1Department of Ophthalmology, Indiana University School of Medicine, Indianapolis, Indiana 46202; and 2Department of Ophthalmology, Kitasato University, Kitasato 228, Japan
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ABSTRACT |
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Suzuki, David A., Tetsuto Yamada, Rebecca Hoedema, and Robert D. Yee. Smooth-Pursuit Eye-Movement Deficits With Chemical Lesions in Macaque Nucleus Reticularis Tegmenti Pontis. J. Neurophysiol. 82: 1178-1186, 1999. Anatomic and neuronal recordings suggest that the nucleus reticularis tegmenti pontis (NRTP) of macaques may be a major pontine component of a cortico-ponto-cerebellar pathway that subserves the control of smooth-pursuit eye movements. The existence of such a pathway was implicated by the lack of permanent pursuit impairment after bilateral lesions in the dorsolateral pontine nucleus. To provide more direct evidence that NRTP is involved with regulating smooth-pursuit eye movements, chemical lesions were made in macaque NRTP by injecting either lidocaine or ibotenic acid. Injection sites first were identified by the recording of smooth-pursuit-related modulations in neuronal activity. The resulting lesions caused significant deficits in both the maintenance and the initiation of smooth-pursuit eye movements. After lesion formation, the gain of constant-velocity, maintained smooth-pursuit eye movements decreased, on the average, by 44%. Recovery of the ability to maintain smooth-pursuit eye movements occurred over ~3 days when maintained pursuit gains attained normal values. The step-ramp, "Rashbass" task was used to investigate the effects of the lesions on the initiation of smooth-pursuit eye movements. Eye accelerations averaged over the initial 80 ms of pursuit initiation were determined and found to be decremented, on the average, by 48% after the administration of ibotenic acid. Impairments in the initiation and maintenance of smooth-pursuit eye movements were directional in nature. Upward pursuit seemed to be the most vulnerable and was impaired in all cases independent of lesioning agent and type of pursuit investigated. Downward smooth pursuit seemed more resistant to the effects of chemical lesions in NRTP. Impairments in horizontal tracking were observed with examples of deficits in ipsilaterally and contralaterally directed pursuit. The results provide behavioral support for the physiologically and anatomic-based conclusion that NRTP is a component of a cortico-ponto-cerebellar circuit that presumably involves the pursuit area of the frontal eye field (FEF) and projects to ocular motor-related areas of the cerebellum. This FEF-NRTP-cerebellum path would parallel a middle and medial superior temporal cerebral cortical area-dorsolateral pontine nucleus-cerebellum pathway also known to be involved with regulating smooth-pursuit eye movements.
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INTRODUCTION |
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The regulation of smooth-pursuit eye movements
involves cortico-ponto-cerebellar circuits, the nature of which has
become clearer during the past several years. Two precerebellar pontine structures of import are the dorsolateral pontine nucleus (DLPN) and
the nucleus reticularis tegmenti pontis (NRTP). Physiological and
anatomic results indicate that DLPN is a major processor of visual
motion and pursuit-related signals presumably from the middle temporal
and superior medial temporal areas of the cerebral cortex
(Mustari et al. 1988; Suzuki and Keller
1984
; Suzuki et al. 1990a
; Thier et al.
1988
). Unilateral, chemical lesions made in DLPN resulted in
significant deficits in both the initiation and the maintenance of
smooth-pursuit eye movements (May et al. 1988
). However,
bilateral lesions to DLPN, while creating significant deficits in
tracking performance, did not permanently impair the smooth-pursuit
eye-movement system (May and Keller 1988
). The lack of
long-lasting effects may have been due to a parallel
cortico-ponto-cerebellar pathway that complements the route involving
DLPN. Because the frontal eye field contains pursuit-related cells
(Gottlieb et al. 1993
, 1994
; MacAvoy et al.
1991
) and is known to project to NRTP (Huerta et
al. 1986
; Künzle and Akert 1977
;
Leichnetz et al. 1984
; Stanton et al.
