Neurons in the Posterior Interposed Nucleus of the Cerebellum Related to Vergence and Accommodation. I. Steady-State Characteristics

Hongyu Zhang and Paul D. R. Gamlin

Vision Science Research Center, University of Alabama at Birmingham, Birmingham, Alabama 35294

    ABSTRACT
Abstract
Introduction
Methods
Results
Discussion
References

Zhang, Hongyu and Paul D. R. Gamlin. Neurons in the posterior interposed nucleus of the cerebellum related to vergence and accommodation. I. Steady-state characteristics. J. Neurophysiol. 79: 1255-1269, 1998. The present study used single-unit recording and electrical microstimulation techniques in alert, trained rhesus monkeys to examine the involvement of the posterior interposed nucleus (IP) of the cerebellum in vergence and accommodative eye movements. Neurons related to vergence and ocular accommodation were encountered within a circumscribed region of the IP and their activity during changes in viewing distance was characterized. The activity of these neurons increased with decreases in vergence angle and accommodation (the far-response) but none showed changes in activity during changes in conjugate eye position and we therefore term them "far-response neurons." Far-response neurons were found within a restricted region of the IP that extended ~1 mm rostrocaudally and mediolaterally and 2 mm dorsal to the fourth ventricle. Microstimulation of this far-response region of the IP with low currents (<30 µA) often elicited divergence and accommodation for far. The behavior of 37 IP far-response neurons was examined during normal binocular viewing, during monocular viewing (blur cue alone), and during binocular viewing with accommodation open-loop (disparity cue alone). The activity of all cells was modulated under all three conditions. However, the change in activity of some of these neurons was significantly different under these three viewing conditions. The behavior of 70 IP far-response neurons was compared during normal binocular viewing and during viewing in which the accommodative response was significantly dissociated from the vergence response. The data from these two conditions was pooled and multiple regression analyses for each neuron generated two coefficients expressing the activity of the neuron relative to the vergence and to accommodative response respectively. On the basis of these coefficients, the overall activity of the neurons were classified as follows: 34 positively correlated with divergence, 11 positively correlated with far accommodation, 14 positively correlated with divergence and far accommodation, 9 positively correlated with divergence and accommodation, and 2 positively correlated with convergence and far accommodation. The results of this study demonstrate the involvement of a specific region of the posterior interposed nucleus of the cerebellum in vergence and accommodation. IP far-response neurons are active for vergence and accommodation irrespective of whether or not these eye movements are elicited by blur or disparity cues. The data in the present study strongly suggest that this cerebellar region is a far-response region that is involved in vergence as well as accommodative eye movements.

    INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References

Experimental and clinical studies of the cerebellum have shown that it is involved in the dynamic and adaptive control of eye movements (see Lewis and Zee 1993 for a review). It has been shown in primates that the dorsal vermis (lobules VI and VII) and the underlying fastigial nuclei are involved in the regulation of saccades and smooth pursuit (e.g., Fuchs et al. 1993, 1994; Kase et al. 1979; Keller 1989; Noda et al. 1988; Optican and Robinson 1980; Suzuki and Keller 1988a,b). The dentate nucleus and adjacent y-group possess neurons that discharge in relation to vertical pursuit eye movements (Chubb and Fuchs 1982). The cerebellar flocculus and paraflocculus are involved in the vestibulo-ocular reflex, optokinetic nystagmus, and smooth pursuit (e.g., Büttner and Waespe 1984; Lisberger and Fuchs 1978; Miles et al. 1980). In general, however, these oculomotor studies have focused on the role of the cerebellum in controlling conjugate eye movements and relatively little is known about the contribution of the cerebellum to the control of vergence and ocular accommodation.

Single-unit recording studies in alert, trained rhesus monkeys have previously reported the presence of neurons related to vergence and accommodation in the midbrain in the vicinity of the oculomotor nucleus (Judge and Cumming 1986; Mays 1984; Mays et al. 1986; Zhang et al. 1992). We have shown that neurons within the fastigial and interpositus nuclei of the cerebellum have reciprocal connections with this midbrain region (May et al. 1992). We have also shown that some neurons within a medial zone of a precerebellar nucleus, the nucleus reticularis tegmenti pontis (NRTP) are related either to the near-response or to the far-response (Gamlin and Clarke 1995). Furthermore, Westheimer and Blair (1973) reported transient vergence deficits in primates after cerebellectomy. Thus there is some circumstantial evidence that the cerebellum plays a role in vergence and ocular accommodation in primates.

There is further evidence that the deep cerebellar nuclei are involved in ocular accommodation. Recording studies in paralyzed, anesthetized cats have reported cells in the interpositus nucleus that are related to accommodation, and electrical microstimulation of this region orthodromically activated accommodation-related cells in the midbrain (Bando et al. 1979a,b). In addition, stimulation of the posterior interposed nucleus bilaterally and the fastigial nucleus contralaterally is reported to elicit positive accommodation (Hosoba et al. 1978).

Lesions in humans also suggest a role for the cerebellum in vergence and accommodative eye movements. Some patients with cerebellar lesions have been reported to show convergence excess or divergence failure (Leigh and Zee 1991). More recently Kawasaki and coworkers reported a case in which a patient with cerebellar signs manifested a slow release of accommodation (Kawasaki et al. 1993). Taken together, these studies indicate a role for the cerebellum in vergence and ocular accommodation. However, the evidence described above is fragmentary. The present study was therefore aimed at providing more detailed information on cerebellar involvement in vergence and ocular accommodation. With the use of single-unit recording techniques, we have identified neurons in a specific region of the posterior interposed nucleus (IP) of the rhesus monkey that are related to divergence and ocular accommodation for far. Electrical microstimulation of this region produces divergence eye movements and accommodation for far. The activity of these IP far-response neurons is related to vergence and accommodation regardless of whether or not these movements are elicited during normal viewing conditions, during blur-driven monocular viewing (disparity open-loop), or during disparity-driven viewing (accommodation open-loop). Also, with dissociation of vergence from accommodation we found that, as is the case for midbrain neurons (e.g., Judge and Cumming 1986; Zhang et al. 1992), the activity of some neurons is related predominately to vergence, some predominantly to accommodation, and some to both vergence and accommodation. In addition, the firing rate of IP far-response cells is related not only to the steady-state far-response, but is also correlated with the dynamics of the movement. Some cells display an apparent sensory signal related to the appearance of the far target. These dynamic and sensorimotor characteristics of IP far-response cells will be presented separately. Preliminary results of some of these findings have been presented elsewhere (Gamlin 1991; Gamlin et al. 1996; Zhang and Gamlin 1994).

