Cerebral Cortex Advance Access originally published online on August 22, 2007
Cerebral Cortex 2008 18(5):1042-1057; doi:10.1093/cercor/bhm143
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Involvement of the Cerebellar Dorsal Vermis in Vergence Eye Movements in Monkeys
1 Department of Physiology, 2 Department of Ophthalmology Hokkaido University School of Medicine, Sapporo 060-8638, Japan
Address correspondence to email: kikuro{at}med.hokudai.ac.jp.
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Abstract |
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Frontal-eyed primates use both smooth pursuit in frontoparallel planes (frontal pursuit) and pursuit-in-depth (vergence pursuit) to track objects moving slowly in 3-dimensional (3D) space. To understand how 3D-pursuit signals represented in frontal eye fields are processed further by downstream pathways, monkeys were trained to pursue a spot moving in 3D virtual space. We characterized pursuit signals in Purkinje (P) cells in the cerebellar dorsal vermis and their discharge during vergence pursuit. In 41% of pursuit P-cells, 3D-pursuit signals were observed. However, the majority of vermal-pursuit P-cells (59%) discharged either for vergence pursuit (43%) or for frontal pursuit (16%). Moreover, the majority (74%) of vergence-related P-cells carried convergence signals, displaying both vergence eye position and velocity sensitivity during sinusoidal and step vergence eye movements. Preferred frontal-pursuit directions of vergence + frontal-pursuit P-cells were distributed in all directions. Most pursuit P-cells (73%) discharged before the onset of vergence eye movements; the median lead time was 16 ms. Muscimol infusion into the sites where convergence P-cells were recorded resulted in a reduction of peak convergence eye velocity, of initial convergence eye acceleration, and of frontal-pursuit eye velocity. These results suggest involvement of the dorsal vermis in conversion of 3D-pursuit signals and in convergence eye movements.
Key Words: cerebellar dorsal vermis • monkey • Purkinje cell • smooth pursuit • vergence eye movements • visual response
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Introduction |
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There are 4 types of voluntary eye movements: saccades, smooth pursuit, vergence eye movements, and fixation. Both saccades and smooth pursuit rotate the left and right eyes in the same direction. Compared with fast saccadic eye movements, smooth-pursuit movements are slow and are used to track moving targets in frontoparallel planes (frontal pursuit). Vergence eye movements rotate the 2 eyes in opposite directions. Primates use the vergence system to track a slowly moving target in sagittal planes or to change the gaze point in-depth. Because a target could move in 3-dimensional (3D) direction, frontal pursuit and vergence eye movements must be coordinated to track slowly moving objects in 3D space.
In the cerebral cortex, 3 areas are known to be closely related to pursuit eye movements: the medial superior temporal (MST) visual area, the supplementary eye fields (SEFs), and the caudal part of the frontal eye fields (FEFs) in the fundus of the arcuate sulcus (for a review, see Leigh and Zee 2006
). Recent studies have shown that the majority of pursuit neurons in MST and SEF discharge either for frontal pursuit or for vergence pursuit (Fukushima et al. 2004; Akao, Mustari, et al. 2005). In contrast, the majority of pursuit neurons in the caudal FEF discharge not only for frontal pursuit but also for vergence pursuit with preferred directions that are the sum of 2 components and thus represent 3D-pursuit signals (Fukushima K, Yamanobe, Shinmei, Fukushima J, Kurkin, and Peterson 2002; Akao, Kurkin, et al. 2005). These results indicate that signals for these 2 eye movement subsystems are represented independently in MST and SEF (for prearcuate vergence neurons, see also Gamlin and Yoon 2000), but these signals are primarily synthesized in the caudal FEF. Moreover, the majority of FEF pursuit neurons discharge before the onset of frontal and vergence pursuit (Gottlieb et al. 1993; Tanaka and Fukushima 1998; Akao, Kurkin, et al. 2005; Akao et al. 2007), suggesting the importance of the caudal FEF in synthesis of 3D-pursuit signals. In the brain stem, signals for these 2 eye movement subsystems are processed separately (Mays 1984), and during vergence, convergence and divergence neurons are found in the separate areas (also Mays et al. 1986). These results suggest that 3D-pursuit signals carried by FEF neurons must be converted into frontal and vergence components and that vergence signals must further be converted into convergence and divergence signals by the downstream pathways. However, it is unknown where and how these conversions occur.
Pursuit signals in the caudal FEF are sent to the nucleus reticularis tegmenti pontis (NRTP) and dorsolateral pontine nucleus (Ono et al. 2004). These areas project to the cerebellar dorsal vermis and floccular region (for a review, see Leigh and Zee 2006). The question we address in the present study is whether the dorsal vermis carries 3D-pursuit signals and how it is involved in vergence pursuit. This area has been known as a saccade-related area and is also known as a region situated in the frontal-pursuit pathways (for a review, see Robinson and Fuchs 2001). Vermal Purkinje (P) cells modulate their activity during frontal pursuit (Kase et al. 1979; Suzuki et al. 1981; Suzuki and Keller 1988a; Sato and Noda 1992a; Shinmei et al. 2002). Although there is no detailed study on neuronal activity in the dorsal vermis for vergence pursuit, vergence eye movement signals are most probably found there because vergence signals have been found in the NRTP that projects to the dorsal vermis and also in the fastigial nucleus that receives projections from the dorsal vermis (Gamlin and Clarke 1995; Gamlin and Zhang 1996; Zhang and Gamlin 1996). In particular, these studies reported that vergence signals and frontal-pursuit signals are represented in the NRTP and fastigial nucleus independently of each other.
To elucidate the role of the dorsal vermis in pursuit eye movements, in this study, we first asked whether vergence-pursuit and frontal-pursuit signals are represented separately in vermal P-cells. Second, to examine the properties of vergence-pursuit signals in the dorsal vermis, we asked whether P-cells discharge during convergence or divergence and whether that discharge encodes vergence eye velocity and/or position. Third, we tested whether P-cells carry target motion-in-depth information. Fourth, to clarify whether the dorsal vermis could be involved in initiation of vergence pursuit, we examined the latencies of P-cell discharge with respect to the onset of vergence eye movements. Finally, we injected muscimol into the region where we recorded vergence pursuit–related P-cells to examine how chemical inactivation of the vermal-pursuit area affected pursuit eye movements. Our results indicate that, although 3D-pursuit signals were found in a significant proportion (41%) of vermal-pursuit P-cells, in the majority of vermal-pursuit P-cells (59%), vergence signals and frontal-pursuit signals are represented independently. Moreover, the majority of vergence-related P-cells carry convergence signals. These results suggest partial conversion of FEF 3D-pursuit signals in the dorsal vermis and its involvement in vergence, especially convergence, eye movements. Some of the results have been presented in preliminary form (Nitta et al. 2006).
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General Procedures
Three Japanese monkeys (A, C, K; Macaca fuscata; 4–5 years old; 4.2–5.1 kg) provided data for this study. All procedures were performed in strict compliance with the guidelines for the Care and Use of Animals of National Institutes of Health. Our specific procedures were approved by the Animal Care and Use Committee of Hokkaido University School of Medicine. Each monkey was sedated with ketamine hydrochloride (5 mg/kg, intramuscularly) and then anesthetized with sodium pentobarbital (25 mg/kg, intraperitoneally [i.p.]). Additional anesthesia (0.5–1.0% halothane mixed with 50% nitrous oxide and 50% oxygen) was administrated as necessary. Under aseptic conditions, head holders were affixed to the skull. A scleral search coil was implanted on each eye to record the vertical and horizontal components of eye movement for both eyes (Fuchs and Robinson 1966; Judge et al. 1980). Analgesics and antibiotics were administered postsurgically to reduce pain and prevent infection.