1988
), it has been hypothesized that such a parallel
smooth-pursuit pathway could be composed of projections from the
pursuit cells of the frontal eye field to NRTP (Suzuki et al.
1990b
). Completion of the cortico-ponto-cerebellar pathway would involve the NRTP projections to vermal lobules-VI, VII, the
flocculus, and the fastigial nucleus (Brodal 1980
, 1982
;
Thielert and Thier 1993
; Yamada and Noda
1987
), cerebellar structures that are involved with
smooth-pursuit control (e.g., Fuchs et al. 1994
; Lisberger and Fuchs 1978
; Miles and Fuller
1975
; Noda and Suzuki 1979
; Suzuki and
Keller 1988a
,b
).
Recent findings support the notion that NRTP and specifically the
rostral portion of NRTP (rNRTP) is involved with smooth-pursuit eye
movement control. Single-cell activity that is modulated during smooth-pursuit eye movements has been observed (Suzuki et al. 1991) and slow, pursuit-like eye movements have been evoked
with microstimulation of rNRTP (Yamada et al. 1996
).
This study was initiated to provide behavioral support for the
physiologically based conclusion that rNRTP is involved with the
regulation of smooth-pursuit eye movements. Pharmacological lesions in
rNRTP consistently resulted in significantly impaired smooth-pursuit eye movements. The results implicate rNRTP involvement in both the
initiation and the maintenance of smooth-pursuit eye movements.
A preliminary report of these results has appeared in abstract form
(Suzuki et al. 1990b).
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METHODS |
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Preparation
Three monkeys (1 Macaca fascicularis and 2 M. nemestrina) were prepared with scleral eye coils and head implant
hardware under aseptic conditions and anesthesia. Pentobarbital sodium
anesthesia was used for the M. fascicularis and isoflurane
gas anesthesia for the two M. nemestrina. The eye coils
(Teflon-covered stainless steel wire) were implanted surgically under
the conjunctiva at the junctions of the eye muscle insertions of the
right eye (Judge et al. 1980). A stainless steel chamber
(17 mm diam), centered at stereotaxic anterior 6.5 mm with a
dorsolateral-to-ventromedial 20° off vertical angle in the coronal
plane, was attached to the skull with dental acrylic cement. To
immobilize the head relative to the primate chair, two small-diameter
transverse, stainless steel cylinders also were attached to the skull
with dental acrylic cement. All sutured incisions were treated with
antibiotic ointments. During the postsurgical recovery period, buprenex
was administered for analgesia and penicillin or cefazolin was
administrated for prophylaxis.
The monkeys were trained, before surgery, to enter a primate chair from their home cages. After recovery from the surgical procedure, the monkeys voluntarily entered the primate chair and were placed in the experimental setup on a daily basis. The monkeys were returned to their home cages for the remainder of the day after each daily recording session. The cages were provided with swings, "toys," and a foraging board. To ensure the good health of the monkeys, parameters that included weight, liquid and food intake, agility, and general level of activity were monitored daily. Periodic physical examinations were conducted by the veterinarian staff of the animal facilities. The experimental and surgical protocols were approved by the Indiana University School of Medicine Animal Care and Use Committee and complied with U. S. Public Health Service policy concerning the care and use of laboratory animals.
Recording and delivery of pharmacological agents
For recording, tungsten microelectrodes insulated with Isonel-31 were lowered within and together with a 23-gauge stainless steel guide tube to a point 10 mm dorsal to the oculomotor (IIIrd) nucleus. The microelectrode was hydraulically driven out of the guide tube by a stepper-motor, microdrive system (Kopf 650). The location of the IIIrd nucleus first was determined to establish a stereotaxic reference structure that aided the subsequent localization of NRTP. Tracks aimed at NRTP-traversed paths that were lateral and ventral to the oculomotor nucleus.
