optokinetic and vestibular responsiveness in the macaque rostral

Optokinetic and Vestibular Responsiveness in the Macaque Rostral Vestibularand Fastigial Nuclei

Ayanna S. Bryan and Dora E. AngelakiDepartment of Neurobiology, Washington University School of Medicine, St. Louis, Missouri

Submitted 27 May 2008; accepted in final form 4 December 2008

Bryan AS, Angelaki DE. Optokinetic and vestibular responsive-ness in the macaque rostral vestibular and fastigial nuclei. JNeurophysiol 101: 714–720, 2009. First published December 10,2008; doi:10.1152/jn.90612.2008. We recorded from rostral vestibu-lar (VN) and rostral fastigial nuclei (FN) neurons that did not respondto eye movements during three-dimensional (3D) vestibular andoptokinetic stimulation (OKS). The majority of neurons in both areas(76 and 69% in VN and FN, respectively) responded during bothrotational and translational motion. Preferred directions scatteredthroughout 3D space for translation but showed some preference forpitch/roll over yaw for rotation. VN/FN neurons were also testedduring OKS while monkeys suppressed their optokinetic nystagmusby fixating a head-fixed target. Only a handful of cells (VN: 17%, FN:6%) modulated during 0.5-Hz OKS suppression, but the number ofresponsive cells increased (VN: 40%, FN: 48%) during 0.02-Hz OKS.Preferred directions for rotation and OKS were not matched onindividual neurons, and OKS gains were smaller than the respectivegains during rotation. These results were generally similar for VN andFN neurons. We conclude that optokinetic-vestibular convergencemight not be as prevalent as earlier studies have suggested.

I N T R O D U C T I O N

Many studies have qualified the responses of primate ves-tibular nuclei (VN) and rostral fastigial nuclei (FN) neuronswith no eye movement–related activity during rotation andtranslation (VN: Angelaki and Dickman 2000; Chen-Huangand Peterson 2006; Dickman and Angelaki 2002; Musallamand Tomlinson 2002; FN: Buttner et al. 1991; Gardner andFuchs 1975; Shaikh 2004; Shaikh et al. 2005a; Siebold et al.1997, 1999; Zhou et al. 2001). With the exception of Chen-Huang and Peterson (2006), characterization of responses totranslation was limited to the horizontal plane. Responsivenessto rotation and translation in these earlier studies was taken asthe signature of otolith/canal convergence. More recently,however, it has become increasingly clear that the two types ofconvergence are not necessarily identical. In particular, there islittle argument that responses to both yaw rotation and trans-lation suggest otolith/canal convergence. However, modulationto translational but not rotational stimuli does not necessarilyexclude semicircular canal inputs to the cell. This occursbecause otolith afferents respond during both translation androtation (e.g., roll and pitch from an upright orientation). Thusthe only way for central neurons to respond exclusively totranslation is by canal signals canceling out otolith activationduring tilt movements relative to gravity (Angelaki et al. 2004;Shaikh et al. 2005b; Yakusheva et al. 2007, 2008).

In contrast to vestibular stimulation, studies testing theresponses of central vestibular neurons during visual or opto-kinetic stimulation (OKS) have been limited. Waespe andHenn (1977a,b, 1978, 1979) were the first to show modulationof macaque VN neurons during constant velocity yaw optoki-netic nystagmus (OKN) and optokinetic after-nystagmus(OKAN). These results were generally in agreement to those inthe goldfish (Allum et al. 1976; Dichgans et al. 1973), rabbit(Dichgans and Brandt 1972), rat (Precht 1981), cat (Precht1981), and guinea pig (Azzena et al. 1974). Using sinusoidalstimulation, Boyle et al. (1985) showed that yaw OKS modu-lation of VN neurons was limited to low frequencies. Althoughnot quantified in detail, rostral FN neurons also modulateduring OKS (Buttner et al. 1991). Most OKS studies werelimited to a single axis of stimulation and characterized neuralresponses while the animals elicited an optokinetic nystagmus.As a result, the retinal stimulus was continuously varying, thuspreventing quantification of the true visual responsiveness ofthe neurons.