Each monkey was seated in a primate chair in darkness with the head firmly restrained in the stereotaxic plane facing a 22-inch computer display (Mitsubishi, RDF 221S) placed 65 cm away from the eyes. The interocular distance of the monkeys in this study varied between 20 and 21 mm. Visual objects (spot, see below) were presented in the central 30° by 20° of the visual field. As schematically illustrated in Figure 1A, 3D virtual stimuli were generated using images viewed alternatively by the right and left eye through polarized shutter glasses switching at 120 Hz, and the refresh rate of the computer monitor was synchronized with the shutter glasses. A red spot of 0.2° angular size was used as the main target in all experiments. To elicit vergence pursuit, a stereo virtual target was moved in the midsagittal plane sinusoidally. Throughout the text, "sinusoidal vergence or target movement" refers to sinusoidal oscillation of the vergence target angle. Our setup enabled us to induce pure vergence pursuit or frontal pursuit in both humans and monkeys (Kurkin et al. 2003). In this study, a virtual spot was moved between 10 and 65 cm from the eyes and that movement required vergence eye movements of 
10°. Vergence eye position was defined as the difference between left horizontal eye position and right horizontal eye position (LHE–RHE). Frontal-pursuit (horizontal) eye position was defined as the mean of LHE and RHE ([LHE+RHE]/2)(Kurkin et al. 2003). Spot motion in the frontoparallel plane was generated at the screen distance. The monkeys were rewarded for pursuing the moving target. Eye position signals were calibrated for each eye separately by requiring the animal to fixate the stationary target or pursue a slowly moving one. The monkeys were trained in 2 tasks; in one, when the reward target was moved in frontal plane or in-depth, the monkeys were rewarded for pursuing it. In the other, when the target was stationary, the monkeys had to fixate it, even when a probe stimulus (i.e., second spot, see below) appeared and moved in various directions. After the animals were trained in these tasks, a recording chamber was installed over a craniotomy to enable single-neuron recording in the cerebellar dorsal vermis bilaterally as described previously (Shinmei et al. 2002).
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Recording Procedures and Behavioral Paradigms
For our search stimulus, the target was moved sinusoidally at 0.5 Hz along oblique trajectories in 3D virtual space that were generated by combinations of frontal and depth target motions (Fig. 1A). Monkeys were required to pursue the spot with a combination of frontal and vergence pursuit of equal amplitudes (i.e., 5° for each, peak velocity 15.7°/s). P-cells were identified by the existence of complex spikes (e.g., Lisberger and Fuchs 1978; Kase et al. 1979; Suzuki et al. 1981; Suzuki and Keller 1988a; Sato and Noda 1992a; Fukushima et al. 1999). Figure 1B shows an example of vergence-related P-cell discharge with complex spikes (asterisk). Simple spike discharge rate was modulated during vergence pursuit (Fig. 1B). Once responsive single P-cells were encountered as judged visually and on the audio monitor, simple spike activity of P-cell responses was tested during frontal pursuit in 8 directions (vertical, horizontal, and 2 diagonal directions at 45° and 135° polar angles) and during vergence pursuit in the midsagittal plane from 65 to 10 cm in-depth.
To examine velocity sensitivity to vergence eye movements for vergence-related P-cells, the spot was moved in-depth with a variety of frequencies (0.3–1.0 Hz) at constant amplitude (5°) (peak velocities 9.5–31.4°/s). Visual responses of vergence-related P-cells to target motion-in-depth were examined by moving a probe stimulus (i.e., second spot, 0.6° diameter) while the monkeys fixated a 0.2° stationary spot (first spot). The first spot was presented at 65 cm in front of the eyes, 0.5° above the second spot that was moved in-depth from 65 to 10 cm in the midsagittal plane. The first spot was occasionally extinguished, whereas the second spot was presented continuously, and the monkeys were then required to track the second spot (Fukushima et al. 2000; Fukushima K, Yamanobe, Shinmei, and Fukushima J 2002; Akao, Kurkin, et al. 2005; Akao, Mustari, et al. 2005). This procedure was used to reward the monkeys for pursuing the second spot, so that this spot would not become behaviorally meaningless and so that they would, presumably, attend to it. The second spot was also moved in the frontal plane in the above 4 directions to examine visual motion responses.
To examine latencies of P-cell discharge relative to the onset of vergence eye movements and also to examine vergence eye velocity and position sensitivity further, the target was stepped in-depth between 65 and 10 cm from the eyes because it is well known that such a disparity stimulus is appropriate to induce vergence (but not frontal saccadic) eye movements (Collewijn and Erkelens 1990; Gonzalez and Perez 1998; Leigh and Zee 2006). The intertrial intervals were set random between 1 and 3.5 s to reduce potential predictive responses. The monkeys were required to make rapid convergence and divergence eye movements to the target.
P-cell discharge was also tested while the target moved in-depth aligned with either the left or the right eye. This task required abduction or adduction of one eye, whereas the other eye remained relatively motionless.
To inactivate vergence-related P-cells, the recording electrode was replaced by a Hamilton syringe, and 0.5–1.0 µl of 
-aminobutyric acid agonist muscimol dissolved in physiological saline (10 µg/1 µl) was infused into the identified sites. Effects of muscimol injection upon the initiation of eye movements were examined by moving the target in-depth between 65 and 10 cm from the eyes in the step trajectory as described above and also in a ramp trajectory with a constant velocity of 10°/s. The target was also moved in frontal plane 10° right to 10° left stepwise or with a constant velocity of 20°/s. Muscimol infusions were repeated on different days.
Data Analysis
The data were analyzed off-line as previously described (e.g., Fukushima et al. 2000; Akao, Kurkin, et al. 2005; Akao, Mustari, et al. 2005). Neuronal discharge was discriminated with a dual time–amplitude window discriminator and digitized together with eye position and target position signals at 500 Hz using a 16-bit A/D board. Eye position signals were differentiated by analog circuits (DC-100 Hz, –12 dB/octave) to obtain eye velocity. Stimulus position signals for sinusoidal task conditions were differentiated by software to obtain velocity. Saccades were identified and removed using the interactive computer program utilizing a maximum likelihood ratio criterion (Singh et al. 1981). Pursuit P-cells that we analyzed in this study did not exhibit clear burst or pauses during saccades. Spike data were not removed for purposes of analysis in this study.
Sinusoidal Target Motion
All traces were aligned with stimulus velocity for 10–30 cycles, and raster and histograms of neuronal responses were constructed. Eye velocities during frontal pursuit and vergence pursuit were calculated by fitting them with a sine function after deleting saccades. Gains of vergence eye movements were calculated as the peak amplitude of the fundamental component fitted to the desaccaded vergence eye velocity divided by the peak amplitude of the target velocity. To quantify neuronal responses, each cycle was divided into 64 equal bins. A sine function was fitted to averaged velocities and cycle histograms of cell discharge, excluding the bins with zero spikes, by means of a least-squared error algorithm. Responses that had a harmonic distortion (HD) of less than 50% or a signal to noise ratio (S/N) of higher than 1.0 were accepted for further analysis according to Wilson et al. (1984); S/N was defined as the amplitude of the fundamental frequency component divided by the amplitudes of the third to eighth harmonic and HD as the amplitude of the second harmonic divided by that of fundamental. The phase shift of the peak of the fitted function relative to target or eye velocity was calculated as a difference in degrees. Amplitude of discharge modulation was calculated as the peak amplitude of the fundamental component fitted to the cycle histograms. P-cells that responded to both frontal pursuit and midsagittal vergence pursuit were classified as vergence + frontal-pursuit P-cells. P-cells that responded only during vergence pursuit or only during frontal pursuit were classified as vergence-only P-cells and frontal-pursuit–only P-cells, respectively.
As described in Results, the majority of vermal-pursuit P-cells had both eye position and eye velocity sensitivity during vergence pursuit. We, therefore, calculated eye velocity sensitivity (r) and eye position sensitivity (k) during vergence pursuit at 0.5 Hz using the equations provided by Chubb and Fuchs (1982). Briefly, r = [Au sin (90 – pv)]/Aev and k = [2
*f*Au cos (90 – pv)]/Aev, where Au was the amplitude of discharge modulation, pv was the phase shift (in degrees) of discharge modulation with respect to the peak vergence eye velocity, Aev was the amplitude of vergence eye velocity, and f was stimulus frequency.
A similar correction for phase shift was also made to calculate eye velocity sensitivity during frontal pursuit and sensitivity to second spot motion during fixation (see below). For analysis of neuronal responses that had diagonal direction preferences during frontal pursuit, radial eye or target velocity was first calculated as the square root of the sum of the squares of the vertical and horizontal components. The phase shift of the neuronal responses that had diagonal preferred directions was calculated relative to the rightward component of eye-or target velocity.
