Short- and Long-Term Consequences of Canal Plugging on Gaze Shifts in the Rhesus Monkey. I. Effects on Gaze Stabilization

Shawn D. Newlands,1 Leo Ling,2,3 James O. Phillips,2,4 Christoph Siebold,5 Larry Duckert,4 and Albert F. Fuchs2,3

 1Department of Surgery (Otolaryngology), University of Mississippi Medical Center, Jackson, Mississippi 39212;  2Regional Primate Research Center,  3Department of Physiology and Biophysics, and  4Department of Otolaryngology, Head and Neck Surgery, University of Washington, Seattle, Washington 98195; and  5Department of Neurology, Center for Sensorimotor Research, Ludwig Maximilians University, D81377 Munich, Germany


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Newlands, Shawn D., Leo Ling, James O. Phillips, Christoph Siebold, Larry Duckert, and Albert F. Fuchs. Short- and long-term consequences of canal plugging on gaze shifts in the rhesus monkey. I. Effects on gaze stabilization. To study the contribution of the vestibular system to the coordinated eye and head movements of a gaze shift, we plugged the lumens of just the horizontal (n = 2) or all six semicircular canals (n = 1) in monkeys trained to make horizontal head-unrestrained gaze shifts to visual targets. After the initial eye saccade of a gaze shift, normal monkeys exhibit a compensatory eye counterrotation that stabilizes gaze as the head movement continues. This counterrotation, which has a gain (eye velocity/head velocity) near one has been attributed to the vestibuloocular reflex (VOR). One day after horizontal canal plugging, the gain of the passive horizontal VOR at frequencies between 0.1 and 1.0 Hz was <0.10 in the horizontal-canal-plugged animals and zero in the all-canal-plugged animal. One day after surgery, counterrotation gain was ~0.3 in the animals with horizontal canals plugged and absent in the animal with all canals plugged. As the time after plugging increased, so too did counterrotation gain. In all three animals, counterrotation gain recovered to between 0.56 and 0.75 within 80-100 days. The initial loss of compensatory counterrotation after plugging resulted in a gaze shift that ended long after the eye saccade and just before the end of the head movement. With recovery, the length of time between the end of the eye saccade and the end of the gaze movement decreased. This shortening of the duration of reduced gain counterrotation occurred both because head movements ended sooner and counterrotation gain returned to 1.0 more rapidly relative to the end of the eye saccade. Eye counterrotation was not due to activation of pursuit eye movements as it persisted when gaze shifts were executed to extinguished targets. Also counterrotation was not due simply to activation of neck receptors because counterrotation persisted after head movements were arrested in midflight. We suggest that the neural signal that is used to cause counterrotation in the absence of vestibular input is an internal copy of the intended head movement.


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Large gaze shifts are composed of rapid head and saccadic eye movements. Once the eye saccade lands, the eye begins a slow movement in the direction opposite to the head movement, i.e., a counterrotation. The occurrence of a compensatory eye counterrotation once the target is acquired terminates the movement of the eye in space and stabilizes gaze while the head continues to move. In fact, stabilization of gaze requires either that eye counterrotation is perfectly compensatory, i.e., has a gain (eye velocity/head velocity) equal to one, or that the head movement has ended. For most large gaze shifts in the primate, gaze is stabilized by a compensatory counterrotation until the head movement stops. Thus the timing of the initiation of the counterrotation, which appears coordinated with the end of the eye saccade (Lefevre et al. 1992), effectively controls the termination of the gaze shift. Because the counterrotation takes some time to increase to a gain of one, the gaze control system must anticipate this delay and turn on the counterrotation before the target is reached (Lefevre et al. 1992).

In the normal animal, counterrotation depends on the vestibular system because unexpectedly arresting the head movement stops the compensatory eye movement (Bizzi et al. 1971) and removal of vestibular input by bilateral labyrinthectomy results in an immediate loss of counterrotation (Dichgans et al. 1973). Thus complete bilateral labyrinthectomy initially produces hypermetric gaze shifts, in part because compensatory counterrotation is eliminated. The eye saccade ends near the target, but the head movement without the compensatory counterrotation carries the gaze well past the target. In time, counterrotation reappears and increases. Four months after bilateral labyrinthectomy, counterrotation recovers to ~90% of its previous amplitude, and gaze accuracy is restored (Dichgans et al. 1973). Recovery of counterrotation occurs despite the persistent absence of a passive vestibuloocular reflex (VOR).

In labyrinthectomized animals, the time course and details of this recovery have not been documented quantitatively. Furthermore, labyrinthectomy not only eliminates dynamic inputs during head turning but also the robust tonic input to the vestibular nuclei due to the high resting rate of vestibular afferents when the head is still (Goldberg and Fernandez 1971). Therefore to eliminate this objection to labyrinthectomy, we plugged the canals bilaterally. The goals of our study are to document quantitatively the acute effect of canal plugging on counterrotation, to quantify any recovery of counterrotation after canal plugging, to investigate possible mechanisms underlying counterrotation recovery in the absence of semicircular canal input, and to explore how disruption of the normal relationship between the end of the saccadic eye movement and the end of the gaze movement disrupts the termination of gaze shifts.

A preliminary report of this data has appeared in abstract form (Newlands et al. 1996).


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Gaze measurement and behavioral paradigms

Three Macaca mulatta monkeys, HC1, HC2, and AC, were prepared for recording by implantation of a scleral search coil under the insertions of the extraocular muscles (Fuchs and Robinson 1966) and a metal post on the skull to stabilize the head (Phillips et al. 1995). The axis of the head post was aligned with the C1-C2 joint. All surgery was performed under deep halothane inhalation anesthesia after induction with intramuscular ketamine (10 mg/kg).

