Department of Neurology, The University of Connecticut Health Center, Farmington, Connecticut 06030
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ABSTRACT |
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Waitzman, David M., Valentine L. Silakov, Stacy DePalma-Bowles, and Amanda S. Ayers. Effects of Reversible Inactivation of the Primate Mesencephalic Reticular Formation. II. Hypometric Vertical Saccades. J. Neurophysiol. 83: 2285-2299, 2000. Electrical microstimulation and single-unit recording have suggested that a group of long-lead burst neurons (LLBNs) in the mesencephalic reticular formation (MRF) just lateral to the interstitial nucleus of Cajal (INC) (the peri-INC MRF, piMRF) may play a role in the generation of vertical rapid eye movements. Inactivation of this region with muscimol (a GABAA agonist) rapidly produced vertical saccade hypometria (6 injections). In three of six injections, there was a marked reduction in the velocity of vertical saccades out of proportion to saccade amplitude (i.e., saccades fell below the main sequence). This was associated with a moderate increase in saccade duration. Inadvertent inactivation of the INC could not account for these observations because vertical, postsaccadic drift was not observed. Similarly, pure downward saccade hypometria, the hallmark of rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) inactivation, was always preceded by loss of upward saccades in our experiments. We also found a downward and ipsiversive displacement of initial eye position and evidence of a contraversive head tilt following piMRF injections. Saccade latency was shorter after two of six injections. Simulation of a local feedback model provided three possible explanations for vertical saccade hypometria: 1) a shift in the input to the model to request smaller saccades, 2) a reduction of LLBN input to the vertical saccade medium lead burst neurons (MLBNs), or 3) an increase in the gain of the feedback pathway. However, when the second hypothesis was coupled to a shortened duration of the saccade trigger (i.e., the discharge of the omnipause neurons), the physiological observations of piMRF inactivation could be replicated. This suggested that muscimol had targeted structures that provided both long-lead burst activity to the MLBNs in the riMLF and were critical for reactivation of the omnipause neurons. Evidence of markedly reduced vertical saccade amplitude, curved saccade trajectories, increased saccade duration, and saccades that fall below the amplitude/velocity main sequence in these monkeys closely parallels the oculomotor findings of patients with progressive supranuclear palsy (PSP).
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INTRODUCTION |
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Recent data have demonstrated neurons with
vertical on-directions located in the rostral portion of the
mesencephalic reticular formation (MRF) adjacent to the interstitial
nucleus of Cajal (INC) (Fukushima et al. 1990;
Fukushima and Kaneko 1995
; Handel and Glimcher
1997
; Scudder et al. 1996
; Silakov and
Waitzman 1994
). These MRF neurons had low-frequency long-lead
burst activity that began 150 ms before vertical saccades
(Handel and Glimcher 1997
). A subsequent high-frequency
burst began 8-32 ms before saccade onset (King et al.
1981
; Scudder et al. 1996
).
Intracellular filling of these neurons has shown them to be located
just lateral to the INC, dorsal-medial to the reticulotectal long-lead
burst neurons (RTTLNs) (Scudder et al. 1996a
), the focus of
inactivation occurring in the previous paper (Waitzman et al.
2000
). Cells in the peri-INC MRF (piMRF) region have axons that
descend toward the pons and innervate the raphe nuclei [raphe pontis
(RP), nucleus raphe interpositus (RIP), raphe obscuris (RO)] and the
medullary reticular formation (primarily the inhibitory burst neuron
region caudal to the abducens nucleus) (Scudder et al.
1996a
). This pattern of arborization is distinct from the cells
located in the INC per se. Cells within the INC project primarily to
the oculomotor and trochlear nuclei and provide only a small projection
to the spinal cord. The primary afferents to the INC arise from the
rostral interstitial nucleus of the medial longitudinal
fasciculus (riMLF). To the contrary, the piMRF receives direct
projections from the superior colliculus (SC; primarily vertical and
caudal regions) and provides strong innervation to the cervical spinal
cord (Chen and May 2000
; Kokkoroyannis et al.
1996
; May et al. 1997
). Another interesting
aspect of these neurons is that they do not project back to the SC as
do the RTTLNs in the more caudal portion of the MRF (see
Waitzman et al. 2000
). Thus cells in the piMRF are
positioned both anatomically and physiologically to provide a vertical
eye and possibly head (i.e., gaze) signal directly to the cervical
spinal cord and to the cerebellum via projections to the nucleus raphe pontis.