Before delivery of lidocaine or ibotenic acid, NRTP sites were
identified by the presence of modulations in neuronal discharge rate
that were correlated with smooth-pursuit eye movements. The microelectrode then was withdrawn and replaced with an injection cannula (28-gauge stainless steel hypodermic tubing), which was positioned with the microdrive system. To avoid effects on structures ventral to NRTP and to minimize damage to fibers of passage, the cannula tip was driven to a depth just at the dorsal edge of the region
where the smooth-pursuit-related unit responses previously were
observed. The cannula was connected to a 10-µl syringe (Hamilton) with the syringe, connecting tubing, and cannula previously filled with
lidocaine or ibotenic acid. Sterile, 4% lidocaine hydrochloride (5 µl) was employed to induce reversible functional lesions (May et al. 1988). The lidocaine was infused at a rate of 0.5 µl/min.
The ibotenic acid (Sigma Chemical) was dissolved in 0.1 M sodium phosphate buffer to yield a final concentration of 15 µg/µl. To facilitate the dissolving of the ibotenic acid, small drops of 1. O N sodium hydroxide were added to make the solution slightly basic. Approximately 1.2 µl of ibotenic acid were slowly infused during 30 min. The injection cannula was left in place for an additional 20 min before it was slowly withdrawn and the animal returned to its cage. Eye movement recording sessions began 18-20 h later. No side effects e.g., poor balance, paresis or paresthesia of a limb, were observed.
Paradigms
The monkeys were trained with standard conditioning techniques
to fixate and track a 0.25°, back-projected, laser-generated red
light spot that was moved with an X,Y-mirror galvanometer system. The
90 × 90° tangent screen was located ~60 cm in front of the
monkeys in a dark room. An Apple Macintosh IIfx computer (programmed
with LabView) controlled visual target presentation, monitored
behavior, and rewarded correct task performance. Liquid intake was
controlled for 5 days each week and freely available on weekends.
During the experimental sessions, the extent of liquid intake was
self-determined by the monkeys because they received performance-dependent liquid rewards until they were satiated. Reward
delivery was contingent on eye position being within a small electronic
"window" surrounding the position of the red fixation spot. Window
size was dependent on target speed and was usually set for 2-4°.
After chemical lesions were made, larger spatial window sizes were
employed to accommodate poor eye movement performance. Window size was
varied with the degree of task difficulty and the magnitude of the
pursuit deficit. It was set large enough not to frustrate a motivated
animal that had difficulty initiating or maintaining pursuit and to a
size small enough to maintain motivated task performance. The
lesion-induced changes in pursuit gain were not thought to be due to
changes in window size because the deficits were often direction
specific. Eye position was determined with the magnetic-field
search-coil method (Robinson 1963).
Monkeys were required to both initiate and maintain smooth-pursuit eye
movements. For pursuit initiation, a "step-ramp" or Rashbass
(Rashbass 1961)-type task was employed, whereby the
target first stepped in the direction opposite to the direction of
subsequent constant velocity target movement. This minimized the
occurrence of corrective saccades during the initiation period.
Maintained smooth-pursuit eye movements were studied by requiring the
monkeys to track periodic target movements of 10-20° amplitudes and
0.2- to 1.0-Hz frequencies. Target motions were usually constant
velocity but sometimes sinusoidal.
Recording and data analysis
Analogue signals were filtered at 202 Hz (Frequency Devices,
8-pole Bessel) and sampled at 1 kHz per channel. Horizontal and vertical eye and target positions were stored on a magneto-optical disk
(Pinnacle Micro REO-650) for later off-line analysis. Data was analyzed
with a 486 microcomputer employing interactive, analytic software that
was developed with ASYST (Keithley). For eye velocity traces such as
those in Fig. 5, eye position signals were differentiated with an ASYST
supplied algorithm that interpolated a second-degree polynomial through
consecutive (eye position) data points and then differentiated the
polynomial (Blahaut 1984). The resulting eye velocity
signal was smoothed with an ASYST algorithm that employed a Blackman
window (Blackman and Tukey 1958
). The smoothing resulted
in an 11-ms advance in the velocity signal relative to the eye position
signal. This did not affect determination of eye speeds, which were
quantified in a different manner described in the following text.