    METHODS
Abstract
Introduction
Methods
Results
Discussion
References

Animal preparation and surgery

Three juvenile rhesus monkeys (Macaca mulatta) were used in this study. All experimental procedures were approved by the University of Alabama at Birmingham Institutional Animal Care and Use Committee and complied with the United States Public Health Service Policy on Human Care and Use of Laboratory Animals. All the described surgical procedures were performed under sterile conditions using sodium pentobarbital anesthesia. Postsurgically animals received analgesics (Numorphan, 0.15mg/kg or Buprenex, 0.01 mg/kg) to minimize pain. Initially, animals were implanted with four fixation plates attached to the skull by bone screws. One end of each plate was bent up ~1 cm and a headpost cemented between them with dental acrylic. The scalp was sutured to tightly approximate the acrylic and the plates thus covered completely by skin. With this system the head can be held stationary during training and subsequent recording sessions. Subsequently, scleral search coils for measuring eye movements (Fuchs and Robinson 1966) were implanted under the conjunctiva of each eye according to Judge et al. (1980). Finally, two recording cylinders, one on each side of the skull, were implanted stereotaxically over the cerebellum at a 15° angle to the sagittal plane.

Visual display

The visual display system (Fig. 1) is similar to one used previously by us (Gamlin et al. 1989). It has a field of view of ±18° and is based on the original design of Crane and Clark (1978). It has however been modified to allow video displays to be used for target presentation, with each having its own independent accommodative and vergence demand. By switching between two pairs of video displays, step changes in both accommodative and vergence demand can be produced, with real targets denoted by positive values of accommodation and vergence demand. This modification also allows a stationary fixation target with a well-defined vergence and accommodative demand (range -4 D to +12 D) to be presented on one pair of video displays at the same time as either static or moving visual probe stimuli with different vergence and accommodative demands are presented on the other pair of displays. Two important aspects of this visual stimulator are that 1 cm of motion of lenses L1-4 produces a 4 D change in target focus and that, because the optical arrangement closely approximates a Badal lens system, changes in accommodative demand produce no significant change in target size or luminance. To facilitate the presentation and analysis of accommodation and vergence, we calculate them in the equivalent units of diopters (D) and meter angles (MA) respectively. One meter angle corresponds to the reciprocal of the target distance in meters. In this scheme, vergence angle measured in MA is related to the vergence angle measured in degrees by the interpupillary distance. For example, with an interpupillary distance of 3.2 cm, the average for our animals, 1 MA is approximately equal to 1.83°.


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FIG. 1. Schematic view of visual display apparatus. L1-4 are 150-mm focal length moveable camera lenses. L5-8 are 50 mm focal length camera lenses. L9-12 are achromatic doublets with an effective focal length of 106 mm. MM1 and MM2 are moveable mirror galvanometers conjugate with the center of rotation of left and right eye, respectively. M1-4 are front surface mirrors. M5 and M6 are 50/50 beamsplitters. P1 and P2 are pellicles. TV1-4 are super VGA monochrome monitors.

Standard behavioral tasks

TARGETS STEPPED IN DEPTH. In this trial type the animal was required to make a far- or a near-response to a visual target that was stepped in depth. Except for accommodation open-loop trials, all of these trials and tracking in depth trials used a checkered target subtending 1.8°. Because of the independent video displays, the initial target position could range from -4 D to +12 D and from -4 to +12 MA and the required target step could be presented over the same ranges. Also, because accommodative and vergence demands could be varied independently in this optical system, targets could be presented with either appropriate or conflicting vergence and accommodative demands as described below.

SACCADIC TRIALS. Animals were required to make saccades to visual targets over a range of ±18° horizontally or vertically.

PURSUIT TRIALS. Animals were required to pursue a target that moved in depth with either a ramp or a predictable sinusoidal trajectory of different frequencies to elicit vergence and accommodative tracking. These target could also move in the frontoparallel plane so that smooth pursuit eye movements could be elicited.

In general, both saccadic and smooth pursuit eye movements in the frontoparallel plane were used to test whether a putative far-response neuron displayed activity that was influenced by conjugate eye position or velocity.

Further characterization of vergence and accommodation characteristics

As shown in Fig. 2, the vergence and accommodation systems are cross-coupled, i.e., disparity-driven vergence signals influence accommodative output and, similarly, blur-driven accommodative signals influence vergence output (e.g., Cumming and Judge 1986; Fincham and Walton 1957; Hung and Semmlow 1980; Schor 1992; Westheimer 1963). Thus to fully examine the relationship of the cell to its inputs and outputs, additional experiments are required that characterize the activity of the cell under viewing conditions other than those imposed by normal viewing of a target that moves in depth.


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FIG. 2. Dual interaction model of accommodation and vergence control (based on Hung and Semmlow 1980 and Schor 1992). AC, accommodative convergence; BA, blur-driven accommodation; CA, convergence accommodation; DC, disparity-driven convergence.

As described below, we used the visual apparatus to put disparity into an open-loop condition by having the animal view a target monocularly. This apparatus was also used to put accommodation into an open-loop condition. Finally, the apparatus was used for conflict viewing experiments.

BLUR-DRIVEN ACCOMMODATION AND VERGENCE WITH DISPARITY OPEN-LOOP. In this trial type, the animal views the target monocularly placing disparity in an open-loop condition. The target is stepped or ramped from one accommodative demand to another and the animal is required to make an accurate accommodative movement to the new target position. By measuring the accommodative response and vergence angle, one can calculate the response AC/A [i.e., the ratio of accommodative convergence (AC) measured in meter angles as a function of accommodation (A) measured in diopters].

