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. I. Hypermetric Goal-Directed Saccades. J. Neurophysiol. 83: 2260-2284, 2000. Single-neuron recording and electrical microstimulation suggest three roles for the mesencephalic reticular formation (MRF) in oculomotor control: 1) saccade triggering, 2) computation of the horizontal component of saccade amplitude (a feed-forward function), and 3) feedback of an eye velocity signal from the paramedian zone of the pontine reticular formation (PPRF) to higher structures. These ideas were tested using reversible inactivation of the MRF with pressure microinjection of muscimol, a GABAA agonist, in four rhesus monkeys prepared for chronic single-neuron and eye movement recording. Reversible inactivation revealed two subregions of the MRF: ventral-caudal and rostral. The ventral-caudal region, which corresponds to the central MRF, the cMRF, or nucleus subcuneiformis, is the focus of this paper and is located lateral to the oculomotor nucleus and caudal to the posterior commissure (PC). Inactivation of the cMRF produced contraversive, upward saccade hypermetria. In three of eight injections, the velocity of hypermetric saccades was too fast for a given saccade amplitude, and saccade duration was shorter. The latency for initiation of most contraversive saccades was markedly reduced. Fixation was also destabilized with the development of macrosaccadic square-wave jerks that were directed toward a contraversive goal in the hypermetric direction. Spontaneous saccades collected in total darkness were also directed toward the same orbital goal, up and to the contraversive side. Three of eight muscimol injections were associated with a shift in the initial position of the eyes. A contralateral head tilt was also observed in 5 out of 8 caudal injections. All ventral-caudal injections with head tilt showed no evidence of vertical postsaccadic drift. This suggested that the observed changes in head movement and posture resulted from inactivation of the caudal MRF and not spread of the muscimol to the interstitial nucleus of Cajal (INC). Evidence of hypermetria strongly supports the idea that the ventral-caudal MRF participates in the feedback control of saccade accuracy. However, development of goal-directed eye movements, as well as a shift in the initial position following some of the cMRF injections, suggest that this region also contributes to the generation of an estimate of target or eye position coded in craniotopic coordinates. Last, the observed reduction in contraversive saccade latency and development of macrosaccadic square-wave jerks supports a role of the MRF in saccade triggering.
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INTRODUCTION |
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Three possible oculomotor roles have been
suggested for the central mesencephalic reticular formation (cMRF)
(Waitzman et al. 1996). Subthreshold low-frequency
electrical microstimulation and single neuron recording of a
low-frequency, long-latency (15-100 ms) discharge before saccades
support the idea that the cMRF participates in saccade triggering
(Cohen et al. 1985
, 1986
;
Handel and Glimcher 1997
; Waitzman 1982
,
1992
; Waitzman et al. 1996
). Second,
existence of cMRF neurons with contralateral movement fields that
increase their discharge with the horizontal but not the vertical
component of movement suggests that these cells could serve as a
spatial filter extracting the horizontal component of movements from
the superior colliculus (SC) output (Sparks 1986
;
Sparks and Mays 1990
; Waitzman et al.
1996
). Cells in the rostral portion of the MRF (see
accompanying paper) may participate in the generation of the vertical
component of saccadic eye movement (Handel and Glimcher
1997
). Third, by virtue of a burst of activity that peaks just
before and during saccades and dynamics of the neural discharge that
correlate closely with either eye velocity and/or displacement, we have
hypothesized that cMRF neurons could participate in the feedback
control of saccades (Waitzman et al. 1996
).
The impact of each of these hypotheses on saccade generation is
illustrated with the help of two feedback models of oculomotor control
(Fig. 1). The eye position
model shown in Fig. 1A was based on the original,
local-feedback model of Robinson (1975). Current eye
position in space (Eye) was subtracted from target position in space
(Targ) by the retina, to produce a retinal error signal
(Rerr). Robinson's major contribution
was to suggest that retinal error was added to an internal copy of eye
position (i.e., efference copy, or corollary discharge, E') in
craniotopic coordinates to create an estimate of target position with
respect to the head not the retina (Tarest). In a
subsequent step, efference copy (E') was subtracted from a delayed copy
of target position to generate a motor error signal
(em) used to drive the burst neurons in the pontine reticular formation (B). Integration of the velocity output of the burst neurons (Vc) by the neural integrator
(NI) produced an eye position signal used to drive the ocular
motoneurons. Two unique properties emerged from this model. First, by
virtue of local feedback, burst output continued for as long as
necessary to get the eyes onto the target and explained many aspects of the relationship between saccade amplitude and duration. Second, the
input to the oculomotor system was a target position with respect to
the head signal. This property in particular made it easy to
incorporate vestibular inputs (Robinson 1975
). However, since its proposal, a number of objections have been raised to this
model and question its applicability to the oculomotor system. One
primary concern has been that few regions of the brain contain eye
position activity [i.e., nucleus prepositus hypoglossi (NPH) and the
interstitial nucleus of Cajal (INC)]. More importantly these regions
do not project back to areas such as the SC, which should receive
feedback of this efference copy of eye position.
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The eye displacement model shown in Fig. 1B
addressed these issues by placing a resettable integrator (RI) into the
local feedback pathway. This modification transformed the inputs of the
model into retinotopic coordinates (Jurgens et al. 1981;
see Waitzman et al. 1991
, 1996
for
further discussion of this model). Briefly, the input to the model,
desired eye displacement (
E), was thought to arise from
the frontal eye fields (FEF) and dorsomedial frontal cortex (DMFC). The
desired displacement was compared with current eye displacement
(
E') to produce a motor error (em) that was thought to reside in the long
lead burst neurons (LLBNs) of the paramedian zone of the pontine
reticular formation (PPRF). This motor error signal was then relayed
through a switch (controlled by a trigger signal) to the medium-lead
burst neurons (B) in the PPRF. The output of the burst neurons was a velocity command (Vc) that was
directed to both the NI and a RI (
E' or the efference copy) whose
output was reset to zero at the end of each saccade. The purpose of the
NI was to hold the eyes steady following the occurrence of each saccade
while the output of the RI was used to update higher structures of the
current displacement of the eyes. The NI for the horizontal
saccade component is generated in the NPH (Cannon and Robinson
1987
), and the NI for the vertical component of
saccades is thought to originate from the INC (Crawford et al.
1991
). The source of the trigger signal used to initiate
saccades is thought to be the omnipause neurons located in the nucleus
raphe interpositus (RIP).
Predictions about the specific oculomotor deficits, which may occur
after inactivation of brain stem structures, are easier to understand
by reference to these models. Shifts in the input to either model, that
is a more distant orbital position (EP model), or larger eye
displacement (ED model), would result in saccades that overshoot the
goal (Fig. 1, A and B, Hyper
#1). A shift in input could occur if cMRF neurons performed a
spatial filter role for the SC and FEF output (Sparks
1986; Sparks and Mays 1990
; Waitzman et
al. 1996
). Simulations of these various aspects of the models
are presented in the DISCUSSION.
Reduction or damage to the pathways within the feedback
loop would eventually produce a reduction in either the current eye position (EP model) or eye displacement (ED model) feedback signals. This reduction would increase the duration of the motor error signal
(em), and the eyes would continue to move beyond their goal, albeit at a slower velocity (Fig. 1, A and
B, Hyper #2). Thus, in the ED model if
the reticulotectal, long lead burst neurons (RTLLBNs) of the cMRF
provide a conduit for a velocity signal from the PPRF to the SC, or
participate in the process of integrating eye velocity (i.e., the RI),
loss of these cells should produce saccade hypermetria. This result
would correlate well with the feedback hypothesis (Waitzman et
al. 1996). However, damage to the feedback mechanism of the ED
model could not produce a change in initial eye position or generate a
saccade goal.
