Role of the Cerebellar Flocculus Region in the Coordination of Eye and Head Movements During Gaze Pursuit

Timothy Belton and Robert A. McCrea

Department of Neurobiology, Pharmacology and Physiology, University of Chicago, Chicago, Illinois 60637


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Belton, Timothy and Robert A. McCrea. Role of the Cerebellar Flocculus Region in the Coordination of Eye and Head Movements During Gaze Pursuit. J. Neurophysiol. 84: 1614-1626, 2000. The contribution of the flocculus region of the cerebellum to horizontal gaze pursuit was studied in squirrel monkeys. When the head was free to move, the monkeys pursued targets with a combination of smooth eye and head movements; with the majority of the gaze velocity produced by smooth tracking head movements. In the accompanying study we reported that the flocculus region was necessary for cancellation of the vestibuloocular reflex (VOR) evoked by passive whole body rotation. The question addressed in this study was whether the flocculus region of the cerebellum also plays a role in canceling the VOR produced by active head movements during gaze pursuit. The firing behavior of 121 Purkinje (Pk) cells that were sensitive to horizontal smooth pursuit eye movements was studied. The sample included 66 eye velocity Pk cells and 55 gaze velocity Pk cells. All of the cells remained sensitive to smooth pursuit eye movements during combined eye and head tracking. Eye velocity Pk cells were insensitive to smooth pursuit head movements. Gaze velocity Pk cells were nearly as sensitive to active smooth pursuit head movements as they were passive whole body rotation; but they were less than half as sensitive (approx 43%) to smooth pursuit head movements as they were to smooth pursuit eye movements. Considered as a whole, the Pk cells in the flocculus region of the cerebellar cortex were <20% as sensitive to smooth pursuit head movements as they were to smooth pursuit eye movements, which suggests that this region does not produce signals sufficient to cancel the VOR during smooth head tracking. The comparative effect of injections of muscimol into the flocculus region on smooth pursuit eye and head movements was studied in two monkeys. Muscimol inactivation of the flocculus region profoundly affected smooth pursuit eye movements but had little effect on smooth pursuit head movements or on smooth tracking of visual targets when the head was free to move. We conclude that the signals produced by flocculus region Pk cells are neither necessary nor sufficient to cancel the VOR during gaze pursuit.


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The vestibuloocular reflex (VOR) smoothly rotates the eyes in the opposite direction of head movements so that images are stabilized on the retina when the head is perturbed. The pathways that produce the VOR are also utilized to produce smooth pursuit and optokinetic eye movements that track images moving in space with respect to the head (Collewijn et al. 1982; McCrea and Cullen 1992; Precht et al. 1985; Robinson 1981; Waespe and Henn 1985). Smooth tracking head movements can also be used to pursue moving images. When both eye and head movements are utilized to stabilize images on the retina, the VOR must be suppressed or canceled in order for image stability to be maintained.

Several ideas have been advanced for how the VOR might be canceled during active voluntary head movements. One idea is that the brain produces a common gaze velocity command utilized by both oculomotor and head movement circuits, with the VOR functioning to coordinate the movements of the eyes and the head by canceling the gaze motor command at the level of the extraocular motoneuron (Lanman et al. 1978). A second idea is that the central pattern generators that produce smooth visual following eye movements carry signals that are gaze velocity commands. These gaze velocity commands add with vestibular reafferent signals in the vestibular nuclei so that the oculomotor signals carried by central VOR pathways are, on balance, canceled (Barnes 1993; Barnes and Eason 1988; Lisberger and Fuchs 1978a). A third idea is that the central pattern generators that produce smooth eye movements and smooth head movements are controlled by separate central motor programs and that the VOR is canceled by the addition of an internal estimate of the active head movement (Cullen et al. 1991; Robinson 1982; Tomlinson and Robinson 1981).

One way to assess which of these mechanisms is involved in coordinating eye and head movements when both are used to stabilize images on the retina is to examine the activity in neural circuits that are involved in producing visuomotor reflexes and in modifying the VOR during smooth eye and head movements. The cerebellar flocculus region (FLR, flocculus and ventral paraflocculus) is an essential part of the neural substrate for producing visual ocular following reflexes, and for modifying the VOR (Büttner and Waespe 1984; Lisberger and Fuchs 1978a; Lisberger et al. 1994a,b; Miles et al. 1980; Noda and Suzuki 1979). In the accompanying paper (Belton and McCrea 2000) we showed that the firing behavior of Purkinje cells in the squirrel monkey FLR was strongly modulated during smooth pursuit eye movements and during cancellation of the VOR when a visual target moved with the head during passive whole body rotation (WBR). One class of FLR Purkinje (Pk) cells, the so-called gaze-velocity Pk cells (Gv Pk cells) were modulated in phase with eye velocity during smooth ocular pursuit and with head velocity when the VOR was canceled by fixation of a visual target that moved with the head during passive WBR (Büttner and Waespe 1984; Lisberger and Fuchs 1978a; Miles et al. 1980). The other major class of FLR Pk cells, the eye velocity Pk cells (Ev Pk), was strongly modulated during smooth ocular pursuit but was insensitive to passive head movements during VOR cancellation. The signals produced by these Pk cells were apparently essential for producing smooth ocular pursuit eye movements and for VOR cancellation during passive head rotation, since inactivation of the FLR using injections of muscimol profoundly disrupted both behaviors. Similar observations have been made in other primates (Takemori and Cohen 1974; Waespe et al. 1983; Zee et al. 1981).

Since the FLR produces signals that are essential for smooth tracking of visual targets during passive head movements, it might also be the source of central gaze motor commands for coordinating eye and head motor control systems (Bizzi 1981; Lanman et al. 1978); or it could produce a gaze velocity signal that adds with vestibular signals on VOR pathways to cancel vestibular reafferent inputs (Barnes 1993). On the other hand, if the FLR produces signals that are related primarily to smooth eye movements, regardless of the contribution of active head movements to gaze pursuit, then it is likely that smooth eye and head movements are controlled by separate motor programs, and that an internal estimate of active head movements is used to cancel the VOR (Chambers and Gresty 1983; Tomlinson and Robinson 1981).