DISPARITY-DRIVEN VERGENCE AND ACCOMMODATION WITH ACCOMMODATION OPEN-LOOP. In this trial type, the animal views binocularly with accommodation open-loop. The target is stepped or ramped from one disparity to another and the animal is required to make accurate vergence movements to the new target position. By measuring the accommodative response and vergence angle, one can calculate the response CA/C (i.e., the ratio of convergence accommodation (CA) measured in diopters as a function of convergence (C) measured in meter angles). In our optical system we placed accommodation into an open-loop condition by presenting a small (0.3°), circular image to the animal that was defocused to such an extent (at least -4 D) that it was composed of spatial frequencies of <0.5 cycles/deg. Because of the low spatial frequency content of this stimulus it did not elicit accommodative eye movements (Charman and Tucker 1977; Tsuetaki and Schor 1987), but did elicit robust vergence responses. In confirmation of these previous reports, during monocular viewing of this specific stimulus in our optical system, we found that its movement in depth elicited no changes in accommodation.

CONFLICT VIEWING. This trial type was designed to better characterize the response characteristics of the far-response cells. This approach has been used in studies of the midbrain near-response and far-response cells (Judge and Cumming 1986; Morley et al. 1992; Zhang et al. 1992). Under normal viewing conditions, the vergence and accommodation demands of real targets are matched in a predictable fashion determined by the interpupillary distance. Because of this, vergence and accommodation responses are also closely matched. Therefore to determine more precisely how a cell is related to each of these two components, the required vergence and accommodation responses are put into conflict, with the animal required to increase its vergence response while increasing its accommodation by significantly less. So that animals could achieve better dissociation of vergence from accommodation during conflict viewing, most data were obtained during trials in which the target ramped in depth for 1-3 s and then maintained a steady vergence and accommodative demand for the remainder of the trial. An example of the behavior seen during this type of trial type is shown in Fig. 3. The gain of the accommodative demand of the target was set to be 0.25 of that appropriate for normal viewing. Thus for example, the animal is required to converge 5 MA while only accommodating 1.25 D. Figure 3 shows that the animals were able to closely match their vergence and accommodative responses to these conflicting demands and thus dissociate these normally tightly coupled responses. We therefore used this paradigm to compare the activity of a far-response cell during binocular viewing in which the vergence and accommodation responses were matched to the activity of the cell when these responses were dissociated.


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FIG. 3. Typical behavior during normal binocular viewing (triangle ) and conflict viewing condition (black-triangle). In conflict viewing condition, accommodative demand was 25% of that in normal viewing condition. The dotted line(- - -) represents vergence and accommodative demands under 2 conditions. Note that response closely matches demand under these 2 viewing conditions.

Eye movement recording

The horizontal and vertical gains of each eye were calibrated independently at the beginning of each recording session. This was done by requiring the animal to fixate targets that appeared at various horizontal and vertical positions using either eye alone. Animals showed little variability in fixation from trial to trial and saccades of <0.2° could be reliably detected. Accommodation of the left eye was measured by using a continuous recording, infrared optometer, which was calibrated during normal binocular viewing as described previously (Zhang et al. 1992). The positions of both the right and left eyes, and accommodation were sampled at 1 KHz and stored on computer disk for later analysis.

Data analysis

The recorded data were analyzed off-line by using a Sun workstation equipped with interactive graphics. Individual trials showing vergence demand, accommodation demand, accommodation response, the positions of both eyes, and the velocities of vergence and accommodation were displayed. Below these traces, concurrent single-unit activity was also displayed.

Averages of firing rate, vergence angle, and accommodation were computed for sequential 200-ms periods during steady fixation (1-4 s in duration) at various vergence angles. Scatterplots of firing rate as a function of vergence angle during normal viewing were produced and correlation coefficients and regression parameters were determined for each plot. The slopes of these regression lines yielded a measure of the normal viewing coefficient (knv) for each cell. The firing rate for fixation of a distant target (0 vergence and accommodation demand; Ronv) was computed from the intercept of the regression line with the y-axis. The threshold (T) for the neuron was computed from the intercept of the regression line with the x-axis.

The activity of some far-response cells was examined during blur-driven accommodation and vergence (disparity open-loop) and during disparity-driven vergence and accommodation (accommodation open-loop). To allow for comparisons between these conditions and normal viewing, firing rate was related to vergence angle for all cells and viewing conditions. Scatterplots of firing rate as a function of vergence angle during blur-driven accommodation and vergence (disparity open-loop) were produced and correlation coefficients and regression parameters were determined for each plot. The slopes of these regression lines yielded a measure of the sensitivity of the cell to blur-driven movements (kb). In addition, scatterplots of firing rate as a function of vergence angle during disparity-driven vergence and accommodation (accommodation open-loop) were produced and correlation coefficients and regression parameters were determined for each plot. The slopes of these regression lines yielded a measure of the sensitivity of the cell to disparity-driven movements (kd). To test for differences between the values for kb, kd, and knv F-tests were used with an alpha -level of P < 0.001. This conservative alpha -level was chosen throughout to minimize type I errors.

To determine the degree to which the activity of a far-response cell is associated with changes in vergence or accommodation, we used the conflict viewing strategy as described above. To quantify this procedure we used the same methods as reported by Zhang et al. (1992). It assumes the following linear interaction between vergence and accommodation:
FR = <IT>R<SUB>o</SUB>+ k<SUB>da</SUB></IT>⋅AR + <IT>k<SUB>dv</SUB></IT>⋅CR (1)
where FR is the firing rate of the far-response cell, Ro is the predicted firing rate for viewing a distant target (i.e., AR = 0,CR = 0), kda is the accommodation coefficient, kdv is the vergence coefficient, AR is the accommodation response, and CR is the vergence response. Averages of firing rate, vergence angle, and accommodation were computed for sequential 200-ms periods during steady fixation (1-4 s in duration) for various levels of vergence and accommodation. Data from the normal viewing and conflict viewing conditions were combined and the coefficients for vergence and accommodation were generated by using multiple regression analyses. To determine whether or not the kdv and kda values were significantly different from zero, t-tests were performed with a conservative alpha -level of P < 0.001.