To measure gaze position, we placed the animals in a chair, which in turn was placed within alternating magnetic fields in spatial and temporal quadrature. We measured gaze position (eye position in space) to an accuracy of 1° over ±40° with an electromagnetic technique (Robinson 1963). Gaze position was measured both with the head completely restrained and the head free to move about a vertical axis. Head movements were constrained to rotate in the horizontal stereotaxic plane by inserting the head post into a collar attached to a potentiometer, which was fastened to a framework that contained the field coils and held the chair. Head movement, as measured by the potentiometer, was subtracted from gaze movement to provide eye movement in the head. Throughout the text, "eye movement" refers to eye movement with respect to the head.

Target movements were generated by sequential illumination of an array of light-emitting diodes (LEDs), which faced the monkey at a distance of 47 cm. Diodes were located between ±40° at 5° increments in dim light. The monkeys were rewarded with applesauce for fixating a LED target with an accuracy of 2° for 1-2 s. The LEDs were illuminated in a pseudorandom order such that a mixture of gaze amplitudes with different start and end positions resulted and also so that prediction was eliminated (Bizzi et al. 1972). When the head was fixed in space, we illuminated LEDs to a maximum of only ±30° from straight ahead.

In some experiments, we eliminated all visual feedback during the gaze shift by turning the target LED off at the onset of the eye saccade while the animal was otherwise in total darkness. The LED was held off for 500 or 750 ms so that the target remained off during the entire eye counterrotation. Data were gathered using this "target extinguished" paradigm before plugging in HC2 and AC and on postplugging days 1, 3, 5, and 74 in HC1, days 1, 2, 4, 8, 12, and 21 in HC2, and on all postoperative days in AC.

Application of head and body movement stimuli

PASSIVE VOR TESTING. To provide vestibular stimuli, we rotated the entire framework including the monkey and the field coils about the interaural axis to produce pitch or about a vertical axis midway between the two ears to produce yaw. The monkeys' heads were secured to the framework through the head post. The horizontal VOR (HVOR) was tested at frequencies of 0.1, 0.3, 0.5, 0.8, and 1.0 Hz with peak displacement held constant at 10°. The HVOR was tested both in the stereotaxic plane, in which the gaze shifts were made, and with the nose pitched down 22° to rotate the animal in a plane orthogonal to the vertical canals (Blanks et al. 1985). The vertical VOR (VVOR) was tested at 0.3, 0.5, 0.8, and 1.0 Hz. All VOR testing was done in the dark. Monkeys were kept alert by taps on the walls of the test chamber and by air puffs blown unexpectedly on the back of their necks. The VOR was measured in the dark approximately biweekly after canal plugging.

COR TESTING. To test the cervicoocular reflex (COR), we fixed the animal's head in space by means of a retractable arm and oscillated its body in yaw by rotating the chair in which it sat. The body was rotated ±10° at frequencies from 0.1 to 1.0 Hz. The coil frame moved with the chair, so eye position was calculated by adding the chair position and eye position signals. The validity of this method was verified by placing a stationary test coil within the moving coil frame. COR gain was calculated as (-eye velocity/chair velocity).

ALTERING SENSORY INPUT DURING COUNTERROTATION. In monkey AC, we stopped the head movement at various times during eye counterrotation by means of the electromagnetic clutch attached to the head post. The application of the clutch was triggered by the initiation of the eye saccade with a delay adjusted by the experimenter. Trials were examined with the brake applied at various times, so that head stoppages were produced at all times of the eye counterrotation.

Data analysis

Analog voltages proportional to eye, head, gaze, chair, and target position, as well as signals indicating target illumination (or blanking) were converted to video signals with a Vetter PCM and recorded on video tape. The signals then were digitized off-line at 1 kHz with a Macintosh computer. Data were analyzed with the aid of interactive programs that displayed the data and automatically identified epochs for analysis.

ANALYSIS OF THE GAZE SHIFT. The start and end of the target step and the gaze, eye, and head movements of a gaze shift were identified by the computer using a velocity criterion of 20°/s, as were head-restrained saccades. The computer also identified the peak velocity of each movement. Each of these time points then was used to determine latencies and movement durations and was used as a reference for measurement of comparable velocities and positions of the other movement components. The results of this analysis could be displayed graphically or exported for further analysis using commercially available statistical and graphics packages.

As will be seen in a companion paper, gaze shifts after canal plugging were quite variable and often not directed at the target. Because the current paper deals specifically with the effects of canal plugging on eye counterrotation of targeted gaze shifts, we excluded from the analysis all gaze shifts that were not within 20% of the target amplitude.

ANALYSIS OF THE COUNTERROTATION. The period of counterrotation was taken as the interval between the end of the eye saccade and either the termination of the gaze shift or the occurrence of a correction saccade, whichever occurred first. We considered the onset of counterrotation as the time when the eye began rotating slowly in the direction opposite to the eye saccade. In the normal animal, counterrotation usually begins immediately after the eye saccade. On some trials, the eye holds its position (i.e., exhibited a plateau) (Phillips et al. 1995) before counterrotation; in those cases, counterrotation again was considered to begin when the eye rotated slowly in a direction opposite to the eye saccade. The inclusion of such trials did not materially effect our results since gaze shifts with long plateaus were rare in these experiments.

In the normal monkey, counterrotation gain starts at zero (when E = 0) and builds to unity by the time that the gaze shift ends. The time from the onset of counterrotation until it is fully compensatory, i.e., has a gain of 1.0 (the noncompensatory counterrotation period), averages ~40 ms in the normal monkey. In the majority of the gaze shifts in this study, gaze had reached its final position, i.e., gaze velocity was zero, by 100 ms. Therefore in this study, counterrotation gain was measured in the interval between 40 and 100 ms after the end of the eye saccade.

To measure the counterrotation gain (-E/&Hdot;), we determined the eye and head velocity 40, 60, 80, or 100 ms after the end of the eye saccade for many trials. The absolute value of eye velocity was plotted against the absolute value of the head velocity in a scatter plot and fitted (95% confidence intervals) with a regression line without intercept, i.e., forced through the origin. The slope of the regression line is taken as counterrotation gain. Calculation of the slope and all statistics in this study were done using a commercial program (Statview) on a Macintosh computer. A fast Fourier transform was run on the head movement profile using a script written in Matlab.