Reduction of feedback or damage to the neural integrator itself in the
EP model would also produce hypermetric, slow saccades (Fig. 1A,
Hyper #2). However, in this instance, shifts in initial position and generation of a saccade goal relative to the head could
result. Moreover, damage in the second portion of the feedback pathway
of the EP model (Fig. 1A, Hyper #3) could increase
delays in the generation of the Tarest and cause repeated
saccades to a virtual target that continues to reappear (see
DISCUSSION). Finally, making the saccade trigger easier to
flip from opened to closed and vice versa could make saccade latency
shorter. This might occur if excitatory activity from cMRF neurons
important for maintaining the tonic firing of omnipause neurons was
removed (i.e., the triggering hypothesis) (Cohen et al.
1985; Hepp and Henn 1982
, 1983
;
Waitzman et al. 1996
). Providing clear
neurophysiological evidence to support each of these hypotheses of MRF
participation in oculomotor control has proven difficult. The midbrain
tegmentum contains both cells and fibers in passage from the superior
colliculus and other structures. As a result, the destruction or
activation of the collicular output may have biased previous
electrolytic lesion and electrical microstimulation experiments
(Cohen et al. 1982
, 1985
,
1986
; Komatsuzaki et al. 1972
).
The current group of experiments has been designed to circumvent some
of these difficulties. Following electrical microstimulation and single
and multiunit identification of the MRF, we have made microinjections
of muscimol, a GABAA agonist. We demonstrate that the MRF
can be divided into two separate regions. Inactivation of a
ventral-caudal region, which corresponds to the nucleus subcuneiformis (the cMRF), leads to oblique (contraversive and up) saccade
hypermetria, higher saccade velocity, reduced saccade duration, and
marked instability in fixation with the development of macrosaccadic square-wave jerks to a specific goal in the orbit (the current paper).
Inactivation of the rostral portion of the MRF results in severe
hypometria primarily of vertical, but not horizontal saccades (see
accompanying paper, Waitzman et al. 2000). The
implications of these findings are discussed with reference to the two
models and three possible hypotheses for cMRF function just presented. Abstracts of these findings have appeared previously (Silakov and Waitzman 1996
; Waitzman and Silakov 1994
;
Waitzman et al. 1997
).
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METHODS |
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The methods for recording eye movements and single neurons,
electrical microstimulation, and data analysis in awake behaving primates in these experiments are essentially the same as those described in detail elsewhere (Waitzman et al. 1991,
1996
). All procedures were approved by the University
Animal Care and Use Committee.
Injection and recording procedures
In brief, four male rhesus monkeys (G, C, K, and
T) were surgically prepared under isoflurane inhalational
anesthesia with two eye coils (Judge et al. 1980), a
head restraining device, and two stainless steel chambers to allow
separate access to the MRF and the SC. The MRF cylinder was positioned
over the posterior portion of the cerebral cortex tilted 15° off the
sagittal plane (Waitzman et al. 1996
). The MRF, located
just lateral to the oculomotor nuclei, was identified by the
characteristic features of single neurons that discharge with
contraversive saccadic eye movements and electrical microstimulation
that elicited contraversive, conjugate saccades at short latency
(Silakov et al. 1995
; Waitzman et al. 1996
). Eye movements were recorded using the magnetic search
coil technique and were accurate to 0.1° (Judge et al.
1980
). In two monkeys, a series of guide tubes were placed
parallel to each other and sampled the rostral, mid, and caudal
portions of the MRF. The tubes were semipermanently positioned using a
grid (spacing of 1 mm) fixed within the stainless steel recording
chamber. In the third and fourth monkeys, only the caudal portion of
the MRF was sampled. The arrangement of a rostral-caudal orientation of the guide tubes allowed for repeated testing and subsequent permanent identification of the sites of muscimol injections. A customized microinjection/recording needle (Crist et al. 1988
)
attached to a Hamilton syringe allowed for physiological confirmation
of neuronal activity related to saccades before an injection and
monitoring of neuronal activity after the injection. In monkey
T, a picospritzer apparatus was substituted for the Hamilton
syringe (Dias and Segraves 1997
).
Behavioral paradigms
Figure 2A shows the
fixation paradigm, and Fig. 2, B-D, illustrates the
visually guided saccade (VGS) paradigm. Visual targets were positioned
at eight different directions [0° (position 0), 45° (1), 90°
(2), 135° (3), 180° (4), 225° (5), 270° (6), and 315° and/or
45° (7)] and five amplitudes (5, 10, 15, 20, and 25°) along each
of these directions for a total of 40 different target locations. The
monkey was trained to fixate a central light-emitting diode. After a
variable interval of 200-400 ms, the light was extinguished, and a new
target light appeared that was the cue for the monkey to shift his eyes
and fixate the new visual target (15° saccades are shown in Fig.
2B and 20° in Fig. 2C). The monkey was rewarded
for moving the eyes to within ±2° window of the visual target. After
the injections this window was relaxed to ±7° and in some cases
±12° so that all attempted saccades to the visual target would be
collected. The trajectories in each direction of Fig. 2C
show five repetitions. Note the regularity, accuracy, and straightness
of the trajectories. Following a control injection of saline in another
monkey, the trajectories of the saccades were unchanged from baseline
(compare Fig. 2D, saline, to Fig. 4A, same monkey
25° saccades, no injection). Filled circles show the average of all
endpoints of control saccades to the same visual target. Saccades of
five different amplitudes (5-25°; 8 randomized directions × 5 repetitions of each saccade + errors) were collected into separate
files for each injection. Each file took ~6-15 min for the monkey to
complete. A "complete" set of data covering all 40 positions was
comprised of 5 files (1 for each amplitude, total collection time of
30-50 min). The amplitudes sampled during the first two or three files
were repeated at the end of the sequence to document changes that
occurred while the drug had diffused.
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Data analysis
Following each experiment raw eye movement records were
processed by software that identified the beginning and end of each eye
movement using a template matching algorithm (Waitzman et al.
1991). Each trial was visually inspected, and marks indicating the beginning and end of horizontal and vertical components of each
saccade were corrected as needed. Corrective saccades following the
primary movement were specifically excluded from the current analysis.
Variations in saccade amplitude and direction following the injections
were evaluated in a number of ways. One analytic technique calculated
the fractional change in saccade amplitude and direction following the
injection (Fig. 4D). A "difference coefficient" for each
of these metrics was calculated by taking the difference between post-
and pre-parameters and then expressing this as a fraction of the
prevalue. This technique effectively normalized the data so that
changes in eye movements of different amplitudes or directions could be
compared. A negative value for the difference coefficient for amplitude
(Diff. Amp.) indicated saccade hypometria, and a positive value
reflected saccade hypermetria. Difference coefficients were plotted
against target direction. In the analysis of changes following an
injection, we also tested the "null" hypothesis that saccades in a
particular direction were not deviated from their normal trajectory
(Fig. 4E). If direction was not modified, then the direction
at saccade end should be no different from target direction. The
absolute difference between the angle at saccade end and target
direction was the saccade deviation that was plotted as a function of
target direction. If the deviation was positive (i.e., the
postinjection angle was larger than target direction), then by
definition this was plotted as a counterclockwise deviation (CCW), and
if the deviation was negative (i.e., postinjection angle smaller than
target direction), then this was scored as a clockwise (CW) deviation.