Unit recording and electrical microstimulation

By using a Kopf microdrive, a parylene-insulated tungsten microelectrode was advanced to the deep nuclei through a premeasured 26-gauge cannula that just penetrated the tentorium and a 21-gauge hypodermic needle used to puncture the dura. The electrode was advanced to the far-response region of the interposed nucleus (IP), which was often indicated by encounters with saccadic and blink cells that are found nearby. Sometimes the drive was continued until reaching the fourth ventricle and, as the electrode was subsequently retracted, far-response cells were isolated. Unit activity was filtered sharply above 5 KHz and the occurrence of a spike was detected with a window discriminator and recorded to computer disk to the nearest 0.1 ms. When needed, the microelectrode could be switched to electrical microstimulation, which was produced by using biphasic stimuli of 0.2 ms total duration generated by a Grass S88 stimulator and two stimulation isolation units.

Histology and verification of recording site

Animals were used for several months and marking lesions could not be made at all relevant sites. However, records were kept of the location of familiar landmarks, the X-Y location of the micropositioner, and the depth at which cells of interest were located. To verify the location of our electrodes, marking lesions were made during the last 2 wk of recording at the site of responsive neurons by passing 30 µA anodal current through the microelectrode for 20 s. Subsequently, monkeys were anesthetized deeply with sodium pentobarbital and perfused transcardially with saline followed by 4% paraformaldehyde in 0.1 M phosphate buffer. The brains were cut coronally at 40 µm thickness and a Nissl series was prepared. The marking lesions were retrieved from these histological sections.

    RESULTS
Abstract
Introduction
Methods
Results
Discussion
References

Initial identification of far-response neurons

We examined the single-unit activity of all cells encountered in the deep nuclei during static vergence and ocular accommodation. Neurons with activity specifically related to the far-response were found in the posterior interposed nucleus. Figure 4A shows the behavior of a cell (cell 32 in Table 1) recorded in this nucleus during a decrease in vergence angle and accommodation that occurred in response to a step change in target distance from 4 to 1 MA. This cell shows a clear increase in its firing rate when looking from the near to the far target. Figure 4E summarizes the overall relationship between vergence angle and firing rate of this cell during normal binocular viewing. The response of the cell during sine-wave tracking of a target moving in depth from 1 to 4 MA was also examined (Fig. 4B). Under these conditions, the firing rate of the cell is modulated as a function of viewing distance as would be expected from change in firing rate during the static trial shown in Fig. 4A. In addition however, on the basis of the phase lead of the cell during this trial and on the transient increase in firing rate that is apparent during the divergence movement shown in Fig. 4A, it is evident that this cell also shows a strong correlation between firing rate and vergence velocity and a scatterplot of this relationship is shown for this particular cell in Fig. 4F. Such a vergence velocity sensitivity is a common feature of IP far-response neurons and will be addressed at length in a future communication.


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FIG. 4. A: behavior of an IP far-response cell for a symmetrical far-response movement in response to a step change in accommodative and vergence demand (normal binocular viewing). In this and subsequent figures, convergence, accommodation for near, and rightward eye movements are represented as upward on traces. B: response of this cell during sine-wave tracking of a target moving in depth. C: activity of this cell is unaffected by smooth pursuit of a horizontally moving target at optical infinity. D: activity of this cell is unaffected by saccadic eye movements made between targets at optical infinity. E: summary plot of firing rate as a function of vergence angle for this cell. F: summary plot of firing rate as a function of vergence velocity for this cell. ACC, accommodation; HL, horizontal left eye position; HR, horizontal right eye position; VA, vergence angle; VR, vertical right eye position. Scalebars = 4 meter angle (MA) in A and B; 4° in Cand D.

 
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TABLE 1. Summary data for IP far-response neurons

Each far-response neuron was also examined for its sensitivity to changes in conjugate eye position. The activity of each cell was tested during trials in which the animal made smooth pursuit eye movements and other trials in which it made saccadic eye movements. Figure 4, C and D show the behavior of the same far-response cell as is shown in Fig. 4, A and B during these eye movements while viewing a target with 1 MA of vergence demand. Note that the activity of this cell is not altered during smooth pursuit eye movements (Fig. 4C) or saccadic eye movements (Fig. 4D). Thus, on the basis of its responses during these standard trials, the cell in Fig. 4 would satisfy our definition of a far-response cell. The conjugate sensitivity of all cells was examined on-line and none were modulated during saccadic or smooth pursuit eye movements. To further document this finding, nine far-response neurons were examined systematically during vertical and horizontal saccades and during horizontal smooth pursuit. Summary data plots from these cells are shown in Fig. 5. Figure 5A shows the vergence sensitivity of these cells and demonstrates that their firing rates decrease significantly during convergence eye movements. In contrast, Fig. 5, B-D shows that the firing rate of these cells remains essentially constant for vertical and horizontal saccades and for horizontal smooth pursuit eye movements over an even greater oculomotor range than that shown for the vergence eye movements.


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FIG. 5. Firing rate of IP far-response neurons increases as a function of increases in vergence angle (A). No such change in firing rate is seen for horizontal saccades (B), vertical saccades (C), or horizontal smooth pursuit (D). Horizontal scale: A, meter angles; B-D, degrees.

Location of far-response neurons

A total of 185 cells with the characteristics described above were recorded from the posterior interposed nucleus of three animals. Sufficient data were collected from 70 of these cells for the steady-state analyses described below. These far-response neurons were found within a restricted region of the posterior interposed nucleus immediately above the fourth ventricle. On the basis of marking lesions, X-Y locations, and microdrive depths, this region extended ~1 mm rostrocaudally, 1 mm mediolaterally, and 2 mm dorsoventrally, i.e., much of the dorsoventral extent of this nucleus at this level (Fig. 6).


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FIG. 6. A: schematic illustration of a coronal section through cerebellum. Enclosed area indicates region of deep nuclei from which photomicrograph was taken. B: photomicrograph of a coronal section at level of deep cerebellar nuclei showing site of a marking electrolytic lesion in posterior interposed nucleus. F, fastigial nucleus; IA, anterior interposed nucleus; IP, posterior interposed nucleus; D, dentate nucleus. Scale Bar in A and B = 2 mm.