In this study we examined the firing behavior of Pk cells in the FLR in squirrel monkeys that were free to pursue visual targets by generating a combination of head and eye pursuit movements. We compare the signals those Pk cells generate during active head pursuit movements to the signals generated during smooth pursuit eye movements and passive WBR. In addition the effects of muscimol inactivation of the FLR on the monkeys ability to generate smooth pursuit head movements and to cancel the VOR during active gaze pursuit are described. Although we found that many FLR Pk cells were weakly sensitive to smooth active head movements, we argue that these signals are neither necessary nor sufficient to cancel the VOR generated during active head movements. We conclude that the FLR does not produce gaze velocity signals that function to coordinate gaze when both eye and head movements are used to stabilize images on the retina. A preliminary report of these results has been published (Belton and McCrea 1999).


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The methods used for recording and analyzing eye movements and single-unit activity in squirrel monkeys and for injecting muscimol into the FLR were described in the accompanying paper (Belton and McCrea 2000). They are briefly described here along with the methods for recording from the head-free monkey.

Methods for recording combined eye and head movements

The experimental recording apparatus is illustrated in Fig. 1. Monkeys were seated in a Plexiglas chair with the C1-C2 axis concentric with the turntable's rotational axis. The torso was somewhat restrained by a plate abutting the chest, which restricted arm raising. The head was attached to a delrin rod that rotated within a double ball bearing assembly (Barden Precision Bearings; Fig. 1, b) mounted to the turntable. The rod's axis of rotation was coincident with the axis of turntable rotation and passed through the intersection of the midsagittal and interaural planes and within 5 mm of the C1-C2 axis. Head movements were permitted in the plane of the horizontal semicircular canal (±45°, 15° nose down from the stereotaxic plane). A universal joint (Fig. 1, e) was placed in-line with the vertical rod above the animal's head (~5 cm) to permit slight postural adjustments. A clamp (Fig. 1, a) attached to the chair-mounted end of the rod could be quickly tightened for restraining head movements, although the flexibility of the delrin rod permitted small head movements even when its rotation was prevented. Liquid reinforcement was delivered through a headset-like tube that moved with the head (Fig. 1, d).



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Fig. 1. Experimental setup. Squirrel monkeys were seated on a vestibular turntable with the monkey's head attached to a rod that permitted head movements (±45°, a) in the plane of the horizontal semicircular canals. The rotational axis of the head and turntable was positioned to be within 5 mm of the C1-C2 juncture at the level of the external auditory meatus. The rod rotated within a low-friction double bearing assembly (b) that was fixed to the table; it contained a universal joint (c) that permitted small pendular movements of the head for postural adjustments. The head could be fixed to the turntable by disabling the universal joint and reversibly clamping the axis rod, preventing its rotation. Visual targets were generated using a HeNe laser and directed by galvanometer-controlled mirrors onto a circumferential screen 90 cm distant from the turntable's center. Liquid reinforcement was delivered via a headset-like tube (d). Tungsten microelectrodes were advanced into the floccular region by reversibly fixing the slave cylinder (e) of a hydraulic microdrive to the head.

The position of the right eye with respect to the turntable was recorded with a magnetic search-coil system (40 cm diam, Neurodata Instruments). The animal's head position was determined using a second search coil (Fig. 1, c) placed on the vertical rod below the universal joint. Table velocity, gaze, head, and target position signals were low-pass filtered (Frequency Devices, 7 kHz, 7-pole Butterworth) and sampled at 200 or 500 Hz (Cambridge Electronics, model CED1401).

Single-unit activity was recorded using Epoxy-insulated Tungsten microelectrodes (4-7 MOmega impedance) that were stereotaxically inserted into the cerebellum through a guide tube (22G) using a micromanipulator. After placement, the electrode and guide tube, attached to the slave cylinder of a hydraulic microdrive, were secured to the skull. The electrode was then advanced within the cerebellum using the hydraulic microdrive. The methods of searching for, identifying, and mapping the location of Pk cells were described in the accompanying paper (Belton and McCrea 2000).

Experimental protocols

The responses of each Purkinje cell were studied using the following four paradigms when the head was free to move in the yaw plane.
1) Sinusoidal smooth gaze pursuit (0.5 Hz, ±12.7°, 40°/s peak target velocity). Monkeys were rewarded for accurate smooth gaze tracking (angular gaze position maintained within 2° of the target for approximately 1 s). Although rewards were delivered regardless of whether eye or head movements were used to pursue the target, approximately 60-80% of gaze velocity was produced by active head movements.
2) WBR during fixation of an earth stationary target (0.5 Hz, 40°/s peak table velocity). In this condition the vestibular stimulus produced by passive WBR often evoked a compensatory vestibulo-collic reflex or a smooth compensatory head movement that reduced head velocity in space.
3) WBR evoked during fixation of a head stationary target (0.5 Hz, 40°/s peak velocity). In this condition active head movements were usually minimal (<1.5°), and the VOR produced by passive WBR was canceled.

These responses were compared with those recorded when the head was restrained from moving.

The following experiments were carried out in a fraction of the Pk cells.
4) WBR in the light without a target present (0.5 Hz, 40°/s turntable rotation)
5) Procedures 1-3 above at 2.3 Hz, 20°/s peak stimulus velocity
6) Combined passive and active head movements: Passive WBR at 2.3 Hz (20°/s) during head-free pursuit of a target moving sinusoidally at 0.5 Hz with respect to the turntable or moving at a constant velocity (40°/s).

Inactivation of flocculus region using muscimol

The methods of muscimol inactivation were described in the accompanying paper (Belton and McCrea 2000). The head-free experiments reported here were interleaved with experiments reported in that paper that were carried out when the head was restrained.

Data analysis

ANALYSIS OF PURKINJE CELL SENSITIVITY TO HEAD POSITION. Sensitivity to static head position was assessed by multiple regression analysis of the mean firing rate recorded during 20 or more periods of stable gaze and stable head position in the absence of a target.