The vector term "velocity" was used when referring to
differentiated eye position traces, whereas eye "speed" was
specifically applied to the measured eye velocity amplitudes. Eye speed
was determined by employing an interactive program that allowed the user to select segments of the constant velocity smooth-pursuit eye
position signal for analysis. These pursuit segments were selected from
instances when a monkey appeared to be exerting its best effort. The
results, therefore represent a conservative quantification of the
behavioral deficits. The duration of the selected segments were 100
ms and routinely
200 ms. Eye position values within a selected
segment were fitted with a linear regression line and the slope taken
as eye speed. At least 5 and typically >10 pursuit segments were
averaged to obtain a time-weighted, average eye speed for specific
pursuit parameter settings.
Eye acceleration was calculated by first differentiating the eye
position signals. From the display of the eye velocity trace, the
operator marked the beginning of the smooth-pursuit eye movement, and a
linear regression line then was fitted through the eye velocity points
during the 80 ms after the designated onset time of pursuit. The slope
of the regression line was taken as the average eye acceleration during
the 80 ms. This duration corresponds to the open-loop portion of
pursuit initiation that has been well studied by Lisberger and his
colleagues (e.g., Lisberger and Westbrook 1985). At
least 5 and typically
10 episodes of pursuit initiation per parameter
were analyzed and the results averaged. The effects of the lesions on
corrective saccade metrics and predictive pursuit were not investigated.
Histology
The monkeys were anesthetized deeply and perfused rapidly through the left ventricle with 1 l of buffered isotonic saline followed by a rapid perfusion of 2 l then slow perfusion of two additional liters of buffered 10% formalin. The brains were blocked, frozen, and cut in the coronal plane in 40-µm sections. The resulting sections were stained with cresyl violet. Ibotenic acid lesion sites were identified by a decrease in the number of stained neuronal cells and an increase in number of glial cells.
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RESULTS |
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The effects on smooth-pursuit eye movements of lidocaine and ibotenic acid were studied in three monkeys. In one monkey, only lidocaine was administered to study the effects of transient functional lesions. In the following text, this site is referred to as site 1. In a second monkey, both lidocaine (site 2L) and ibotenic acid (site 2) injections were made. The ibotenic acid lesion was made 4 days after the lidocaine injection at the same NRTP site. The third monkey received ibotenic acid injected at two different NRTP locations (sites 3 and 4). The two ibotenic acid injections were made 5 days apart. Injection site 4 was 0.8 mm anterior to site 3 and in the same medial-lateral location.
Location of lesions
Histological examination indicated that ibotenic acid site 2 was
in the right rNRTP and sites 3 and 4 were in the left rNRTP (Fig.
1). There was some uncertainty as to the
exact borders of the lesions, which were not as clear as the edges of
the hatched areas in Fig. 1 would seem to imply. However, the center of
all three lesion sites were located within the rostral end of NRTP, anterior to 2 mm of normal-appearing portions of NRTP. Because the
ibotenic acid injections were made just dorsal to observed pursuit-related unit activity, the core of the lesions were within NRTP. To a limited degree, the edges of the affected region may have
intruded ventrally into the dorsal and dorsomedial pontine nuclei.
Results obtained for site 4 (Fig. 1, bottom), which was centered on the dorsal aspect of rNRTP and seemed to have excluded involvement of neighboring pontine nuclei, appeared similar to those
obtained from sites 2 and 3. This would support the notion that the
observed results can be attributed primarily to lesions of rNRTP.
Although it was impossible to reconstruct the extent of the functional
lesion created by lidocaine, both right and left rNRTP were probably
affected. This presumption is based on estimations of injection site
location and volume of lidocaine injected.
