Journal of Neurology, Neurosurgery, and Psychiatry 1 992;55: 1125-1131

Coordination of eye and head movements duringsmooth pursuit in patients with vestibular failure

John A Waterston, Graham R Barnes, Madeleine A Grealy, Linda M Luxon

AbstractDuring pursuit of smoothly moving tar-gets with combined eye and head move-ments in normal subjects, accurate gazecontrol depends on successful interactionof the vestibular and head movementsignals with the ocular pursuit mecha-nisms. To investigate compensation forloss of the vestibulo-ocular reflex duringhead-free pursuit in labyrinthine-deficient patients, pursuit performancewas assessed and compared under head-fixed and head-free conditions in fivepatients with isolated bilateral loss ofvestibular function. Target motion con-sisted of predictable and unpredictablepseudo-random waveforms contalningthe sum of three or four sinusoids. Com-parison of slow-phase gaze velocity gainsunder head-free and head-fixed condi-tions revealed no significant differencesduring pursuit ofany of the three pseudo-random waveforms. The finding of sig-nificant compensatory eye movementduring active head movements in dark-ness in labyrinthine-deficient patients,which were comparable in character andgain to the vestibular eye movementelicited in normal subjects, probablyexplains the similarity of the head-fixedand head-free responses. In twoadditional patients with cerebellardegeneration and vestibular failure, nocompensatory eye movement responsewas observed, implying that the cere-bellum is necessary for the generation ofsuch responses in labyrinthine-deficientpatients.

MRC HumanMovement andBalance Unit, NationalHospital for Neurologyand Neurosurgery,Queen Square,LondonJ A WaterstonG R BarnesM A GrealeyNeuro-otology UnitL M LuxonCorrespondence to:Dr G R Barnes, MRCHuman Movement andBalance Unit, NationalHospital for Neurology andNeurosurgery, QueenSquare, LondonWCIN3BG.Received 1 November 1991and in revised form26 February 1992.Accepted 10 March 1992

(_ Neurol Neurosurg Psychiatry 1992;55: 1125-1131)

During coordinated movements of the headand eyes to track smoothly moving targets, anumber of physiological mechanisms help toprevent retinal image slip, including thevestibulo-ocular, optokinetic and smooth pur-suit responses. Dysfunction in one or more ofthese systems has the potential to degraderetinal input under certain conditions, produc-ing visual blurring or illusory movement of theenvironment (oscillopsia). The vestibulo-ocular reflex (VOR) helps to maintain the eyesstationary in space during motion of the headand, following bilateral loss of vestibular func-tion, patients frequently complain of visualdisturbances which are associated with head orbody movement. Such systems have been aptly

compared to the unstable images produced bya movie camera which is moved about ran-domly at high frequency.' Despite these prob-lems, many patients who are initially quitedisabled by loss of vestibular function, subse-quently demonstrate remarkable adaption andare able to return to near normal lifestyles withfew visual complaints.

Studies of gaze control in labyrinthine-deficient (LD) patients and monkeys haverevealed a variety of adaptive mechanismswhich may be used to compensate for loss ofthe VOR during self-motion or rapid gazeshifts. These include potentiation of the pur-suit,2 optokinetic,'-4 and cervico-ocularreflexes,`8 the use of somatosensory cues,9and the central pre-programming of eye move-ments.2568 There have been relatively fewstudies of head-free pursuit performance inthese subjects. During pursuit with the headfree it is necessary to suppress theVOR whichwould normally produce eye movements ofopposite polarity to head movement. It hasbeen postulated that, during head-free pursuit,a central suppression mechanism incorporat-ing an efference copy of the planned headvelocity signal, is used to cancel the VOR.'0One prediction of this model is that, in patientswith loss of vestibular function, tracking gainsshould exceed unity during head-free pursuit ifcancellation were still in operation, eventhough there is no significant vestibularresponse.'0 Previous studies in normal subjectsover limited ranges of motion have shown littledifference in tracking performance betweenhead-free and head-fixed pursuit.11-'4 In con-trast, it has been shown that head-free pursuitof predictable sinusoidal target motion pro-duced better slow-phase tracking gains com-pared with pursuit with the eyes alone inlabyrinthine-deficient patients. " The differ-ences demonstrated were not marked and thegains never exceeded unity because of thepresence of compensatory eye movements(CEM) which simulated the normal VOR.Performance during pursuit of unpredictabletarget motion has not been assessed in thesepatients.To explore the mechanisms that compensate

for loss of the VOR during head-free pursuit,we have examined the responses to pursuit ofpseudo-random target motion in deficientpatients, under head-free and head-fixed con-ditions, using a stimulus consisting of the sumof three or four sinusoids. By adjusting thevelocities and frequencies of the componentsin the pseudo-random stimulus it is possible toproduce both predictable and unpredictable