Two midbrain structures could be influenced by inactivation of the MRF
by muscimol: 1) the nuclei of the optic tract (NOT) and
2) the INC. Contraversive slow phases of nystagmus develop after inactivation of the NOT, and position-dependent vertical postsaccadic drift occurs after inactivation of the INC (Cohen et al. 1992; Crawford and Vilis 1993
;
Crawford et al. 1991
). To calculate the slow-phase eye
velocity, instantaneous eye velocity was averaged from the end of the
current saccade to just before the beginning of the subsequent saccade.
This was done for the horizontal component of all spontaneous saccades
(in total darkness) that occurred just before the paradigm began
(including control injections of saline). The horizontal slow-phase eye
velocity plotted for a single time point represented the average of all intersaccadic intervals for a particular file (~70-100 movements per
file spanning 5-10 min). Time points were collected starting just
before the injection and for each subsequent file following each
injection until recording ended.
Drift amplitude [the amplitude of slow movement from saccade offset to
the end of the drift as per Crawford and Vilis (1993)] was measured for at least 10 spontaneous eye movements occurring in
each file. A running average (Student's t-test) was used to decide when significant vertical drift had occurred.
Duration is directly proportional to vectorial amplitude for pure
horizontal saccades (Fuchs 1967). However, for oblique
saccades component stretching occurs to produce saccade trajectories
that are straight. As a result, component (horizontal or vertical) duration is proportional to vectorial amplitude (King et al.
1986
) and is used to display the duration data here. Comparison
of the slopes of saccade duration versus vectorial amplitude was made by t-test to determine whether a change in component
duration had occurred after muscimol injections. In a similar fashion, the log relationship between vectorial amplitude and velocity (Fuchs 1967
; King et al. 1986
)
was compared before and after muscimol to decide whether saccades had
been displaced off this main sequence.
Histology
Once all data were collected and the most productive eye movement regions identified, a pressure injection of 1-2 µl of fluorescent labeled microspheres (green and red, LumaFluor, ~0.05 µm diam; blue, Polyscience, BB19773, 0.05 µm diam) was made to positively localize the sites of microinjection in three monkeys. The location of the electrode tracks in one monkey was identified by placement of a small electrolytic lesion. At the conclusion of the experiments, monkeys were deeply anesthetized with pentobarbital sodium and perfused. The brains were removed, and 50 µm vibratome sections were made through the brain stem. Unstained sections (with fluorescent beads) were mounted wet and photographed under both white and fluorescent light. Alternating sections were stained with thionin and drawn onto paper using an inverted microscope. Drawings were then scanned into the computer and traced to produce the final anatomic representation of injection sites.
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RESULTS |
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Neuronal effects of muscimol: areas of inactivation
Eight injections of the GABAA agonist muscimol were made in four monkeys at sites in the MRF where eye movement-related cells were recorded (Table 1). Of the eight injections, seven were made in head-fixed animals, and these injections were used to summarize the effects of muscimol inactivation. The eighth injection was done in the head-free animal to demonstrate the interaction between head and eye initial position shift. Besides these eight injections, two injections of inactive muscimol (determined empirically) produced no changes in eye movements and were used as controls. Changes in eye movements were noted as early as 5 min after a 1.0 µg injection of muscimol (Sigma, 0.5 µg/µl in sterile NaCl) into the MRF and could last for up to 7 h. Typically, electrical silence was noted at 20-30 min, and thus early time points were repeated after this initial inactivation period. Data collection began with the start of the injection and continued for as long as the monkey could perform the behavioral tasks.
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We made parallel tracks 1 and 2 mm away from a 1.0 µg/µl muscimol injection in one monkey. Data from these tracks showed that the blocked region (electrical silence) extended no greater than 1.5 mm laterally from the site of injection. Cell activity 2 mm lateral to the injection was normal. Monitoring of activity above and below this site demonstrated a vertically blocked region of 2.5 mm above the site of the injection. No eye movements could be elicited from within the blocked region using electrical microstimulation at 3 times threshold, but eye movements could be elicited below the blocked region. Twenty-four hours later, neuronal activity in the blocked area had recovered, electrical microstimulation could elicit saccades, and eye movements had returned to normal. These experiments suggested that an injection of 1.0 µg/µl of muscimol inactivated an ellipsoid portion of the brain stem 2.2 mm in diameter and 1.5-2.5 mm in length. After a control injection of saline, neuronal activity was suppressed for ~3 min (Fig. 2D, monkey G), but returned to normal levels within 5-10 min. Following this control injection, saccades to the eight different target positions located 20° from primary position were straight, accurate, and thus unaffected by the injection (Fig. 2D).
Our initial hypothesis was that the MRF [corresponding to
nucleus cuneiformis and nucleus subcuneiformis of Olszewski and Baxter (1954)] was physiologically a homogeneous region. As
our experiments progressed, it was clear that some division of the "MRF" was necessary, because the effects on eye movements were quite different if injections were made rostral or caudal to the posterior commissure. Specifically, an analysis of caudal injection sites showed that oblique, upward, contraversive saccades became hypermetric (Fig. 3, all caudal
injections), whereas vertical saccades became hypometric after rostral
injections (Fig. 4A of accompanying paper, Waitzman
et al. 2000
).
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Inactivation of the ventral-caudal MRF (the cMRF): synopsis
Muscimol inactivation of the ventral-caudal MRF produced seven primary oculomotor effects in the monkey: 1) hypermetria of contraversive oblique saccades; 2) reduced saccade latency; 3) a moderate reduction in saccade duration, with an increase in saccade velocity following many injections; 4) repetitive macrosaccadic square-wave jerks to a specific goal in the orbit; 5) spontaneous saccades in the dark directed toward the same specific goal in space (relative to the head); 6) straight trajectories of saccades directed toward the orbital goal and curved trajectories of saccades directed toward adjacent locations to the goal; and 7) contraversive head-tilt following five of eight ventral-caudal injections.
These effects were consistent across monkeys and did not occur after inactivation of adjacent locations in the brain stem. The effects on saccade metrics (duration, velocity, and latency) will be illustrated for seven injections. The eighth injection was made in a monkey free to move its head, and thus the data for saccade metrics were not comparable to the head-fixed case. Careful examination of saccade duration will point to which portions of the oculomotor models could account for the observed changes. No change in saccade duration would suggest the input to the local feedback loop had shifted, whereas increased duration would suggest loss of feedback. Shorter duration and higher saccade velocity suggest a combination of effects on model parameters.
Analysis of square-wave jerks and the goal-directed nature of postinjection saccades are presented for all injections made in head-fixed animals. Each of these injections had a different goal to which spontaneous saccades were directed repeatedly. Data from two injections will be presented in detail, one in the left and the other in the right MRF. The rest of the data are presented in summary format to illustrate the range of effects observed.
Inactivation of the cMRF: changes in saccade metrics
Seven injections (c0416, c0419, c0521, g0217, k0329, k0331,
and k0403) placed into the caudal MRF of three head-fixed monkeys produced hypermetric saccades. The results of one muscimol injection (1 µg/2 µl) placed at the site of cMRF long-lead burst neurons that
discharged before contraversive (rightward) saccades is shown in Fig.
4. Multiunit contraversive eye movement
related activity was registered through the recording syringe at a
similar depth at which the single cells of Fig. 4F (movement
fields) had previously been recorded. Electrical microstimulation was
not performed at this site. Within 5 min after the end of the injection
(duration of 20 min), 25° saccades up and to the contraversive side
became hypermetric (Fig. 4B, positions 1 and 2;
, averaged endpoints of control saccades). During the next hour of
observation (5-30 min shown) all visually guided saccades up and to
the right became hypermetric (Fig. 4, B-D). This
hypermetria affected the vertical more than the horizontal component of
movement (Fig. 4D). For a 15° oblique saccade the
horizontal component of movement was increased by 20%, whereas the
increment in the vertical component approached 50% (Fig.