Microstimulation

We investigated the effect of electrical microstimulation (biphasic, 0.2-ms pulse width, 20-100-ms duration) at the site of some of these far-response neurons. Microstimulation with low currents (<30 µA) often elicited a far-response (Fig. 7). This effect was specific in that when the animal was viewing at far, microstimulation had no effect on accommodation or vergence (Fig. 7A), but clear far responses were elicited if the animal was viewing a near target (Fig. 7B). The latencies of these responses varied little and were 15-20 ms for the vergence response and 80-90 ms for the accommodative response. In many cases, in addition to a well-defined far-response, microstimulation also elicited small saccadic eye movements.


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FIG. 7. A: electrical microstimulation with low current (30 µA; 100 ms; 500 Hz) at location of a far-response cell elicits no vergence or accommodation response when eyes are diverged. B: microstimulation with same parameters elicits well-defined decreases in convergence and accommodation when eyes are converged and accommodating for near. Scale bar = 4° and 2 diopters.

Behavior of cells during the far-response with blur or disparity open-loop

In addition to normal binocular viewing, we also examined the behavior of IP far-response neurons during trials in which blur or disparity were open-loop. The behavior of 53 IP far-response neurons were studied during monocular viewing. Of these neurons, 37 were also studied during binocular viewing with accommodation open-loop. A summary of the results obtained during these trials is presented in Table 1. The activity of all tested cells was modulated under both of these viewing conditions. The response of one neuron (Cell 58 in Table 1) is shown in Fig. 8 during normal binocular viewing (Fig. 8A), viewing with disparity open-loop (Fig. 8B), and viewing with blur open-loop (Fig. 8C) ofa target that stepped from an apparent distance of 20 cm to an apparent distance of 1 m. Overall, the relationship between the activity of this particular neuron and vergence angle is not significantly different under these three conditions (Fig. 8D). This is also true for the population averages as is indicated in Table 1. However, as can also be seen in Table 1, the activity of some cells is significantly different between these two open-loop conditions. In some cases, it is also different between normal viewing and these open-loop conditions. Summary plots in Fig. 9 illustrate these differences.


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FIG. 8. A: behavior of an IP far-response cell (cell 58 in Table 1) during normal binocular viewing. B: behavior of this cell during monocular accommodative vergence. C: behavior of this cell during binocular viewing with accommodation open-loop. D: scatter plot of firing rate as a function of vergence angle for 3 viewing conditions. (), normal viewing; (···), monocular viewing; (- - -), binocular viewing condition with accommodation open-loop. Scale bar = 4 MA and 4 diopters.


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FIG. 9. A: plot of 37 far-response cells comparing their response during normal viewing (knv) and disparity open-loop (kb). B: plot of same far-response cells comparing their response during normal viewing (knv) and accommodation open-loop (kd). C: plot of same far-response cells comparing their response during disparity open-loop (kb) and accommodation open-loop (kd).

Determination of far-response cell type

After the initial identification of a far-response cell, we determined its relationship to vergence and to ocular accommodation under conflict viewing conditions. These cells were classified into different groups using a classification scheme similar to that developed by us previously (Zhang et al. 1992). Table 1 summarizes the response properties based on multiple regression analyses for these far-response cell during normal binocular and conflict viewing conditions.

Divergence cells (-V cells)

Thirty-four far-response cells had a negative kdv coefficient that was significantly different from zero and a kda value that was not significantly different from zero (Table 1; Cells 1-34). During normal binocular viewing, Fig. 10A shows the decrease in activity of one of these cells (cell 5 in Table 1) in response to an increase in convergence of 6 MA and in accommodation of 6 D. Figure 10B shows the behavior of the same cell during conflict viewing in which there is a convergence movement of 6 MA (a similar amount to that seen in Fig. 10A) but an increase of accommodation of only 2 D. This reduced accommodative response can be appreciated by referring to Fig. 10B(- - -), indicating the approximate level of the accommodative response in Fig. 10A. Under both viewing conditions, the change in firing rate is the same and is closely related to the vergence angle. This is confirmed by the scatterplots in Fig. 10C of the relationship between the vergence angle and the cell's firing rate under the two viewing conditions. Clearly neither the slopes nor intercepts of the regression lines are significantly different from one another. Figure 10D shows scatterplots of the cell's firing rate as a function of accommodation for the same two viewing conditions. In contrast to 10C, there is a pronounced difference in the relationship between firing rate and accommodation under these two conditions. This cell would therefore be characterized as a divergence cell (-V cell) on the basis of the classification scheme previously introduced by us (Zhang et al. 1992).


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FIG. 10. Response properties of a divergence (-V) IP far-response cell (cell 5 in Table 1) during normal binocular viewing and partial dissociation of vergence from accommodation. Initial VA and ACC of 1 MA and 1 D respectively in A and B. A: during normal binocular viewing activity of neuron decreases as a function of near response. B: during conflict viewing condition, in which accommodative demand and responses are less than normal, both vergence amplitude and activity of neuron is similar to that in A. - - -: accommodative response seen in A. C: scatterplot and linear regression of firing rate as a function of vergence angle for normal viewing (black-triangle), and conflict viewing (triangle ). D: scatterplot and linear regression of firing rate as a function of accommodation for normal viewing (black-triangle) and conflict viewing condition (triangle ). Scale bar = 4 MA and 4 diopters.