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Table Clinical features of labyrinthine-deficient patients

Number Age/sex Aetiology Duration (years)

1 49/M Post-infectious 142 59/F Syphilis 103 59/M ?Vascular 34 52/F Idiopathic 95 25/M Idiopathic 18

target motion.'5 16 have also measured theCEM gain in the patients during active headmovements in darkness because, if these eye

movements are of sufficiently high gain, theywill influence head-free pursuit performance.The study of two patients with degenerativeneurological diseases, characterised by cer-

ebellar degeneration in addition to loss ofvestibular function, provided an opportunity toexplore the possible relationship between cer-ebellar pathways and the genesis of the CEMresponses.

Materials and methodsPatientsFive patients with isolated bilateral loss ofvestibular function, as demonstrated by caloricand rotational testing, were studied. Theirdetails are summarised in the Table. Patientswere excluded if there was any central disorderof eye movement evident, such as smoothpursuit breakdown or slowing of saccades, or ifthey were taking sedatives or anticonvulsantdrugs, which are known to impair smooth eyemovement control. One subject with absentresponses on caloric testing was later excludedbecause of the finding of significant residualfunction on sinusoidal rotation, an apparentdisparity alluded to by previous authors.3 17 18All patients had longstanding vestibular failureand were clinically well compensated.Two additional subjects with degenerative

neurological conditions characterised by rela-tively pure combined loss of vestibular and

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Figure I Traces of eye, head, gaze and target displacement, and gaze velocity duringhead-free pursuit ofpseudo-random target motion from LD patient. The stimuluscontained 0 11, 0-24, 037 (peak velocity 8°/s) and 1 56 Hz sinusoids (peak velocity16°/s, ie. velocity ratio of two). The fast phases have been removedfrom the gaze velocitytrace. The arrows below the eye displacement trace mark clear examples of unsuppressedCEM activity, similar in character to the vestibular activity observed in normal subjects.

cerebellar function were also investigated.Some of their clinical features were reminis-cent of multiple system atrophy or olivoponto-cerebellar atrophy, but there was no otherclinical evidence of brain stem, cortical,extrapyramidal or autonomic involvement, andfamily history was negative. The exact natureof their disease was unknown, but CT of thebrain showed evidence of cerebellar atrophy inboth patients. A group of normal, naive, age-matched subjects acted as controls, and allexperiments were carried out with local ethicscommittee approval.

ApparatusSubjects were seated comfortably in front of asemicircular screen of radius 1-5 m. A motor-driven mirror situated above the subject's headwas used to drive the target, a small white crosswithin a circle whose diameter subtended 70minutes of arc at the eye. Eye movements wererecorded using an infrared limbus reflectiontechnique (Iris 6500 system, Skalar Medical)with a resolution of 10 minutes of arc and alinear range of at least plus or minus 200. Therecording equipment was mounted on a hel-met assembly which was attached firmly to thesubjects head, so that no movement occurredbetween the eye recorders and the head. Asingle turn potentiometer, attached to the topof the helmet via a flexible assembly, was usedto record head movements. Vestibular functionwas assessed during sinusoidal oscillationabout an earth vertical axis on a motorisedturntable (Toennies, 200 Nm).

Experimental designEach experiment was presented in a balancedrandomised fashion to control for practice orfatigue effects. Calibrations were performedbefore each test condition. All experimentswere carried out in a room which, apart fromtarget illumination, was otherwise completelydark.

Experiment 1 A pseudo-random stimuluscontaining the sum of three or four sinusoids(0-11, 0-24, 0 37 and 1-56 Hz) was used forthe pursuit task (fig 1). The velocity of thethree lower frequency sinusoids remained con-stant at 8°/s, whereas the velocity of the 1-56Hz component was varied as a ratio of thelower frequency velocity (velocity ratio)between zero and two. Thus the 1-56 Hzcomponent was absent when the velocity ratiowas zero. Previous studies have demonstratedthat, when a pseudo-random stimulus containsfrequency components which are all less than acritical level of04 Hz, the target motion is verypredictable. The addition of a sinusoid whosefrequency is greater than 0 4 Hz, however,results in a reduction in gain of all the lowerfrequency components. Further breakdown atthe lower frequencies occurs when the velocityof the high frequency is increased.16 Thesechanges are accompanied by preferentialenhancement of gain at the highest frequency,such that the gain at this frequency is similar tothe gain obtained during pursuit of a singlesinusoid of the same frequency.'" The break-down in the low frequency responses occurs