ANALYSIS OF DATA OBTAINED WITH SINUSOIDAL STIMULI. The methods used for analyzing unit responses to sinusoidal stimuli are described in the accompanying paper (Belton and McCrea 2000). Behavioral and unit responses to periodic sinusoidal stimuli were analyzed by fitting desaccaded, averaged records with a sinusoidal function whose frequency was fixed to that of the stimulus. The desaccaded records excluded periods when gaze deviated from target position by more than 3° and periods that contained head and eye movements related to saccades.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Smooth pursuit head movements in squirrel monkeys

When the head is free to move, squirrel monkeys pursue moving targets with a combination of eye and head movements over a wide range of stimulus frequencies and velocities (Cullen and McCrea 1990). Even though the target remained well within oculomotor range and could be pursued equally well with either eye or head movements, both of the squirrel monkeys used in this study chose to pursue targets primarily with smooth head movements at 0.5 Hz, which was the stimulus frequency that was most commonly used. Head movements also contributed to smooth tracking at higher stimulus frequencies, including the highest stimulus frequencies that were used (2.3 Hz).

The behavioral responses recorded concomitantly with single-unit recordings in three behavioral conditions at 0.5 Hz are summarized in Table 1. On average, smooth head movements contributed nearly three-quarters (74%) of the gaze velocity during sinusoidal gaze pursuit, and nearly two-thirds of the compensatory gaze shift evoked during WBR when the monkey fixated an earth stationary target. Head movement also contributed significantly to compensatory gaze velocity when no target was present during WBR. On the other hand, the head movements evoked by WBR when the monkey fixated a head stationary target were negligible or absent.


                              
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Table 1. Purkinje cell responses during 0.5-Hz smooth head-free tracking

Single-unit recordings from FLR Pk cells in head-free squirrel monkeys

Single-unit recordings were obtained from 121 FLR Pk cells that were sensitive to horizontal eye movements during combined eye and head smooth tracking. Most of the Pk cells were studied during passive WBR as well as during active smooth pursuit head movements. All of the units whose firing rate was modulated during active head movements or during passive WBR in the horizontal plane were also modulated during smooth pursuit eye movements. Consequently no Pk cells were found in the FLR whose firing behavior was related to head movements but not eye movements. A systematic effort was made to record from Pk cells in both the flocculus and the ventral paraflocculus, but no significant differences were found between the Pk cells located in the two regions; so the results obtained from cells in both regions will be combined. Many Pk cells were sensitive primarily to vertical eye movements or had firing behavior that was neither related to eye nor head movements, but these units were not systematically studied and are not included in the following discussion. The response gain during smooth pursuit of 6 of the 121 Pk cells was <0.2 spikes/s/deg/s. These signals were judged to be too small to allow accurate quantitative analysis and were dropped from further consideration.

The remaining 115 horizontal eye movement-related FLR Pk cells were grouped into two categories based on their responses during ocular pursuit and VOR cancellation (Belton and McCrea 2000). The majority of the cells (n = 66, 55% of the sample) were sensitive to horizontal eye velocity during smooth pursuit eye movements and to slow phase eye velocity during the VOR, but were insensitive to head velocity during passive WBR. These were categorized as eye velocity Pk cells (Ev Pk cells). When monkeys canceled their VOR by fixating a head stationary target during passive WBR, the firing rate of Ev Pk cells was related only to residual, unsuppressed eye velocity. The remaining cells generated signals related both to eye velocity during smooth ocular pursuit and to head velocity during passive WBR, and were termed gaze velocity Pk cells (Gv Pk cells; n = 55). A detailed description of the firing behavior of both classes of Pk cell in the squirrel monkey in the absence of active head movements is available in the accompanying paper (Belton and McCrea 2000).

Responses of Pk cells during head-free pursuit

During head-free gaze pursuit the firing rate of Ev Pk cells was related to pursuit eye movements but not to smooth pursuit head movements. Figure 2 illustrates the responses of an Ev Pk cell during smooth pursuit of a visual target when the head was restrained from moving (Fig. 2A) and when the head was free to move (Fig. 2B). Sample records of unit activity during smooth tracking are illustrated on the left side of the figure, and the averaged, desaccaded responses are illustrated on the right side. The dashed trace superimposed on the average firing rate histogram in B is the predicted response based on the cell's sensitivity to eye movements during ocular pursuit. The modulation of this Ev Pk cell, like every other Ev Pk cell examined, was dramatically reduced when head movements rather than only eye movements were used to pursue moving targets.



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Fig. 2. Firing behavior of an eye velocity Purkinje (Ev Pk) cell during head-restrained (A) and head-free pursuit (B) of a target moving sinusoidally at 0.5 Hz, 40°/s peak velocity. The records on the left show the responses of the cell during 3 stimulus cycles. Desaccaded and averaged responses are illustrated on the right. The 3 traces above the unit firing rate histogram in A are head velocity (Hv; blue trace), target velocity (Tgv; dashed trace), and eye velocity (Ev; red trace). In B gaze velocity (Gv; maroon trace) is also added. Ipsilateral movements are upward. Predicted responses superimposed on averaged responses (dashed traces) were based on the cells sensitivity to eye velocity during ocular pursuit (2.47 spikes/s/deg/s) and to head velocity during vestibuloocular reflex (VOR) cancellation (0.0 spikes/s/deg/s) when the head was restrained from moving.

The pursuit responses of most Gv Pk cells were also attenuated during combined eye and head pursuit. The Gv Pk cell illustrated in Fig. 3 was typical. When the monkey's head was restrained, the unit's firing rate was strongly modulated in phase with ipsilateral eye velocity during ocular pursuit (Fig. 3A). When the head was free to move, the monkey used primarily head movements to track the target (Fig. 3C), and the firing rate modulation was reduced. The green dashed trace superimposed on the average in C is the predicted response based on the cells sensitivities to eye velocity during ocular pursuit and passive head velocity during cancellation of the VOR (Fig. 3B).



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Fig. 3. Firing behavior of a Gv Pk cell during head-restrained pursuit (A), VOR cancellation (B), and head-free pursuit (C). The records on the left show the responses of the cell during 3 stimulus cycles. Averaged, desaccaded responses are illustrated on the right. The predicted responses superimposed on averaged responses (dashed traces) were based on the cells sensitivity to eye velocity during ocular pursuit (1.37 spikes/s/deg/s) and to head velocity during VOR cancellation (0.57 spikes/s/deg/s) when the head was restrained from moving. Abbreviations and labeling conventions are the same as in Fig. 2.