ANALYSIS OF PASSIVE VOR GAIN. Data from passive sinusoidal rotations was analyzed on a cycle by cycle basis. Digitizing was done on a Macintosh II computer, which adjusted the sampling rate to provide a constant number of samples (600) per cycle. The computer identified and displayed individual cycles of head and eye velocity and position, which then were desaccaded by one of the investigators based on a visual inspection of the eye velocity trace. All data during the saccade were removed from all subsequent analyses. The program then fit a sine wave to each cycle excluding the desaccaded segments by means of a least squares algorithm and provided the gain and phase of the eye velocity relative to the velocity of the chair and therefore the head. Finally, after >= 10 cycles had been analyzed at each frequency, the program determined, displayed, and saved the average eye and head velocity data for all cycles.

Analysis of head movement frequencies

We calculated the frequency distribution of head movements during gaze shifts by estimating the power spectral density of sample head traces (e.g., a 35° head movement with a duration of ~700 ms). Records of average head movements were joined back to back and repeated to a length of 2.048 s and then subjected to a standard spectral analysis. This procedure eliminates the transients resulting from the discontinuity of the signal and improves the frequency resolution of the Fourier transform.

Surgical manipulations

CANAL PLUGGING. A postauricular incision exposed the mastoid cortex and a simple mastoidectomy was performed to reveal the semicircular canals. Each canal was individually entered, and the lumen of the canal was packed with fascia and bone dust. The defects then were sealed with bone wax. The wound was closed in two layers. In HC1, initially only the two horizontal canals were plugged, and then later all four vertical canals were plugged in a separate procedure. In HC2, only the horizontal canals were plugged. In AC, all six canals were plugged in one procedure. Immediately after canal plugging, the monkeys were placed in the dark overnight and tested the next day. Subsequently, the animals were housed with normal light/dark cycles.

DORSAL RHIZOTOMY. In one animal, bilateral cervical dorsal rhizotomy (C1-C3) was performed after all six canals were plugged. This procedure was performed via a dorsal approach with laminectomy of the cervical vertebrae to expose the spinal cord and dorsal roots and the dorsal roots were cut. The animal was allowed to survive only until the next day.

All experiments were carried out in strict accordance with the guidelines of the National Institutes of Health and the Society for Neuroscience and all surgical and behavioral protocols were approved by the Animal Care and Use Committee of the University of Washington.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Normal counterrotation

As already described in several other studies, after the eye saccade of a normal head-unrestrained gaze shift ends but before the head movement is over, the eye counterrotates in the head (Fig. 1, preplug) so gaze remains stable in the world. Stable gaze is the result of a compensatory eye counterrotation the gain (-E/&Hdot;) of which is near 1.0 throughout the head movement. The technique for calculating eye counterrotation is illustrated in Fig. 2. In Fig. 2, instantaneous eye velocity 80 ms after the eye saccade is plotted against the head velocity at that same time. Throughout this paper, most often data at 80 ms are illustrated because this time is well after the end of the eye saccade but well before the end of the head movement. The slope of the linear regression determines the counterrotation gain. For monkey HC1 (Fig. 2), the slope (based on 375 gaze shifts) was 0.97 ± 0.03 (95% confidence interval for the slope) whereas for monkey AC the slope was 1.00 ± 0.02 (201 gaze shifts; mean ± SD). All calculations of counterrotation gain are based on an assortment of gaze shifts between 20 and 80° unless otherwise noted.



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Fig. 1. Representative eye, gaze, and head movement traces for 60° head-unrestrained gaze shifts in monkey AC before (preplug), 1 day after (acute postplug), and 103 days after plugging (late postplug) of all the semicircular canals. Acute data from HC1 also are shown. In AC, counterrotation disappears after plugging but returns with time. All traces are aligned on the end of the eye saccade and the vertical lines are located 20, 40, 60, 80, and 100 ms later.



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Fig. 2. Eye velocity 80 ms after the end of the eye saccade of a gaze shift as a function of head velocity for monkeys HC1 and AC. For each monkey, data were taken before plugging (×), on postoperative day 1 (), and on the last day (+) recorded (day 74 in HC1 and day 101 in AC). Lines for each set of data are regressions without intercepts (i.e., forced through 0). Slopes of the lines before, just after and long after plugging for HC1 are 0.97 ± 0.03 (95% confidence interval), 0.27 ± 0.02, and 0.68 ± 0.01, respectively, and for AC are 1.00 ± 0.02, -0.01 ± 0.01, and 0.48 ± 0.02, respectively.

The preplugging counterrotation gains varied from one animal to another (Fig. 3), counterrotation gains in monkey HC1 tended to average <1.0, and consequently it exhibited occasional forward slides in gaze position after an eye saccade had ended. In contrast, counterrotation gains in monkey HC2 were greater than one, on average, and it exhibited occasional backward slides in gaze position after the eye saccade. Finally, counterrotation gains in monkey AC all were ~1.0 so its counterrotations produced the most stable gaze holding.



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Fig. 3. Counterrotation gain (eye velocity/head velocity) measured at several times (40, 60, 80, and 100 ms) after the end of the eye saccade of head-unrestrained gaze shifts as a function of the number of elapsed days after plugging. Animals with only bilateral horizontal canals plugged (HC1 and HC2) and all canals plugged at once (AC) are considered separately. Leftmost data points in each panel (Pre) were obtained before plugging. Error bars denote the confidence interval of the slope of the regression line fitting eye velocity vs. head velocity. Data also are separated according to whether the target remained on (open circle , triangle , , diamond ) or was extinguished at the initiation of the gaze shift (, black-triangle, , black-lozenge ).

For all pre- and post canal plugged gaze shifts that involved a head movement (usually gaze shifts >25°) (Phillips et al. 1995), we first considered whether counterrotation gain was related to either gaze or target amplitude. In the late postplugging trials, counterrotation gains tended to be slightly higher (by <15%) for smaller gaze and head movements. This weak relation with head velocity is considered later. Otherwise, counterrotation gain did not depend on whether gaze shifts were centrifugal, centripetal, or across the primary direction of gaze. Therefore we merged the data from all gaze shifts unless otherwise noted.