4D, compare
with
).
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There was a counterclockwise, upward deviation of the endpoints of contraversive, horizontal (position 0), and oblique, upward saccades (position 1, 45°) after this injection. The endpoints of pure upward movements (position 2, 90°) were deviated downward (i.e., negative direction, clockwise in Fig. 4E). A similar, albeit smaller reversal of saccade endpoint deviation occurred between positions 5 (225°) and 6 (270°; Fig. 4E). These reversals of saccade deviation correspond with zero crossings from counterclockwise to clockwise (Fig. 4E, arrows). This defined a plane tilted ~25° from the vertical toward which saccade endpoints were deviated (Fig. 4B). This plane also influenced the trajectory of the saccade. Saccade trajectories close to the plane remained almost straight, whereas saccade trajectories in other directions became curved. For example, the trajectories of upward vertical saccades to position 2 were bowed away from this plane (but their endpoints were closer to the plane), whereas the trajectories of oblique saccades to position 1 and those of horizontal saccades were bowed upward toward this plane. Similarly, downward saccades to position 6 were bowed away from the tilted vertical plane (Fig. 4B). Such curvature suggests discoordination in the generation of the horizontal or vertical components of the saccade such that the vertical component reached peak velocity before the horizontal component.
Details of the upward saccade hypermetria following this injection (g0217) are shown in Fig. 5. Hypermetric oblique saccades (position 1) had an increase in peak velocity compared with preinjection movements (Fig. 5A, horizontal; Fig. 5B, vertical). Saccades in the opposite direction (down and to the left, position 5) were only slightly hypermetric (Fig. 5D). In both cases (position 1 and 5), the amplitude and velocity of the vertical component was affected more than the horizontal component. In fact, horizontal component amplitude for ipsiversive saccades (position 5) was slightly hypometric (Fig. 5C, solid line). The overall increases in vectorial peak velocity for both directions (positions 1 and 5) were matched by a commensurate increase in saccade amplitude and duration. As a result, these postinjection saccades remained on the amplitude versus peak velocity main sequence (Fig. 5E, Table 2, P > 0.05).
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The increased amplitude of postinjection saccades was matched by a commensurate increase in horizontal saccade duration. This maintained the same linear amplitude-duration relationship as before injection [Fig. 6A, slopes (m) not different]. However, duration of the vertical saccade component was longer than the associated vectorial amplitude would have required (Fig. 6B), while the slope of postinjection vertical duration versus amplitude relationship rose. The difference in slope did not reach statistical significance (see Fig. 10C). On the other hand, the latency to onset for saccades of all amplitudes was significantly reduced following this injection. Contraversive, upward saccades were initiated the fastest and some latencies (150 ms) approached that of express saccades (Fig. 6C).
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To determine whether muscimol had spread to include the NOT, dorsal to
the MRF, horizontal slow-phase eye velocity (slow movements between
saccades) was calculated after the injection (see METHODS) (Cohen et al. 1992). Control preinjection files
demonstrated <3°/s of contraversive, slow-phase eye velocity (Fig.
6D,
). Following this muscimol injection, no
contraversive horizontal nystagmus was found (Fig. 6D,
).
These results suggested that the changes in saccade amplitude,
velocity, and latency could not be accounted for by spread of the
muscimol to involve the nucleus of the optic tract.
Inactivation of the cMRF: square-wave jerks and changes in initial position
At the end of 25 min after the injection, the monkey developed pronounced contraversive, upward macrosaccadic square-wave jerks (Fig. 7). The requirement of this particular paradigm was for the monkey to maintain stable fixation (Fig. 7, control, dotted line: see also Fig. 2A). After the injection the monkey made repeated saccades that were in the same direction as the previously described hypermetria. Each eye movement was separated by a minimal intersaccadic time interval of 150-200 ms. These movements were very stereotypic and brought the eye to a specific location in the orbit (Fig. 8A).
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Although changes of visually guided saccades with shifts in initial
position were not specifically studied in this monkey, spontaneous
saccades made in total darkness were collected just after this fixation
file. The trajectories of the spontaneous saccades (whose vectors are
shown in Fig. 8B) demonstrate that the eyes were directed to
a specific goal in the orbit located 13° to the right and
19° up (error bars are ±1 SD). We compared an average of the
endpoints of the macrosaccadic square-wave jerks from the fixation
paradigm with the location of the endpoints of all of the spontaneous
saccades and found an extensive overlap (compare rectangular boxes in
Fig. 8, A and B). The dependence of postinjection
spontaneous saccades on initial eye position was assessed by
calculation of an "orbital perturbation index." This reflects the
slope of the regression line relating component saccade amplitude with
initial position (Russo and Bruce 1993). The indexes for
horizontal and vertical saccade components were markedly elevated,
supporting a strong effect of initial eye position on saccade
amplitude. In summary, this injection demonstrated that inactivation of
the cMRF was critical for the generation of saccade hypermetria. Within
1 h of this injection the monkey generated repetitive
macrosaccadic square-wave jerks that brought the eyes to a specific
goal in the orbit. Two hours after the injection, the monkey's head
was released and a contraversive head tilt was noted.
Six other cMRF sites in two additional monkeys produced essentially the
same results but to varying degrees. The results of an injection on the
opposite side of the brain stem of another monkey are shown in Fig.
9. The first visually guided saccades were collected 88 min after the injection (Fig. 9B). The
monkey could still make 25° saccades in all directions; however, the initial position of the eyes was displaced up and slightly to the
contraversive side. Downward saccades were hypometric missing the
target, even when the shift in initial eye position was taken into
account. Contraversive (left) upward saccades were displaced clockwise
toward the earth vertical. Thirty minutes later (127 min
postinjection), saccades intended for the 5° contraversive target
position (position 3) were markedly hypermetric and drawn toward a
specific position in the orbit (Fig. 9C). Other
contraversive movements were either very hypermetric or could not be
generated. Ipsiversive 5° saccades were normal. Fixation was
persistently interrupted by macrosaccadic square-wave jerks toward a
contraversive upward goal (x = 11.4°;
y = 19°, Fig. 9D). One hundred forty-two minutes after the end of the injection, spontaneous saccades were directed toward this same location, 11.8° left and 19.4° up (Fig. 9E). Slopes of the regression of initial eye position on
component saccade amplitude were markedly elevated. This confirmed a
strong effect of initial position on the amplitude and direction of
spontaneous saccades (Kh = 0.61;
Kv =
1.6). Last, postinjection
saccades (particularly those to position 3) were displaced above the
main sequence relating vectorial amplitude and velocity. However, the slope of the log regression for all postinjection saccades while higher
did not reach statistical significance compared with preinjection control (Table 2). This result would not be expected from displacement of the saccade input (Hyper #1, Fig. 1), or interruption of
the feedback loop (Hyper #2, Fig. 1), which would have left
saccade velocity normal or lower, respectively.
|
Summary of results: caudal injections
All but one muscimol injection showed a significant reduction in saccade latency (Fig. 10). The higher control latency for monkey C was most likely the result of difficulty with down gaze in the right eye (the recorded eye) following surgical procedures to correct tethering of the eye-coil in this eye. However, the reduction in latency was significant, and control latency returned back to 600 ms the day following the c0416, c0419, and c0521 injections. Taken together, these latency results suggest a role for the ventral-caudal MRF in saccade initiation and maintenance of fixation.
|
Because of the differential effect on saccade velocity following the two injections shown in detail, we also examined the duration of the saccades for each of the seven injections. Note that in six of seven injections, horizontal saccade duration was reduced and in five of the seven injections, vertical saccade duration was reduced compared with control. These trends reached significance in only two of the horizontal and one of the vertical measurements (Fig. 10, B and C). This corresponded closely to an increase in saccade velocity that was higher than expected for amplitude and positioned these movements above the main sequence. These results suggest that inactivation of the ventral-caudal MRF could influence portions of the saccade burst generator.