Unilateral inactivation of the INC with muscimol produces an almost
immediate loss of vertical gaze holding characterized by
position-dependent, vertical, postsaccadic drift and a contralateral head tilt (Crawford and Vilis 1993). Unilateral loss or
inactivation of the riMLF (the site of the medium lead vertical burst
neurons) causes a reduction in downward eye velocity and loss of
ipsitorsional saccades, whereas bilateral destruction eliminates all
saccades in the vertical plane (Suzuki et al. 1995
). We
show here that unilateral inactivation of the piMRF
produces a striking reduction in the gain of both up and down eye
movements similar to the effect of bilateral riMLF
lesions. Saccade duration was also prolonged. At the same time,
position-dependent, vertical postsaccadic drift had not developed when
saccade hypometria was evident. Simulations using a one-dimensional eye
displacement model (see Waitzman et al. 2000
, and Fig.
7B) suggested three possible explanations for saccade
hypometria: 1) a shift in the input to the local
feedback loop from a region coding large to a different region coding
small eye displacement (Hypo #1), 2)
combined reduction in the output of long-lead burst neurons (LLBNs) to
vertical medium lead burst neurons (MLBNs) and shortened saccade
triggering (Hypo #2), and 3) increased
gain in the feedback path (Hypo #3). Only
hypothesis #2 was able to reproduce saccade hypometria
and increased saccade duration. The oculomotor (i.e., hypometric
vertical saccades with longer durations) and postural difficulties of
monkeys following piMRF inactivation closely parallel those seen in
patients with progressive supranuclear palsy (PSP). These findings
suggest that the bilateral input of the piMRF to nucleus reticularis
pontis oralis (NRPo) and RIP is critical for the generation of
accurate, rapid, vertical saccades. Abstracts of this work have
appeared recently (Waitzman et al. 1997a
,b
).
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METHODS |
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The methods for recording eye movements and single neurons,
electrical microstimulation, and performing muscimol injections in
three monkeys (G, C, and S) were the same as
those described in detail in the accompanying paper (Waitzman et
al. 2000).
Data analysis
A major analytic tool employed here was calculation of
postsaccadic drift. This analysis was critical to our hypothesis that the piMRF and not the INC or riMLF was responsible for the vertical saccade hypometria reported here. The importance of the drift analysis
is reflected by the use of two different methods used to calculate
postsaccadic drift. As per the recommendation of Crawford and
Vilis (1993) "drift amplitude," the amplitude of drift
following each saccade, is a very sensitive measure of integrator failure. These changes were noted within a few minutes of muscimol injection in the INC (Crawford and Vilis 1993
). First,
we calculated when significant drift amplitude appeared. This was done
by comparing control drift amplitude (usually <2°) to drift
amplitude calculated from
10 centrifugal spontaneous saccades
selected from sequential files collected after the injection (typically
every 5-10 min, Student's t-test, P < 0.05). Large centrifugal spontaneous saccades could be most easily
assessed for postsaccadic drift because they brought the eye to an
eccentric location in total darkness. The time at which drift amplitude
became significantly different from control was defined as the onset of
partial inactivation of the INC by muscimol (Table
1). Data collection usually continued for
1 or 2 h after the appearance of significant drift. A second method examined the dependence of drift amplitude on eye position. Onset of postsaccadic drift from INC inactivation is characterized by a
linear relationship between eye position and drift amplitude (e.g.,
Fig. 3D, filled symbols). Because we were unable to measure three-dimensional eye movements (i.e., no separate torsion signal), we
also monitored the horizontal eye channel for evidence of
vertical eye position-dependent drift that occurred at a similar time
to that seen in the vertical channel (Fig. 3, C and
D).
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Variations in saccade amplitude and direction following the
injections were evaluated using a "circle" plot. The amplitudes (final initial radial eye position) and radial directions for each group of saccades to each of the 40 possible target positions were
calculated. The amplitude and direction of control saccades were
obtained either immediately before or during the 24-h period before the
injection of muscimol (
, Fig.
1, C, D, and E, for 25, 20, and 15° saccades, respectively). The amplitude and direction of the postinjection (
) movements were then overlaid on the
preinjection data (Fig. 1, C-E). If the injection had
produced no change in saccade metrics, these plots should exactly
correspond. All other data analysis methods are the same as in the
preceding paper (Waitzman et al. 2000
).
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RESULTS |
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Effects of muscimol: area of inactivation
Six injections of the GABAA agonist muscimol were made in three monkeys (G, C, and S) at sites in the rostral MRF where vertical eye movement related cells were located (Table 1). All but two injections produced the same constellation of findings: 1) hypometria of both up and down saccades, 2) shift in the initial position of the eyes to the ipsiversive side and down, 3) curvature of saccade trajectories toward the horizontal or a plane tilted ~22° from the horizontal, 4) the duration of vertical saccades was prolonged and a small horizontal component appeared in five of six injections, and 5) a contraversive head tilt. In two of six injections, the latency of vertical saccades was moderately shortened. Vertical postsaccadic drift was carefully monitored to determine when the INC was inactivated by spread of the muscimol (Table 1). Data were collected until the monkey could no longer perform the visually guided saccade paradigm.