With recovery, the length of time between the end of the eye saccade and the end of the gaze movement decreased. This shortening of counterrotation duration was due both to faster head movements and to an increased counterrotation gain.

Effects of canal plugging on the passive VOR

As expected, plugging of the semicircular canals produced a substantial reduction in the HVOR elicited by passive yaw rotations. One day after horizontal canal plugging, the gain of the HVOR was <0.10 at all frequencies (0.1-1.0 Hz) in both HC1 and HC2 whether their heads were in the normal upright stereotaxic position or were positioned so rotation occurred in the plane of the horizontal canals (Fig. 4A). In monkeys HC1 and HC2, horizontal canal plugging did not affect the pitch VOR, whose gain after plugging showed an average reduction from 0.84 to 0.81, which was not significant. Plugging of all canals in monkey AC produced postoperative HVOR gains <0.07 at all frequencies tested with the head in the stereotaxic position (Fig. 4A). Postoperative vertical VOR gains fell to <0.08 at all frequencies.



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Fig. 4. Changes in the gain of the horizontal vestibulo-ocular reflex, HVOR, as a function of the number of elapsed days after canal plugging. HVOR was produced by passive rotations in the stereotaxic plane about a vertical axis in the dark. A: data from all monkeys at a stimulus frequency of 0.5 Hz. B: data from monkey HC1 at a variety of stimulus frequencies. On day 121, HC1's vertical canals were plugged.

In the two animals with only horizontal canals plugged, HVOR gain increased with time. In HC2, who was last tested 73 days after horizontal canal plugging, the gain at 0.5 Hz increased to 0.21 with the head in the stereotaxic plane (Fig. 4A). The gain increase was greater in HC1 because we followed him for 110 days by which time HVOR gain at 0.5 Hz was 0.45 with the head held in the stereotaxic plane (Fig. 4A) and 0.27 with the head pitched 22° down (not shown). The highest HVOR gain in HC1 was 0.61 at 1.0 Hz (Fig. 4B). In this monkey, subsequent vertical canal plugging at 121 days dramatically reduced the HVOR (Fig. 4A), which showed no evidence of recovery <= 50 days later (not shown). In monkey AC, who had all canals plugged at once, there was no significant recovery of either the HVOR (Fig. 4A) or VVOR (not shown) after 63 days (gains at all frequencies <= 0.13).

It has been argued that the VOR at high frequencies survives canal plugging (R. D. Rabbitt, R. Boyle, and S. M. Highstein, unpublished observation). To determine whether the passive frequencies we tested were representative of those found in normal active head movements, we subjected representative head movements made by our monkeys to a fast Fourier transform (FFT). Less than 2% of the energy was at frequencies >2 Hz.

Acute effects of canal plugging on counterrotation gain

On postplug day 1, counterrotation velocities were well below normal in all animals (Fig. 1, acute postplug). Eighty milliseconds after the eye saccade, counterrotation gain for 39 gaze shifts was 0.27 ± 0.02 for monkey HC1 (Fig. 2, open squares) and 0.39 ± 0.02 for monkey HC2 for 141 gaze shifts (not shown). Counterrotation gain for 420 gaze shifts was essentially zero for monkey AC (Fig. 2). The values of counterrotation gain at 80 ms were similar to those measured at 40, 60, and 100 ms (Fig. 3). Counterrotation gain did not depend on either the amplitude of the head movement or the gaze movement. Because counterrotation gain was quite low, the gaze shift did not end until well after the eye saccade. In HC1, gaze shifts often were accomplished by multiple small eye movements with one large slow head movement. This pattern accounted for the lower number of accurate, typical shifts recorded in this animal.

Recovery of counterrotation

After plugging of either the horizontal canals or all canals, eye counterrotation gradually recovered (Fig. 1, late postplug). Some recovery of counterrotation gain was seen within 10 days of canal plugging in all three animals (Fig. 3). After 10 days, counterrotation gain remained relatively constant in monkey HC2 and continued to show a modest increase in monkeys HC1 and AC. In the two horizontal canal plugged animals, counterrotation gain was largest on the last days tested. In monkey HC1, counterrotation gain 80 ms after the eye saccade was 0.68 ± 0.01 (652 gaze shifts, Fig. 2) after 74 days, and in monkey HC1 it was 0.54 ± 0.01 (512 gaze shifts) after 47 days. After all canals had been plugged in monkey AC, the largest counterrotation gain at 80 ms was 0.60 ± 0.01 on day 49 (757 gaze shifts).

Source of counterrotation after canal plugging

After canal plugging, eye counterrotation could be the result of a variety of different sensory drives. These include visual, somatosensory from the neck, and vertical semicircular canal inputs. We now address each of these in turn.

ROLE OF DIRECT VISUAL FEEDBACK. To eliminate visual feedback, we turned off the target spot at the initiation of the eye saccade while the animal was otherwise in total darkness. Extinction of the target affected control saccades in different monkeys in different ways. In control experiments on HC2, eye counterrotation was the same whether the target was extinguished for 500 ms after initiation of the eye saccade or was left on continuously. In either case, eye counterrotation gain 80 ms after the eye saccade was 1.15 ± 0.01 (target on trials, 141 gaze shifts; target extinguished trials, 331 gaze shifts). In control experiments on AC, however, turning off the target caused a decrease in eye counterrotation gain before plugging. At 80 ms after the saccade, counterrotation gain was 1.00 ± 0.02 (201 gaze shifts) with the target spot on for the entire gaze shift and 0.89 ± 0.05 (140 gaze shifts) with the target light extinguished. Monkey HC1 was not tested with the target extinction paradigm.

For all three animals, counterrotation gains after canal plugging also were similar with the target light on or off. As seen in the data of Fig. 5 gathered 80 ms after the eye saccade, the slope of the regressions of eye velocity on head velocity are essentially identical whether the target was switched off () or remained on (+) for the entire gaze shift. For monkey HC1 on the 74th day after plugging, the slope of the linear regressions was identical (target off, 185 gaze shifts; target on, 517 gaze shifts). On the last day that monkey AC was tested, counterrotation gain was 0.48 ± 0.02 (382 gaze shifts) with the target on and 0.47 ± 0.02 (436 gaze shifts) with the target extinguished.