Far accommodation cells (-A cells)

Eleven far-response cells had a negative kda coefficient that was significantly different from zero and a kdv value that was not significantly different from zero (Table 1; cells 35-45). Figure 11A shows the decrease in activity of one of these cells (cell 38 in Table 1) in response to convergence of 6 MA and accommodation of 6 D during normal binocular viewing. Figure 11B shows the behavior of the same cell during conflict viewing in which there is a convergence movement of 6 MA (a similar amount to that seen in Fig. 11A) but an increase of accommodation of only 2 D. This reduced accommodative response can be appreciated by referring to the dashed line above the accommodative trace (ACC) in Fig. 11B, indicating the approximate level of the accommodative response in Fig. 11A. Reference to the dotted lines above the frequency (FREQ) traces in Figs. 11, A and B, indicate that the change in firing rate is very different under these two viewing conditions and that it is closely related to the accommodative response. This is confirmed by the scatterplots in Fig. 11D of the relationship between accommodation and the cell's firing rate under the two viewing conditions. Neither the slopes nor intercepts of the regression lines are significantly different from one another. Figure 11C shows scatterplots of the cell's firing rate as a function of vergence angle for the same two viewing conditions. In contrast to 11D, there is a pronounced difference in the relationship between firing rate and vergence angle under the two viewing conditions. This cell would therefore be characterized as a far-accommodation cell (-A cell) on the basis of the classification scheme previously introduced by us (Zhang et al. 1992).


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FIG. 11. Response properties of a far accommodation (-A) IP far-response cell (cell 38 in Table 1) during normal binocular viewing and partial dissociation of vergence from accommodation during conflict viewing. Initial VA and ACC of 1 MA and 1 D respectively in A and B. A: during normal binocular viewing, activity of this neuron decreases as a function of near response. B: response of this cell during a convergence movement similar in amplitude to that in A and an accommodative response substantially lower than that in A. This difference is best appreciated by reference to upper of 2 dashed lines that indicates accommodative response in A. For reference, lower of 2 dashed line is located at same relative level as it is in A. Note that decrease in firing rate is much less than in A. C: scatterplot and linear regression of firing rate as a function of vergence angle for normal viewing (black-triangle) and conflict viewing (triangle ). D: scatterplot and linear regression of firing rate as a function of accommodation for normal viewing (black-triangle) and conflict viewing condition (triangle ). Scale bar = 4 MA and 4 diopters.

Divergence plus far accommodation cells (-- A cells)

Fourteen far-response cells had negative kdv and kda coefficients, both of which were significantly different from zero (Table 1, cells 46-59). During normal binocular viewing and conflict viewing the change in firing rate of these cells was not related entirely either to vergence angle or to the accommodative response. This is shown for one cell (cell 49 in Table 1) in the scatterplots in Fig. 12A of the relationship between vergence angle and firing rate under the two viewing conditions. The slope of the regression line during conflict viewing is significantly less steep from that seen during normal viewing. Thus the firing rate of the cell is not entirely related to vergence angle. Figure 12B shows scatterplots of the cell's firing rate as a function of accommodation for the same two viewing conditions. As in 12A, the relationship between firing rate and accommodation is significantly different under the two viewing conditions. This cell would therefore be characterized as a divergence plus far-accommodation cell (-- A cell) on the basis of the classification scheme previously introduced by us (Zhang et al. 1992).


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FIG. 12. A and B: divergence plus far accommodation (-- A) IP far-response cell (cell 49 in Table 1). A: scatterplot and linear regression of firing rate as a function of vergence angle for normal viewing (black-triangle) and conflict viewing (triangle ). B: scatterplot and linear regression of firing rate as a function of accommodation for normal viewing (black-triangle) and conflict viewing condition (triangle ). C and D: divergence minus far accommodation (-V + A) IP far-response cell (cell 63 in Table 1). C: scatterplot and linear regression of firing rate as a function of vergence angle for normal viewing (black-triangle) and conflict viewing (triangle ). D: scatterplot and linear regression of firing rate as a function ofaccommodation for normal viewing (black-triangle) and conflict viewing condition (triangle ). Scale bar = 4 MA and 4 diopters.

Divergence minus far accommodation cells (-V + A cells)

Nine far-response cells had negative kdv and positive kda coefficients that were significantly different from zero (Table 1; cells 60-68). During normal binocular and conflict viewing the change in firing rate of these cells was not related entirely either to vergence angle or to the accommodative response and their firing rates decreased more with conflict viewing than with normal viewing. This observation is confirmed for one such cell (cell 63 in Table 1) by the scatterplots in Fig. 12C of the relationship between the vergence angle and firing rate under the two viewing conditions. The slope of the regression line during conflict viewing is significantly steeper from that seen during normal viewing. Figure 12D shows scatterplots of the firing rate of the cell as a function of accommodation for the same two viewing conditions. As in 12C, the relationship between firing rate and accommodation is significantly different under the two viewing conditions and the decrease in firing rate of this cell during conflict viewing is more than would be expected for a far-response cell that was solely related to vergence angle (i.e., a -V cell). Indeed, as described above, this was confirmed by the multiple regression analysis, which yielded a negative kdv and a positive kda for this cell. Thus this cell would be characterized as a divergence minus far-accomodation cell (-V + A cell) on the basis of the classification scheme previously introduced by us (Zhang et al. 1992).

Far accommodation minus divergence cells (+V - A)

Only two far-response cells had positive kdv and negative kda coefficients that were significantly different from zero and these were classified as far accommodation minus divergence (+V - A) cells (Table 1, cells 69 and 70).

Relationship of kda and kdv to knv

During normal viewing the accommodative and vergence responses are matched and therefore Eq. 1, which was used in our multiple regression analysis linking the activity of the cell independently to the accommodative and vergence responses by their respective gain factors of kda and kdv, reduces to:
FR = <IT>R<SUB>o</SUB></IT>+ (<IT>k<SUB>da</SUB>+ k<SUB>dv</SUB></IT>)⋅CR (2)
Equation 2 is directly comparable to a single linear regression of firing rate as a function of vergence angle in which
FR = <IT>R<SUB>onv</SUB>+ k<SUB>nv</SUB></IT>⋅CR (3)
Thus for normal viewing, knv should be more closely matched to the sum of the kda and kdv coefficients than to either of these two coefficients alone. As can be seen from Fig. 13, A-C, this is the case. The similarity is confirmed by statistical analysis showing that the correlation between knv and kdv is low (r = 0.55) and that the correlation between knv and kda is insignificant, while that between knv and (kda + kdv) is robust (r = 0.74). Comparison of Eqs. 2 and 3 also implies that the multiple regression analysis will yield an Ro value that approximates the Ronv value generated by single linear regression. As illustrated in Fig. 13D, the correlation between these two values is strong (r = 0.84).