Effects of muscimol inactivation of the caudal MRF on the amplitude of
postinjection saccades are shown in Fig.
11. To compare the degree of
hypermetria of one injection to that of another, the difference
coefficient data were plotted so that the direction of hypermetria was
up and to the right (i.e., 45°). Thus all injections were displayed
as if they had occurred on the left side of the brain stem. Difference
coefficients for the time point for which the monkey demonstrated the
greatest hypermetria but was still capable of making saccades in all
other directions are illustrated. All MRF injections caudal to the
posterior commissure produced contraversive saccade hypermetria, albeit
to a small degree in two injections (k0329 and k0403). The direction of
saccade hypermetria was primarily up and to the contraversive side. In
one case, horizontal saccades were hypermetric (c0419, ), but most
often oblique upward saccades were hypermetric. This family of curves
illustrates two points. First, the saccade hypermetria following
ventral-caudal MRF inactivation ranged from ~5% of the control value
to >50%. Second, there was a small secondary peak in saccade
hypermetria 180° in the opposite direction of the primary
hypermetria. This occurred for all injections except k0403, which had a
small degree of hypometria in the opposite direction.
|
A final aspect of the hypermetria was that it directed saccades toward a specific region in the orbit regardless of initial eye position. This was demonstrated by analyzing the direction and final endpoints of files of spontaneous saccades recorded toward the end of the monkey's ability to generate visually guided saccades. The different regions to which spontaneous saccades were directed are summarized in Fig. 12 for seven injections and two control days. Regions determined by right-sided MRF injections are shown by open symbols, and those following left-sided injections are filled. Note that all regions were located in the top half plane of movement and most (6 of 8) were contraversive. This trend of spontaneous saccades toward a specific goal in the orbit was confirmed by calculating an orbital perturbation index for both the horizontal and vertical components of spontaneous saccades. A clear effect (P < 0.05) of initial eye position on the vertical component of spontaneous saccades was found in three of seven injections (data not shown). The horizontal orbital perturbation indexes were increased over control (5 of 7) but did not reach statistical significance.
|
Head tilt and shift of initial eye position
Three of eight injections in the ventral-caudal MRF were associated with a shift in initial eye position (Fig. 13). The shift in initial eye position increased over time, and typically the monkey made no attempt to compensate for the shift. The shift was contraversive in two injections (c0419 and c0521) and ipsiversive and up after one injection (c0416).
|
One key to a better understanding of the changes noted in initial eye position came from studying the contralateral head tilt generated after the ventral-caudal muscimol injections. One possibility was that the shifts in initial position noted with the head fixed were compensatory for an attempted head movement in the opposite direction. Another possibility was that the MRF contributed to maintenance of initial eye position via its connections to the omnipause region of the PPRF. Loss of a fixation signal would produce destabilization of fixation similar to the macrosaccadic square-wave jerks shown above (Fig. 7). To examine whether the shift in initial eye position was compensatory, we measured the head tilt in one monkey free to move its head following a muscimol injection in the right ventral-caudal MRF.
With the use of an additional coil fixed to the head in the coronal
plane, horizontal and vertical, but not torsional displacement of the
head could be recorded (only 2 coils, 1 eye, and 1 head could be
monitored). An almost immediate contralateral head roll of ~30° was
confirmed visually and via photographs. The coronal coil demonstrated a
contraversive and downward head displacement (Fig.
14, D, E, and G).
This was associated with a compensatory shift of the initial
position of the eyes up and to the ipsilateral (right) side (Fig.
14D, ; F, horizontal,
; H,
vertical,
). This combination of head tilt and shift in eye position
resulted in gaze (combination of head and eyes) being directed toward
the center of the screen (Fig. 14B). This injection was
performed using <0.5 µl of muscimol from a picospritzer apparatus,
limiting spread of muscimol to adjacent structures. This suggests that
the shift of initial eye position seen after the muscimol injections
could have been compensatory for an intended head tilt.
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DISCUSSION |
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To better characterize the oculomotor function of neurons in
the MRF, injections of the GABAA agonist,
muscimol, were placed at the sites of midbrain neurons that discharged
with, or where electrical microstimulation induced, contraversive,
conjugate saccades (Cohen et al. 1985; Handel and
Glimcher 1997
; Waitzman et al. 1996
). Two
previous, careful studies of single-neuron activity in behaving monkeys
have revealed only a gross topographic arrangement of the oculomotor
functions in this region. In particular, cMRF neurons, adjacent to the
oculomotor nuclei, began to discharge 150 ms and peaked 8-10 ms before
saccades with a contraversive horizontal or downward oblique
component of movement (Waitzman et al. 1996
). More
rostrally located MRF neurons, adjacent to the INC also had long-lead
activity, but were most sensitive to contraversive oblique and
vertical saccades (Handel and Glimcher 1997
).
The movement fields for both groups of neurons were large and could
extend for up to 40°.
The primary findings of this study were that inactivation of the ventral-caudal MRF 1) generated conjugate, contraversive, upward saccade hypermetria; 2) reduced saccade latency; and 3) produced a moderate increase in saccade velocity accompanied by a moderate reduction in saccade duration. Many ventral-caudal injections also produced macrosaccadic square-wave jerks that repetitively brought the eyes to a fixed place in the orbit. Similarly, spontaneous saccades executed in total darkness were directed toward the same specific orbital position. The distribution of the orbital goals was across the contralateral upper field of movement. Interestingly, three of the muscimol injections induced a displacement of the initial position of the eyes. These findings suggested a number of possible roles for cells in the cMRF: 1) participation in feedback of an eye position or displacement signal, 2) stabilization/maintenance of fixation, and 3) activation of the saccade burst generator. These results are discussed in light of the various anatomic connections of the MRF and how these cells could participate in the circuitry needed for the control of saccades and stabilization of fixation. Simulations of the two models presented in the INTRODUCTION are used to demonstrate that an eye position and not eye displacement model can best explain the current observations.
Localization of muscimol inactivation
GABA containing interneurons have been localized to the MRF
(Nagai et al. 1983). Our basic assumption was that
muscimol activation of GABAergic synapses on saccade related long-lead
burst neurons in the MRF produced the observed oculomotor effects and
left fibers in passage (i.e., the central tegmental tract) unaffected
(Andrews and Johnston 1979
; Krogsgaard-Larsen et
al. 1979
; Ritchie 1979
). Single-unit recordings
through and around the region blocked by muscimol demonstrated no
neural activity in a sphere of radius 1.1 mm centered on the site of
injection and support this view. Saline injections produced no
oculomotor effects, thus eliminating a mechanical pressure gradient as
the source of our findings (Fig. 2). Inadvertent inactivation of a
number of oculomotor structures adjacent to the MRF including the
nucleus reticularis tegmenti pontis (NRTP), the SC, and the NOT, could
color the above interpretation of our results. However, muscimol
inactivation of these regions has produced little or no saccade
hypermetria (Aizawa and Wurtz 1998
; Hikosaka and
Wurtz 1985a
; Lee et al. 1988
;
Munoz and Wurtz 1993
; Quaia et al. 1998
;
van Opstal et al. 1996
). Destabilization of fixation has
occurred following rostral SC inactivation, but macrosaccadic
square-wave jerks directed toward a specific location in the orbit were
not generated (Munoz and Wurtz 1993
). Possible inactivation of the NOT could produce horizontal,
contraversive nystagmus (Cohen et al. 1992
). In five of
seven injections, no nystagmus was found (Table 1). In one of the other
two remaining injections, onset of hypermetria occurred before
contraversive nystagmus developed. Taken together this constellation of
findings suggests that inactivation of the MRF and not adjacent
structures produced saccade hypermetria.