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Fig. 5. Effect of target visibility on eye counterrotation gain (eye velocity/head velocity). Eye velocity was measured 80 ms after the end of the eye saccade and targets either remained visible (+) or were extinguished at the initiation of the gaze shift (). Data were gathered 74 days after plugging the horizontal canals in monkey HC1 and 103 days after plugging all the canals in monkey AC. For HC1, the linear regressions have identical slopes of 0.68 ± 0.01. For AC, the slope with the target on is 0.48 ± 0.02 and 0.47 ± 0.02 with the target off.

The open circle , triangle , , and diamond  in Fig. 3 reflect the counterrotation gains present with the target extinguished on all the days where this paradigm was utilized. The data with the target extinguished are always similar to those with the target on (Fig. 3, , black-triangle, , and black-lozenge ). Therefore pursuit of the target spot contributes little to eye counterrotation after canal plugging.

VERTICAL CANAL INPUT. The role of the vertical canals in the recovery of horizontal counterrotation was investigated by plugging them in monkey HC1, the horizontal canals of which had been plugged 110 days earlier and which already had recovered some eye counterrotation. The day after we had plugged its remaining vertical canals, both the passive HVOR and VVOR were reduced dramatically either with the nose 22° down or with the head in the stereotaxic position. Immediately after vertical canal plugging, counterrotation gain 80 ms after the saccade was reduced to 0.51 ± 0.02 based on 225 gaze shifts (Fig. 6, A and B, ), and it ranged from 0.39 ± 0.02 to 0.59 ± 0.03 at 40 and 100 ms, respectively, after the eye saccade. Forty-seven days after vertical canal plugging, horizontal counterrotation gains had improved somewhat to 0.61 at 80 ms and to 0.41 ± 0.02 and 0.72 ± 0.02 at 40 and 100 ms, respectively, after the eye saccade (not shown). Indeed counterrotation gains had almost returned to those seen with only the horizontal canals plugged. Clearly other nonvestibular mechanisms must contribute to the control of counterrotation.



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Fig. 6. Effect on eye counterrotation gain of plugging the vertical canals after the horizontal canals had been plugged in monkey HC1. A: 4 representative eye and head traces 1 day after plugging the vertical canals aligned on the end of the saccade (short vertical bar). B: eye velocity, measured 80 ms after the end of the eye saccade, as a function of head velocity, 74 days after horizontal canal plugging (×) and 1 day after subsequent vertical canal plugging (). Plugging the vertical canals reduced the counterrotation gain from 0.68 ± 0.01 to 0.51 ± 0.02.

SOMATOSENSORY FEEDBACK. One possible source of sensory information during eye counterrotation is the afferent signal from cervical joint and neck muscle afferents. We assessed the contribution of these sensory signals in two ways. First, eye movements during trunk rotation under a stationary head (i.e., the cervicoocular reflex, COR) were measured in all three animals before and after canal plugging. The gain of the COR was not consistent between animals, ranging from 0.17 to 0.35. Eye rotations moved in phase with trunk rotation in all three animals. Over the course of >= 50 days, two of the three monkeys exhibited a slight increase in COR gain (Fig. 7A). Similar changes were seen across all tested frequencies (0.1-1.0 Hz) of trunk rotation (Fig. 7B). Although such an improved COR would provide compensatory eye counterrotation during gaze shifts with the head unrestrained, COR recovery cannot explain a gradual recovery of counterrotation in monkey AC because he exhibited none.



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Fig. 7. Changes in the gain of the cervicoocular reflex as a function of the number of elapsed days after plugging the canals. Data were gathered during whole body rotations about a stationary head in the dark. A: cervicoocular reflex (COR) gain determined at 0.5 Hz in all 3 monkeys. B: COR gain at several frequencies in monkey HC1. Data on day 281 were obtained after partial cervical rhizotomy. On day 121, vertical canals were plugged.

As can be seen in Fig. 7B, subsequent vertical canal plugging in monkey HC1 caused a transient reduction of COR gain. We currently have no explanation for this lowering of COR gain to preplug values.

In a second test, 50 days after its vertical canals were plugged and 1 day before it was killed, we sectioned the dorsal roots of monkey HC1 from C1 to C3, sparing the afferents serving the arms. The day after the rhizotomy, the animal had difficulty maintaining its head steady but could still perform horizontal head-unrestrained gaze shifts. During these gaze shifts, eye counterrotation still occurred but was reduced at 80 ms after the eye saccade from 0.61 ± 0.02 for 664 gaze shifts to a gain of 0.43 ± 0.01 for 175 gaze shifts. The counterrotation gains from 40 to 100 ms ranged from 0.42 ± 0.02 to 0.44 ± 0.02 after the rhizotomy. COR gain was reduced by the same procedure to a gain of 0.32 (Fig. 7B). The concurrent reduction of COR gain with counterrotation gain suggests some contribution of the COR to the head-unrestrained eye counterrotation.

EFFECT OF HEAD STOPPAGE ON COUNTERROTATION. A more direct measure of the contribution of any sensory signal to the counterrotation is to perturb the ongoing head movement and observe changes in eye counterrotation. We engaged an electromagnetic brake on some trials of monkey AC and a few trials in monkey HC2 to arrest the head movement completely in midflight. The time between the start of the eye saccade and the application of the brake was varied to produce arrests of the head movement during all portions of eye counterrotation. In all, hundreds of brakes were applied at various times during counterrotation. In none of these cases did application of the brake during counterrotation stop it. Small, transient perturbations in counterrotation, consisting of a brief deceleration and reacceleration, were noticeable, but they did not significantly alter the overall trajectory of the observed eye movement. We assume that they reflect a small influence of sensory signals from the neck or otolith organs. Despite stoppage of the head movement, eye counterrotation continued as is shown in Fig. 8.



