton et al. 1992; Ikegami et al. 1994). In fact, several recent studies have concluded that most of the otolith-activated vestibular nuclei neurons seem to participate in vestibulospinal (rather than vestibuloocular) reflexes (Bolton et al. 1992; Ikegami et al. 1994; Kushiro et al. 2000; Sato et al. 1996, 1997; Uchino et al. 1997). These vestibulospinal otolith-activated neurons project not only to neck flexors and extensors, but also to other muscles like the sternocleidomastoid, whose main function is head rotation (Kushiro et al. 1999). Interestingly, unlike the reciprocal sacculospinal connectivity, utricular projections to extensor and flexor motoneurons of the same side are of the same polarity (Ikegami et al. 1994; Uchino et al. 1997). Thus whereas stimulation of one sacculus activates neck extensors bilaterally and inhibits neck flexors bilaterally (suggesting a clear role in antigravity posture), stimulation of one utricle causes excitation of both flexors and extensors on the ipsilateral side. In fact, this differential organization of utriculospinal and sacculospinal connectivity parallels a similar difference in the spinal projections between horizontal and vertical semicircular canals (Isu et al. 1988; Shinoda et al. 1994; Suguichi et al. 1995; Uchino et al. 1990, 1997; Wilson and Maeda 1974). Thus one could speculate that utriculo–neck pathways could be better suited for horizontal than vertical head movement control in response to linear accelerations in the horizontal plane. Nevertheless, the functional significance of these differences has yet to be explored. To our knowledge, the only otolith–neck reflex that has been investigated is a gravity-orientation reflex previously studied in the EMG activity of neck muscles in decerebrate, canal-plugged animals during roll rotations (Schor and Miller 1981; see also Wilson et al. 1995).
Given such an extensive, short-latency connectivity between the utricle and neck muscles, we had hypothesized (see also Hirasaki et al. 1999; Moore et al. 1999) that a specific otolith vestibulocolic reflex (referred to as translational vestibulocolic reflex, or TVCR) might exist. Such a postulated TVCR could function, in parallel to the otolith–ocular reflexes, to provide gaze stabilization during translation. Despite strong neurophysiological support for the TVCR hypothesis, we found no evidence for a compensatory horizontal TVCR under the present experimental conditions. In fact, the horizontal head velocity elicited during passive lateral translation was anticompensatory for gaze stabilization; that is, the head rotated in the same direction as the head translation and oppositely to the direction of gaze rotation. Coupled with the fact that this observation was limited to high frequencies, these head movements most likely represent inertial rather than vestibular reflex components. Moreover, the slight dependency of head rotation on viewing distance observed here was opposite to that reported during active translation in humans. Although the head was only partially unrestrained, because motion was restricted along the interaural axis and the targets had zero vertical elevation, if a compensatory TVCR existed, a purely horizontal head rotation should have been called for. Yet, we did not find such evidence in the present experiments. Thus the compensatory head rotation observed in active human studies might more likely represent an active strategy rather than a vestibularly driven reflex (Medendorp et al. 2002). Alternatively, it is possible that the gain of the TVCR might be low under the present experimental conditions, but higher in more natural situations, e.g., when the head is allowed to both rotate and translate in the horizontal plane or during active movements just as the vestibuloocular gain is modulated during active head movements (Guitton and Volle 1987; Roy and Cullen 1998; Tabak et al. 1996; Tomlinson and Bahra 1986).
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ACKNOWLEDGMENTS |
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The work was supported by National Eye Institute Grant EY-12814.
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FOOTNOTES |
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Address for reprint requests and other correspondence: D. Angelaki, Department of Neurobiology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110 (E-mail: angelaki{at}pcg.wustl.edu).
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ABSTRACT
INTRODUCTION
180°) from what would have been compensatory for gaze stabilization (0°). Data from each of 4 animals are shown with different symbols. Open symbols: Target On. Filled symbols: Target Off. Error bars in B (plotted only for filled symbols) illustrate SDs. C and D: scaling of the translational vestibuloocular reflex (TVOR) (defined as eye-in-space motion) as a function of inverse distance. C: data from one animal during lateral motion with the head fixed (black symbols) or free to rotate in the horizontal plane (gray symbols). Horizontal axis plots the inverse viewing distance, as defined by the vergence posture during each cycle of whole-body motion. Solid and dashed lines indicate the best linear fits to the data. D: TVOR regression lines during lateral translation with the head fixed (black solid lines) or free (gray dashed lines) are shown for all animals during motion while fixating a central target at different viewing distances. Dotted lines in C and D show the expected behavior if viewing distance scaling was ideal (slope of 0.57).