To examine vergence eye velocity sensitivity further, discharge modulation was tested at different frequencies (0.3–1.0 Hz) at constant amplitude (5°). By plotting the amplitude of discharge modulation against peak vergence eye velocity for each P-cell, vergence eye velocity sensitivity was calculated as the slope of least-square fit linear regression.
Visual responses to the probe motion-in-depth were analyzed by aligning all the traces with the second spot cycles. Traces that contained saccades or slow eye movements were removed because they were indicative of the monkey's failure to fixate the stationary spot, and only those traces with eye position changes of less than 1° during each cycle were selected and analyzed, as previously described (Fukushima et al. 2000; Fukushima K, Yamanobe, Shinmei, and Fukushima J 2002; Akao, Kurkin, et al. 2005; Akao, Mustari, et al. 2005).
Response to Step and Ramp Target Motion-In-depth
To examine neuronal sensitivity to vergence eye position/velocity, further, we analyzed P-cell responses during step target motion. If vermal P-cells are indeed involved in vergence eye movements by sending motor signals, neuronal sensitivity to vergence eye position/velocity should be consistent irrespective of the stimulus dynamics. To examine vergence eye position sensitivity, the monkeys fixated a near or far target for >1 s and mean discharge rates during static convergence and divergence eye positions for 0.5 s were compared in each P-cell over 10 trials. Differences in discharge rates were evaluated (2-tailed paired t-test). Vergence eye position sensitivity was calculated as the difference in mean discharge rates divided by the difference in vergence eye position.
To characterize discharge during vergence eye movements to the target stepped in-depth, we first calculated mean discharge rate before, during, and after vergence eye movements for 0.3–0.5 s in the preferred direction of each P-cell as illustrated in Figure 7A,B. We then calculated the difference in discharge rate during vergence eye movements and the prevergence period (burst components) and the difference in discharge rate during post- and prevergence period (tonic components). Vergence-related P-cells that had burst components significantly larger than the tonic components but that had tonic components not significantly different from the prevergence discharge rates were classified as burst P-cells. P-cells that had tonic components significantly larger than the prevergence discharge rates but that had tonic components not significantly different from the burst components were classified as tonic P-cells. P-cells that had burst components significantly larger than the tonic components and that also had tonic components significantly larger than the prevergence discharge rates were classified as burst tonic P-cells. Two-tailed paired t-tests were used for this analysis. Vergence eye velocity sensitivity was calculated as burst components divided by peak vergence eye velocity for individual burst tonic and burst P-cells.
Latencies of eye and neuronal responses to vergence target steps were analyzed by aligning at least 20 trials with the onset of target motion-in-depth. Traces in which saccades appeared within 200 ms of the target onset were omitted. Standard deviations (SDs) of the mean responses were calculated for the 200-ms interval immediately before target motion onset, and these values were used as the baseline. Onset of vergence eye movements in response to target steps was determined as the time at which mean vergence eye velocity deviated by 2 SD of the baseline value. Similarly, onset of P-cell responses to target steps was determined as the time at which the mean discharge rate exceeded 2 SD of the baseline value. Latencies of discharge modulation of individual P-cells relative to the onset of eye movements were calculated by subtraction. Traces were also aligned with the onset of vergence eye velocity, and peak vergence eye velocity was averaged to obtain mean values (see above).
To analyze the effect of muscimol injection into the unilateral dorsal vermis upon the initiation of eye movements induced by step and ramp target motion-in-depth, at least 20 trials were aligned with the onset of target motion to obtain mean ± 2 SD responses for each injection.
Histological Procedures
Near the conclusion of recordings in monkey A and K, the recording sites were marked by electrolytic lesions made by passing current through the microelectrode (20 µA for 60 s). After recording was completed, each monkey was deeply anesthetized with sodium pentobarbital (50 mg/kg, i.p.) and perfused with physiological saline followed by 3.5% formalin for reconstruction of recording sites. After histological fixation, sagittal sections were cut at 100-µm thickness on a freezing microtome. The sections were then stained for cell bodies and fibers using the Klüver–Barrera method, and the locations of recording sites were verified microscopically.
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Results |
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We recorded simple spike discharge of a total of 112 P-cells that responded during our search task, which required both vergence pursuit and frontal pursuit (e.g., Fig. 1B). Of these, 12 P-cells responded during vergence pursuit but were not tested for frontal pursuit. They were excluded from our analysis. Overall mean (±SD) eye velocity gains during midsagittal vergence pursuit and frontal pursuit at 0.5 Hz were 1.04 (±0.05) and 0.85 (±0.12), respectively. The slightly lower gains for frontal pursuit were due mostly to the low gain in vertical pursuit. The mean (±SD) gains for horizontal, vertical, and diagonal pursuit were 0.94 (±0.07), 0.73 (±0.09), and 0.91 (±0.04), respectively. Thus, gains for horizontal pursuit and midsagittal vergence pursuit were >0.9, indicating that the monkeys performed our pursuit tasks reasonably well while recording vermal-pursuit P-cells.
Classification of Vermal P-cells during 3D Pursuit
We analyzed a total of 100 pursuit P-cells that were tested during both frontal pursuit and midsagittal vergence pursuit. These P-cells were classified as 1 of 3 groups (i.e., vergence + frontal pursuit, vergence only, frontal-pursuit only) (see Data Analysis). Representative P-cell discharge from each group is illustrated in Figure 2, and Figure 3 plots vergence eye velocity sensitivity against frontal eye velocity sensitivity for each P-cell. Forty-one pursuit P-cells (41/100 = 41%) were vergence + frontal-pursuit P-cells (Fig. 2A). In Figure 3, these cells are plotted as those that had both vergence eye velocity sensitivity and frontal eye velocity sensitivity. Forty-three pursuit P-cells (43/100 = 43%) responded only during vergence pursuit (Fig. 2B), and 16 (16/100 = 16%) responded only during frontal pursuit (Fig. 2C). In Figure 3, these 2 groups of P-cells are plotted on the vertical and horizontal axes, respectively. Table 1 (dorsal vermis) summarizes the percentage of each group. These results indicate that 3D-pursuit signals were found in a significant proportion of vermal-pursuit P-cells (41%; Table 1 and Fig. 3). However, in the majority of vermal-pursuit P-cells (59%), vergence signals, and frontal-pursuit signals are represented independently of each other. Moreover, most pursuit-related P-cells (84/100 = 84%) in the dorsal vermis responded during vergence pursuit (Table 1). In the following sections, we compared discharge characteristics of vergence + frontal-pursuit P-cells and vergence-only P-cells during vergence pursuit in detail. The number of neurons tested varied between task conditions due to the occasional degradation or loss of neural recordings.
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Response Phase and Sensitivity of Vergence-Related P-cells during Sinusoidal Vergence Pursuit
Figure 4 plots sensitivity (re vergence eye velocity) of 84 vergence-related P-cells against their response phase during vergence pursuit at 0.5 Hz. Data points within the solid oval indicate that response phase of these P-cells was around peak convergence eye velocity (0 ± 45°, n = 45). Whereas, points within the dashed ovals indicate that response phases of these P-cells were around peak divergence eye velocity (135–180°, n = 16). Response phases of the majority of P-cells were scattered around vergence eye velocity, especially around convergence eye velocity. The percentage of P-cells that exhibited response phase near convergence velocity was significantly higher than that of P-cells that showed response phase near divergence velocity (45/61 = 74% vs. 16/61 = 26%, P < 0.01). Mean (±SD) sensitivities (re vergence eye velocity) of these 2 groups of P-cells were similar (1.8 ± 1.3 vs. 1.8 ± 1.1 sp/s/°/s, respectively, P > 0.1). Response phases of vergence-only P-cells (open circles) and vergence + frontal-pursuit P-cells (filled circles) were similarly distributed. However, some vergence-only P-cells had sensitivities that were much higher than those of vergence + frontal-pursuit P-cells. Mean (±SD) sensitivities of vergence-only P-cells and vergence + frontal-pursuit P-cells were 1.8 (±1.3) and 0.9 (±0.7) sp/s/°/s, respectively. The difference was significant (P < 0.01).
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This figure also plots some P-cells with the response phase around 90°; they had very low eye velocity sensitivity. We calculated vergence eye position sensitivity during 0.5-Hz vergence pursuit (see Data Analysis). The mean (±SD) sensitivity for all P-cells was 2.3 (±1.8, n = 84) sp/s/°. Sensitivity was similar in vergence-only P-cells and vergence + frontal-pursuit P-cells (2.2 ± 1.9, 2.3 ± 1.6 sp/s/°, respectively).