Inactivation of the rostral MRF: vertical hypometria
Muscimol injection into the left MRF rostral to the posterior
commissure and lateral to the INC produced the rapid onset of vertical
saccade hypometria (Fig. 1). Quickly (within 10-25 min) upward (45 and
90°, positions 1 and 2) and downward saccades
(270 and 315°, positions 6 and 7) became
hypometric (Fig. 1, C-E). The progressive effect of
muscimol over time was reflected by the increasingly negative
difference coefficients (deeper troughs; Fig. 1F; see
METHODS) (Waitzman et al. 2000). Note that
oblique saccades up and to the contraversive (right) side were more
hypometric than comparable oblique saccades for the ipsilateral side.
The trajectories of horizontal saccades were straight, but the
trajectories of oblique or vertical movements were curved toward the
horizontal plane (Fig. 1B). The endpoints of vertical
saccades were deviated toward the horizontal plane as reflected by the
clockwise (downward) deviation of contraversive oblique upward
movements, and the counterclockwise (upward) deviation of oblique
downward movements (315°; Fig. 1G). One other feature
evident in Fig. 1 was that initial eye position was shifted down and
toward the ipsilateral side (Fig. 1B). This 4° shift did
not account for the significant vertical hypometria observed after this injection.
The metrics of the postinjection saccades were analyzed by comparing saccade amplitude, peak velocity, latency, and duration before and after the muscimol injections. The peak velocity of vertical saccades was reduced by >50% of the preinjection value following the muscimol injection (Fig. 2, B and D). One way the oculomotor system could compensate for the reduced vertical saccade amplitude would be to increase saccade duration and amplitude in the horizontal direction. Within 20 min after the muscimol injection, a small ipsiversive horizontal component of movement appeared in pure upward saccades (Figs. 1D and 2A, position 2). This was not evident for pure downward saccades (Fig. 2C).
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The relationship between the peak velocity and saccade amplitude
typically saturates for larger saccade amplitudes, the so-called main
sequence (Fuchs 1967). The main sequence for
preinjection control saccades was fit with a log regression (Fig.
2E, dotted line, r2 = 0.90). Most postinjection saccades obeyed the main sequence. However,
vertical saccades to positions 2 and 6 fell below
the control regression (open circles and squares, respectively).
Separate regressions were performed for the entire postinjection group of saccades (data not shown) and those to positions 2 and
6 (solid line). The latter were different from control
(P < 0.01, Table 2). Thus the
velocity of postinjection, vertical saccades was lower than expected
for their vectorial amplitude despite the appearance of the small
horizontal component of movement. This velocity reduction was evident
for vertical saccades in the amplitude range of 7-25°.
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Saccade latencies were unchanged after this injection despite separate
analysis for pure vertical movements (Figs.
3A and 5C). On the
other hand, the duration of postinjection vertical movements was longer
than control after this injection (Fig. 3B). The duration of
downward saccades (position 6) was slightly greater than
upward movements (position 2), probably reflecting the
appearance of a new horizontal component for upward and not downward
saccades (compare Fig. 2, A with C, Fig.
3B, position 6 vs. position 2 regression). This stretching of the vertical saccade components and the
addition of a new horizontal component contributed to the bending of
saccades toward a plane tilted ~22° from the horizontal (Fig.
1D). Similar longer duration oblique saccades in response to
vertically placed visual stimuli have also been observed in patients
with PSP (Pierrot-Deseilligny et al. 1989;
Rottach et al. 1996
).
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The appearance of position-dependent, postsaccadic drift after the injection heralded the spread of muscimol to involve nearby structures such as the INC. Typically there was little drift after each control saccade (Fig. 3, C and D, dotted regression lines, before injection). After the injection, drift amplitude remained within the ±95% confidence intervals of control up to 55 min for the horizontal channel and 30 min for the vertical channel (Fig. 3, C and D, open symbols). A relationship between postsaccadic drift and eye position did not fully develop in either channel until 55 min (Fig. 3, C and D, closed symbols; see METHODS). Evidence of drift amplitude in the horizontal channel suggested that a torsional eye signal had appeared, possibly from INC inactivation.
Summary of rostral MRF muscimol injections
Vertical saccade hypometria developed to varying degrees
after all injections (Fig. 4). Increased
saccade duration was noted in four injections (Fig.
5, A and B), and
two injections showed a moderate reduction in saccade latency (Fig.
5C). Shifts in the initial position of the eyes were noted
in five of six injections (Fig.
6B). The details are shown in
the next three figures. All injections from the three monkeys within
the piMRF region were projected onto a single section of the rostral
midbrain (Fig. 4A). Two injections (c0430 and s0714) that
were closer to the INC produced more upward than downward saccade
hypometria (Fig. 4B, and
).