Here we have evaluated the responsiveness of macaque VNand FN neurons to three-dimensional (3D) optokinetic stimu-lation at two frequencies: 0.5 and 0.02 Hz (one above and theother within the frequency range of the velocity storage mech-anism; Cohen et al. 1977). In contrast to previous studies, wecharacterized neural responses during OKS cancellation (i.e.,while monkeys fixated a head-fixed target), thus providing acontrolled retinal stimulus. In addition, we tested VN and FNresponses during OKS and vestibular stimulation in 3D, allow-ing for the first time a comparison between preferred vestibularand OKS responsiveness.

M E T H O D S

Experiments were performed on three juvenile macaque monkeys(Macaca mulatta), weighing 5–7 kg. Our general experimental pro-cedures were similar to previous studies (Angelaki and Dickman2000; Angelaki et al. 2001; Gu et al. 2006; Meng et al. 2005). Briefly,animals were chronically implanted with a circular, delrin ring andscleral search coils for measuring horizontal and vertical eye move-ments using a three-field magnetic search coil system (Robinson1963). All surgical and experimental procedures were performedunder sterile conditions, in accordance with the Institutional AnimalCare and Use Committee at Washington University and NationalInstitutes of Health guidelines.

Sinusoidal 3D motion was delivered using a six degrees of freedommotion platform (6DOF2000E, MOOG, East Aurora, NY). In allexperiments, the head was positioned such that the horizontal stereo-taxic plane was earth-horizontal, with the axis of rotation always

Address for reprint requests and other correspondence: D. Angelaki, Dept.of Anatomy and Neurobiology, Box 8108, Washington Univ. School ofMedicine, 660 South Euclid Ave., St. Louis, MO 63110 (E-mail: [email protected]).

The costs of publication of this article were defrayed in part by the paymentof page charges. The article must therefore be hereby marked “advertisement”in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

J Neurophysiol 101: 714–720, 2009.First published December 10, 2008; doi:10.1152/jn.90612.2008.

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centered at the center of the head. Because we did not have access toa 3D optokinetic sphere, the visual stimuli used here were generatedby a three-chip DLP projector (Mirage 2000, Christie Digital Systems,Cypress, CA) that was mounted on top of the motion platform andrear-projected images onto a 60 � 60-cm tangent screen that wasviewed by the monkey from a distance of 30 cm (thus subtending90 � 90° of visual angle; see Gu et al. 2006 for details). The tangentscreen was mounted on the front of the field coil frame. The sides, top,and back of the coil frame were covered with black enclosures suchthat the monkey’s field of view was restricted to visual stimulipresented on the screen.

The optokinetic stimuli simulated yaw, pitch, and roll rotationinside a sphere (70 cm radius) whose inner surface consisted of spotsof varying sizes (density of 0.01 dots/cm2 and dot sizes randomlychosen from a Gaussian distribution centered at 7° with an SD of1.5°). This was achieved by generating stereoscopic images of thesimulated sphere (using an OpenGL accelerator board; nVidia QuadroFX 3000G), which were projected onto the flat screen and wereviewed binocularly through Kodak written filters (29 and 61, Kodak,Rochester, NY) mounted on custom-made goggles. Two OpenGLcameras were required for accurate rendering of sphere rotation,binocular disparity, and texture cues (for details, see Gu et al. 2006).This provided an effective optokinetic stimulus that generated a robustnystagmus during both low and high frequencies.

Behavioral control and data acquisition were controlled by Spike2scripts and the CED system (Cambridge Electronic Design, Cam-bridge, UK). During all neural recordings, animals suppressed theirOKN/VOR by fixating on a central target that was front-projectedonto the screen using a head-fixed laser. The behavioral performanceof the animal was continuously monitored using electronic windows,which ensured that right and/or left eye positions were maintainedwithin 2.5° of ideal target fixation. This “eye-in-window” signal wasmonitored by the CED for on-line juice reward delivery and wassaved for off-line analyses. Juice rewards were typically given at afrequency of once every �3 s, as long as eye position was within thespecified behavioral windows. To ensure that these stimuli wereadequate to generate optokinetic nystagmus, horizontal and verticaleye movements were also collected in the absence of any behavioralcontrol.