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Fig. 8. Effect on eye counterrotation of stopping the head unexpectedly during a gaze shift. Data taken from 13 attempted 60° gaze shifts in monkey AC on the 103rd postplug day. All trials are aligned on the end of the head movement (vertical line). Eye counterrotation continues despite arrest of the head.

Termination of gaze shifts

By definition, the gaze movements ended when counterrotation gain reached 1.0. In our experiments, the interval between the end of the eye saccade and the end of the gaze shift, i.e., the duration of reduced gain counterrotation, increased immediately after plugging when counterrotation gain was low and shortened as counterrotation gain recovered (Fig. 9). However, the recovery of counterrotation gain to less than unity cannot account for the shortening. Part of the shortening can be attributed to a decrease of head movement duration, particularly in monkey HC1. However, the bulk of the remaining shortening must be due to a return of counterrotation gain to one, allowing termination of the gaze movement before termination of the head movement. We now consider the timing of the return of this fully compensatory counterrotation and whether head velocity and orbital eye position are instrumental in producing it.



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Fig. 9. Average head velocity at the end of the gaze shift (, black-triangle, ) and average duration of reduced gain eye counterrotation (open circle , triangle , and ) as a function of the number of elapsed days after plugging just the horizontal canals (HC1: open circle ; HC2: triangle ) or all canals (AC: ). Only gaze shifts with head movements >20° were used. Bars represent SEs.

HEAD VELOCITY. For a gaze movement to end before the head stops rotating, eye counterrotation must match head velocity. To determine the head velocities at which the gaze shift was terminated, we used gaze shifts with a substantial head movement of >= 20°. The amplitudes of these gaze shifts ranged from 40 to 80°.

Immediately after canal plugging, the head velocity at the end of a gaze shift was quite low. Head velocity when gaze was stable in space averaged 20 ± 22°/s (mean ± SD; n = 28) and 25 ± 10°/s (n = 108) for monkeys HC1 and HC2, respectively. In AC, which had all canals plugged, the average head velocity at the end of the gaze movement was 14 ± 9°/s (n = 220). Since 20°/s was our velocity criterion for termination of the gaze shift, we conclude that gaze and head movements ended at the same time or, equivalently, that gaze continued moving forward until the head stopped moving. However, as counterrotation velocity was recovering, head velocities at the end of the gaze shift increased (Fig. 9) such that the head was still moving after the gaze shift had ended. Head velocity at the end of the gaze movement increased to an average of 70 ± 20°/s (n = 270) and 99 ± 39°/s (n = 360) after 74 and 47 days after horizontal canal plugging in HC1 and HC2, respectively. In contrast, in normal intact animals, the gaze shift typically ends at head velocities >300°/s, often at or near the time of peak head velocity. Forty-nine days after all of the canals in monkey AC had been plugged, average head velocity had increased to 73 ± 32°/s (n = 423). Therefore gaze duration is shortened because the gaze shift can end earlier in the head movement, i.e., at higher head velocities (Fig. 9). This ability of the animal to match higher head velocities with eye counterrotations in combination with faster head movements of shorter durations accounts for the modest reduction of noncompensatory counterrotation duration (Fig. 9).

ORBITAL POSITION. Because the eye position at the end of the saccadic eye movement is eccentric (Freedman and Sparks 1997), we considered the possibility that passive orbital properties contribute to the counterrotation from these eccentric orbital positions. To test this, we compared head fixed saccades of 0-30° with the horizontal head-unrestrained gaze shifts. We found that in canal plugged animals with the head fixed, there are small amplitude (generally <3°) and low peak velocity (generally <30°/s) postsaccadic drifts back toward the origin of the saccade (Fig. 10C). The amplitude and velocity of this drift are much smaller than those seen in the horizontally head-unrestrained gaze shift counterrotations (Fig. 10, A and B).



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Fig. 10. Effect of orbital eye position at the end of the eye saccade on either eye counterrotation with the head unrestrained (+) or postsaccadic backward drift with the head restrained (open circle ). Data are for monkey AC (all 6 canals plugged) on postplug day 103. Peak counterrotation or backward drift eye velocity (A) and eye amplitude (B) are plotted against eye position at the beginning of the counterrotation or drift. C: 8 representative centrifugal gaze shifts with the head held to illustrate the modest backward drift.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The most striking finding of our study is the persistence of the eye counterrotation despite the absence of inputs from the semicircular canals. After only the horizontal canals were plugged in two monkeys, counterrotations already were present on the first test day after surgery. Furthermore, counterrotation gain increased with time to reach values between 0.51 and 0.75 (Fig. 3). After all canals were plugged in monkey AC, counterrotation gain initially dropped to zero; however, in only 2 or 3 days, counterrotations again were visible. In time, counterrotation gain in this monkey also improved to as much as 0.67.

In this DISCUSSION, we will consider the characteristics of the early, postplugging counterrotation, how the counterrotations improved with time, what mechanisms contributed to this recovery, and why a recovery of counterrotation is desirable for the task of shifting the direction of gaze. But first, we need to consider whether the canals indeed were inactivated functionally and whether they remained inactivated as counterrotation gain increased.

Our canal plugging eliminated the passive VOR

We are confident that our surgical intervention successfully plugged the canals because passive yaw rotation in all three animals and pitch rotation in the animal with all canals plugged produced horizontal and vertical VOR gains of <0.07 at frequencies between 0.1 and 1.0 Hz (Fig. 4). However, it is still possible that the natural head rotations produced in our behavioral situation consist of frequency components that are higher than those we could test with our passive vestibular stimulator and that those frequencies were spared after canal plugging. Indeed, after the canals of the toadfish are plugged, primary afferents still respond normally to 10-Hz rotations (Rabbitt et al. 1999). Because the radius of the horizontal canal in the rhesus monkey is smaller than that of the toadfish (compare Blanks et al. 1985 with Rabbitt et al. 1999), one might expect a significant VOR in canal-plugged rhesus monkeys at frequencies of ~5 Hz. A very recent study published after our paper was accepted has shown that this is indeed the case (Yakushin et al. 1998). Because our apparatus could produce yaw rotations at a maximum of only 1.0 Hz, we could not test the passive VOR at this frequency in our animals.