Oculomotor functions of cMRF: anatomic connections
Intracellular filling of MRF neurons has shown them to be of at
least three types (Scudder et al. 1996). RTLLBNs located
within the central MRF (i.e., the nucleus subcuneiformis) direct their axons toward the ipsilateral SC where they arborize within the intermediate and deep layers. These cells provide a collateral branch
that crosses the intracollicular commissure to innervate the
contralateral SC (Moschovakis et al. 1988
). This group
of neurons could influence the generation of saccades in the SC. A
second group of MRF LLBNs [probably reticulospinal LLBNs (RSLLBNs)] is located lateral to the INC, rostral to the RTLLBNs just described (Scudder et al. 1996
). The RTLLBNs have contralateral
movement fields, whereas the RSLLBNs have vertical movement fields
(Handel and Glimcher 1997
; Scudder et al.
1996
; Waitzman et al. 2000
). These cells have
axons that descend toward the pons to innervate the raphe nuclei (raphe
pontis, nucleus RIP, raphe obscuris) and the medullary reticular
formation (primarily the inhibitory burst neuron region caudal to the
abducens nucleus) (Scudder et al. 1996
). The RSLLBNs
could interact with both saccade and head generation networks in the
pons and spinal cord (see accompanying paper, Waitzman et al.
2000
). Scudder and colleagues (1996)
have also described a third group of saccade-related neurons whose cell bodies
are probably located within the caudal MRF (i.e., again nucleus subcuneiformis) and whose discharge is similar to the RTLLBNs.
However, the axons of these cells (cRSLLBNs) were directed caudally
within the predorsal bundle and innervated the NRTP, RIP, the nucleus
reticularis pontis oralis and caudalis (NRPo-NRPc; including the
excitatory and inhibitory burst neurons) and sent a descending axon to
cervical levels of the spinal cord. An ipsilateral projection from the
cMRF and cuneate reticular nucleus to the ventral horn of the cervical
spinal cord, separate from that of the INC, has been confirmed
repeatedly in both cat (Castiglioni et al. 1978
) and
monkey (Crawford and Villis 1993
; Fukushima
1987
; Fukushima et al. 1987
;
Kokkoroyannis et al. 1996
; Robinson et al.
1994
; Scudder et al. 1996
). Moreover, the
descending MRF projections were more numerous than the projections from
the INC to the cervical spinal cord (Robinson et al.
1994
; Scudder et al. 1996
). Furthermore, the MRF
also receives head-related proprioception via direct afferents from the
cervical spinal cord and dorsal column nuclei (Bjorkland and
Boivie 1984
; Pechura and Liu 1986
). In sum, this
pattern of connectivity suggests that caudal MRF neurons could
participate in the generation of combined head and eye movements. The
output of cRSLLBNs to the omnipause neurons in the RIP and the
precerebellar saccade generating machinery in the NRTP and NRPo-NRPc
could account for the marked reduction in saccade latency found after
cMRF inactivation (J. Buttner, personal communication;
Buttner-Ennever and Buttner 1988
; Horn et al.
1994
; Scudder et al. 1996
). In the normal state, MRF activity could enhance the tonic firing rate of omnipause neurons
thereby suppressing unwanted saccades as has been suggested by the
results of electrical microstimulation (Cohen et al.
1985
). A likely candidate for this function would be high
background cMRF neurons described previously (Waitzman et al.
1996
). Loss of this excitation would lead to fixation instability.
However, loss of fixation would not necessarily generate macrosaccadic square-wave jerks per se. More likely, the muscimol inactivation produced two possible effects. First, it could produce a loss of a suppression signal (i.e., reduced excitation) to the omnipause neurons, permitting more frequent and longer pauses. Second, during saccades, the MRF could provide a feedback signal (i.e., of current eye displacement or eye position) needed to stop the saccade accurately. A reduction or complete loss of this feedback signal would cause a persistent motor error that the saccadic system would try to correct, thus generating the repeated macrosaccadic jerks at short latency. The interesting part of the square-wave jerks observed here were that they brought the eyes to a particular position in the orbit that was similar to the final positions of many of the spontaneous saccades made in total darkness. This could not be accomplished by feedback of an eye displacement signal.
Caudal MRF: craniotopic not retinotopic organization
Five findings in the current study suggest that MRF neurons are
organized in a spatial not a retinotopic frame of reference. First,
electrical stimulation at sites where muscimol was injected generated
contraversive saccades. Recent results have demonstrated that
electrical stimulation and single-unit recording at similar sites
elicited goal-directed saccades (Waitzman et al. 1998). Muscimol inactivation of this region left horizontal saccades for the
most part intact. The endpoints of horizontal saccades were deviated
upward and in a few instances, the horizontal component of movement was
modestly hypermetric. At the same time, contraversive upward, oblique
saccades were markedly hypermetric. If the ventral-caudal MRF were
retinotopically coded, then horizontal saccades should have been
rendered hypometric and the vertical component of saccades should have been unaffected. This would be similar to the results of
muscimol inactivation of the retinotopic map found in the superior colliculus (Aizawa and Wurtz 1998
; Hikosaka and
Wurtz 1985a
; Lee et al. 1988
;
Munoz and Wurtz 1995a
,b
; Quaia et al.
1998
).
A second finding following MRF inactivation was that spontaneous
saccades were not executed to random locations in the orbit. Instead,
saccade endpoints defined a specific "goal" in the orbit. This goal
was typically displaced upward from the pure horizontal locus defined
by prior electrical stimulation and single-unit recording (see Fig.
15). Third, an analysis of the
endpoints of macrosaccadic square-wave jerks during fixation showed
that the eyes were directed to the same locus defined by spontaneous
saccades executed in total darkness. The amplitude and direction of
these repetitive movements varied in a predictable way with shifts in the initial position of the eyes, such that their endpoints coincided with the same region of the orbit delineated by the final positions of
spontaneous saccades. Fourth, a moderate shift in the initial position
of saccades was noted after three of eight injections. Last, a
contraversive head tilt was observed after nearly all the caudal MRF
injections. Electrical stimulation in the MRF of dogs and monkeys has
produced contraversive saccades in association with head movements
(Bender and Shanzer 1964; Silakov et al.
1999
; Szentagothai 1943
). Electrolytic lesions
in the MRF produced an ipsilateral gaze preference, in
which monkeys did not make gaze movements (head and eyes) to visual
stimuli on the contralateral side (Komatsuzaki et al.
1972
). Taken together, these data are most consistent with the
idea that MRF neurons participate in the generation of a final eye or
target position in the orbit (craniotopic, Eye Position Model), rather
than an eye displacement signal (retinotopic, Eye Displacement Model).
|
The concept that the MRF specifies final orbital position would fit
with a number of other physiological and anatomic findings. The cMRF
receives projections from both cortical and subcortical regions where
goal-directed saccades have been elicited. Electrical microstimulation
and single-neuron recording in the SC and the FEF, both of which
provide projections to the cMRF, have demonstrated a moderate effect of
initial eye position on the size of elicited saccades (Cowie and
Robinson 1994; Segraves and Goldberg 1991
). Probably more important are the large numbers of projections from the
supplementary eye fields (SEF) (Huerta and Kaas 1990
;
Shook et al. 1990
) and posterior parietal cortex
(Kunzle and Akert 1977
; Leichnetz and Goldberg
1988
) to the MRF. Neurons in the SEF discharge in response to
purposeful movements and have open and closed movement fields similar
to those found in the MRF (Schall 1991
). Recent data
suggest that the discharge of some SEF neurons is object oriented and
initial eye position can have a moderate effect on discharge
(Olson and Gettner 1995
; Russo and Bruce
1993
). Microstimulation of the SEF produces contraversive
saccades that bring the eyes to a "termination zone" (Russo
and Bruce 1993
; Tehovnik et al. 1994
). This is
similar to the variable amplitude and/or goal-directed saccades
generated after MRF microstimulation (Cohen et al. 1985
; Silakov et al. 1995
; Waitzman et al.