However, position-dependent, postsaccade drift did not appear until
>20-30 min after the onset of vertical saccade hypometria (Table 1).
This suggested that the reduction in upward saccade amplitude occurred
first and then the muscimol spread to inactivate a portion of the INC.
Vertical hypometria ranged from 80% to almost 40% of preinjection
vertical saccade amplitude (Fig. 4B). Note that these
numbers are underestimates because we have shown in Fig. 4B
the time point at which the monkey could still make saccades in all
directions.
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In four of six injections, saccade duration of the vertical component of eye movement was markedly prolonged (Fig. 5B). A new horizontal saccade component appeared in five of six injections (Fig. 5A). Last, the latency of vertical (position 2 and 6) saccade onset following four of six injections was modestly shorter and reached significance in two experiments (Fig. 5C).
Effects on spontaneous saccades
After each of the injections, a file of spontaneous saccades
made in total darkness was collected. In contrast to the effects found
in the accompanying paper (Waitzman et al. 2000), no
specific goal of eye movement was noted (Fig. 6A). The
average locus of the endpoints for all spontaneous saccades is shown in
Fig. 6A. The monkey made saccades toward the central
fixation point. In one injection (s0714) the average position shifted
in the same direction as the shift in initial eye position.
Head tilt and shift in initial position
Inactivation of the INC with muscimol has been reported to produce
a position-dependent, postsaccadic drift and a contralateral head-tilt
in monkeys (Crawford and Vilis 1992; Fukushima et
al. 1987
). Five of the six rostral injections reported here
produced position-dependent, postsaccadic drift within 70 min of
initiation of injection (Table 1). However, within 10-20 min all but
one of the rostral MRF muscimol injections demonstrated an ipsilateral and downward shift in the initial position of the eyes (Fig.
6B). As demonstrated in the head-free monkey (see
accompanying report, Waitzman et al. 2000
), an
ipsilateral shift in the initial position of the eyes in the head-fixed
monkey could in fact be compensatory for an attempted contralateral
head tilt in the head-free monkey. This compensation brought gaze
(combined head and eye signal) close to zero (Waitzman et al.
2000
). Three of six rostral injections produced a contralateral
head tilt of 20-30° after the monkey's head was released from the
head fixation device 2-3 h after the injection. No experiment was
performed during which the head was released immediately after a
muscimol injection in the rostral MRF. Postural abnormalities were also
observed and included an inability to maintain an upright posture after
the injection without hemiparesis or sensory loss. Gait and posture
returned to normal within the next 24 h.
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DISCUSSION |
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The results presented here support the idea that the MRF
is divided into at least two portions. Inactivation of the
ventral-caudal MRF, which corresponds to nucleus subcuneiformis,
produced saccade hypermetria (Waitzman et al. 2000).
Single cell recordings in the rostral MRF, which we designate the
peri-INC MRF (piMRF), have demonstrated cells related primarily to the
vertical component of eye movement (Handel and Glimcher
1997
; Silakov and Waitzman 1994
). Inactivation
of the piMRF with muscimol produced marked hypometria of vertical
saccades. We will discuss the current findings with respect to refining
the circuitry and signals used to generate the vertical component of saccades.
Inactivation of the riMLF or INC
One possibility that might account for the results of peri-INC MRF
injections was that either the INC or riMLF, nearby structures mediating vertical eye movements, were also inactivated by the muscimol. Inactivation of the riMLF is unlikely for a number of reasons. First, unilateral lesions in the riMLF produce little deficit
in upward eye movements (Suzuki et al. 1995).
All of our piMRF muscimol injections produced upward saccade hypometria
(Fig. 4B). After unilateral riMLF lesions there was an
~50% reduction in the amplitude of downward saccades. A
similar reduction has also been observed after muscimol injections in
the INC (Crawford and Vilis 1993
). We compared results
of riMLF lesions to the inactivation of the piMRF by performing a curve
fit of the saccade velocities of the Suzuki et al.
(1995)
data and derived saccade amplitude by integration
(results not shown). The amplitude of upward saccades was normal,
whereas that of downward saccades was reduced by 30% and duration was
modestly increased (Suzuki et al. 1995
). These effects
on downward saccades following riMLF lesions are quite different from
the 50% reduction of vertical saccade amplitude noted following the
current muscimol injections into the piMRF (e.g., Fig. 1).
Three pieces of evidence eliminate the possibility that the vertical
hypometria demonstrated here was the result of inactivation of the INC.
First, both up and down saccades [except for s0714 and c0430, Fig. 4
(summary), which initially affected upward saccades only] quickly
became hypometric. This would be atypical for INC inactivation because
it affected downward saccades to a greater degree than upward saccades
(Fukushima and Kaneko 1995; Helmchen et al.