The sensitivity of Ev and Gv Pk cells to gaze velocity in the head-restrained and head-free conditions is compared in Figs. 4 and 5. Since most Pk cells were less sensitive to head velocity than to eye velocity, the firing rate modulations during head-free pursuit were significantly smaller than those recorded during ocular pursuit, although the response phase in the two conditions was comparable (Fig. 4, A2 and B2). In Fig. 5A Ev and Gv Pk unit sensitivity to gaze velocity in the head-free condition during 0.5-Hz, 40°/s pursuit is plotted as a function of their sensitivity to gaze velocity during ocular pursuit in the head-restrained condition. Most units were significantly less sensitive to gaze velocity during head-free pursuit than during ocular pursuit, although a few Gv Pk cells generated comparable responses in both conditions. On average, Pk cells were about half as sensitive to gaze velocity during 0.5-Hz pursuit when the head was free to move as compared with when the head was restrained.



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Fig. 4. Gain and phase of flocculus region (FLR) Pk cell responses during ocular pursuit (A1 and A2) and combined eye-head pursuit (B1 and B2) with respect to gaze velocity. Units sensitive to ipsilateral gaze velocity are plotted to the right of zero, while those sensitive to contralateral gaze velocity are plotted to the left in A1 and B1.



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Fig. 5. A: gaze velocity sensitivity of FLR Pk cells during combined eye-head pursuit plotted as a function of gaze velocity sensitivity during ocular pursuit. , Gv Pk cells; open circle , Ev Pk cells. Dashed line indicates equal sensitivity in both conditions. Ipsilateral and contralateral: unit maximal response direction during pursuit. B: predicted response for Gv Pk cells during combined eye-head pursuit, as derived from their sensitivity to head velocity during VOR cancellation and to eye velocity during ocular pursuit, is plotted as a function of the observed combined eye-head pursuit response. The dashed line indicates equality between predicted and recorded responses. The correlation between predicted and recorded responses was 0.95.

Pk unit responses during head-free pursuit were predictable from the responses measured during ocular pursuit and VOR cancellation. The reduction in Ev Pk cell responses was roughly proportional to the reduction in the contribution of eye velocity to gaze pursuit (mean reduction in modulation of 58% compared with a mean concomitant reduction in peak eye velocity of 66%), which suggests that the signals generated by Ev Pk cells were related primarily to eye movements during both head-restrained and head-free pursuit. A prediction of Gv Pk unit responses during head-free pursuit was made by decomposing the averaged, desaccaded gaze velocity record into eye and head movement components, and modeling each unit's expected response by summation of its sensitivity to eye movements during ocular pursuit and to head velocity during cancellation of the VOR. In Fig. 5B the predicted Gv Pk response during head-free pursuit is plotted as a function of the observed response. The mean predicted modulation of Gv Pk cells during head-free pursuit was 23.8 spikes/s, or 0.78 spikes/s/deg/s re gaze velocity, compared with a mean observed modulation of 21.5 spikes/s, or 0.71 spikes/s/deg/s re gaze velocity.

The sensitivity of Gv Pk cells to head velocity during smooth head pursuit was estimated by vector subtraction of the estimated ocular pursuit component from each unit's head-free pursuit response. Figure 6 plots this estimated active head movement sensitivity versus sensitivity to passive head velocity as measured during VOR cancellation. Since the VOR was usually not entirely suppressed in the VOR cancellation paradigm (see Table 1), the estimate of passive head velocity sensitivity was obtained with the algorithm described in the accompanying paper (Belton and McCrea 2000), which corrected for the influence of residual eye velocity on unit firing behavior. Although most Gv Pk cells were less sensitive to head velocity during active gaze pursuit than to passive head velocity during VOR cancellation, some units were more sensitive to active pursuit head movements than to VOR cancellation. On average, Gv Pk cells were less sensitive to head velocity during active gaze pursuit than to passive head velocity during VOR cancellation. The slope of a linear regression fit to the data in Fig. 6 was 0.76. 



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Fig. 6. The estimated head velocity sensitivity of Gv Pk cells during head-free pursuit is plotted as a function of head velocity sensitivity during cancellation of the VOR during passive whole body rotation (VORc). Unit responses in both conditions were estimated after vector subtraction of the eye movement-related component of the response (the coefficient estimated from ocular pursuit). The dashed line has a slope of 1.0. Note that most Gv Pk cells were less sensitive to active head movements than to passive head movements.

In sum, only a few FLR Pk cells were as sensitive to gaze velocity during gaze pursuit as they were during ocular pursuit. The unequal modulation of FLR Pk cells during comparable types of smooth pursuit suggests that this region of the cerebellar cortex does not contribute as much to the control of gaze velocity during combined eye-head pursuit as it does to eye velocity during ocular pursuit.

Responses of Pk cells during passive WBR with the head free

In head-restrained squirrel monkeys, passive WBR evokes eye movements that compensate for head movement in space both in the presence and absence of visual targets. Most FLR Pk cells are sensitive to the eye velocity evoked by passive WBR, although the sensitivity is significantly enhanced by the presence of an earth stationary visual target (Belton and McCrea 2000). With their head free to move, the monkeys generated smooth compensatory head movements as well as eye movements during passive WBR. The combined gaze velocity produced by eye and head movements was nearly sufficient to stabilize gaze in space (Table 1). Since most Pk cells were more sensitive to eye velocity than to head velocity, the firing rate modulation observed during passive WBR when the head was free was typically smaller than that observed when the head was restrained. Examples of the responses of typical Ev and Gv Pk cells are illustrated in Fig. 7A. The traces at the top of Fig. 7A show averaged, desaccaded responses evoked by 0.5 Hz WBR when the head was restrained (Fig. 7A, left) and when the head was free to move (right). In both cases the monkey fixated an earth stationary target and maintained relatively stable gaze (Gv traces). When the head was restrained, gaze stability was maintained by smooth compensatory eye movements (Ev) alone. However, when the head was free, stability was maintained with a combination of smooth eye movements and head on trunk, or neck movements (Nv). The neck movements reduced the head-in-space movement (Hv) that would otherwise have been induced by the stimulus.