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FIG. 13. A: comparison of kda with knv for a sample of 70 cells. B: comparison of kdv with knv for same group of cells. C: comparison ofkdv + kda with knv for same group of cells. D: comparison of Ronv with Ro for same group of cells.

Threshold for IP far-response neurons

Figure 14 presents the threshold data for all 70 IP far-response neurons. The curve represents a logistic fit to the sample of thresholds for this population of neurons. This curve shows clearly that with as little convergence as 5 MA some IP far-response neurons become inactive and that with 9.5 MA of convergence half the population of neurons is inactive. The entire population is inactive by 18 MA. As discussed below, this decrease in number of active far-response neurons with increases in vergence angle may have important consequences for vergence control.


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FIG. 14. Plot of number of IP far-response neurons that would reach threshold (T) as viewing distance is decreased. Data points were fitted with a logistic function of form
<IT>y</IT> = <FR><NU>(<IT>a − b</IT>)</NU><DE>1 + (<IT>x</IT>/<IT>c</IT>)<SUP><IT>d</IT></SUP></DE></FR> + <IT>b</IT>
where a is maximum y value, b is minimum y value, c is inflection point, and d is a slope function. Function shown here had following parameters: a = 73, b = -4.0, c = 9.6, d = 4.4.

    DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References

To our knowledge, this is the first detailed report of the steady-state activity of neurons in the posterior interposed nucleus of the cerebellum in the alert, behaving primate during vergence and ocular accommodation. Previous reports linking cerebellar activity to these oculomotor functions have been conducted in paralyzed, anesthetized cats (e.g., Bando et al. 1980; Hosoba et al. 1978). Our results raise a number of important questions regarding the role of the posterior interposed nucleus in the eye movements

Location and connections of IP far-response neurons

The region of the IP from which we have recorded far-response neurons coincides closely with the region of the nucleus that we have previously shown, using anterograde and retrograde techniques, to be reciprocally connected with the midbrain near-response region (May et al. 1992). This region of the interposed nucleus is also reported to receive inputs from the nucleus reticularis tegmenti pontis (NRTP) (Mihailoff 1993; Noda et al. 1990) and from the intermediate cerebellar cortex (e.g., Bishop et al. 1979; Haines and Rubertone 1979). Neurons within this region of the interposed nucleus project to the NRTP (e.g., Noda et al. 1990), to the midbrain near-response region (May et al. 1992) and presumably to the ventrolateral thalamic nucleus (e.g., McCrea et al. 1978; Tolbert et al. 1977). The afferent and efferent connections of IP that are relevant to this discussion are summarized in Fig. 15. More specifically, the afferent connections of the IP are arranged so that the NRTP is in a position to relay a signal related to vergence and accommodation to this nucleus. Because the NRTP is reported to receive input from the prearcuate cortex (Leichnetz et al. 1984), it is possible that a far-response signal reaches NRTP neurons from the far-response neurons that have recently been reported in this region (Gamlin and Yoon 1995; Gamlin et al. 1996). The connections of the IP with neurons in the midbrain near-response region are such that these midbrain neurons could relay a signal related to motor output (corollary discharge) to the IP. The efferent projections from the IP to the midbrain near-response neurons are in a position to modulate the activity of these midbrain neurons on the basis of mismatches between the desired and actual motor output signals impinging on the IP. Furthermore, these signals, if supplemented by signals related to visual errors (i.e., disparity, blur) are precisely those that are required for the roles that have generally been ascribed to the cerebellum such as the continuous adjustment of movement parameters, predictive control, and long-term adaptation (see Stein and Glickstein 1992 for a review).


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FIG. 15. Summary diagram of known efferent and afferent connections of posterior interposed nucleus potentially involved in vergence and accommodation. EW, Edinger-Westphal nucleus; MRMN, medial rectus motoneurons; SOA, Supraoculomotor area (midbrain near-response region).

Latency of vergence and accommodation responses to electrical microstimulation of IP

The latency to elicit vergence changes from the IP of15-20 ms is consistent with the shortest latency of 13 ms required to elicit convergence from electrical microstimulation of the midbrain near-response region (Judge and Cumming 1986). Similarly, the latency to elicit an accommodative response of 80-90 ms from IP stimulation is consistent with the average latency of 75 ms reported for stimulation of the Edinger-Westphal nucleus (Gamlin et al. 1994) and the shortest latency of 75 ms reported for stimulation of the midbrain near-response region (Judge and Cumming 1986).

Relationship to single-unit studies of primate midbrain neurons

Previous studies have reported that the activity of ~75% of the neurons in the midbrain near-response region is correlated with the near-response, whereas the activity of ~25% is related to the far-response (Judge and Cumming 1986; Mays 1984). Just as is the case for the IP far-response neurons, neither the midbrain near-response or far-response neurons display a conjugate signal. They are active for both blur-driven and disparity-driven movements as well as for normal binocular movements. Figure 16 compares the kda and kdv values from the IP far-response neurons with those reported for midbrain near-response neurons (Zhang et al. 1992). The distributions form mirror images of one another that only differ in the absolute values of these two coefficients. This quantitative difference is to be expected because the sensitivity of IP far-response neurons to accommodation and vergence is, on average, approximately three times less than that of midbrain near-response neurons i.e., on average kdv and kda values are three times higher for midbrain near-response neurons than for IP far-response neurons. Also, the relative numbers of neurons related to vergence as opposed to accommodation are approximately the same between the two populations of cells. As pointed out by Zhang and colleagues (1992), the behavior of the near-response cells during conflict viewing depends not only on whether or not they are in the vergence or accommodation output pathways but also on their relative blur and disparity inputs. These authors demonstrated that a cell identified as a vergence-only cell by conflict viewing might in fact be in the accommodative output pathway if the blur and disparity controller inputs to it were sufficiently mismatched. However, Morley and colleagues (1992) pointed out that, for midbrain near-response neurons to be ascribed to the wrong output pathway, large and specific mismatches between the blur and disparity controller inputs would be required, such that this would be a rare occurrence. Although our study of IP far-response neurons found more cells with mismatches between their kb and kd values than were reported for midbrain neurons, these mismatches were not sufficiently large or specific that many IP far-response neurons would have been ascribed to the wrong output pathway. It is therefore safe to assume that, in the present study, the majority of vergence-only neurons are related to the vergence output pathway and the majority of accommodation-only neurons are related to the accommodative output pathway.