1998
). Second, the reduction in vertical saccade amplitude described previously after INC inactivation occurred after bilateral, not unilateral inactivation. Third, all reported INC inactivations were
associated with difficulty with vertical saccade holding, i.e.,
position-dependent, postsaccadic drift. In all six peri-INC MRF
injections, vertical saccade hypometria occurred before the development of position-dependent, postsaccadic vertical drift (Table
1, Fig. 3). Taken together, the above analysis provides strong evidence
that a critical region of the rostral MRF, lateral to the INC and riMLF
and rostral to the posterior commissure, was responsible for the
vertical saccade hypometria. This area, as defined by the centers of
all injection sites, has been projected onto a single rostral MRF
section. It encompasses a region 2 mm wide, 2 mm deep, and 2.5 mm in
rostral-caudal dimensions and is designated the peri-INC MRF (piMRF,
Fig. 4A, dotted region).
Unilateral or bilateral muscimol injections
Before exploring the functions of the piMRF further, we provide a
number of observations that suggest that all six muscimol injections
were unilateral. First, the onset of vertical saccade hypometria was
very rapid requiring <20 min following rostral injections to become
apparent. Second, the histological sections show that only one of the
tracks (s0721) crossed the midline within the brain stem. Third, the
rate of spread of the muscimol was directly measured in control
experiments (see accompanying paper, Waitzman et al.
2000), and a spherical region of inactivation 2 mm in size was
found 3 h after the injection. Inactivation of MLBNs in the riMLF
produces downward saccade hypometria and an ipsitorsional shift in
Listing's plane (Crawford 1994
; Crawford and
Vilis 1992
, 1993
; Helmchen et al.
1998
; Suzuki et al. 1995
; Vilis et al.
1989
). Combined paralysis of upward and downward movements has
not been observed following unilateral inactivation or
destruction of riMLF (Crawford 1994
; Crawford and
Vilis 1992
, 1993
; Suzuki et al.
1995
; Vilis et al. 1989
). In a similar vein, inactivation of the INC quickly (within 15 min) generates a 50% reduction in vertical eye velocity and typically position-dependent, postsaccadic drift, the hallmark of damage to the vertical neural integrator (Crawford 1994
; Crawford and Vilis
1992
, 1993
; Helmchen et al. 1998
;
Suzuki et al. 1995
; Vilis et al. 1989
).
In the current experiments, clear vertical saccade hypometria was
evident 4-40 min before the appearance of vertical,
postsaccadic drift (Table 1).
Last, after the end of each experiment, the monkey's head was
released. In all experiments in which a prominent head tilt was
observed, it was contraversive (to the right shoulder following a left
side injection, Table 1) and not backward or to the ipsilateral side.
This suggested that unilateral, not bilateral inactivation of the brain
stem had occurred (Crawford 1994; Crawford and
Vilis 1992
, 1993
; Fukushima et al.
1987
). In sum, these results support the idea that unilateral
inactivation of the rostral MRF led to rapid deficits in vertical
saccades. This raises two questions regarding vertical eye movement
organization. First, if the INC and/or the riMLF were not responsible
for the observed changes in vertical saccades, how does inactivation of
the piMRF lead to a vertical gaze palsy and prolonged duration of
vertical saccades? Second, the prevailing dogma has stipulated that
bilateral inactivation or destruction of the cerebral cortex or brain
stem structures is required to produce significant impairment of
vertical saccades (Christoff 1974
; Christoff et
al. 1962
; Pasik et al. 1969
). How can a
unilateral inactivation of the rostral MRF with muscimol lead to
effects on both up and down saccades?
Role of piMRF in oculomotor control: anatomic connections
The anatomic connections and previously described physiological
characteristics of piMRF neurons (Handel and Glimcher
1997) suggest answers to both questions. Consider first saccade
durations and bilateral riMLF inactivation. The MLBNs in the riMLF
burst 20-30 ms before the onset of saccades with either upward,
downward, or torsional components of movement (King and Fuchs
1977
, 1979
; King et al. 1981
;
Vilis et al. 1989
). Despite the presence of both up and
down bursters on each side of the brain stem, both riMLFs must be
activated to produce purely vertical saccades (Bender and
Shanzer 1964
; Christoff et al. 1962
). To
generate horizontal saccades, a cascade of LLBNs in the SC and
elsewhere eventually activate MLBNs in the paramedian zone of the
pontine reticular formation (PPRF) (Hepp and Henn 1983
;
Moschovakis et al. 1996
). A loss of these LLBNs will
produce a reduction in the peak discharge of the burst in pontine MLBNs
and thus a stretched (longer duration) horizontal component. We
(Waitzman et al. 1997a
) as well as others (King et al. 1981
; Kokkoroyannis et al.