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Fig. 7. Responses of FLR Pk cells to passive whole body rotation (WBR) during fixation of an earth stationary target. Typical behavioral and single-unit responses are illustrated in A. In the head-restrained condition (left column) the monkeys produced smooth compensatory eye movements (Ev) that stabilized gaze in space (Gv). In the head-free condition (right column), gaze was stabilized primarily with smooth compensatory head on neck movements (Nv) that reduced head velocity in space (Hv). The bottom histograms show the averaged responses of an Ev and a Gv Pk cell in the 2 conditions. Note that modulation in the responses of both types of cells was reduced in the head-free condition. B: the sensitivity to WBR in the head-free condition is plotted as a function of sensitivity to WBR in the head-restrained condition. Dashed line indicates equal sensitivity to passive rotation in each condition.

The histograms at the bottom of Fig. 7A illustrate the averaged responses of an Ev and Gv Pk cell in both conditions. Both cell types were sensitive to WBR in the head-restrained condition, but the modulation in the Gv Pk cell's firing rate was less due to the oppositely directed eye and head velocity-related signals being out of phase and thus canceling in this paradigm. When the head was free to move, the modulation in firing rate of both Pk types was reduced. Figure 7B summarizes the relative sensitivity of both cell types to the stimulus velocity during WBR in the presence of an earth stationary target in the head-restrained and head-free conditions. On average, the modulation of FLR Pk cells was reduced by more than half (i.e., the slope of a linear fit to the data illustrated in Fig. 7B was 0.43), which corresponded well to the reduction in VOR eye velocity observed in the head-free condition (i.e., a 58% reduction). Thus FLR Pk cells are preferentially sensitive to eye velocity during a task that requires stable gaze to be maintained during passive WBR as well as during smooth gaze pursuit.

Responses of Pk cells during simultaneous gaze pursuit and passive WBR

One question that arises is whether the head movement sensitivity of FLR Purkinje cells during VOR cancellation and head-free pursuit is due to vestibular sensory inputs or to active pursuit motor commands. Interactions of active and passive head movements were studied by rotating the turntable at one frequency (2.3 Hz) while the monkey pursued a target in the head-free condition that was moved at a second, nonharmonic frequency (0.5 Hz). The responses of an Ev and a Gv Pk cell during combined active and passive head movements are illustrated in Fig. 8. The target was projected from a turntable-mounted laser, so that the passive oscillation of the turntable did not perturb the 0.5-Hz target motion with respect to the monkey's head (dashed trace, Tgv, in Fig. 8B). Thus the movement of the target in space (dashed trace in Fig. 8A) was a combination of the 2.3-Hz turntable motion and the 0.5-Hz movement of the target with respect to the head and eyes.



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Fig. 8. Firing behavior of an Ev Pk (A1-C1) and Gv Pk cell (A2-C2) during combined active and passive head movements. The visual target (dashed lines in A and B) was projected from the turntable, and thus moved in tandem with turntable motion (2.3 Hz, 20°/s). Simultaneously the target moved with respect to the monkey (0.5 Hz, 40°/s peak velocity), which was rewarded for tracking the target with eye and/or head movements. A: typical behavioral and single-unit records obtained from Pk cells during smooth tracking of the moving target during ongoing whole body rotation. Target, horizontal gaze (green trace), and head position (blue trace) are superimposed in the top half of A1 and A2. Table position and firing rate histograms of an Ev Pk cell and Gv Pk cell are shown in the bottom half of A1 and A2. The traces in B1 and B2 are cycle averaged, desaccaded records at the frequency of target motion with respect to the turntable (0.5 Hz). The traces in C1 and C2 are cycle-averaged, desaccaded records at the frequency of turntable motion in space (2.3 Hz). Note that head movements contributed significantly to active target following in this circumstance. The head movements were a combination of smooth pursuit head movements (Hv re trunk, blue traces in B1 and B2) and compensatory vestibulocollic reflex movements evoked by passive whole body rotation at 2.3 Hz (blue traces in C1 and C2, head velocity in space). The latter high-frequency head movements evoked a VOR that was not effectively suppressed. The responses of Pk cells to both active and passive head movements was predictable using the sensitivities to eye and passive head velocity measured during ocular pursuit and VOR cancellation (dotted red traces superimposed on firing rate histograms in B and C).WBR, whole body rotation; Tgv, target velocity; Tbv, table velocity.

The passive head oscillation induced by higher frequency turntable rotation was reduced by compensatory vestibulocollic reflex head movements, but the residual passive head movement (blue traces in Fig. 8C) did drive gaze off the target when the VOR was not completely suppressed. The responses of Pk cells to this combination of active and passive head rotation were analyzed by averaging cycles at the frequency of target movement (B1 and B2) and turntable movement (C1 and C2).

The modulation in the firing rate of both Pk cell types with respect to target velocity and with respect to passive head velocity were similar to responses predicted from their firing behavior during ocular pursuit and VOR cancellation (dotted traces superimposed on the averaged, desaccaded histograms in Fig. 8, B and C). Ev Pk cells were nearly equally sensitive to eye velocity related to ocular following movements evoked at 0.5 Hz as they were to the compensatory (but retinal slip producing) eye movements evoked by turntable rotation at 2.3 Hz. Their responses were similar to those predicted from their sensitivity to ocular pursuit in the absence of active or passive head movements (dotted trace superimposed on unit firing rate in Fig. 8, B1 and C1). The firing behavior of Gv Pk cells during combined head-free pursuit and WBR were also predictable from the responses recorded in the head-restrained condition during ocular pursuit and VOR cancellation (dotted traces in Fig. 8, B2 and C2).

The eye and head velocity-related signals observed during 0.5-Hz ocular pursuit and VOR cancellation were evident at a similar gain and phase both in the firing rate modulation associated with active head-driven pursuit and also with the smaller modulation (or lack of modulation) related to passive WBR. The equivalent sensitivity of both types of Pk cells to eye and head movements that both aided and inhibited target tracking suggest that the head movement signals they produce in response to head rotation are more likely related to vestibular, or to neck reafferent (Wilson et al. 1976) signals during head-free pursuit than to an internal estimate or prediction of target motion.