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Effects of NRTP lesions on maintained smooth-pursuit eye movements
Transient functional lesions created by the injection of lidocaine into rNRTP resulted in clear deficits in the ability to maintain smooth-pursuit eye movements as shown in Fig. 2. Normally, a monkey can maintain a fairly high gain for pursuit over the 13-38°/s target velocity range associated with the pursuit epochs in Fig. 2, left. When 5 µl lidocaine was infused into NRTP, the ability to maintain smooth-pursuit eye movements was significantly degraded as exemplified by the pursuit epochs in Fig. 2, right. In this case, upward smooth pursuit was severely affected, though downward pursuit gain remained fairly high. To be sure, there were instances where downward pursuit was less than optimal, but these instances might have been due to the influence of the large retinal errors that existed at the turnabout points after the poor upward pursuit. In contrast, the first and third half-cycles of downward pursuit at 0.4 Hz appeared to be close to normal, suggesting that the ability to maintain downward smooth-pursuit eye movements remained relatively unimpaired by the lesion.
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For the same lidocaine session represented by the results in Fig. 2, both ipsilaterally (rightward) and contralaterally (leftward) directed maintained pursuit also were impaired. The deficit for contralaterally directed pursuit was greater than for maintained pursuit in the ipsilateral direction (Fig. 8A). With respect to these lidocaine results, the terms "ipsilateral" and "contralateral" must be viewed with caution. Although the injection site was known to be in the right NRTP, the exact borders of the lidocaine induced lesion were not known.
To quantify the effects of NRTP lesions on maintained smooth-pursuit
eye movements, pursuit gains were calculated as the ratio of pursuit
eye speed to target speed and plotted as a function of target speed as
shown in Fig. 3. Control eye movements
were elicited without the placement of the injection cannula. The
pursuit epochs shown in Fig. 2 were included in the analysis for Fig. 3A. In addition, gain values were determined after the
injection of saline into NRTP and are shown in Fig. 3, A and
B. For a similar volume (5 µl) of saline injected into
about the same site as the lidocaine injection, no significant
differences in maintained pursuit were observed between the
cannula-less control conditions () and following the saline
injection (
). This indicates that the lidocaine-induced pursuit
deficits were not due to nonspecific mechanical stress effects on
pontine structures nor to potential tissue damage that may have
resulted from the injection of the volumes employed. The results can
more specifically be attributed to an effect on a
smooth-pursuit-related component structure. Furthermore effects on
pursuit of the injection cannula also could be ruled out, indicating
that data obtained in the absence of the injection cannula could be
used for controls. As noted above, but not shown in Fig. 2, maintained
pursuit in the ipsilateral and contralateral directions also were
impaired by lidocaine. The magnitudes of these impairments are shown in
Fig. 3B. Standard errors for most points in Fig. 3,
A-D, were within the vertical dimensions of the symbols
used in the graphs and were omitted for clarity.
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Deficits in maintained smooth-pursuit eye movements also were observed
after the administration of ibotenic acid, a compound that specifically
destroys cell somata while sparing fibers passing through the region
(Schwarcz et al. 1979). Because lidocaine affects both
cell bodies and fibers passing through the region, the use of ibotenic
acid was necessitated to more definitively argue that the observed
impairment of maintained pursuit was specifically due to lesioning of
cells in rNRTP. As shown in Fig. 3C, an
ibotenic-acid-induced rNRTP lesion did result in deficits in maintained
smooth-pursuit eye movements. For this site, upward maintained
smooth-pursuit was significantly impaired (
). Downward, ipsilateral,
and contralateral maintained pursuit appeared normal (not shown). The
normality of maintained smooth-pursuit eye movements in directions
other than up, indicates that the deficits in upward tracking were
specific and not due to any global effects on the motor system or
pontine functions.