Extracellular recordings were obtained using epoxy-coated, etchedtungsten microelectrodes using standard electrophysiological tech-niques. We recorded from vestibular only (VO) cells in the rostral FNand rostral VN (mostly rostral medial and caudal superior nuclei), asidentified by the absence of eye movement–related activity during0.5-Hz, �10° horizontal and vertical pursuit eye movements. Once aVO cell was identified, we tested its responsiveness during thefollowing protocols: 1) 0.5-Hz, �7° yaw, pitch, and roll rotation;

2) 0.5-Hz, �10 cm lateral, fore-aft, and vertical translation; and 3)yaw, pitch, and roll OKS at two frequencies: 0.5 (�7°) and 0.02 Hz(�175°). For both frequencies, peak velocity was kept constant at22°/s, a stimulus sufficient to elicit robust OKN and OKAN (Cohenet al. 1977; Waespe and Henn 1977a). Note that, because we wantedto characterize neural responses during OKN cancellation (i.e., in theabsence of reflexive eye movements, for a consistent retinal stimulus),constant velocity OKN and OKAN were not used in these experi-ments.

The data were analyzed off-line using custom-written MATLABscripts (MathWorks, Natick, MA). Eye position was calibrated andslow phase eye velocity computed by filtered differentiation afterremoval of fast phases of nystagmus through a semiautomated com-puter algorithm (see Angelaki 1998; Angelaki and Hess 1994 fordetails). Neural activity was expressed as instantaneous firing rate(IFR), calculated as 1/interspike interval, and assigned to the middleof the interval. IFRs from multiple stimulus cycles were folded in timeinto a single cycle instantaneous frequency response for each stimuluscondition. This procedure provides no averaging, because all spikeoccurrences are represented in time (Fig. 1; see Meng et al. 2005 fordetails). For OKN/VOR cancellation tasks, only portions of data inwhich the eye position was within 2.5° of the target were included inthe folding and further analyses.

Gain and phase were computed by fitting a sine function (1st and2nd harmonics, and a DC offset) using a nonlinear least-squaresalgorithm (see Dickman and Angelaki 2002; Meng et al. 2005; Shaikhet al. 2005a for details). To assign a statistical significance in theresponse modulation for each neuron, we also computed averagebinned histograms (40 bins per cycle) and a Fourier ratio (FR), definedas the ratio of the fundamental over the maximum of the first 20harmonics. The statistical significance of FR was based on a permu-tation analysis. Briefly, the 40 response bins were shuffled randomly,thus destroying the systematic modulation in the data but maintaining theinherent variability of the responses. The FR was computed fromthose randomly permuted histograms, and the randomization processwas repeated 1,000 times. If the FR for the original data exceeded thatfor 99% of the permuted data sets, we considered the temporalmodulation to be statistically significant (P � 0.01).

Preferred response directions are described in spherical coordinatesas combinations of azimuth and elevation. A spatio-temporal conver-gence (STC) function was used to calculate each cell’s preferreddirection (Angelaki 1991, 1992; Schor and Angelaki 1992). This wasdone by first computing the preferred direction in the horizontal plane.The polar angle of this vector defined the azimuth of the 3D-preferredresponse. We computed its elevation by applying the spatio-temporalmodel in a vertical plane defined by the preferred direction in thehorizontal plane and the vertical axis. This procedure allows compu-

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FIG. 1. Responses from a rostral vestibular nuclei (VN)neuron during 3-dimensional (3D) vestibular stimulation. Smalldots show instantaneous firing rate during (A) translation and(B) rotation at 0.5 Hz (12–16 cycles superimposed). Motiondirections are indicated by the cartoon drawings. The solid graylines show the sinusoidal fits. The stimulus traces (Stim) showlinear acceleration (A) or angular velocity (B). Response gainswere as follows—A: 209 (P �� 0.001, lateral), 55 (P � 0.05,fore-aft), and 74 sp/s/G (P � 0.01, vertical); B: 0.75 (P ��0.001, yaw), 0.43 (P �� 0.001, pitch), and 1.19 sp/s/deg/s(P �� 0.001, roll).

715VESTIBULAR AND OPTOKINETIC RESPONSES IN VESTIBULAR NUCLEI

J Neurophysiol • VOL 101 • FEBRUARY 2009 • www.jn.org

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tation of the azimuth and elevation of the 3D-preferred direction inspherical coordinates, the gain and phase of the 3D-preferred direc-tion, and the tuning ratio (i.e., ratio of the minimum over maximumresponse) in each of the horizontal and vertical planes.