1998
). In conclusion, we suggest that muscimol inactivation of
the cMRF uncovered an underlying craniotopic organization of neurons
that contributes to the generation of an estimate of a final eye or
target position in the orbit signal.
Generation of vertical saccade components from a "horizontal" region
Single-unit recordings and microstimulation have strongly implied
that neurons in the caudal MRF participate in the generation of the
horizontal component of saccades (Cohen et al. 1985;
Waitzman et al. 1996
). Our expectation was that
inactivation of this caudal region would have generated hypometric
horizontal not the hypermetric oblique saccades observed. This
prediction was based on two primary assumptions. First, we hypothesized
that the MRF was coded in retinotopic coordinates, and, second, we
thought MRF neurons provided signals for generation of eye movements
alone, not a combination of head and eye movements. The data just
reviewed support the idea that MRF neurons are organized in craniotopic
not retinotopic coordinates. We now present one example of how
inactivation of a brain stem region that specifies final horizontal
position of the eyes in the orbit could produce oblique saccades.
An averaged representation of the movement fields from nine caudal MRF
neurons from one monkey was constructed by normalizing all neuronal
activity to the peak firing of each cell (see also Fig. 9C
of Waitzman et al. 1996). The peak activity of this
averaged movement field was greatest for contraversive eye positions
(Fig. 15, positive horizontal axis is contraversive). However,
contraversive movements, with vertical components were also associated
with increased (albeit lower than peak) neuronal discharge (note
elevated shelf of neural activity on the right half of the movement
field). Moreover, there was no clear cutoff of activity for saccades of very large amplitude (i.e., the movement field was open). Ipsiversive movements were associated with little or no activity. Unfortunately these MRF neurons were not studied during shifts of initial position, but for the purposes of this discussion we assume that changes in
initial position would lead to activation of cells when the eye reached
a particular final position. We also made the assumption that muscimol
inactivation would eliminate neural activity in this averaged
"movement field" of all saccades with final positions, which landed
on the contraversive side 10° up to 40° down. The resultant
averaged activity is shown in Fig. 15B. Note that a relative peak activity of the grouped MRF activity appears up and to the contraversive side. We further assume that, as more of the map is
inactivated, final eye position will be shifted further upward (e.g.,
arrow moving the mountain toward the upward region). This idea would
explain the upward displacement of the final position of spontaneous
and visually guided eye movements observed after the injection of muscimol.
Evidence of a contraversive head tilt following MRF inactivation
suggests that interaction of the MRF with either the vestibular or head
movement control system could produce vertical saccade components.
Interactions with the vestibular system would have to occur via
indirect pathways because there are no direct projections from the MRF
to the vestibular nuclei or vestibular cerebellum (flocculus). On the
other hand, projections from the MRF to regions of the brain stem and
spinal cord that participate in head movements have been described. Two
regions could have particular importance. First, the loss of MRF input
to the cervical spinal cord (Castiglioni et al. 1978;
Robinson et al. 1994
) could produce an intended
contraversive head tilt (i.e., the monkeys were head fixed). This could
generate secondary changes of proprioceptive inputs to the vestibular
cerebellum and would result in a reduction of floccular output. This
scenario would result in the generation of a compensatory upward
vertical output signal. A more direct route might target the NRTP,
which also receives significant projections from the MRF as well as the
SC and cerebellar nuclei (Brodal 1980
; Gerrits
and Voogd 1986
; Harting 1978
;
Scudder et al. 1996
; Thielert and Their
1993
). Inactivation of the caudal portion of NRTP would produce
a loss of torsional eye control and could result in the generation of
an upward eye movement component (van Opstal et al.
1996
). A similar mechanism has been proposed for the upward
deviation of saccade trajectories observed after inactivation of the SC
(Aizawa and Wurtz 1998
; Quaia et al.
1998
, 1999
). In sum, generation of the vertical
saccade component after muscimol inactivation would be better
understood in head-free monkeys whose head and eye movements were
recorded in three dimensions.
Generation of saccade hypermetria: models
What physiological mechanisms are responsible for saccade hypermetria, increased saccade velocity, and goal-directed macrosaccadic square-wave jerks observed after muscimol inactivation of the MRF? Simulation has demonstrated that changes at three sites (Hyper #1, #2, and #3) in the Eye Position model (Fig. 1A) and two sites in the Eye Displacement model (Fig. 1B) could generate saccade hypermetria. Manipulation of two locations (Hyper #1 and #2), the model input and the local feedback pathway, could generate saccade hypermetria in both models (Hyper #1 and #2). A shift in the input to either model (i.e., a new eye displacement, or desired final eye position) to a higher value resulted in saccade hypermetria (Fig. 16A, new input).
|
In the one-dimensional case shown here, this would be a switch from a 12° to a 24° eye displacement site or a similar shift in final orbital position. Such a shift might occur via inactivation of a particular portion of the saccade map in the MRF that projects to either the SC or the NRTP.
To utilize the better understood SC saccade map, assume that
"quasi-visual" and "build-up" neurons of the SC provide a
desired displacement signal (E) to the medium lead burst
neurons in the PPRF (Mays and Sparks 1980
; Munoz
and Wurtz 1995a
,b
). Then, MRF RTLLBNs with presumed inhibitory
projections to a region of the SC that mediates large, upward movements
would be inactivated. This loss of inhibition would shift the
distribution of activity on the collicular map toward the medial
(upward eye movement) and caudal portion of the SC, thus generating
upward, hypermetric saccades.
An alternative route for shifting model input utilizes projections from
the MRF to the NRTP (Crandall and Keller 1985;
Edwards 1975
; Edwards and Olmos 1976
;
Lefevre et al. 1998
; Suzuki et al. 1994
).
Inactivation of the fastigial nuclei, to which the NRTP projects,
generates a significant degree of ipsilateral hypermetria and contralateral hypometria (Robinson et al. 1993
;
van Opstal et al. 1996
). Although no changes in latency
were observed after muscimol inactivation of the fastigial nuclei, the
inaccuracies of saccade endpoints and curvature of saccade trajectories
were similar to those found after caudal MRF inactivation. This
suggests that inactivation of the caudal MRF RSLLBNs could shift
activity in a NRTP
Cerebellum (fastigial nuclei)
PPRF and/or
SC loop in a similar way as in the proposed MRF
SC pathway
(Lefevre et al. 1998
; Quaia et al. 1999
).
In either scenario, changes in model inputs would preserve the
relationships between amplitude and duration, as well as amplitude and
velocity (Fig. 16A, new input) (Robinson et
al. 1993
). Hypermetric saccades with a normal amplitude/velocity relationship but longer duration for a given amplitude were noted after one injection (g0217, Figs. 4 and 10). Longer saccade duration would not be observed after a shift in model
input. Moreover, shifts in saccade inputs could not account for the
goal-directed nature of postinjection spontaneous saccades.