To test whether the head movements of our monkeys had substantial high-frequency components that might account for the postplugging counterrotations, we performed a FFT on the averaged head movement in monkey AC 107 days after all his canals were plugged. This animal had relatively high velocity head movements after plugging and a robust counterrotation. We found that 99% of the energy of the head movement is at frequencies <3 Hz, suggesting that canal plugging is effective at the frequencies present in this task. Certainly if the plugged canals were still participating in the counterrotation, we would expect that stoppage of the head would halt the counterrotation but, as we saw in Fig. 8, this was not the case.

Counterrotation is related to head velocity

If our plugging was indeed effective, then eye counterrotation during natural gaze shifts in a lighted room must be coming from sources other than the peripheral vestibular apparatus. The first question, therefore, is whether the counterrotation is related to head movement at all or is just a "ballistic" return of eye position in the orbit. Figures 2 and 4 show that counterrotation velocity indeed exhibits a monotonic, even linear, increase with head velocity in monkeys HC1 and HC2 when measured at 80 ms after the end of the rapid gaze shift. Indeed there is a linear relation (average correlation coefficient = 0.8) between eye and head velocity at all times between 40 and 100 ms after the eye saccade is over. Finally a linear relation is a good description of counterrotation gain from just after the canal-plugging surgery until as long as the counterrotation was measured (from 71 to 110 days).

For monkey AC, the slope of the eye versus head velocity ratio begins at zero on the day after the surgery (Fig. 2). However, when counterrotations begin to appear, as soon as day 2 or 3, eye velocity is seen to increase monotonically with head velocity and a straight line fits the data quite well.

Now that we have satisfied ourselves that eye counterrotation velocity is dependent on head velocity even though there is no canal input, we ask how a counterrotation related to head velocity could be generated. For animals with only horizontal canals plugged, the possibilities included smooth eye movements caused by movement of the target spot on the retina, contributions from the intact vertical canals and eye movements generated by stimulation of the neck receptors. Here it also would be useful to consider whether eye movements attributable to any of these possible sources changed in parallel with the gradual improvement in counterrotation gain with time (Fig. 3).

Possible sources of eye counterrotation after canal plugging

VISUALLY ELICITED PURSUIT MOVEMENTS. Counterrotation gain was unaffected by whether the target light remained on or whether it was extinguished at the onset of the rapid gaze shift in the two animals tested (Fig. 4). This suggests that retinal slip cannot be solely responsible for driving the eye counterrotation.

CONTRIBUTION OF THE VERTICAL CANALS AFTER HORIZONTAL CANAL PLUGGING. In both animals with plugged horizontal canals, HVOR gain increased with time after plugging. This recovery was seen even when the head was tipped to align the horizontal canals with earth horizontal. This HVOR recovery after plugging of the horizontal semicircular canals could depend on signals from the vertical semicircular canals, which would be sensitive to horizontal rotation even with the nose pitched down by 22° (Angelaki and Hess 1996). When the head is completely free, the animal might choose to move its head so as to maximize the vertical canal signal. Unfortunately, we could not test this possibility as our animals were constrained to make horizontal gaze shifts about a vertical axis.

Even though the monkeys could use vertical canal signals to facilitate head-unrestrained gaze shifts when the horizontal canals are plugged, they don't do so in our paradigm. We reach this conclusion because counterrotations appear to be just as vigorous whether an animal is deprived of vertical canal input from the outset, as in AC, or the vertical canals are subsequently disabled well after the horizontal canals, as in HC1. The modest drop of counterrotation gain after vertical canal plugging in HC1 does show that vertical canal input could have some effect on counterrotation recovery, at least in this animal. However, even in this animal, the major driver of counterrotation recovery must be looked for elsewhere.

INPUTS FROM NECK ACTIVATION. We investigated the role of cervical somatosensory pathways in counterrotation recovery in two ways: the COR was tested in all three monkeys and restricted dorsal roots were transected in one monkey.

The cervicoocular reflex was quite variable from trial to trial in a particular monkey and from one monkey to another. For example, both animals with only their horizontal canals plugged exhibited a long-term increase in COR gain, but the animal with all canals plugged showed a slight decrease in gain (Fig. 7). Furthermore, the animal (HC1) who underwent plugging of the vertical canals after the horizontal canals already had been plugged for 110 days, showed an increase in gain unlike the steady gain or slight decrease exhibited by monkey AC. Because eye rotations when the body was rotated under a stationary head moved in phase with the trunk rotation in all three animals, the COR, if it were consistent, would provide compensatory counterrotations for gaze shifts with the head unrestrained.

Others have suggested that the COR helps substitute for the loss of vestibular inputs in labyrinthine-defective subjects. In monkeys, 3 mo after labyrinthectomy, the COR gain has increased from a value near 0 to ~0.3 (Dichgans et al. 1973). In three bilaterally labyrinthine-defective patients, there was a 25% potentiation of the passive COR (Kasai and Zee 1978). However, as with our monkeys, the COR is variable from patient to patient and even between trials based on the instructions given (Jürgens and Mergner 1989; Kasai and Zee 1978). For example, if labyrinthectomized patients imagine a fixed target in the dark, their CORs can be enhanced.

The COR in our monkeys also showed up to a 30% increase with time but, unlike the former study of Dichgans et al. (1973), the COR started from ~0.3 and "improved" to 0.6. Whereas there is little data on COR gain in canal-plugged rhesus monkeys, labyrinthine-deficient humans show a variable but often high COR gain [e.g., 0.3-0.5 (Kasai and Zee 1978); 0.3-1.0 (Bless et al. 1984); <0.72 (Huygen et al. 1991)]. Normal COR gain in humans is also quite variable [0.5 (Doerr et al. 1981); <0.25 (Huygen et al. 1991); <0.20 (Jürgens and Mergner 1989); <0.20 (Takemori and Suzuki 1971)]. In our monkeys, the high initial COR gain could be a strategy developed to facilitate gaze shifts in the laboratory environment. Unfortunately we did not measure the COR as the animal was learning the gaze shifting task so we cannot address this suggestion.