Another indication of the ability of the pursuit system to match eye to
target velocity is the plateau in eye velocity observed during
responses to step-ramp target motion (e.g., Fig. 5). For comparison,
pursuit gains calculated from such plateau values of eye velocity
(divided by target velocity) are presented in Fig. 3D. For
site 4, comparable decrements in pursuit gains were observed for the
eye velocity-plateau of pursuit initiation () and for pursuit
maintained for periodic target motion (
, · · ·). When there were difficulties in initiating
pursuit, i.e., when the initial eye acceleration was less than normal,
there were always decrements in the amplitude of the eye velocity
plateau. However, problems with initiating pursuit could affect the
initial velocity plateau without affecting subsequent maintained
pursuit after the first cycle (cf. Fig. 8, B and
E). Thus periodic target motion was used to characterize the
ability to maintain pursuit rather than step-ramp target motion and the
associated eye velocity plateaus observed during pursuit initiation.
To obtain a representative indicator for the decrement in maintained smooth-pursuit eye movements, gains for maintained pursuit elicited to periodic, constant velocity target motion were calculated and the results presented in Fig. 4. Decrements in maintained pursuit gains were evident for all lesion sites and ranged from 38 to 53% for the direction of pursuit that was most affected at each site. The average reduction in maintained smooth-pursuit gain following a rNRTP lesion was 44%. The results for site 3 were obtained from only two cycles of pursuit for a target velocity of 16°/s because the monkey had difficulty maintaining pursuit at this target velocity. This monkey could not maintain pursuit at higher target velocities. Thus the average 44% gain reduction was a conservative minimum value reflecting the process of selecting for the best pursuit responses. The average gain reduction would have been even greater if site 3 results obtained for target velocities >16°/s were included.
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In Fig. 4, the control gains for sites 1, 3, and 4 were <0.9. With horizontal sinusoidal tracking, gains of >0.9 are typical. The slightly lower control gains observed for these sites may have been due to using constant velocity tracking and the fact that these gains were for upward tracking, which was usually not as good as horizontal tracking. This latter explanation may be the most valid because the data for site 2 were for contralateral, i.e., horizontal maintained smooth pursuit and yielded an average control gain of 0.95.
Effects of ibotenic acid on the initiation of smooth-pursuit eye movements
Lesions created by ibotenic acid in rNRTP resulted in significant
deficits in eye acceleration during the initiation of smooth-pursuit eye movements. Representative epochs of smooth-pursuit initiation are
shown in Fig. 5 for step-ramp target
movements. The diverging average eye velocity traces for control versus
postlesion behavior (Fig. 5, right) indicate a decrement in
initial eye acceleration. Contralateral eye acceleration averaged over
the first 80 ms of pursuit initiation, the open-loop portion
(Lisberger and Westbrook 1985), was
251°/s2 for controls and
151°/s2 after lesion formation. Average upward
eye acceleration decreased from 183°/s2 for
controls to 34°/s2 for pursuit initiation after
lesion formation by ibotenic acid. Ipsilateral and downward eye
accelerations were also decremented after rNRTP lesion formation from
285 to 115°/s2 and 187 to
112°/s2, respectively. Because maintained
smooth-pursuit eye movements for ipsilateral, contralateral, and
downward target movements were approximately normal, at least at
16°/s, these pan-directional deficits in pursuit initiation were
presumably not due to nonspecific effects on the eye-movement system.
|
For the same site, the effects of ibotenic acid on pursuit initiation
were studied over a range of target velocities and the results are
presented in Fig. 6. Smooth-pursuit eye
accelerations initiated in response to step-ramp target motion were
averaged during the initial 80 ms to obtain the data points in Fig. 6. Each point represents the average of 5 and typically 10 trials of
pursuit initiation. All four cardinal directions of pursuit initiation
were affected. Significant recovery was observed 2 days after the
administration of ibotenic acid, and essentially normal values for eye
acceleration were obtained for pursuit initiation studied 3 days
postlesion.