R E S U L T S

Neural responses to 3D rotation and translation

We recorded from 53 neurons in the rostral VN (rostralmedial and caudal superior nuclei) and 89 neurons in the rostralFN during pitch/roll/yaw rotation and/or lateral/fore-aft/verti-cal translation at 0.5 Hz. Data were collected exclusively fromneurons without any eye movement sensitivity. An exampleresponse from a VN neuron that showed some modulationduring motion along all six axes (lateral/fore-aft/vertical trans-lation and yaw/pitch/roll rotation) is shown in Fig. 1, A and B.Based on these responses, the cell was classified as a conver-gent neuron. Note that the present classification of VN/FNneurons as convergent or nonconvergent applies to their re-sponsiveness during 3D translation and 3D rotation and not to

their presumed canal and otolith inputs. As shown previously,many central neurons that selectively modulate during transla-tion, but not during rotation, receive signals from both otolithand semicircular canals (Angelaki et al. 2004; Shaikh et al.2005a,b; Yakusheva et al. 2007).

The majority of VN and FN neurons (32/42, 76% and 59/85,69%, respectively) were significantly modulated for at leastone of the three cardinal directions during both translation androtation; thus they were considered convergent cells. In con-trast, 7 cells (17%) in the VN and 20 cells (24%) in the FNwere only significantly modulated during translation. The re-maining three (7%) VN and six (7%) FN neurons were onlymodulated during rotation.

Using responses from all three axes, we computed the3D-preferred direction of 47 VN and 82 FN neurons signifi-cantly tuned to translation, as well as 38 VN and 66 FNneurons significantly tuned to rotation (Fig. 2, A and B, respec-tively; definitions of azimuth and elevation are shown in Fig.2C). The corresponding gain distributions along each cell’spreferred direction are shown in Fig. 2, E and F. Preferred

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FIG. 2. Summary of rotation/translation responses in 3D.A and B: distributions of preferred directions (shown in spher-ical coordinates as azimuth and elevation) during translationand rotation, respectively. Each data point in the scatter plotscorresponds to the preferred azimuth (abscissa) and elevation(ordinate) of a single neuron with significant responses along atleast 1 motion direction. The data are plotted on Cartesian axesthat represent the Lambert cylindrical equal-area projection ofthe spherical stimulus space (see Gu et al. 2006 for details).Histograms along the top and right sides of each scatter plotshow the marginal distributions. Filled symbols indicate cellswith significant modulation along at least 1 direction duringboth rotation and translation (convergent). Open symbols shownonconvergent cells that only modulate during either translation(A) or rotation (B). Cells that were not tested during both rotationand translation are shown with open triangles. C: definition ofazimuth and elevation: Top and side view. Straight arrows depictthe direction of translation, whereas curved arrows show thedirection of rotation around each of the movement axes (basedon the right-hand rule). D: scatter plot comparing preferredazimuths for rotation and translation (shown only for conver-gent cells). For tilt-aligned rotation/translation preferences, datapoints should fall along the �90° diagonals (e.g., forwardtranslation is defined with an azimuth of 0°, but a pitch rotation-tilt has an azimuth of 90/270°). E and F: distribution of neuralresponse gains along the 3D preferred direction for translationand rotation, respectively. Fastigial nuclei (FN) neurons, redsymbols/bars; VN neurons, blue symbols/bars.

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directions, plotted in the form of azimuth and elevation scatterplots, were distributed throughout the 3D space. Translationresponse preferences were uniformly distributed in both hori-zontal and vertical planes (uniformity test, P � 0.05). Tuningratios in each of the horizontal and vertical planes were broadlydistributed, with 45% of the VN and 22% of the FN neuronshaving both tuning ratios �0.2. Approximately 19% (VN) and28% (FN) had tuning ratios �0.2 in both horizontal andvertical planes. For rotational response preferences, there wasa weak departure from uniformity for both azimuth and eleva-tion in FN cells (Fig. 2B, red; P � 0.03) and for elevation inVN cells (Fig. 2B, blue; P � 0.01). When comparing theazimuth of the 3D-preferred directions for rotation and trans-lation (Fig. 2D), data points were scattered throughout therange, suggesting the absence of tilt-aligned rotation/transla-

tion preferences, as previously described in rats (Angelaki et al.1993; Bush et al. 1993). That is, cells that prefer forward(lateral) motion do not necessarily prefer pitch (roll) rotationsand vice versa.