1996
; Moschovakis et al. 1996
) have proposed a
similar physiological cascade for the vertical system. If this were so,
then the muscimol injections would reduce the peak discharge of the
LLBNs in the piMRF neurons and secondarily cause similar reductions in
the discharge frequency of the MLBNs in the riMLF bilaterally. This
would lead to prolonged but accurate vertical saccade components.
Despite the simplicity of this idea, direct connections between the
peri-INC LLBNs and the riMLF have not been demonstrated
(Buttner-Ennever and Buttner 1988
; Kokkoroyannis et al. 1996
; Robinson et al. 1994
).
At least three indirect, alternative paths exist that could carry LLBN
activity from the piMRF to MLBNs in the riMLF (Fig. 7A). A disynaptic pathway
from the piMRF to the SC and then to the riMLF (Chen and May
2000; May et al. 1997
) is the fastest route for
modulation of riMLF neurons by MRF LLBNs (pathway 1, Fig. 7A). These connections have been confirmed in both
the cat (Nakano et al. 1985
) and the monkey
(Kokkoroyannis et al. 1996
). A second pathway that may
include longer disynaptic or multisynaptic delays could be mediated via
bilateral projections from the peri-INC LLBNs to NRPo of the pontine
reticular formation (location of pontine LLBNs) as well as to raphe
interpositus (rip, omnipause neurons; pathway 2)
(Buttner-Ennever, personal communication). Both of these regions
provide dense ascending projections to the riMLF (Nakano et al.
1985
; Scudder et al. 1996
). A third route for
modulation of the riMLF output could be mediated via a piMRF
cerebellum
SC loop (Lefevre et al. 1998
). Cells in
the piMRF provide descending projections to RP, RO, and bilaterally to
the inhibitory burst neuron region of the medullary reticular formation just caudal to the PPRF (IBN-med RF; pathway 3). Raphe
pontis and obscuris also project to lobule VII and the flocculus of the cerebellum and are known to carry saccade-related signals
(Blanks and Precht 1983
; Blanks et al.
1983
; Langer et al. 1985
; Nakano et al.
1985
; Scudder et al. 1996b
). The flocculus is
also important in gaze holding and is critical in the adaptive control
of postsaccadic drift (Optican et al. 1986
; Zee
et al. 1981
). The primary cerebellar input to the SC arises
from the fastigial nuclei that receive a strong projection from the
flocculus and the cerebellar vermis, lobules VI and VII. Again like
pathway 1, the SC would mediate activity in this path.
Seen in this light, the role for the bilateral projections from the
piMRF to NRPo, medullary reticular formation, and raphe interpositus
(rip; pathway 2) is accentuated because it provides a
route for the generation of spontaneous saccades without the SC acting
as an intermediary. It is the loss of such bilateral inputs that would
be expected to produce the longer saccade durations observed after
piMRF inactivation.
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The answer to the question of reduced saccade amplitude is less clear. As noted earlier, changes in the input to the local feedback loop could produce saccade hypometria. This could arise via the pathway from the piMRF to the SC (pathway 1, Fig. 7A). Thus a shift in activity from a caudal to a more rostral portion of the SC could produce hypometric vertical saccades with normal duration. Changes within the feedback pathway while affecting saccade dynamics most often produce minimal reduction in saccade amplitude, the controlled variable. As a result, reduced LLBN output to MLBNs in the riMLF would yield longer duration, normal amplitude vertical saccades. However, if reduced LLBN output were coupled to changes in the triggering mechanism of saccades, significant reduction in saccade amplitude could result. For instance, suppose the omnipause neurons could more easily flip from on to off and vice versa. This would lead to shorter saccade latency, which was observed in four injections (Fig. 5C). In addition, the tendency to return to a tonic firing level would also increase. Reactivation of omnipause neurons is dependent on how close the saccade is to target (i.e., residual motor error). If reactivation occurred at larger residual error, saccades would be shorter because the pause would be truncated before the plant reached the target. Timed correctly, this could account for vertical saccades that fall short of their goal despite longer durations. Such effects could be mediated via pathway 2 leading from the piMRF to RIP and NRPo. Recording of activity in the NRPo and rip during piMRF inactivation would permit these ideas to be tested physiologically.
Role of the piMRF in oculomotor control: models
In addition to physiology and anatomy, simulation of the
one-dimensional eye displacement model (retinotopic coordinates) described previously was used to examine these hypotheses
(Jurgens et al. 1981; Waitzman et al.
1996
; see also accompanying paper, Waitzman et al.
2000
) (Fig. 7B). As noted above, reduction in the
amplitude of vertical saccades could occur in three different ways.