|
To facilitate comparison, the effects of rNRTP lesions on the initial eye acceleration of pursuit initiation were studied at or near a target speed of 30°/s for all lesion sites. Results were obtained for 12-20 trials of pursuit initiation in the ipsilateral direction for site 2 and in the upward direction for the remaining sites. Because a representative indication of the deficit in eye acceleration was sought, the results are for the direction of pursuit initiation that was most impacted for a given site. Initiation-related pursuit eye accelerations were normalized relative to control values which were set equal to one. The results are presented in Fig. 7 as the ratio of postlesion initial eye accelerations to prelesion control accelerations. Decrements in eye acceleration ranged from 30 to 82% and averaged 48%.
|
Directional effects
Maintained smooth-pursuit eye movements in the upward direction appeared to be the most susceptible to lesions made in rNRTP as shown in Fig. 8. In contrast, downward maintained smooth pursuit seemed to be relatively resistant, though not impervious, to the effects of rNRTP lesions. Deficits in horizontal maintained pursuit were observed with the presumably large lesions associated with lidocaine injections (Fig. 8A) but not always with the more circumscribed ibotenic acid lesions (Fig. 8C). For ibotenic acid lesion site 3 (Fig. 8B), horizontal pursuit was normal for a target velocity of 16°/s but could not be maintained for the higher target velocity of 30°/s commonly used for constructing other panels in this figure. Thus a more profound and multi-directional deficit in maintained pursuit was actually observed for site 3, even though only measured results were plotted in Fig. 8B. Testing of vertical maintained pursuit behavior inadvertently was missed for the lesion at site 2 because attention was focused on collecting pursuit initiation data.
|
The results for initiating smooth-pursuit eye movements are qualitatively similar to those for maintaining pursuit. The directional effects of ibotenic acid on average eye acceleration during the initial 80 ms of pursuit initiation were plotted in Fig. 8, D-F. Upward pursuit initiation seemed to be the most consistently impaired, whereas downward pursuit initiation seemed to be more resistant to the effects of lesion formation. For site 3, it already has been noted that the monkey was unable to maintain pursuit for targets moving at 30°/s, yet the monkey was able to initiate pursuit at this target velocity (Fig. 8E). Although pursuit could be initiated, it is clear that there was a severe deficit in the initial eye acceleration.
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DISCUSSION |
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The results provide behavioral support for the conclusion
that rNRTP is part of the neural substrate that regulates
smooth-pursuit eye movements. Chemical lesions, induced by lidocaine or
due to ibotenic acid excitotoxicity, caused deficits in both the
maintenance and the initiation of smooth-pursuit eye movements (Figs. 2
and 5). The functional implications of these deficits complement the observation of smooth-pursuit-related neuronal responses in rNRTP (Suzuki et al. 1991). Presumably rNRTP receives
smooth-pursuit-related signals from, at least, the frontal eye field
where pursuit-related activity has been recorded (Gottlieb et
al. 1993
, 1994
; MacAvoy et al. 1991
) and lesions
resulted in impaired pursuit performance (Keating 1991
;
Keating and Gooley 1988
; Lynch 1987
;
MacAvoy et al. 1991
).
Directional pursuit deficits after rNRTP lesions
The average 44% decrease in the gain of maintained pursuit
indicated that lesions in rNRTP can have a profound effect on
smooth-pursuit performance. The observed deficits always involved
upward pursuit and sometimes involved other directions of
smooth-pursuit eye movements (Fig. 8). The seemingly greater
susceptibility of upward pursuit to rNRTP lesions is consistent with
microstimulation results. When microstimulation was delivered to rNRTP,
slow pursuit-like eye movements were evoked in the upward direction in
89% of the cases (Yamada et al. 1996). Upward eye
movements were evoked even when the microelectrode recorded down
smooth-pursuit unit responses at the same site just before microstimulation.