Neural responses to 3D optokinetic stimulation

Forty-one of the vestibular-sensitive cells in the VN and FNwere also tested with optokinetic stimuli. In the absence of thehead-fixed fixation target during yaw and pitch OKS, animalsgenerated a robust horizontal and vertical optokinetic nystag-mus (Fig. 3). Figure 3, A and B, shows representative examplesof the nystagmus evoked during 0.5- and 0.02-Hz OKS, re-spectively. Figure 3, C and D, summarizes the distributions ofpeak horizontal and vertical eye velocities evoked during yaw

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and pitch OKS, respectively [mean amplitude: 17.7°/s for yaw,23.3°/s for pitch OKN (0.5 Hz) and 18.9°/s for yaw, 24.0°/s forpitch OKN (0.02 Hz)]. To have a greater control over theretinal stimulus, VN/FN neurons were tested while the animalsuppressed its optokinetic nystagmus by fixating a head-fixedtarget. Only a small percentage of VN/FN neurons weresignificantly modulated along at least one stimulus direction;VN: 2/12 (17%) at 0.5 Hz and 4/10 (40%) at 0.02 Hz; FN: 2/31(6%) at 0.5 Hz and 15/31 (48%) at 0.02 Hz.

Even for responsive cells, modulation amplitude was small,as shown with the example in Fig. 4A that shows one of themost responsive FN neurons to yaw, pitch, and roll OKS at 0.5(top) and 0.02 Hz (bottom). Like the vestibular responses, wecomputed the preferred direction and gain of the OKS re-sponses of VN/FN neurons. Figure 4B compares the respectivegains for all cells that were significantly modulated to at leastone direction during rotation and optokinetic stimulation (VN:5 cells; FN: 16 cells). Data at both 0.5 Hz (filled symbols) and0.02 Hz (open symbols) fall above the unity line, indicatingpoorer response modulation to OKS than vestibular stimula-tion. Preferred directions were not matched, as shown in Fig.

4C, which plots the difference in 3D-preferred directionsbetween 0.5-Hz rotation and 0.02-Hz OKS responses (becauseof the small number of OKS-responsive cells at 0.5 Hz, thiscomparison is not included). Distributions were not signifi-cantly different from uniform (uniformity test, P � 0.05).

D I S C U S S I O N

We characterized vestibular and OKS responsiveness during3D stimulation in the rostral VN and rostral FN. Unlikeprevious reports that have implied that all units that wereactivated by vestibular stimulation could also be activated bymoving optokinetic patterns (Buettner and Buttner 1979; Hennet al. 1974; Waespe and Henn 1977a,b), we found that, at most,one half of the vestibular-responsive neurons modulated duringsinusoidal OKS at 0.02 Hz. The fact that this number decreaseddrastically down to only less than a handful of responsiveneurons at 0.5 Hz is in agreement with the findings of Boyleet al. (1985), who showed that most VN neurons only modulateduring low-frequency yaw OKS. In particular, only a fewneurons responded at 0.2 Hz and none �1 Hz (Boyle et al.

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during yaw, pitch, and roll OKS at 0.5 (top, �15 cyclessuperimposed) and 0.02 Hz (bottom, �3 cycles superimposed).Response gains were as follows—0.5 Hz: 0.20 (P � 0.6, yawOKS), 0.12 (P � 0.5, pitch OKS), and 0.28 sp/s/deg/s (P � 0.3,roll OKS); 0.02 Hz: 0.07 (P � 0.09, yaw OKS), 0.34 (P ��0.001, pitch OKS), and 0.33 sp/s/deg/s (P � 0.03, roll OKS).B: scatter plot comparing 3D neural response gains duringrotation and OKS, shown separately for FN (red symbols) andVN (blue symbols) at 0.5 (filled symbols) and 0.02 Hz (opensymbols). Data are only shown for cells with significant mod-ulation during both rotation and OKS. C: distribution of thedifference in 3D preferred directions for responses to 0.5-Hzrotation and 0.02-Hz OKS stimulation. A difference of 180°signifies alignment of preferred directions; 0° representsanti-alignment.