Either the desired change in eye displacement (
E, the
input to local feedback loop) could be reduced (Hypo #1),
the input to the MLBNs in the riMLF could be reduced in combination
with changes in saccade triggering (Hypo #2), or feedback
gain could be increased (Hypo #3). Changes in the
input to the feedback loop (Hypo #1) could occur by shifting
activity in the SC from a region coding large amplitude to a nearby
region coding small saccades. Typically, changes within the local
feedback loop do not affect the amplitude of saccades. However, we
provide two different "within-loop" scenarios that generate saccade
hypometria, and the resultant saccades have very different dynamics.
Hypothesis #2 suggests that reduction of the motor error
signal input (presumably from reduced LLBN output) to the MLBNs is
coupled to early reactivation of the omnipause neurons (i.e., removal
of the trigger) before the saccade reached the target. In this scheme,
saccade duration would most likely be lengthened because the eyes were
moving more slowly (reduced MLBN input). However, saccade amplitude
would be inaccurate because the omnipause neurons were reactivated
before the eyes reached the target. Long duration, but accurate
horizontal saccades have been observed clinically following damage to
the cerebellum (Zee et al. 1975
). In this situation, the
omnipause neurons were held off until motor error went to zero. In
Hypothesis #3, increased gain would produce a series of
smaller, but short duration saccades that eventually brought
the eyes onto target.
The results of simulating these ideas are shown in Fig.
8. A 50% reduction
in the desired change in position (E) resulted in a 50%
smaller, but normally executed saccade (i.e., Hypo #1, normal saccade dynamics, Figs. 7B and 8, A and
B,
). Within the feedback loop the motor error
input to the MLBNs of the riMLF was reduced by 80%, and the omnipause
neurons (i.e., the trigger) were reactivated when the saccade reached
within 1° (previously 0.1°) of the proscribed motor error
(Hypo #2, Fig. 7B). This produced a 60%
reduction in saccade amplitude and a 20% increase in saccade duration
(Fig. 8, C and D,
). However, desired
change in eye position (the input to the local feedback loop) remained
at its original value, leaving an unresolved motor error. This
generated a staircase of saccades that brought the eyes onto target
(Fig. 8E). This scenario corresponds closely with the
results of the injections shown here. Increasing the gain of the
feedback pathway by 50% (Hypo #3, Fig. 7B)
produced a 50% reduction in the amplitude of the initial saccade (Fig.
8, A and B). Again a staircase of smaller
saccades was generated to bring the eyes eventually onto target (Fig.
8E). However, each saccade of the staircase was of shorter
duration and slightly higher velocity then a saccade executed without
the change in the gain of the feedback pathway (Fig. 8, A
and B, - · - and
, respectively). Shortened
saccade duration was evident following one piMRF inactivation (Fig.
5B, s0714). Possibly this injection could have changed
feedback gain. On the other hand, four of six piMRF injections produced
hypometric, longer duration saccades. This suggested that
hypothesis #2, reduced LLBN activity coupled with changes in
saccade triggering, could best account for our observations.
|
MRF: participation in head control
Two findings in the current study extend the results of the
previous paper (Waitzman et al. 2000), suggesting a
strategic role of the MRF in the control of head movement and posture.
First, three of six injections resulted in a contralateral head tilt and postural instability (Table 1), albeit observed 2-3 h after the
muscimol injection. At the same time, there was a clear offset of the
initial position of the eyes to the ipsilateral side within 20 min of
all rostral injections (including that of g021494). We
suggest that this ipsilateral and often downward shift in eye position
would almost exactly compensate for a contralateral and upward head
tilt. A similar ipsilateral offset of primary position was noted in the
previous paper with the onset of a contralateral head tilt in the
head-free monkey (Fig. 14 in Waitzman et al. 2000
).
Taken together these observations suggest that inactivation of the
peri-INC MRF produced a compensatory ipsilateral shift in primary eye
position before the onset of inactivation of the INC.
Some controversy surrounds the exact role of the INC in head control
(Crawford 1994; Crawford and Vilis 1992
,
1993
; Crawford et al. 1991
;
Fukushima 1987
; Fukushima et al. 1987
). A
number of studies agree that there are ipsilateral descending
connections from the INC to the vestibular nuclei (Fukushima et
al. 1987
; Kokkoroyannis et al. 1996
;
Robinson et al. 1994
). However, there is disagreement
regarding the direct projections from the INC to the cervical spinal
cord. One study using retrograde transport from the cervical segments
suggested a sparse descending projection from the INC (Robinson
et al. 1994
). A more recent study using intracellular recording
and biocytin neuroanatomy demonstrated that INC neurons of the squirrel
monkey do terminate in the ipsilateral ventral horn of the upper
cervical segments (Kokkoroyannis et al. 1996
). However,
all studies demonstrate much larger ipsilateral projections from the
cMRF, peri-INC MRF, and cuneate reticular nucleus to the upper cervical
cord than from the INC (Castiglioni et al. 1978
;
Kokkoroyannis et al. 1996
; Robinson et al.