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.11 24 27 1.S6

Frquency

Figure 2 Slow-phase gaze velocity gain and phase in labyrinthine-deficient subjectsduring a comparison of head-fixed (solid line) and head-free (broken line) pursuit of apseudo-random target containing the sum of three (0-11, 0-24 and 0-37 Hz); or four(0 11, 0-24, 0 37 and 1 -56 Hz) sinusoids. The peak velocity of the three lowest frequencycomponents was held constant at 801s, whereas the peak velocity of the 1 -56 Hzcomponent was varied as a velocity ratio of the lowerfrequency velocity between zero andtwo (resulting in peak velocities of 0, 8 or 16°/s). Each single line plot represents theindividual gains for each of the frequency components contained in one stimulus (total ofthree head-fixed, and three head-free stimuli). Mean (SE 1) (n = 5).

essentially as a result of making the waveformless predictable, and allows for comparisons tobe performed during both predictable andunpredictable target motion at normal fre-quencies of eye and head movement. Subjectswere asked either to track the target using eyemovements only, or to use the eyes and headtogether in a natural fashion.

Experiment 2 During this experiment, thegain of the CEM response was measured byrecording the eye movements associated withactive head movements in darkness. Sinusoidalresponses at each of the three lower fre-quencies used in the pseudo-random stimulus(0-11, 0-24, and 0-37 Hz) were assessed. Eachhad a peak velocity of 16°/s to approximate theRMS velocity of the pseudo-random wave-form. Subjects were initially asked to track thetarget with the head free, in time with asinusoidally modulated tone placed above thesubject's head. The target was then extin-guished and subjects were asked to imaginethat they were still tracking the target in thedark in time with the tone. Previous studieshave demonstrated that this paradigm resultsin little, if any, suppression of the VOR.'2 14 19An observer monitored head movements on an

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Figure 3 Head displacement gain during head-free pursuit of the pseudo-randomstimulus for labyrinthine-deficient patients (solid line) and control subjects (broken line).Mean (SE 1) for deficient patients (n = 5).

oscilloscope screen and prompted the subject ifthe movements were inappropriately large orsmall. The order of the above conditions wasmaintained, but the frequencies were pre-sented in random fashion. A measure ofresidual VOR gain was also obtained forcomparison, during whole body turntable rota-tion in the dark at the same frequencies andvelocities, while subjects performed mentalarithmetic to maintain alertness. In addition,recording of saccadic velocities in the twopatients with cerebellar degeneration was per-formed at amplitudes of up to 30° using avisually elicited saccade paradigm, to look forpossible evidence of brain stem involvement.

Experimental control and analysisAll experiments were controlled and analysedoff line by computer (Hewlett-Packard360CH) using techniques described previ-ously."5 Gaze displacement was calculated bysumming eye and head displacement, and gazevelocity was obtained by differentiating gazedisplacement. A computer graphics procedurewas used to remove the saccadic componentsfrom the gaze velocity trace.20 The resultantmeasure of slow-phase gaze velocity was thencorrelated with target velocity to produce gazevelocity gain and phase. All subsequent refer-ences to gaze velocity refer to slow-phase gazevelocity. Statistical significance was assessed byanalysis of variance.

ResultsExperiment 1During head-fixed pursuit of the pseudo-random stimulus composed of the three lowestfrequency sinusoids (0-11, 0-24 and 0-37 Hz,there was no significant difference between theeye velocity gains for labyrinthine-deficient(mean 0-91) and control subjects (mean 0-94).The gains for each frequency component forthis combination of frequencies have pre-viously been shown to be similar to the gainsobtained during pursuit of single sinusoidaltarget motion at the same frequencies, as longas all the components are less than a criticallevel of 0-4 Hz."5 When the 1-56 Hz sinusoidwas added to the stimulus, and when itsvelocity was increased with respect to the otherfrequency components, there was a progressivedecrease in gain at all the lower frequencies,and again there was no significant differencebetween the patient and control groups (forexample, mean gain was 0-34 for the patientgroup and 0-38 for controls when the velocityratio was two). The phase changes were alsosimilar to those reported previously in normalsubjects, with an increasing phase lead at thelowest frequency, and increasing phase lags atall other frequencies except the highest fre-quency which showed a decreasing phase lag asthe velocity ratio was increased (fig 2).15 16During head-free pursuit of an identical

stimulus, head displacement gain tended to beslightly higher in the control group but thedifferences were not significant. Mean headdisplacement gain in the LD group was alwaysgreater than 0-62 for the three lowest fre-