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1985). The larger proportion of OKS-responding neurons at0.02 Hz, a frequency that is within the range of velocity storagemechanism (Raphan et al. 1979), suggests that non-eye move-ment-sensitive VN/FN neurons might mediate the indirect,slow component, but not the direct, fast component of OKN(Cohen et al. 1977).

Despite these general similarities with earlier studies, evenresponsive cells typically exhibited small modulation duringOKS. This observation differs from the robust responses re-ported previously by Waespe and Henn (1977a,b, 1978, 1979).There are multiple differences between these experiments andthose of previous studies. First, the stimuli are different: weused 0.5- and 0.02-Hz sinusoidal stimulation, whereas thestudies of Waespe and Henn used constant velocity OKN andOKAN. Second, unlike Waespe and Henn (1977a,b, 1978,1979), our animals suppressed reflexive eye movements. In-deed, Buettner and Buttner (1979) also reported reduced neu-ronal responses in the majority of VN neurons during OKSsuppression. Third, previous studies used actual drum rotation,whereas our optokinetic stimuli were simulated using OpenGL.Although we believe that this is an adequate stimulus (becauseit elicits robust optokinetic nystagmus; Fig. 3), we cannotexclude the possibility that neural responses were less robustbecause sphere rotation was not real but simulated. Finally, wewant to emphasize that our aim for these experiments was tocharacterize neural responses during OKN cancellation (i.e., inthe absence of reflexive eye movements) as part of a greatereffort to study the visual motion responses of subcorticalneurons. This could not have been possible by recording (likeprevious studies did) neural activities during OKAN. Perhapsresponses are different during sinusoidal stimulation in theabsence of reflexive eye movements (this study) and OKAN(Waespe and Henn 1977a,b, 1978, 1979). However, the latterdoes not test for visual (sensory) responsiveness. Neuronalmodulation during optokinetic nystagmus and afternystagmuscannot be interpreted as either sensory- or motor-driven. Pre-vious studies have instead shown that VN neurons modulateduring activation of the velocity storage mechanism. In con-trast, these experiments, where we controlled the retinal motionstimulus by suppressing reflexive eye movements, aimed totest for visual (sensory) responsiveness. Results point out that,unlike a widely accepted belief in the field, true visual/vestib-ular interactions might be rather limited in the vestibular androstral fastigial nuclei.

To our knowledge, this is the first study to compare thepreferred OKS and rotational preference for VN/FN neurons.We found no relationship between the two, although thenumber of cells used for this comparison (requiring neuronsresponsive to both) was small. This result is surprising, giventhe strong behavioral relationship between the rotational ves-tibulo-ocular and optokinetic reflexes (Cohen et al. 1977;Raphan et al. 1979). Additional data, with this comparisonavailable for a larger number of neurons, might be necessary toreliably compare these relationships.

Precht (1981) showed that lesions of the nucleus of the optictract (NOT) or nucleus reticularis tegmental pontis (NRTP) inthe cat and rat abolished OKS responses in the VN. In pri-mates, it is generally thought that descending projections fromthe nucleus of the optic tract (NOT) and possibly the dorsalterminal nucleus (DTN) project through the central tegmentaltract to the ipsilateral vestibular complex, where diffuse termi-

nals are seen in the medial vestibular nucleus (Buttner-Enneveret al. 1996). The lateral terminal nucleus (LTN) is also knownto project to the superior and medial VN (Blanks et al. 2000).A similar but smaller projection is also found from the medialterminal nucleus (MTN) to the superior and lateral VN.

Finally, optokinetic signals to the VN/FN can also be carriedby Purkinje cell projections from the ipsilateral nodulus anduvula (Carleton and Carpenter 1983; Voogd et al. 1996; Wylieet al. 1994). Although the OKS responses of nodulus/uvulaPurkinje cells have yet to be quantified in alert primates, thereis evidence that simple spike responses in the anesthetizedrabbit modulate during OKS (Kano et al. 1991a,b). Furtherexperiments are necessary to quantify the properties and originof optokinetic modulation in vestibular-responsive brain stemand cerebellar areas, as well as potential differences in re-sponses between low-frequency sinusoidal versus constantvelocity OKN and OKAN.

A C K N O W L E D G M E N T S

The authors thank H. Meng and T. Yakusheva for assistance with the neuralrecordings.

G R A N T S

This work was supported by National Eye Institute rant R01 EY-12814.

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