Vergence Eye Velocity Sensitivity during Sinusoidal Vergence Pursuit and Frequency Response of Vermal P-cells
To further examine vergence eye velocity sensitivity, the spot was moved in-depth at 0.3–1.0 Hz (±5°). A total of 42 vergence-related P-cells (23 vergence only, 19 vergence + frontal pursuit) were tested. Figure 5 illustrates discharge modulation of a vergence-only P-cell (B) together with vergence eye velocity (A). The phase of peak vergence eye velocity (re target velocity) lagged and peak eye velocity increased as target frequency increased (Fig. 5A). The P-cell illustrated in Figure 5B exhibited mostly constant phase shift (re target velocity), and discharge modulation increased as target frequency increased. To examine how discharge of each P-cell was affected by increasing vergence target frequency, Figure 6 plots phase values (relative to their values at 0.3 Hz) and amplitude of modulation of the 23 vergence-only P-cells (A, B) and the 19 vergence + frontal-pursuit P-cells (C, D). The phases of individual vermal P-cells in the 2 groups were distributed continuously from those that exhibited virtually no phase lag to those that exhibited even larger phase lag than the phase shifts of vergence eye velocity (Fig. 6A,C). The mean phase behavior of the 2 groups was similar to that of the mean vergence eye velocity. Amplitude of modulation of the majority of vermal P-cells increased as target frequency increased (Fig. 6B,D).
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Figure 5C plots amplitude of discharge modulation of the P-cell shown in Figure 5B against peak vergence eye velocity. The 2 were significantly positively correlated, and the slope of the linear regression was 1.90 sp/s/°/s. The majority (28/42 = 67%) of tested P-cells had significant (P < 0.05) linear correlation between amplitude of discharge modulation and peak vergence eye velocity. The percentage of P-cells that exhibited significant linear correlation was similar for vergence-only P-cells (17/23 = 74%) and vergence + frontal-pursuit P-cells (11/19 = 58%). Mean (±SD) slopes for vergence-only P-cells and vergence + frontal-pursuit P-cells were 1.5 (±1.1, n = 17) and 0.7 (±0.5, n = 11) sp/s/°/s, respectively. The difference was significant (P < 0.05). Overall mean (±SD) slope was 1.2 (±1.0, n = 28) sp/s/°/s. These results indicate that vergence eye velocity sensitivity was higher in vergence-only P-cells than vergence + frontal-pursuit P-cells (cf., Fig. 6D).
Vergence Eye Position and Velocity Sensitivity to Step Target Motion-In-depth
We examined position sensitivity in-depth of 40 vergence-related vermal P-cells (26 vergence-only and 14 vergence + frontal pursuit) during vergence step motion (see Materials and Methods). Of the 40 P-cells, the majority (33/40 = 83%) increased discharge during convergence and the minority (7/40 = 17%) during divergence. Figure 7A,B shows representative discharge of convergence and divergence P-cells, respectively. When the monkeys made vergence steps in the cell's preferred direction (average vergence angle, 9.4–10.2°, Fig. 7A,B), mean discharge rates of these P-cells increased from 35 to 111 sp/s (tonic component = 76 sp/s, Fig. 7A, see Data Analysis) and from 39 to 83 sp/s (tonic component = 44 sp/s, Fig. 7B). The vergence eye position sensitivity of the 2 P-cells (Fig. 7A,B) was calculated as the difference in mean discharge rates divided by the difference in vergence angle and was 7.7 and 4.3 sp/s/°, respectively. The majority of tested P-cells (32/40 = 80%) had vergence eye position sensitivity; the mean (±SD) was 3.4 (±2.2) sp/s/°. Of the 32, 26 P-cells (26/32 = 81%) had convergence eye position sensitivity (mean ± SD = 3.6 ± 2.3 sp/s/°). The remaining 6 P-cells (6/32 = 19%) had divergence eye position sensitivity (mean ± SD = 2.6 ± 1.6 sp/s/°). Mean ± SD position sensitivities of vergence-only P-cells and vergence + frontal-pursuit P-cells were 3.4 ± 2.5 and 3.3 ± 1.7 sp/s/°, respectively. Thus, the majority of vermal-pursuit P-cells had both vergence eye position sensitivity and eye velocity sensitivity.
The P-cell illustrated in Figure 7A exhibited burst discharge during convergence (burst component = 143 sp/s, see Materials and Methods) that was significantly higher (P < 0.01) than the tonic component (76 sp/s), whereas the P-cell shown in Figure 7B exhibited no clear burst. Figure 7C,D plots burst and tonic components of convergence P-cells and divergence P-cells, respectively. We classified these P-cells into 3 types according to their discharge patterns (see Data Analysis). In both vergence-only and vergence + frontal-pursuit P-cells, burst tonic P-cells were most frequently observed (25/40 = 63%, 19/26 vergence only and 6/14 vergence + frontal pursuit, Fig. 7C,D) compared with burst P-cells (8/40 = 20%, 4/26 vergence only and 4/14 vergence + frontal pursuit) and tonic P-cells (7/40 = 17%, 3/26 vergence only and 4/14 vergence + frontal pursuit). Representative P-cell responses of the 3 types are illustrated in Figure 8A during vergence step toward their preferred directions. During vergence step toward the opposite directions, their discharge decreased (Fig. 8B).
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Although some convergence P-cells exhibited high burst components compared with those of divergence P-cells (Fig. 7C vs. Fig. 7D), peak vergence eye velocity was higher during convergence step than divergence step (mean ± SD, 109 ± 32.6°/s vs. 51.6 ± 6.8°/s, P < 0.01). As a result, vergence eye velocity sensitivity (i.e., burst components divided by peak vergence eye velocity) for individual burst tonic and burst P-cells in this task was similar between convergence P-cells (n = 28) and divergence P-cells (n = 5); the means (±SDs) for the 2 groups were 0.9 (±0.7) and 0.9 (±0.6) sp/s/°/s, respectively. Vergence-only P-cells tended to have higher velocity sensitivity than vergence + frontal-pursuit P-cells during vergence step, and the mean (±SD) values were 1.0 (± 0.7) and 0.6 (± 0.4) sp/s/°/s (P = 0.11), respectively. These values were slightly smaller than velocity sensitivity calculated by the linear regression (e.g., Fig. 5D, 1.5 ± 1.1 vs. 0.7 ± 0.5 sp/s/°/s for the 2 groups of P-cells) as described in the preceding section.
To compare the response type determined by vergence step (i.e., convergence or divergence P-cells and burst, burst-tonic, or tonic) and phase/sensitivity during sinusoidal vergence pursuit at 0.5 Hz, Figure 8C plots the results for the 40 P-cells in which both tasks were tested. P-cells that exhibited response phase near convergence eye velocity (filled symbols) were indeed convergence P-cells, and they constituted the majority of vergence-related vermal P-cells (see Fig. 4). The majority of burst and burst-tonic P-cells had response phase near the peak vergence eye velocity during sinusoidal vergence pursuit (within the ovals in Fig. 8C). Half of tonic P-cells had response phase near vergence eye position (near 90°). For the 32 burst tonic and tonic P-cells that exhibited significant sensitivity during step target motion, we compared their eye position sensitivity during 0.5 Hz vergence pursuit. The mean was 2.8 ± 2.0 SD sp/s/°/s, similar to eye position sensitivity calculated during vergence steps as described above.
Visual Responses of Vergence-Related P-cells to Spot Motion-In-depth
It has been shown that vermal-pursuit P-cells exhibit a visual response to spot motion in the frontal plane (Kase et al. 1979; Suzuki and Keller 1988a, 1988b; Shinmei et al. 2002). In 41 vergence-related P-cells, we examined whether they exhibited a visual response to spot motion-in-depth (see Materials and Methods). An example is illustrated in Figure 9A. Peak modulation of this cell during vergence pursuit was near peak convergence eye velocity (not shown), and it also exhibited a visual response when the second spot moved toward the monkey during fixation of a stationary spot (Fig. 9A). Of 41 tested P-cells, 34% (14/41) exhibited a visual response to spot motion-in-depth. We compared the sensitivity of visual- and vergence-related responses of these 14 P-cells (11 vergence only and 3 vergence + frontal pursuit). Figure 9B plots sensitivity of visual responses to the velocity of second spot against sensitivity during vergence pursuit. In most P-cells, visual target velocity sensitivity was significantly weaker than vergence eye velocity sensitivity (P < 0.05). Mean (±SD) visual sensitivity and vergence sensitivity of the 14 P-cells were 0.37 (±0.24) and 1.37 (±1.44) sp/s/°/s, respectively (Fig. 9B).