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0.60

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Figure 4 CEM gain for patients (solid line) and control subjects (broken line) during(a) passive sinusoidal turntable motion, and (b) active head movements in darkness. LD= labyrinthine-deficient patients (n = 5), LD + Cer = labyrinthine-deficient patientswith cerebellar degeneration (n = 2). Mean (SE 1).

quencies, but fell to a level of 043 at 1-56 Hzwhen the velocity ratio was two (fig 3). Nosignificant differences were observed whengaze velocity gain was compared under head-fixed and head-free conditions (fig 2). Similarbreakdowns in gain were observed withincreasing velocity ratio under both head con-ditions, and subjects generally did not experi-ence any subjective difference in performancebetween the head-fixed or free conditions.The two patients with cerebellar degenera-

tion showed a similar pattern of breakdown inthe lower frequency responses resulting fromaddition of the high frequency component, butgaze velocity gain was reduced over all condi-

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tions as a result of the severe pursuit deficit(means of 0 65 and 03 1, for velocity ratios ofzero and two respectively). Under head-fixedconditions, the much larger amplitudes of eyedisplacement resulted in a moderate degree ofgaze-evoked nystagmus in both patients, withconsequent reductions in gaze velocity gain, soa meaningful comparison with the head-freegains could not be performed.

Experiment 2The mean VOR gain observed when subjectswere asked to make active head movements totrack an imaginary target in darkness, rangedbetween 0 57 at 0-11 Hz and 074 at 0-24 Hz(mean 068). These measures approximatedthe gains obtained during whole-body turn-table motion at the same frequencies (mean0 66). Despite the absence of any significantturntable VOR response in the LD subjects, asignificant CEM response could be demon-strated during active head movements, whichranged between 0 52 at 0 11 Hz and 0 62 at0-24 Hz (mean 056, fig 4). Generally, thepattern of CEM in the patients resembled anormal vestibular response (fig 5) and,although the gains were not as high as thoserecorded in controls, the differences were notsignificant. When the recordings from deficientpatients during head-free pursuit of pseudo-random target motion were examined, thebreakdown in gaze velocity gain with increas-ing velocity ratio was seen to be associated withinadequate suppression of the CEM response(see fig 1). This activity was often indis-tinguishable from the unsuppressed vestibularactivity observed during similar experiments innormal subjects." 14

In the two patients with cerebellar degenera-tion, no significant CEM response was gen-erated at any frequency (see fig 4). Their eyemovement traces consisted of saccades withoutany major slow-phase components (see fig 5).Examination of the main sequence plots forthese patients demonstrated normal peak sac-cadic velocities at all target amplitudes.

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Figure 5 Eye movement traces during active head movements in darkness at 037 Hzfrom (a) a control subject, (b) a labyrinthine-deficient patient, and (c) alabyrinthine-deficient patient with cerebellar degeneration. Note the presence ofvestibular-like CEM activity present in trace (b) but not trace (c).

DiscussionThese studies have demonstrated that well-

compensated patients with bilateral loss ofvestibular function are able to develop CEM ofsufficient gain to influence smooth gaze controlduring head-free pursuit. As a result, nosignificant difference in gaze velocity gain wasobserved between head-fixed and head-freepursuit. In two patients with additionalinvolvement of cerebellar pathways, the CEMresponses were absent, suggesting that cer-ebellar connections are necessary for the pro-duction of these responses.The pattern of eye movement response seen

in labyrinthine-deficient subjects during activehead movements in the dark is very similar tothat reported by others6 0312 and has beenattributed in part to enhancement of thecervico-ocular reflex (COR), although not allstudies have shown such potentiation." 2Similar changes have been observed withpassive movements, either of the head itself or

a) Passive rotation

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of the trunk with respect to the head, emphas-ising the importance of the COR in the genesisof this response.672425 The gain producedunder these conditions is less than that seenduring active head movements, particularly atfrequencies above 02 Hz,8 so clearly othercompensatory mechanisms are involved, suchas central pre-programming of eye move-ments.5 6 8 22