Because of the difficulty in extrapolating from an inconsistent COR to a possible role for cervical signals in the recovery of counterrotation, we attempted to evaluate the contribution of neck inputs more directly by eliminating them by dorsal rhizotomy in a single animal. This procedure caused substantial instability of the head, but when the head was supported (held upright) in our behavioral situation, the animal could generate ~175 targeting gaze shifts. For these gaze shifts, counterrotation persisted with a gain of 0.43, a reduction of ~30%. Therefore although the counterrotation cannot be attributed to cervical inputs alone, the data from this single animal suggests that they might contribute somewhat.

Efference copy of head velocity?

The experiments in which we arrested the head movement in midflight provide perhaps the most diagnostic test to evaluate "sensory substitution" as a mechanism to compensate for the loss of the vestibular input. Sudden stoppage of the head with the electromechanical brake eliminated any sensory cue of head motion, whether it was provided by a residual labyrinthine, somatosensory, or visual signal. After head motion was stopped, the recovered counterrotation persisted as was seen in Fig. 8, suggesting that an internal copy of the intended head movement, i.e., an "efference copy," is responsible for driving eye counterrotation. Others have reported similar results in bilaterally labyrinthectomized monkeys (Dichgans et al. 1973).

Because we previously had observed changes in counterrrotation after both cervical rhizotomy and vertical canal plugging, it is surprising that counterrotation continues when both those signals are altered during head perturbations. This finding seems to suggest that vertical canal signals and information from the neck are not used on-line to drive the counterrotation. Instead they seem to be important in setting the gain of the counterrotation based on errors in individual trials.

Our data allow us to suggest the nature of the efference copy signal. According to Fig. 3, the efference copy mechanism makes a low, constant gain, estimate of head velocity in the interval between 40 and 100 ms after the rapid eye component of the gaze shift ends. Sometime after 100 ms, the gaze movement ends. The gaze shift ends and the eye remains stable in space because counterrotation gain must have increased to 1.0. Counterrotation gain increases to 1.0 to end the gaze shift when the head is moving at relatively low velocities (Fig. 9). This pattern of a counterrotation gain that initially is low between 40 to 100 ms after the eye saccade (Fig. 3) and then later jumps to 1.0 at low head velocities is quite consistent from trial to trial in the same session for gaze shifts up to 80° in amplitude and peak head velocities as high as 300°/s. Although one animal did exhibit high-velocity counterrotations during the first and second weeks of recovery, the counterrotations did not match head velocity and therefore did not enhance gaze accuracy. This animal gradually abandoned these movements for a pattern like that described above (i.e., an initial constant gain of <1.0 followed by a gain of 1.0).

There are two reliable changes that occur with time after canal plugging. First, the low counterrotation gain in the first 100 ms after the eye saccade increases (Fig. 3). Second, there is a parallel increase in the head velocity at the end of the gaze shift (Fig. 9). If the time course of head velocity is relatively unaffected by adaptation, the second change could cause a decrease in the duration of the gaze shift (Fig. 9) by allowing the gaze shift to end earlier (i.e., at higher velocities) in the head trajectory. But why is there also a modest increase in counterrotation gain to ~0.6 when any gain <1.0 is inadequate to terminate the gaze shift?

We suggest that the two changes are interrelated. The gaze shift ends when counterrotation gain is "switched" to 1.0. Perhaps, this switch can be activated only after gaze velocities have been reduced to a certain low level. Low gaze velocities could be reached either by waiting until head velocities decay to a low level (which occurs early after canal plugging) or by increasing eye counterrotation gain (which occurs with time after canal plugging). Once low gaze velocity has been reached, a final estimate of the required eye counterrotation velocity can be made more accurately, and the gaze shift can be terminated. Therefore increasing counterrotation gain even to values <1.0, can hasten the end of the gaze shift, if the mechanism that halts the gaze shift is engaged only after a relatively low gaze velocity is attained.

There never is complete compensation for the loss of semicircular canal input. The duration of the gaze shift remains long and the variability of the gaze shift remains greater than normal. Thus recovery in this system is limited, like other forms of plasticity in the vestibulooculomotor system such as VOR recovery after labyrinthectomy (Fetter and Zee 1988, Maioli et al. 1983), VOR gain plasticity (Angelaki and Hess 1998; Miles and Fuller 1974), and saccade adaptation (Straube et al. 1997).

The neural substrate of the counterrotation mechanisms remains unknown. In three recent models of gaze control, a head velocity signal arises from hypothetical head burst neurons (Phillips et al. 1995), from spinal afferents (Munoz and Guitton 1991), or is unspecified (Galiana and Guitton 1992). Where this head velocity signal might influence eye movement also is a matter for speculation. Presumably, it may act through the vestibular nucleus as does the semicircular canal input. Indeed evidence in our lab and others (McCrea et al. 1996; Phillips et al. 1996; Roy and Cullen 1998) suggests that the discharge of possible VOR interneurons is shifted away from head velocity during head-unrestrained gaze shifts. Future experiments involving neural recording in the vestibular nucleus during canal plugged gaze shifts will be needed to resolve this issue.


    ACKNOWLEDGMENTS

We thank Dr. Michel Kliot of the Department of Neurological Surgery for performing the dorsal rhizotomy. We are grateful for the excellent support of the veterinary staff at the Regional Primate Research Center and for the superb editorial assistance of K. Elias.

This research was supported by National Institutes of Health Grants EY-00745 and RR-00166.


    FOOTNOTES

Address for reprint requests: S. D. Newlands, University of Mississippi Medical Center, Dept. of Surgery (Otolaryngology), 2500 N. State St., Jackson, MS 39212.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 28 September 1998; accepted in final form 27 January 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

0022-3077/99 $5.00 Copyright © 1999 The American Physiological Society