Although microstimulation and lesion results indicated that upward eye movements are especially affected, it should not be concluded that rNRTP is solely involved with vertical eye movement control. There was ample evidence that rNRTP also is involved with horizontal ocular motility. Deficits in some type of horizontal pursuit eye movements were observed for all sites. Pursuit in both horizontal directions were affected at site 1 (Figs. 3B and 8A), site 2 (Figs. 4 and 8D), and site 3 (Fig. 6 and 8E). For site 4 (Fig. 8F), deficits in ipsilateral pursuit initiation were observed. In contrast with vertical pursuit, definitive conclusions could not be made concerning which direction of horizontal pursuit was more consistently or severely affected by rNRTP lesions. The specific proportion of deficits in ipsilateral versus contralateral tracking may have been more dependent on the specific location and extent of the rNRTP lesion.
Recovery of pursuit behavior
The fairly rapid recovery of tracking ability after the administration of ibotenic acid may be due to functional compensation attributable to the surviving cells in rNRTP. No attempt was made to totally eliminate rNRTP, even unilaterally, thus the effects of complete loss of rNRTP remain undetermined. Because sites 3 and 4 were in the same animal, it was reasonable to expect a greater deficit after the lesion at site 4. Inexplicably, just the opposite was the case. In addition to the possible functional compensation by surviving cells in rNRTP, greater reliance on the cortico-ponto-cerebellar pathway mediated by the dorsolateral pontine nucleus may have contributed to the recovery of pursuit behavior. Bilaterally lesioning both the dorsolateral pontine nucleus and rNRTP would be expected to drastically and possibly permanently impair pursuit, but this remains to be determined.
Relation to cerebellum
The suggested participation of rNRTP in controlling smooth-pursuit
eye movements is consistent with our increased understanding of
cerebellar roles in the regulation of pursuit eye movement. Although
involvement of floccular areas with smooth-pursuit functions has been
known for a number of years (e.g., Lisberger and Fuchs 1978; Miles and Fuller 1975
; Noda and
Suzuki 1979
), the involvement with pursuit functions of the
oculomotor vermis and its target structure, the fastigial nucleus, has
only recently become fully appreciated (Fuchs et al.
1994
; Robinson et al. 1997
; Suzuki and Keller 1988a
,b
; Suzuki et al. 1981
). NRTP
projects to floccular areas, oculomotor vermis, and fastigial nucleus
making it, together with the dorsolateral pontine nucleus, an important
source of pursuit-related information destined for the cerebellum
(Brodal 1980
, 1982
; Gonzalo-Ruiz and Leichnetz
1990
; Gonzalo-Ruiz et al. 1988
; Thielert
and Thier 1993
; Yamada and Noda 1987
).
In summary, the pursuit deficits observed in this study are consistent with the conclusion that the rNRTP is part of the neural substrate for the smooth-pursuit eye movement system. Presumably rNRTP is part of a cortico-ponto-cerebellar pathway that is composed of the frontal eye field, rNRTP, and the oculomotor vermis, fastigial nucleus, and floccular areas. This cortico-ponto-cerebellar pathway parallels an MT/MST to dorsolateral pontine nucleus to oculomotor cerebellum path. Both the initiation and the maintenance of smooth-pursuit eye movements are dependent on rNRTP.
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ACKNOWLEDGMENTS |
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We are indebted to K. Betelak for outstanding technical assistance during surgery, operant conditioning, and histological analysis.
This research was supported by National Eye Institute Grant EY-09082 and by a Development Grant from Research to Prevent Blindness to the Department of Ophthalmology, Indiana University School of Medicine. T. Yamada was supported in part by a Fellowship from Research to Prevent Blindness and a Grayson Ophthalmology Fellowship.
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FOOTNOTES |
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Address for reprint requests: D. A. Suzuki, Dept. of Ophthalmology, Indiana University School of Medicine, 702 Rotary Circle, Indianapolis, IN 46202.
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 6 May 1998; accepted in final form 11 May 1999.
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