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To compare the preferred direction of visual responses with that of vergence-pursuit responses, phases relative to the second spot velocity in-depth during fixation is plotted against phase values during vergence pursuit in Figure 9C. The differences of response phase in these 2 tasks were within ±45° in half (7/14) of the tested P-cells (i.e., they were scattered near the dashed line). This suggests that they had similar preferred directions for their vergence-pursuit responses and visual responses to target motion-in-depth; for example, P-cells that discharged for convergence during vergence pursuit also discharged when the target moved toward the monkey during fixation (Fig. 9C).
Latency of Discharge Modulation of Vergence-Related P-cells to Step Target Motion-In-depth
To understand whether vergence-related P-cells in the dorsal vermis could be involved in the initiation of vergence pursuit, we examined the latencies of discharge of 40 vergence-related P-cells relative to the onset of vergence eye movements induced by step target motion-in-depth (see Materials and Methods). These P-cells were the same cells that were examined for vergence eye position sensitivity (Figs 7 and 8). Figure 7A,B (dashed lines) compares the onset of vergence eye movements and the onset of discharge modulation of the 2 representative P-cells. The convergence P-cell shown in Figure 7A clearly discharged before the onset of vergence eye movements, whereas the divergence P-cell (Fig. 7B) discharged after the onset of vergence eye movements.
Figure 10A,B summarizes latency distributions of the 40 vergence-related P-cells (26 vergence only and 14 vergence + frontal pursuit) with respect to the onset of vergence eye movements (A, dotted line) and of target motion (B). Latencies relative to the onset of vergence eye movements were distributed widely from –105 to 216 ms with the median of –16 ms (minus means that discharge occurred before the onset of vergence eye movements). Twenty-nine P-cells (29/40 = 73%) discharged before the onset of vergence eye movements (Fig. 10A). Latencies relative to target onset were also distributed widely from 73 to 380 ms with the median of 144 ms (Fig. 10B). Latencies of the 2 groups of P-cells (vergence only and vergence + frontal pursuit) were distributed similarly (Fig. 10A,B).
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In Figure 10C,D, we also compared latencies of convergence P-cells (n = 33) and divergence P-cells (n = 7) with respect to the onset of vergence eye movements (C) and of target motion (D). Although the number of divergence P-cells was small, both groups of P-cells contained neurons that discharged before the onset of vergence eye movements (Fig. 10C).
Because one-third of our vergence-related P-cells exhibited visual responses to second spot motion-in-depth (Fig. 9), initial P-cell discharges may have reflected visual responses. Figure 10E,F plots latencies with respect to the onset of vergence eye movements (E) and of target motion (F) during vergence pursuit of 29 P-cells that were tested for visual responses. Of these, 10 P-cells exhibited visual responses in-depth (Fig. 10E,F, visual +), and the remaining P-cells (n = 19) did not (nonvisual). The latency distributions of the 2 groups of P-cells were similar, indicating that the existence of visual response did not substantially affect the response latencies during vergence pursuit.
In Figure 10G,H, we compared latencies of burst, burst tonic, and tonic P-cells (n = 40) with respect to the onset of vergence eye movements (G) and of target motion (H). The majority of burst tonic P-cells discharged before the onset of vergence eye movements (Fig. 10G).
Frontal-Pursuit–Related P-cells
As summarized in Table 1 (dorsal vermis), 57 P-cells responded for frontal pursuit (41 vergence + frontal pursuit and 16 frontal-pursuit only; Figs 2A,C and 3). We classified preferred directions of P-cells during frontal pursuit roughly into the 8 tested directions (see Materials and Methods). Preferred frontal-pursuit directions of vergence + frontal-pursuit P-cells were distributed in all directions and one-third of them (13/41 = 32%) had vertical (but not horizontal) pursuit sensitivity. We calculated the sensitivities of these P-cells during frontal pursuit along preferred directions at 0.5 Hz. The mean sensitivities (±SD) of frontal-pursuit–only P-cells and vergence + frontal-pursuit P-cells were similar (1.05 ± 0.86 and 0.91 ± 0.83 sp/s/°/s, respectively). Of 57 P-cells, 22 were tested for visual responses in the frontoparallel plane while fixating a stationary spot (see Materials and Methods). Four P-cells (4/22 = 18%) responded to visual stimulus, and the visual-preferred directions and frontal-pursuit–preferred directions of these P-cells were similar.
Response of Vermal P-cells during Combination of Vergence Pursuit and Frontal Pursuit
The majority of pursuit neurons in the caudal FEF discharge not only for frontal pursuit but also for vergence pursuit with the preferred directions that are the linear sum of the 2 components (Fukushima K, Yamanobe, Shinmei, Fukushima J, Kurkin, and Peterson 2002). In 3 vermal P-cells, we asked whether similar linear addition occurs during 3D pursuit. Figure 11A–D illustrates responses of a P-cell that discharged for rightward pursuit (A) and divergence during midsagittal vergence pursuit (B). When the monkey tracked a sinusoidally moving target with combination of horizontal pursuit and vergence pursuit (±5°), this cell exhibited stronger modulation for rightward divergence (Fig. 11C). The amplitude of neuronal modulation during sinusoidal frontal pursuit (Fig. 11A), midsagittal vergence pursuit (Fig. 11B), and the combination of the 2 (Fig. 11C) was 10, 12, and 16 sp/s, respectively. Figure 11D compares actual modulation and predicted modulation that was calculated by the linear addition of discharge rates during frontal pursuit (A) and midsagittal vergence pursuit (B). The response phase of the actual modulation (thick line, Fig. 11D) was similar to the predicted modulation (D, thin line), although the amplitude of the actual modulation (16 sp/s) was smaller than that of the predicted modulation (21 sp/s). Two other convergence P-cells showed similar results; the response phase was predicted by linear addition although the mean amplitude of predicted modulation was 34% larger (n = 3) compared with the actual modulation. These results indicate that pursuit signals carried by vermal-pursuit P-cells are combined signals for frontal pursuit and midsagittal vergence pursuit.
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To confirm that modulation of vergence + frontal-pursuit P-cells during vergence pursuit was not specifically related to the motion of a single eye, vergence pursuit was tested by requiring the monkey to pursue a target moving in-depth aligned with either the left or right eye (see Materials and Methods). In similar task conditions, modulation of pursuit neurons in the caudal FEF was well predicted by linear addition of modulation related to vergence and horizontal pursuit (Fukushima K, Yamanobe, Shinmei, Fukushima J, Kurkin, and Peterson 2002
). We examined whether modulation of vermal P-cells during one eye–aligned pursuit could be explained by similar addition. Representative discharge is shown in Figure 11E–H for a P-cell that discharged during rightward pursuit (E) and midsagittal convergence (F). Figure 11H compares actual modulation during right eye–aligned pursuit-in-depth (Fig. 11G) with the predicted modulation that was calculated by adding modulation during horizontal pursuit and midsagittal vergence (H). The response phase of the actual modulation (thick line, Fig. 11H) was similar to the predicted modulation (H, thin line), although the amplitude of the actual modulation (10 sp/s) was half the predicted modulation (20 sp/s). The results were similar for another vergence + frontal-pursuit P-cell tested, suggesting that pursuit signals carried by vergence + frontal-pursuit P-cells are not monocular signals (see Discussion). Chemical Inactivation of the Cerebellar Dorsal Vermis
To further examine whether vergence-related vermal P-cells could be involved in the initiation of vergence eye movements, we injected muscimol into the region in the left or right dorsal vermis (Fig. 13, asterisk) where we recorded many convergence-related P-cells in monkey K. Injections were repeated on 8 different days; in 4 of them, vergence eye movements were tested using step target motion-in-depth, and in the remaining 4, target motion was applied in a ramp trajectory (see Materials and Methods). For each target motion, consistent results were obtained. Representative results during vergence step are shown in Figure 12A before and after muscimol injection into the left dorsal vermis. Before muscimol infusion, the latency of convergence eye movements was 140 ms and the peak vergence eye velocity was 105°/s (Fig. 12A, left). After muscimol injection, the latency of convergence eye movements was slightly longer (166 ms). The monkey made horizontal saccades quite frequently near the onset of convergence movements (typically 200–230 ms after the onset of target step, Fig. 12A, after). Saccade magnitudes of left and right eyes were asymmetric, resulting in a clear vergence component (peak vergence eye velocity 81°/s) despite the fact that saccades were not evoked to this target step before muscimol injection (Fig. 12A, before). The convergence eye velocity at the onset of saccades after muscimol injection was 15–20°/s. These velocities were much lower than the convergence eye velocities (48–87°/s) at the same latency (i.e., 200–230 ms after the onset of target step) before muscimol injection (Fig. 12A, before). Moreover, the peak convergence eye velocity that was observed without interruption of saccades was very low as shown in Figure 12A (left bottom). Clearly, muscimol injection affected the initial convergence eye velocity to step target motion-in-depth, although vergence eye position was maintained similarly before and after muscimol injection.