Effects of muscimol inactivation of the flocculus region on combined eye-head pursuit

The effect of unilateral inactivation of the flocculus region on gaze pursuit was studied in two animals. In each animal the injection of muscimol was into an area of the FLR where the majority of Gv Pk cells were encountered in preceding single-unit recording sessions (see Belton and McCrea 2000 for details). A comparison of the effects of muscimol injection on smooth pursuit when the head was restrained and when the head was free to move are shown in Fig. 9. Prior to muscimol injection, pursuit gaze velocity (red traces in Fig. 9, A1 and A2) was comparable in the head-free and head-restrained conditions, despite the fact that pursuit was accomplished primarily by eye movements in the head-restrained condition and by head movements in the head-free condition. On injection of 1.25 µL of 2% muscimol into the FLR, ocular pursuit was greatly compromised (Fig. 9B1). Head-free pursuit was relatively unaffected, although the frequency of ocular saccades during pursuit ipsilateral to the lesion increased markedly (Fig. 9B2). The preserved ability of the monkey to pursue targets with smooth head movements compensated for the loss of the ability to generate smooth eye movements. Both monkeys were able to suppress VOR eye movements during head-driven target pursuit (Fig. 9, B2 and C2) and were able to match gaze to target velocity well enough to be consistently rewarded in this paradigm during the period of muscimol inactivation of the FLR. Multiple cycles of desaccaded 0.5-Hz pursuit were averaged to quantify the effects of inactivating the FLR (Fig. 9C). Prior to muscimol injection the mean gain of ocular pursuit (solid red filled trace in Fig. 9C1) was approximately 0.84 in one monkey and 0.79 for the other monkey. Following muscimol injection the gain of smooth pursuit eye movements in both directions was reduced by half to approximately 0.4. Ipsilateral ocular pursuit was more affected in one monkey (illustrated in Fig. 9), while in the other monkey contralateral ocular pursuit was more affected. In each monkey the pursuit deficits were superimposed on a spontaneous nystagmus that was present both in the dark and in the light. In one the muscimol injection produced a weak (<= 3°/s) ipsilateral spontaneous nystagmus, while in the other it produced a weak contralateral spontaneous nystagmus. The quick phase of the nystagmus was to the direction in which pursuit was most affected. That is, the asymmetry in the effect of muscimol injection on pursuit in both the head-restrained and head-free conditions was probably due to a concomitantly produced spontaneous nystagmus.



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Fig. 9. Inactivation of the floccular region with muscimol. A1 and A2: preinactivation pursuit in the head-restrained and head-free conditions. Records of gaze velocity (red) and head velocity (blue) are superimposed on target velocity (dashed trace). B1 and B2: sample recordings of pursuit after injection of a 2% solution of muscimol into the floccular region. Orange and blue records are gaze and head velocity respectively. C1 and C2: averaged desaccaded records before and after muscimol injection are superimposed (filled red and orange traces) with averaged records of head velocity (blue trace, Hv) and target velocity (dashed trace).


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The Purkinje cells in the flocculus region of the cerebellum are essential for producing and coordinating gaze velocity when eye movements alone are used to track a target. Most FLR Pk cells are strongly modulated during smooth pursuit eye movements, and inactivation or removal of the FLR compromises both the ability to produce smooth pursuit eye movements and the ability to suppress the VOR when the head is passively perturbed. However, the FLR is less influential in the control of gaze when smooth head movements are used to track a moving target. Half of the cells in the FLR, the Ev Pk cells, were completely insensitive to active, smooth pursuit head movements. Gv Pk cells, which constitute the other half of the output of the FLR, were less sensitive to smooth pursuit head movements than to smooth pursuit eye movements. When the FLR was inactivated using muscimol, there was no effect on pursuit head movements and little effect on the ability to cancel the VOR during smooth head tracking. The signals produced by FLR Pk cells are thus neither sufficient nor necessary for producing smooth pursuit head movements or for canceling the VOR during gaze pursuit.

We conclude that, while the flocculus region of the cerebellum is essential for the production of smooth pursuit eye movements, it is probably not an essential part of the neural substrate for controlling gaze velocity. The implication is that pursuit eye and head movements may not be coordinated by a common gaze velocity command that summates with vestibular reafferent signals in the FLR, the vestibular nuclei, or extraocular motoneurons. We suggest that pursuit eye and head movements are separately programmable and controllable and that they are coordinated by summation of a central estimate of active head movements with vestibular reafferent signal on VOR pathways (Fig. 10).



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Fig. 10. Mechanisms for cancellation of the VOR produced by active and passive head movements during pursuit of visual targets. In this scheme, eye and head pursuit movements are separately programmed. The FLR plays a critical role in constructing ocular pursuit commands (red arrows). Ocular pursuit commands are added with vestibular signals (green/blue arrows) and an internal estimate of the output of VOR pathways (orange arrow) so that image motion with respect to the head and eyes is minimized. When the head is used to pursue targets, an efference copy of active head pursuit movements is used to cancel vestibular reafferent signals on central VOR pathways. The reduction in the output of the VOR also reduces the eye velocity feedback to the flocculus. The absence of ocular pursuit commands and eye velocity feedback signals substantially reduces the output of the flocculus during head pursuit. However, VOR pathways and the flocculus remain sensitive to passive head movements and aid smooth tracking of images when unanticipated perturbations of the head occur.

Absence of gaze velocity signals in the squirrel monkey FLR

The output of the squirrel monkey flocculus and ventral paraflocculus is rarely, if ever, related to gaze velocity. In the accompanying paper we reported that half of the Pk cells in the FLR (the Ev Pk cells) were not sensitive to gaze velocity during VOR cancellation, and that only a few of the FLR Gv Pk cells that were sensitive to passive head movements were equally sensitive to gaze velocity during ocular pursuit and VOR cancellation (Belton and McCrea 2000). We also found that the signals produced by Gv Pk cells were not strictly related to head and eye velocity during passive head movements, since the amplitude of their response was usually dependent on the presence of a target. In this study we found not one Gv Pk cell that was equally sensitive to gaze velocity during ocular pursuit and gaze pursuit. The lack of correlation of Gv Pk cell firing rate and gaze velocity was even more evident during other types of gaze shifts. No FLR Pk cell coded gaze velocity during saccadic gaze shifts (Belton and McCrea 1999) or during cervicoocular reflex eye movements (unpublished observations).