Inactivation in the feedback pathway of either model would also produce
saccade hypermetria (Hyper #2). In this schema we propose that the build-up neurons in the intermediate and deep layers
of the SC in conjunction with MRF RTLLBNS directly participate in the
local feedback pathway (i.e., are in the feedback loop) (Waitzman et al. 1991, 1996
). We
hypothesize that via reciprocal inhibitory connections between the MRF
and the SC, a spatial integration of eye velocity could occur (RI, Fig.
1B). Although the idea that the SC is involved in the
spatial integration of a velocity signal has been suggested previously
(Lefevre and Galiana 1992
; Optican 1994
),
recent data do not support a role for the SC alone as
the resettable integrator for the displacement model. Muscimol
inactivation of the SC produces curved saccade trajectories and
hypometria, but not saccade hypermetria (Aizawa
and Wurtz 1994
, 1998
; Quaia et al.
1998
). Alternatively, MRF neurons could be a part of a velocity
to position integrator that is not reset at the end of each saccade (i.e., part of the NI in the Eye Position model, Fig.
1A). In the latter case, MRF neurons should be sensitive to initial eye position. The eye velocity command signal
(Vc) from the medium lead burst neurons in the PPRF would
be channeled to MRF neurons, where it would be integrated to produce
eye displacement (i.e., the Eye displacement model) or eye position
(Eye position model) (Benevento et al. 1977
;
Buttner-Ennever and Buttner 1988
; Edwards
1975
; Horn et al. 1994
; Scudder et al.
1996
). Loss of the resettable integrator or damage to the
velocity to position integrator of Robinson results in modest
lengthening of saccade duration, commensurate reduction in the velocity
of postinjection saccades, and increased saccade amplitude (Fig.
16A, reduced feedback). In fact, saccade duration was
moderately increased after the muscimol injection of g0217, with
preservation of the amplitude/velocity relationship (Figs. 5,
A and C, and 10C). Thus
effects on saccades following injection g0217 could be explained by a
reduction in the gain of the local feedback pathway of either model
(Hyper #2, in Figs. 1, A and
B, and 16A, reduced feedback, solid line).
However, for three other injections, saccade hypermetria was associated with normal or shorter saccade duration and either normal or higher saccade velocity (c0521, c0419, and c0416). Explanation of increased saccade velocity (over control) necessitated modulation of another model parameter. Increasing the gain of the burst generator causes saccades to be executed faster while accurately reaching their goal (Fig. 16B, i.e., the burst generator is in the local feedback loop). This condition was not observed in our data. In short, we were able to replicate saccade hypermetria and increased velocity in both the Eye Position and Eye Displacement models only by simultaneously increasing the gain of the burst generator while reducing the feedback of current eye position or displacement (Fig. 16C). Note that the simulated saccade amplitude was hypermetric and its velocity was higher, but saccade duration was slightly reduced compared with the amplitude matched control movement. We hypothesize that a combination of saccade hypermetria and increased velocity might occur if muscimol inactivation affected both RTLLBNs (feedback) and caudal RSLLBNs (feed-foward) in the MRF. Loss of the RTLLBNs would severely impair the feedback of current eye position or displacement activity to the SC. Impaired RSLLBNs output could remove inhibition on NRPo and NRPc neurons, thus increasing the number of spikes produced by medium lead burst neurons for a given amplitude saccade.
In summary, reduced feedback would account for the increased amplitude of saccades seen after all injections. However, we must hypothesize an increase in the gain of the pontine burst generator in the PPRF to account for the increased saccade velocity demonstrated in three other injections.
Mechanisms for the generation of square-wave jerks
Two questions arise regarding the generation of the repetitive
macrosaccadic square-wave jerks. First, how do the two models generate
these characteristic movements? Second, can either of the models
produce goal-directed movements? The answer to the first question turns
out to be straightforward. Generation of saccade hypermetria from a
reduction in the gain of feedback in each model leads to an unresolved
motor error (em). If the model is
stable, then oscillations around the final position should slowly die
away and the eye will come to rest at the desired, eccentric location
(E or ed). We observed this
behavior for both models with the proper selection of feedback gain
(i.e., ~60% of normal gain for both models Fig.
17A, 0.6 gain). When
feedback was reduced by ~50% in the eye displacement model, the eye
continued to oscillate around the final desired eye position without
any decay (Fig. 17A, 0.5 gain). With further reductions in
feedback gain, both models could be made very underdamped, in which
case the oscillations increased, completely destabilizing the system (Fig. 17A, 0.25 gain).
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The answer to the second question (goal-directed saccades) was addressed by reducing feedback (i.e., at the Hyper #2 location) by 50% and asking each model to execute saccades from different initial positions to the same final position. Neither model could generate goal-directed saccades in this scheme (data not shown). Rather, each produced saccades that oscillated around the final position without actually reaching the final position. With appropriate choices of feedback gain, the oscillations for each model would eventually die away, and the final position would be achieved (similar to the 0.6 gain shown in Fig. 17A), but these are not goal-directed movements. On the other hand, goal-directed saccades could be produced in some circumstances by careful manipulation of the Eye Position model (Fig. 1A). By placement of a moderate delay (110 ms) in the second limb of the feedback loop directed to summing junction 2 (SJ2, Fig. 1B, Hyper #3, del) macrosaccadic, hypermetric, square-wave jerks could be generated (Fig. 17B, Hyper #3, solid line). Feedback gain in the portion of the feedback loop reaching SJ3 was left intact (i.e., it was 1.0). The interpretation of this result was that the local feedback loop (SJ3) would control execution of the proper initial saccade to the desired orbital position (Fig. 17B, 1). In the meantime, without proper feedback of current eye position to SJ2, retinal error (Rerr) would fall to zero without a commensurate rise in E'. This would result in a sudden drop in ed as if a desired eye position of zero was needed, and a saccade in the opposite direction would be generated (Fig. 17B, 2). However, 110 ms after this "return" saccade ended, the original current eye position signal (E') would suddenly arrive at SJ2. This new "current eye position" would be summed with the residual retinal error (Rerr) remaining after the return saccade. Again a new desired orbital position (ed) would be produced, initiating a third movement (after a normal refractory period) of similar amplitude and direction (+ a small error) toward the original goal (Fig. 17B, 3). The small "overshoot" seen at the end of this movement (Fig. 17B, 4) is the result of the previous retinal error, which disappears quickly as the new residual retinal error goes to zero. Note the difference between this tracing and that obtained with changes at Hyper #2 (Fig. 17A). Oscillations around the goal (with the exception of the overshoot) were abolished. These movements, albeit one dimensional in the model, are very similar to the repetitive goal-directed, macrosaccadic square-wave jerks we observed after inactivation of the caudal MRF. In open loop conditions (i.e., via placement of a long delay in the feedback loop), repetitive saccades were generated, but they did not land on the orbital position goal and became progressively larger in amplitude (Fig. 17B, dashed-dotted line, 5 and 6). Again similar, but not exactly like the square-wave jerks observed experimentally. In conclusion, both models can generate hypermetric, macrosaccadic square-wave jerks, but only the Robinson model could be adapted to generate repetitive goal-directed movements. Clearly new models of higher dimensionality and increased sophistication are needed to more fully characterize the oculomotor behavior found following muscimol inactivation of the caudal MRF.
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ACKNOWLEDGMENTS |
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The authors thank Dr. Douglas Oliver for the use of the 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. Numerous discussions with Drs. Robert Wurtz, Lance Optican, and postdoctoral members of the Laboratory of Sensorimotor Research, National Eye Institute (NEI), have been invaluable in framing the idea of damage to the resettable integrator. The comments of three anonymous reviewers were critical for focusing and simplifying the manuscript.
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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