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During divergence step after muscimol injection (Fig. 12A, right), the monkey always made horizontal saccades near the onset of divergence movements, although the divergence eye velocity at the onset of saccades was similar before and after muscimol infusion. Saccade amplitudes of the left and right eyes were asymmetric, thus resulting in divergence eye velocity that was even higher than the control (Fig. 12A, right bottom). Because of the saccades, we were unable to evaluate muscimol effects on divergence eye movements during step target motion (see Discussion).
The monkey also pursued the spot that moved in a ramp trajectory (see Materials and Methods, Fig. 12B). Before muscimol injection, the average amplitude of these initial vergence eye movements was 5° with the peak convergence eye velocity of 40°/s. After muscimol injection into the right dorsal vermis (Fig. 13, asterisk), the average amplitude of these initial vergence eye movements was similar but the peak convergence eye velocity decreased by 20% compared with peak velocity before injection (from 37.6 to 30.1°/s, Fig. 12B, left) and was delayed. Convergence eye acceleration calculated as the slope of vergence eye velocity decreased by 39% (from 280 to 170°/s2) after muscimol injection. Whereas, divergence eye movements did not change after muscimol injection (Fig. 12B, right). Latencies of convergence and divergence eye movements induced by ramp target motion-in-depth were not affected by muscimol injection.
During frontal pursuit ipsilateral to the injection side (Fig. 12C, rightward), peak eye velocity decreased by 16% (from 18.0 to 15.1°/s) compared with the control. During frontal pursuit in other directions, no obvious change was found. Another striking deficit induced by muscimol injection was the hypometric ipsilateral saccades (Fig. 12D). Saccades toward the contralateral side were only slightly hypermetric. Vertical saccades were shifted toward the left as illustrated in a polar plot (Fig. 12D, right) after injection.
Recording Location
Figure 13 illustrates examples of the recording locations in 2 monkeys. In both monkeys, the 3 types of P-cells (vergence only, vergence + frontal pursuit, and frontal-pursuit only) were intermingled in lobules VI and VII of the dorsal vermis.
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Discussion |
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The present study has revealed for the first time that, although a significant proportion (41%, Table 1) of pursuit P-cells in the cerebellar dorsal vermis carried 3D-pursuit signals, the majority of pursuit P-cells (59%) discharged either for vergence pursuit (43%) or frontal pursuit (16%). Moreover, the majority (74%) of vergence-related P-cells carried convergence signals, displaying both vergence eye position and velocity sensitivity during sinusoidal and step vergence eye movements (Figs 5–7
), and discharged before the onset of vergence eye movements (Fig. 10). Although the dorsal vermis has been known to be related to frontal pursuit (for a review, see Robinson and Fuchs 2001), in the present study, pursuit P-cells that discharged specifically for frontal pursuit were in the minority (16%, Table 1). Our results indicate that the majority of responding P-cells modulated their activity during vergence pursuit. The 2 groups of vergence-related P-cells (vergence only and vergence + frontal pursuit) exhibited similar responses except for the significant difference in velocity sensitivity.
Monocular coding is known for saccade-related burst neurons in the brain stem (Zhou and King 1998). The present study suggests that the modulation of vermal P-cells during vergence pursuit was not a result of their association with monocular movements for the following reasons. First, if modulation during midsagittal vergence reflected modulation to monocular movements, vergence-only P-cells and vergence + frontal-pursuit P-cells should have responded to horizontal pursuit because it is well known that the magnitude of vertical vergence is very small (for a review, see Leigh and Zee 2006). Vergence-only P-cells did not respond to frontal pursuit (e.g., Figs 2B and 3). Preferred frontal-pursuit directions of one-third of vergence + frontal-pursuit P-cells (13/41 = 32%) were vertical but not horizontal, indicating that these P-cells cannot code monocular movements. Second, the modulation of vergence + frontal-pursuit P-cells tested can best be explained by addition of modulation during horizontal pursuit and midsagittal vergence pursuit (Fig. 11). These results taken together suggest that these vermal-pursuit P-cells code primarily disparity vergence eye movements.
Comparison of Vergence Signals
As summarized in Table 1, the percentage of vergence-related neurons (vergence + frontal pursuit and vergence only) in the vermal P-cells was similar to that in the caudal FEF (84% vs. 79%, Akao, Kurkin, et al. 2005), but the percentage was much higher than that in SEF and MST (38% and 39%, Fukushima et al. 2004; Akao, Mustari, et al. 2005). In addition, the dorsal vermis contained neurons that had vergence eye velocity and position sensitivity and discharged before the onset of vergence eye movements, similar to the responses of the majority of pursuit neurons in the caudal FEF (Akao, Kurkin, et al. 2005). Anatomical studies indicate that major outputs of the FEF are sent to the NRTP (Stanton et al. 1988). Pursuit neurons in the caudal FEF were identified to project to the NRTP (Ono et al. 2004). Because the dorsal vermis receives major inputs from the NRTP (Azizi et al. 1981; Thielert and Thier 1993), these previous results and the present findings taken together suggest that 3D-pursuit signals and pure vergence-pursuit signals in vermal P-cells are carried from the FEF via NRTP.
Neurons related to vergence eye movements have been found in the NRTP (Gamlin and Clarke 1995) that projects to both the fastigial nucleus and the cerebellar dorsal vermis. Within the NRTP, near-response neurons and far-response neurons are found equally in number (Gamlin and Clarke 1995). Although Gamlin and Clarke (1995) reported that the activity of near/far-response neurons in the NRTP was unaffected by frontal pursuit, it is not clear how many neurons they specifically tested. It is possible that the tested neurons may have sent signals primarily to our vergence-only P-cells that constituted 43% of our population (Table 1). Preliminary studies reported that the posterior fastigial nucleus contained "near-response neurons" (Zhang and Gamlin 1996). This nucleus receives projections from the dorsal vermis (for a review, see Robinson and Fuchs 2001).
Vergence-related activity has also been reported in the ventral paraflocculus (Miles et al. 1980) and the posterior interposed nucleus (IP) (Zhang and Gamlin 1998). The posterior IP contains "far-response neurons" that discharge during divergence eye movements (Zhang and Gamlin 1998).
In the midbrain, neurons related to convergence and divergence eye movements (near- and far-response neurons; Mays 1984; Mays et al. 1986) are found in the separate areas. The dorsal vermal outputs are sent to these midbrain areas via the caudal fastigial nucleus. These midbrain vergence-related neurons discharge 10 ms before the onset of vergence eye movements and exhibit burst, burst tonic, and tonic discharge (Mays 1984; Mays et al. 1986), similar to vermal-pursuit P-cells in the present study (Fig. 8A). Moreover, most of our vermal-pursuit P-cells (73%) discharged before the onset of vergence eye movements induced by vergence target step; the median lead time was 16 ms. In 6 of these P-cells, we compared latencies using ramp target motion-in-depth; their latencies were similar (Nitta et al. 2006; unpublished data). Thus, our vermal P-cells could provide vergence signals to the midbrain vergence-related neurons.