1994
; Scudder et al. 1996a
). This suggests that
lesions and reversible inactivation that have been directed at the INC
in the past may in fact have inhibited or damaged both the piMRF and
the INC itself. Therefore both the MRF and the INC may be important
sources of descending midbrain activity to the cervical cord related to
the head and postural control and may account for the contralateral head tilt and postural abnormalities noted following rostral MRF injections (Fukushima 1987
; Kokkoroyannis et al.
1996
; Robinson et al. 1994
).
Relationship to mesencephalic disorders of eye movement
PSP is a degenerative neurological disorder in which
neurofibrillary tangles are deposited in the reticular formation and basal ganglia (Steele et al. 1964). These patients
exhibit difficulties with balance, neck movement, and a progressive
restriction in voluntary vertical rapid eye movements. Eventually
patients also develop microsaccadic square-wave jerks and restricted
horizontal eye movements (Fensore et al. 1988
;
Gomez et al. 1990
; Hershkowitz et al.
1989
; Rafal et al. 1988
). Despite these
deficits, conjugate eye movements (including quick phases of nystagmus)
induced by vestibular activation remain intact
(Pierrot-Deseilligny et al. 1989
; Rottach et al.
1996
). Neuropathological changes target vertical and horizontal
eye movement regions of the brain stem, including the riMLF, the SC,
the PPRF, the MRF, but not the oculomotor nuclei (Juncos et al.
1991
; Steele et al. 1964
; Zweig et al.
1987
).
However, destruction of the riMLF or PPRF cannot explain the clinical
features of PSP, because the quick phases of vestibular and optokinetic
nystagmus persist and are mediated by these same structures. Bilateral
removal of the riMLF in both patients and nonhuman primates impairs
all vertical rapid eye movements, i.e., both saccades and
vestibularly induced quick phases (Vilis et al. 1989).
Similarly, destruction of the PPRF on one side abolishes ipsilateral,
horizontal saccadic eye movements and the quick phases of
vestibular nystagmus, whereas smooth pursuit and the slow component of
vestibular nystagmus are preserved in the horizontal direction (Goebel et al. 1971
; Henn et al. 1984
).
These lesion studies suggest that other pontomesencephalic areas are
targeted to produce the characteristic oculomotor findings observed in
patients with PSP.
One of the curious features of PSP is the appearance of vertical eye
movement abnormalities initially that are followed by impairments in
horizontal saccades and the generation of square-wave jerks. These
characteristics are noted in the monkey experiments presented here and
in the accompanying paper (Waitzman et al. 2000).
Initially, the effects of rostral MRF inactivation were exclusively on
vertical eye movements. However, horizontal deficits appeared as the
muscimol injection spread with an inability of the monkey to make
contraversive saccades across the midline. Interestingly, as pointed
out in the accompanying paper (Waitzman et al. 2000
)
macrosaccadic square-wave jerks and a contraversive head tilt appeared
quickly after many caudal injections (e.g., g0217 and c0521)
(Waitzman et al. 2000
). Thus the vertical hypometria of
PSP patients could result from loss of the more rostral MRF, whereas
increased neck tone, imbalance, and square-wave jerks could be
secondary to loss of more caudal portions of the MRF. Taken together,
these two groups of injections suggest that patients with PSP could
have neuropathological involvement that begins rostrally and then
proceeds caudally within the MRF. Neuropathological study of patients
with documented PSP at various stages of the illness could help
determine whether such a hypothesis were true.
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ACKNOWLEDGMENTS |
---|
The authors thank Dr. Douglas Oliver for the use of computing and laboratory facilities, as well as for expertise with fluorescent bead injections, neuroanatomy, and histology. The comments of Drs. Wolfson and Duckrow were instrumental in editing an early version of this manuscript. Discussions with Dr. Robert Wurtz and postdoctoral fellows of the Laboratory of Sensorimotor Research (LSR) have been invaluable in reworking this paper. Discussions with Dr. Lance Optican of the National Eye Institute (NEI)/LSR were instrumental in framing the ideas for saccade hypermetria (i.e., damage to the resettable integrator) and hypometria.
This work was supported by NEI Grant EY-09481 and a Research Advisory Group grant from the Office of Medical Research, Department of Veterans Affairs.
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FOOTNOTES |
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Address for reprint requests: D. M. Waitzman, Dept. of Neurology, MC 3974, The University of Connecticut Health Center, 263 Farmington Ave., Farmington, CT 06030.
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 9 August 1999; accepted in final form 8 October 1999.
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REFERENCES |
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