We referred to Gv Pk cells as gaze velocity Pk cells in our studies primarily because cells with similar characteristics in the rhesus monkey were labeled in this way. In those animals many Gv Pk cells are, on average, equally sensitive to horizontal eye velocity during smooth pursuit eye movements and to head velocity during VOR cancellation (Fukushima et al. 1999; Lisberger and Fuchs 1978a; Miles et al. 1980). Although the choice of nomenclature of rhesus macaque FLR Gv Pk cells was understandable at the time it was given, the appropriateness of the nomenclature is questionable. FLR Pk cells have been reported to produce signals that are related to head movements in many species, but Pk cells that are equally sensitive to eye and head movements are relatively rare in goldfish (Pastor et al. 1994), rabbits (Ghelarducci et al. 1975; Leonard and Simpson 1986), cats (Cheron et al. 1997; Fukushima et al. 1996) squirrel monkeys, and Japanese macaques (Fukushima et al. 1999).

One difference between squirrel monkeys and rhesus monkeys that could be relevant is that squirrel monkeys have a more limited oculomotor range and are presumably less inclined to rely entirely on ocular pursuit to track moving targets. Suppression of the VOR must occur frequently in such arboreal animals. In fact, squirrel monkeys prefer to track moving targets with head movements when given the opportunity, and it is likely that this strategy utilizes other, nonvisual mechanisms for canceling the VOR (Cullen et al. 1991). A second difference between squirrel monkeys and larger primates is that they have a relatively small interocular distance that then requires a smaller vergence angle to be maintained during fixation or pursuit of a target that is located in a small laboratory. If, as seems likely, the FLR were involved in modifying the VOR as a function of viewing distance, the relatively low sensitivity of squirrel monkey FLR Pk cells to head rotation could reflect the comparative absence of required viewing distance-related modifications in the VOR in our experiments.

Recently Fukushima and colleagues (Fukushima et al. 1999) reported that most of the vertically responsive macaque FLR Pk cells were less sensitive to head velocity during VOR cancellation than to eye velocity during ocular pursuit. They referred to FLR Pk cells whose head velocity signals were significantly weaker than their pursuit eye velocity signals as eye-head-velocity Pk cells. In light of the poor correlation between Gv Pk cell firing behavior with gaze velocity in so many behavioral contexts, it would probably be better to refer to all Pk cells in the FLR in every species that are sensitive to head velocity as eye-head-velocity Pk cells in the future.

Coordination of eye and head movements during smooth tracking of visual targets

The problem of coordinating eye and head movements during smooth tracking of visual targets has been studied in both monkeys and humans (Barnes 1993; Barnes and Eason 1988; Grant et al. 1992; Gresty and Leech 1977; Kubo et al. 1981; McKinley and Peterson 1985). One way the output of the two motor systems could be coordinated is by subtraction of a vestibular reafferent signal related to active head movement from a gaze velocity command, produced from an internal estimate of target motion (Bizzi 1981; Lanman et al. 1978; Lisberger et al. 1981). The subtraction could occur on extraocular motoneurons or on central VOR pathways. The difference would be used to produce a smooth pursuit eye movement that kept the image on the retina. However, this scheme does not account for important features of the eye and neck motor systems. The axes of rotation of the eye and the head are usually different; which means that rotation of the head produces translation as well as rotation of images on the retina. More importantly, the VOR cannot be used to coordinate gaze during head movements that require eye rotations larger than the range of the oculomotor plant (±20° in squirrel monkeys). Presumably one of the primary reasons why pursuit head movements are generated rather than eye movements alone is the expectation that the target movement might extend beyond the oculomotor range.

Head movements play a prominent role in gaze control, but they also subserve other, equally important, functions (Fuller 1992). For example, it seems likely that head movements play an important role in auditory, somatosensory, and olfactory tracking. Retinal or oculomotor coordinates seem inappropriate for producing accurate head tracking with those modalities. It may also be useful to separate eye and head control during visual tracking. In this study only angular head movements in the horizontal plane were unrestricted. When active gaze shifts are restricted in this manner, the central programming of the angular rotation of the head and eyes is nearly equivalent. However, the programming of eye and head movements is inherently different in most circumstances. The head/neck motor plant has many degrees of freedom and is capable of significant translation as well as rotation. Eye movements are exclusively angular rotations with three degrees of freedom. A separate computation of linear translation to angular rotation must be carried out to produce adjustments in the VOR related to linear translation of the head (Chen-Huang and McCrea 1999b). Not only are the coordinates of eye and head movement control different, but the range and axes of the two motor systems are significantly different. Thus it is unlikely that the vestibular reafferent signals produced during active head movements would be adequately canceled by a central estimate of target velocity in space or gaze velocity. Even if the brain constructed an internal estimate of gaze velocity in space based on a summation of eye movements and vestibular reafference, the estimate would have to be significantly transformed to take into account the changing distance of the target and the posture of the head and the eye before the signal could be used to program or update a pursuit movement.

A simpler way to coordinate angular eye and head movements during pursuit would be to subtract an efference copy of intended head velocity from vestibular reafferent signals in central VOR pathways (Fig. 10). Any errors in the computation could be corrected using visual feedback. The flocculus region of the cerebellum could adjust the output of central VOR pathways based on visual feedback or unanticipated passive head movements. It would not play a primary role in coordinating eye and head movements, but rather provide the substrate for producing relatively small smooth pursuit eye movements that compensate for both anticipated and unanticipated errors in tracking a moving image. The results of our recent studies in the vestibular nuclei suggest that the summation of active head movement signals with passive vestibular reafferent signals occurs in the vestibular nuclei on secondary vestibular neurons (Gdowski and McCrea 1999; McCrea et al. 1996, 1999). The primary advantage of using an internal estimate of head movement to cancel vestibular signals in VOR pathways rather than a gaze velocity signal or a central estimate of target velocity in space is that it allows eye and head movements to be separately controlled.