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FIG. 16. Scatterplot comparing kda versus kdv values for sample of 70 IP far-response neurons in this study (triangle ) with those from a sample of midbrain near-response neurons (open circle ) (from Zhang et al. 1992).

Another feature of midbrain near-response neurons that may be explained by our data is the tendency for some of these neurons to show "soft-saturation" with increasing vergence angles (Morley et al 1992). As discussed below, these near-response cells may be operating in a push-pull mode with far-response cells. If the activity of the population of far-response cells increasingly decreases above 5 MA (as might be suggested by Fig. 14), then near-response cells would have to decreasingly increase their activity in order to produce the same degree of change in vergence angle. This would appear as soft-saturation in the activity of the near-response neurons with increasing vergence angles.

Potential roles for the cerebellum in the control of vergence and accommodation

The region of the IP from which we have recorded projects to the midbrain near-response region. As stated above, ~25% of neurons in the midbrain region are far-response neurons. If the projections from IP far-response neurons were to the midbrain far-response neurons then these connections could be directly excitatory. Alternatively, the IP far-response neurons could have an inhibitory influence on near-response neurons in this midbrain region. We have recently reported that a region of the posterior fastigial nucleus contains near-response neurons (Zhang and Gamlin 1994). Because this region of the fastigial nucleus also projects to the midbrain near-response region (May et al. 1992), it is possible that the fastigial near-response region and the interposed far-response region represent a push-pull system for modulating vergence and accommodation. Importantly, increases in the tonic activity of the vergence-related cells of the cerebellar near-response region combined with decreases in the tonic activity of vergence-related cells of the cerebellar far-response region would result in an overall increase in the convergence signal, but not the accommodation signal, impinging on the midbrain near-response region. A signal of this nature could underlie the ability of individuals to show phoria adaptation and would be consistent with one report that indicated that cerebellar damage compromises this ability (Milder and Reinecke 1983), although this was not confirmed in a later study (Hain and Luebke 1990)

Relationship to studies in cats

Previous studies in cats have reported that both the fastigial and interposed nucleus are related to positive accommodation (e.g., Bando et al. 1979a,b; Hosoba et al. 1978). Because the animals in these studies were paralyzed and anesthetized, the influence of these cerebellar nuclei on vergence eye movements could not be evaluated. It is possible that slight positive accommodation was obtained with microstimulation of the interposed nucleus because of current spread to the fastigial nucleus or activation of fibers from this nucleus. Alternatively, the small amount (~0.2 D) of positive accommodation that was obtained with microstimulation of the interposed nucleus in cats can be interpreted in light of our results in the primate. We found that the eyes must be converged and accommodating for electrical microstimulation of the interposed nucleus to elicit clear divergence and accommodation for far. Given the physiological state of the cats in the studies described above, it is unlikely that they were accommodating or that their eyes were converged. In our studies, electrical microstimulation of the IP under these conditions had no clear effect and the very small positive accommodative responses obtained in cats under comparable conditions may therefore not have been functionally relevant. For these same reasons, the spontaneous activity of IP neurons in cats that apparently correlated with positive accommodation (Bando et al. 1979a) is hard to interpret.

Relationship to previous studies

The region of the IP from which we have recorded far-response neurons has recently been reported by Van Kan and colleagues (1993) to contain neurons related to eye movements, but the specific eye movement to which these cells were related was not investigated. Apart from the far-response neurons reported here, we have not examined in any detail the activity of other eye movement related neurons in IP, but we have encountered some saccade-related and blink-related neurons within this region, and have found that electrical microstimulation of it often elicits small saccades.

Previous studies of oculomotor control by the deep cerebellar nuclei have concentrated on the role of the oculomotor region of the fastigial nucleus (e.g., Fuchs et al. 1993, 1994; Noda et al. 1988; Ohtsuka and Noda 1991). Single-unit recording studies of the role of the fastigial nucleus in oculomotor control have reported that the tonic activity of saccade-related fastigial neurons is very weakly correlated with eye position and that these neurons, at best, show a relationship of approximately 0.4 spikes/second per degree of change in eye position (Fuchs et al. 1993). Furthermore, those fastigial neurons related to smooth pursuit eye movements are not reported to have any eye position sensitivity (Fuchs et al. 1994). In contrast, as shown by our regression analyses, the tonic activity of many far-response neurons recorded from the IP correlates well with vergence angle and accommodation. Although this finding is different from the findings for fastigial neurons during conjugate eye movements, it is not inconsistent with a report by Thach (1978) on interpositus neuron activity during wrist flexion/extension. He found that the tonic activity of these neurons increased as a function of the force required to overcome either an extensor or flexor load. The force generated by the extraocular muscles and the ciliary muscle must change for divergence and accommodation for far, and the changes in activity of interpositus neurons during the far-response may well be related to these required changes in muscle force. Therefore, our results can be considered analogous to those of Thach and they strongly suggest that for both types of movements the tonic activity of interpositus neurons directly or indirectly contributes to the change in steady-state activity of motoneurons that is required to match force generation to specific loads.

    ACKNOWLEDGEMENTS

  We thank S. Mason and K. Mitchell for technical assistance and S. Hayley for computer programming.

  This research was supported by National Eye Institute Grant R01 EY-07558 to P.D.R. Gamlin and by P30 EY-03039.

    FOOTNOTES

  Address for reprint requests: P.D.R. Gamlin, Vision Science Research Center, University of Alabama at Birmingham, Birmingham, AL 35294.

  Received 30 July 1996; accepted in final form November 4, 1997.

    REFERENCES
Abstract
Introduction
Methods
Results
Discussion
References

0022-3077/98 $5.00 Copyright ©1998 The American Physiological Society