Impairment of Convergence Eye Movements Induced by Chemical Inactivation of the Dorsal Vermis
Chemical inactivation of the dorsal vermis in the present study resulted in reduction of convergence eye velocity and of initial convergence eye acceleration during step and ramp vergence eye movements (Fig. 12A,B). The latencies of convergence eye movements during the vergence step task were also prolonged. These results are consistent with the loss of activity of convergence-related, mostly burst tonic vermal P-cells (Figs 7 and 8), suggesting their involvement in the initial phase of convergence eye movements. Saccades that were observed consistently during convergence step after muscimol injection (Fig. 12A) may have appeared to compensate for the initial low convergence eye velocity. Our results also suggest that sustained components of vergence eye movements were not affected.
We were unable to evaluate muscimol effects on divergence eye movements during vergence step because of the saccades. It seems that the monkey changed the strategy during divergence step most probably because divergence step and convergence step were given alternatively. However, for 2 reasons, we think that divergence eye movements were not affected: 1) the divergence eye velocity at the onset of saccades during vergence step was similar before and after muscimol infusion and 2) divergence eye movements induced by ramp target motion was not affected. Because vergence signals, especially divergence, are also reported in the cerebellar floccular region, this region could be involved in divergence eye movements and also the sustained components of vergence eye movements (Miles et al. 1980; also Tsubuku et al. 2004).
Our results also showed that muscimol injection resulted in reduction of peak eye velocity during frontal pursuit (Fig. 12C) and hypometric saccades toward the ipsilateral side (Fig. 12D). It is well known that hypometric/hypermetric saccades are induced by lesion or chemical blockage of the dorsal vermis (e.g., Sato and Noda 1992b; Takagi et al. 1998; for a review, see Robinson and Fuchs 2001). Although saccade neurons and smooth-pursuit neurons are mostly separate in the dorsal vermis (for review, see Leigh and Zee 2006), saccade-related activity has also been reported in some vermal-pursuit P-cells (Suzuki and Keller 1988b; Shinmei et al. 2002), and microstimulation within the oculomotor vermis evokes both saccades and pursuit-like eye movements (Krauzlis and Miles 1998). In the present study, our pursuit-related P-cells did not respond for saccades, but saccade-related neurons were often observed near these P-cells (Nitta et al., unpublished data). Muscimol that was injected into the pursuit-related vermal area might have spread to the adjacent saccade-related area. Arikan et al. (2002) reported that the effective spread of 2% muscimol injected into the rat cerebellum was roughly 2 mm diameter and that area was surrounded by partially inactivated areas of about 0.5 mm.
Possible Role of Vermal P-Cells in 3D Pursuit and Vergence Eye Movements
Traditionally, the vergence and frontal-pursuit systems are considered to have separate neural substrates, and signals carried by the 2 systems are thought to be combined at the final output stage as suggested by Hering (for a review, see Leigh and Zee 2006). In fact, the 2 signals are combined at the caudal FEF (see Introduction). Because in the brain stem, near- and far-response neurons are found in separate areas and because they do not respond to frontal pursuit (Mays 1984; Mays et al. 1986), 3D-pursuit signals carried by FEF neurons must be converted into vergence- and frontal-pursuit components and vergence signals must further be converted into convergence and divergence signals. It is well known that, in the cerebellar floccular region, omni-directional frontal-pursuit components are converted into horizontal and vertical components (Miles et al. 1980; Shidara and Kawano 1993; Krauzlis and Lisberger 1996).
Comparison of pursuit signals in the caudal FEF and dorsal vermis reveals not only the similarities but also the following differences. First, the percentage of neurons carrying 3D-pursuit signals was significantly lower in the dorsal vermis than in the caudal FEF (Table 1; 41/100 = 41% vs. 106/169 = 63%; P < 0.02). In the majority of vermal-pursuit P-cells, vergence signals and frontal-pursuit signals are represented independently (Table 1; 59%). Second, although preferred directions of vermal-pursuit P-cells for frontal pursuit were distributed in all directions as reported earlier (e.g., Shinmei et al. 2002), convergence signals are predominant in vergence-related vermal P-cells. This is in contrast to the representation of vergence signals in the caudal FEF and also NRTP; the percentages of convergence neurons and divergence neurons are similar (Gamlin and Clarke 1995; Akao, Kurkin, et al. 2005). Third, there was a reduction of the response modulation by combining frontal pursuit with vergence in the dorsal vermis (Fig. 11D,H) but not in the caudal FEF (Fukushima K, Yamanobe, Shinmei, Fukushima J, Kurkin, and Peterson 2002). Reduction in response magnitude could occur if the 2 spatially different inputs activate a common group of neurons at suprathreshold. Because the 2 inputs activate the majority of pursuit neurons in the caudal FEF, the reduction could occur if the dorsal vermis receives 3D-pursuit signals from the caudal FEF. These differences in pursuit signals in the caudal FEF, dorsal vermis, and the brain stem indicate that pursuit signals carried by vermal P-cells are intermediate in signal conversion from the caudal FEF to the brain stem, suggesting processing of FEF pursuit signals in the dorsal vermis with respect to 3D pursuit and the direction of vergence eye movements.
Our results showing that muscimol infusion into the dorsal vermis impaired convergence (but not divergence) eye movements (Fig. 12) may suggest that divergence eye movements are organized primarily separately. Because the posterior IP where divergence signals are found (Zhang and Gamlin 1998) receives projections from the ventral paraflocculus (Nagao et al. 1997) and because divergence signals are also found there (Miles et al. 1980; also Tsubuku et al. 2004), this floccular–posterior IP pathway may be involved in the control of divergence eye movements. A release of convergence eye position holding may also contribute to divergence eye movements. In contrast, because convergence signals are found in the posterior fastigial nucleus in preliminary studies (Zhang and Gamlin 1996), the dorsal vermis–caudal fastigial pathway may be specifically involved in the initiation of convergence eye movements. To further elucidate how the latter pathway is involved in the initial phase of convergence eye movements, future studies must examine discharge characteristics of caudal fastigial neurons in more detail during 3D pursuit.
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Funding |
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Grant-in-Aid for Scientific Research on Priority Areas (System study on higher order brain functions) (17022001); MEXT of Japan (B) (18300130).
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Acknowledgments |
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We thank Dr C.R.S. Kaneko and anonymous reviewers for their valuable comments on the manuscript. Conflict of Interest: None declared.
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References |
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Akao T, Kurkin SA, Fukushima J, Fukushima K. Visual and vergence eye movement-related responses of pursuit neurons in the caudal frontal eye fields to motion-in-depth stimuli. Exp Brain Res (2005) 164:92–108.[CrossRef][Web of Science][Medline]
Akao T, Mustari MJ, Fukushima J, Kurkin S, Fukushima K. Discharge characteristics of pursuit neurons in MST during vergence eye movements. J Neurophysiol (2005) 93:2415–2434.
Akao T, Saito H, Fukushima J, Kurkin S, Fukushima K. Latency of vestibular responses of pursuit neurons in the caudal frontal eye fields to whole body rotation. Exp Brain Res (2007) 177:400–410.[CrossRef][Web of Science][Medline]
Arikan R, Blake NM, Erinjeri JP, Woolsey TA, Giraud L, Highstein SM. A method to measure the effective spread of focally injected muscimol into the central nervous system with electrophysiology and light microscopy. J Neurosci Methods (2002) 118:51–57.[CrossRef][Web of Science][Medline]
Azizi SA, Mihailoff GA, Burne RA, Woodward DJ. The pontocerebellar system in the rat: an HRP study. I. Posterior vermis. J Comp Neurol (1981) 197:543–548.[CrossRef][Web of Science][Medline]
Chubb MC, Fuchs AF. Contribution of y group of vestibular nuclei and dentate nucleus of cerebellum to generation of vertical smooth eye movements. J Neurophysiol (1982) 48:75–99.
Collewijn H, Erkelens CJ. Binocular eye movements and the perception of depth. Rev Oculomot Res (1990) 4:213–261.[Medline]
Fuchs AF, Robinson DA. A method for measuring horizontal and vertical eye movement chronically in the monkey. J Appl Physiol (1966) 21:1068–1070.
Fukushima J, Akao T, Takeichi N, Kurkin S, Kaneko CR, Fukushima K. Pursuit-related neurons in the supplementary eye fields: discharge during pursuit and passive whole body rotation. J Neurophysiol (2004) 91:2809–2825.