Sensitivity of FLR Pk cells to passive and active head movements during pursuit

Summation of gaze pursuit commands with vestibular signals on VOR pathways may not always be appropriate for coordinating pursuit eye and head movements, but it is necessary for modifying signals in VOR pathways when a visual target moves with the head during passive rotation or translation of the whole body. For example, when the substrate on which a primate is standing (e.g., a tree limb or a commuter train) has objects of interest attached to it, the VOR must be canceled to maintain stable vision. In that circumstance, summation of sensory vestibular reafferent signals with ocular pursuit commands is the best way to maintain image stability. In the accompanying paper we reported that most of the Purkinje cells in the FLR were more sensitive to ocular pursuit eye movements than to passive head movements when the monkeys canceled their VOR by fixating a head stationary target. In spite of the relative insensitivity of most Pk cells to head velocity, inactivation of the flocculus compromised smooth pursuit eye movements and VOR cancellation approximately equally, which suggested that the output of flocculus region Pk cells were necessary for both functions. The implication is that the output of the FLR is used to cancel the VOR during passive, but not active head rotation.

Considered as a population, FLR Ev Pk cells were insensitive to head velocity during both active and passive head movements. However, this region of the cerebellum is apparently essential for canceling the VOR when the head is passively moved and the VOR must be canceled (Belton and McCrea 2000; Partsalis et al. 1995; Takemori and Cohen 1974; Waespe et al. 1983; Zee et al. 1981; Zhang et al. 1995). In this circumstance the signals generated by FLR Pk cells are altered in two ways. The eye velocity signals normally present during when an earth stationary target moves relative to the head are absent, and the Gv Pk cells generate a signal related to head velocity in space. The removal of the eye velocity signal represents the larger part of the change in firing behavior, both in individual Pk cells and in the population as a whole. The two changes together constitute a considerable change in the output of the FLR. Since similar changes in FLR Pk cell output occur during head-free pursuit, why was smooth head pursuit essentially unaltered when muscimol was injected into the FLR?

One possible explanation is that two parallel mechanisms for VOR cancellation coexist, and that one mechanism can replace the other when the other is absent or compromised (Barnes 1993). We suggested above that the VOR may be canceled during gaze pursuit by the subtraction of an efference copy of active head movements from a vestibular reafferent signal in the vestibular nuclei. A second mechanism may also be available that relies on subtraction of an internal estimate of target movement with respect to the head from vestibular reafferent signals in the vestibular nuclei. FLR Pk cells are essential for the second mechanism but not the first. The VOR evoked by passive, unpredictable head movements in space could not be suppressed by an efference copy mechanism, and the FLR is clearly essential in that circumstance. During head-free pursuit the VOR is canceled primarily with another, efference copy, mechanism, and the flocculus is not essential for canceling the VOR. A second, related explanation is that a large fraction of the eye movement-related signals produced by FLR Pk cells are derived from inputs that are modulated by the output of VOR pathways (Stone and Lisberger 1989). These efference copy eye velocity signals would in effect be canceled if head movement efference copy signals modified the output of VOR pathways directly.

Implications for functional organization of the cerebellar cortex in motor control

The question of the role of the FLR in gaze control can be considered a test of the more general question of whether individual efferent modules in the cerebellar cortex are organized to coordinate the contribution of disparate muscle groups and motor pattern generators to the control of multiple movement fields, or whether each efferent module is organized to ensure the accurate control of a single central pattern generator with a more limited behavioral objective.

The cerebellar cortex is organized into a series of parasagittal microzones containing Purkinje cells that receive a common climbing fiber input from a group of cells in the inferior olive and which project to a discrete region(s) of the cerebellar or vestibular nuclei (Herrup and Knemerle 1997; Ito et al. 1977; Oscarsson 1979; Voogd et al. 1996). Since the microzones often cross regions of the cerebellar cortex that are involved in controlling movements of several different parts of the body (Brooks and Thach 1981; Ito 1984; Thach et al. 1992), each zone could be organized to modify the output of a set of central pattern generators with a common motor function (e.g., pattern generators related to the extensor phase of locomotion, eye blink, smooth eye rotation). Alternatively, each zone could be organized to control pattern generators that share a common behavioral goal (maintenance of balance during locomotion, prevention of corneal damage, prevention of retinal image slip). In short, an individual efferent zone in the cerebellar cortex could be designed to control central networks that produce particular patterns of movements or it could be organized so that it plays a more general role in coordinating behavior, regardless of specific motor circuits involved in producing it.

There is anatomical and electrophysiological evidence that the efferent zones in the FLR influence brain stem pattern generators that produce smooth eye movements in a particular direction. The FLR is spanned by several zones that each receive climbing fiber inputs related to retinal image slip in a single plane that also corresponds to the horizontal/vertical activation planes of the vestibular canals (De Zeeuw et al. 1994; Sato and Kawasaki 1990, 1991; Tan et al. 1995; Van der Steen et al. 1994). Microstimulation within each zone produces eye movements in one direction, and the Pk cells in each zone tend to have signals related to eye movements in the same direction (Balaban et al. 1984; Belknap and Noda 1987; Ito et al. 1977; Nagao et al. 1985).

The results of our studies suggest that the efferent zones that span the flocculus and ventral paraflocculus are not involved in coordinating the gaze velocity produced by simultaneous eye and head movements but rather are dedicated to regulating the speed of smooth eye movements as a function of behavioral context. The general implication of this result is that the output of one efferent zone in the cerebellar cortex is dedicated to modifying the output of one central motor pattern generator, rather than coordinating the activity of multiple disparate groups of muscles and joints in multiple reference frames.

Conclusion

The flocculus region of the cerebellum is an essential part of the neural substrate that the brain utilizes for modifying signals in VOR pathways and for producing smooth visual following eye movements. However, it does not appear to be either necessary or sufficient to coordinate gaze during smooth pursuit when head movements are used for tracking. Two implications of these observations are that the efferent zones in the FLR primarily function to regulate the speed of smooth eye movements rather than gaze, and that smooth pursuit eye movements are not used to cancel the VOR during smooth tracking.


    ACKNOWLEDGMENTS

This work was supported by National Institutes of Health Grants RO1-EY-08041 and P60-DC-02072.

Present address of T. Belton: Dept. of Physiology and Biophysics, New York University Medical Center, 550 First Ave., New York, NY 10016.


    FOOTNOTES

Address for reprint requests: R. A. McCrea, Dept. of Neurobiology, Pharmacology and Physiology, Abbott 07, University of Chicago, 5830 So. Ellis Ave., MC 0926, Chicago, IL 60637 (E-mail: ramccrea{at}midway.uchicago.edu).

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 3 January 2000; accepted in final form 23 May 2000.


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