Fukushima K, Fukushima J, Kaneko CR, Fuchs AF. Vertical Purkinje cells of the monkey floccular lobe: simple-spike activity during pursuit and passive whole body rotation. J Neurophysiol (1999) 82:787–803.
Fukushima K, Sato T, Fukushima J, Shinmei Y, Kaneko CR. Activity of smooth pursuit-related neurons in the monkey periarcuate cortex during pursuit and passive whole-body rotation. J Neurophysiol (2000) 83:563–587.
Fukushima K, Yamanobe T, Shinmei Y, Fukushima J. Predictive responses of periarcuate pursuit neurons to visual target motion. Exp Brain Res (2002) 145:104–120.[CrossRef][Web of Science][Medline]
Fukushima K, Yamanobe T, Shinmei Y, Fukushima J, Kurkin S, Peterson BW. Coding of smooth eye movements in three-dimensional space by frontal cortex. Nature (2002) 419:157–162.[CrossRef][Medline]
Gamlin PD, Clarke RJ. Single-unit activity in the primate nucleus reticularis tegmenti pontis related to vergence and ocular accommodation. J Neurophysiol (1995) 73:2115–2119.
Gamlin PD, Yoon K. An area for vergence eye movement in primate frontal cortex. Nature (2000) 407:1003–1007.[CrossRef][Medline]
Gamlin P, Zhang H. Effect of muscimol blockade of posterior fastigial nucleus on vergence and accommodation in the primate [Abstract]. Abstr Soc Neurosci (1996) 22:2034.
Gonzalez F, Perez R. Neural mechanisms underlying stereoscopic vision. Prog Neurobiol (1998) 55:191–224.[CrossRef][Web of Science][Medline]
Gottlieb JP, Bruce CJ, MacAvoy MG. Smooth eye movements elicited by microstimulation in the primate frontal eye field. J Neurophysiol (1993) 69:786–799.
Judge SJ, Richmond BJ, Chu FC. Implantation of magnetic search coils for measurement of eye position: an improved method. Vision Res (1980) 20:535–538.[CrossRef][Web of Science][Medline]
Kase M, Noda H, Suzuki DA, Miller DC. Target velocity signals of visual tracking in vermal Purkinje cells of the monkey. Science (1979) 205:717–720.
Krauzlis RJ, Lisberger SG. Directional organization of eye movement and visual signals in the floccular lobe of the monkey cerebellum. Exp Brain Res (1996) 109:289–302.[Web of Science][Medline]
Krauzlis RJ, Miles FA. Role of the oculomotor vermis in generating pursuit and saccades: effects of microstimulation. J Neurophysiol (1998) 80:2046–2062.
Kurkin S, Takeichi N, Akao T, Sato F, Fukushima J, Kaneko CR, Fukushima K. Neurons in the caudal frontal eye fields of monkeys signal three-dimensional tracking. Ann NY Acad Sci (2003) 1004:262–270.[CrossRef][Web of Science][Medline]
Leigh RJ, Zee DS. The neurology of eye movements (2006) 4th ed. New York: Oxford University Press.
Lisberger SG, Fuchs AF. Role of primate flocculus during rapid behavioral modification of vestibuloocular reflex. I. Purkinje cell activity during visually guided horizontal smooth-pursuit eye movements and passive head rotation. J Neurophysiol (1978) 41:733–763.
Mays LE. Neural control of vergence eye movements: convergence and divergence neurons in midbrain. J Neurophysiol (1984) 51:1091–1108.
Mays LE, Porter JD, Gamlin PD, Tello CA. Neural control of vergence eye movements: neurons encoding vergence velocity. J Neurophysiol (1986) 56:1007–1021.
Miles FA, Fuller JH, Braitman DJ, Dow BM. Long-term adaptive changes in primate vestibuloocular reflex. III. Electrophysiological observations in flocculus of normal monkeys. J Neurophysiol (1980) 43:1437–1476.
Nagao S, Kitamura T, Nakamura N, Hiramatsu T, Yamada J. Location of efferent terminals of the primate flocculus and ventral paraflocculus revealed by anterograde axonal transport methods. Neurosci Res (1997) 27:257–269.[CrossRef][Web of Science][Medline]
Nitta T, Akao T, Kurkin S, Fukushima K. Three dimensional (3D) pursuit eye movement signals in cerebellar dorsal vermis [abstract]. Neurosci Res Suppl (2006) 55:S244.
Ono S, Das VE, Mustari MJ. Gaze-related response properties of DLPN and NRTP neurons in the rhesus macaque. J Neurophysiol (2004) 91:2484–2500.
Robinson FR, Fuchs AF. The role of the cerebellum in voluntary eye movements. Annu Rev Neurosci (2001) 24:981–1004.[CrossRef][Web of Science][Medline]
Sato H, Noda H. Posterior vermal Purkinje cells in macaques responding during saccades, smooth pursuit, chair rotation and/or optokinetic stimulation. Neurosci Res (1992a) 12:583–595.[CrossRef][Web of Science][Medline]
Sato H, Noda H. Saccadic dysmetria induced by transient functional decortication of the cerebellar vermis. Exp Brain Res (1992b) 88:455–458.[CrossRef][Web of Science][Medline]
Shidara M, Kawano K. Role of Purkinje cells in the ventral paraflocculus in short-latency ocular following responses. Exp Brain Res (1993) 93:185–195.[Web of Science][Medline]
Shinmei Y, Yamanobe T, Fukushima J, Fukushima K. Purkinje cells of the cerebellar dorsal vermis: simple-spike activity during pursuit and passive whole-body rotation. J Neurophysiol (2002) 87:1836–1849.
Singh A, Thau GE, Raphan T, Cohen B. Detection of saccades by a maximum likelihood ratio criterion. In: Proceedings of the 34th Annual Conference on Engineering in Medicine and Biology (1981) Houston(TX). 136p.
Stanton GB, Goldberg ME, Bruce CJ. Frontal eye field efferents in the macaque monkey: II. Topography of terminal fields in midbrain and pons. J Comp Neurol (1988) 271:493–506.[CrossRef][Web of Science][Medline]
Suzuki DA, Keller EL. The role of the posterior vermis of monkey cerebellum in smooth- pursuit eye movement control. I. Eye and head movement-related activity. J Neurophysiol (1988a) 59:1–18.
Suzuki DA, Keller EL. The role of the posterior vermis of monkey cerebellum in smooth-pursuit eye movement control. II. Target velocity-related Purkinje cell activity. J Neurophysiol (1988b) 59:19–40.
Suzuki DA, Noda H, Kase M. Visual and pursuit eye movement-related activity in posterior vermis of monkey cerebellum. J Neurophysiol (1981) 46:1120–1139.
Takagi M, Zee DS, Tamargo RJ. Effects of lesions of the oculomotor vermis on eye movements in primate: saccades. J Neurophysiol (1998) 80:1911–1931.
Tanaka M, Fukushima K. Neuronal responses related to smooth pursuit eye movements in the periarcuate cortical area of monkeys. J Neurophysiol (1998) 80:28–47.
Thielert CD, Thier P. Patterns of projections from the pontine nuclei and the nucleus reticularis tegmenti pontis to the posterior vermis in the rhesus monkey: a study using retrograde tracers. J Comp Neurol (1993) 337:113–126.[CrossRef][Web of Science][Medline]
Tsubuku T, Akao T, McCrea R, Kurkin S, Fukushima J, Fukushima K. Purkinje cell activity in the cerebellar floccular region during vergence eye movements. Program No. 880.3. Abstract Viewer/Itinerary Planner (2004) San Diego (CA): Society for Neuroscience.
Wilson VJ, Ezure K, Timerick SJ. Tonic neck reflex of the decerebrate cat: response of spinal interneurons to natural stimulation of neck and vestibular receptors. J Neurophysiol (1984) 51:567–577.
Zhang H, Gamlin P. Single-unit activity within the posterior fastigial nucleus during vergence and accommodation in the alert primate. Abstr Soc Neurosci (1996) 22:2034.
Zhang H, Gamlin PD. Neurons in the posterior interposed nucleus of the cerebellum related to vergence and accommodation. I. Steady-state characteristics. J Neurophysiol (1998) 79:1255–1269.
Zhou W, King WM. Premotor commands encode monocular eye movements. Nature (1998) 393:692–695.[CrossRef][Medline]
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