It has been demonstrated previously thatimpairment ofVOR suppression becomes evi-dent as tracking gains fall during head-freepursuit in normal subjects." 12 14 26 The con-tribution to gaze velocity made by the headmovement is effectively nulled by the resultantvestibular signal and consequently, gaze veloc-ity is similar whether or not the head moves. Ittherefore might have been expected that gazevelocity gains would be significantly higherduring head-free pursuit in LD subjects,because of the absence of any significantvestibular response. The eyes would only havehad to make up the difference between headand target displacement, thereby allowing apotential advantage in the use of head move-ment. This conclusion is supported by theresults of a previous experiment in normalsubjects; when the effect of the VOR wasartificially attenuated by using whole bodyturntable motion to oppose head motion dur-ing head-free pursuit, significant increases inslow-phase gaze velocity occurred.'2 In thepresent study, however, there was no sig-nificant difference in gaze velocity gain forhead-fixed and head-free pursuit, even at peaktarget velocities ofup to 400/s. This observationcan be explained by the presence of a sig-nificant CEM response, resembling the normalVOR, during active sinusoidal head move-ments in darkness. The mean gains of thisresponse in the labyrinthine-deficient groupwere only slightly lower than those recorded inthe control group (see fig 4).

Leigh et al'3 previously demonstrated thathead-free gaze velocity gains were significantlygreater than the corresponding head-fixedgains during pursuit of sinusoidal targetmotion in deficient subjects, but only at thehighest frequency tested (1I0 Hz). This appar-ent discrepancy with our results may beexplained by the difference in the frequenciesassessed. Most of the frequencies used in thepresent study were less than 0A4 Hz. Althoughthe highest frequency in the pseudo-randomstimulus was 1 56 Hz, mean head displace-ment at this frequency was much lower (see fig3), and the stimulus velocity was significantlyless than that used by Leigh et al, so that anysmall differences in gaze velocity may not havebeen apparent. We did not measure the gain ofthe CEM responses at frequencies above 0-4Hz, but others have shown that the gain beginsto fall off at around 1 Hz.2 This would explainthe absence of symptoms such as oscillopsiaduring low frequency combined eye and headmovements in deficient subjects, and theirassociation with higher frequency head move-ments such as those produced during walk-ing.27 Compensation is therefore usually onlysatisfactory at lower frequencies, under I Hz,

and this frequency limitation probably explainsthe higher head-free tracking gains found at1-0 Hz in the previously mentioned study.'3The differences found were almost certainlynot explained by the pseudo-random stimulusbeing less predictable than the equivalentsinusoidal waveforms as, when the velocityratio of the pseudo-random waveform waszero, the stimulus should have been as predict-able as each of the individual sinusoidal wave-forms contained within the stimulus.'5 16

In common with the findings ofLeigh et al,'3head-free pursuit gain never exceeded unity.Gains greater than unity might be predictedfrom the central suppression model'0 if headdisplacement gain was high and patients withloss of vestibular functions were not able todispense with what, to them, would be aninappropriate suppressive mechanism. Analternative model proposes that the visualfeedback mechanisms active during smoothpursuit are responsible for suppression of thevestibular response." 2 This model predictsthat the head velocity signal will be effectivelynulled by the CEM signal if the gain of thelatter response is near unity. Within certainlimits gaze control will therefore be similarwhether or not the head is free to move. 1 '-14Wewere still unable to demonstrate a differencebetween head-fixed and head-free perform-ance despite the fact that the CEM gain wassignificantly less than unity in both deficientpatients and control. This can be explained bythe results of previous studies in normalsubjects, which demonstrated that the under-lyingVOR gain during head-free pursuit actu-ally correlated better with the higher gainsobserved during active head movements indarkness when subjects were asked to imaginean earth-fixed target.'3 14 These gains areusually around a level of 0-9 and this level ofgain is very similar to the level of VOR gainobserved in normal subjects during the first150-180 ms of transient rotations in the lightor dark.30

It therefore seems that, in compensating forthe loss of the VOR, labyrinthine-deficientpatients effectively modify their gaze control insuch a way as to reduce any difference inperformance characteristics between head-freeand head-fixed pursuit. The neural signaldriving gaze during smooth pursuit wouldtherefore be very similar whether or not thehead is moving. Possible neurophysiologicalcorrelates of such a signal have been docu-mented in neurons which discharge in propor-tion to gaze, rather than eye or head velocity, inthe primate flocculus.3" 32 We therefore con-tend that no practical advantage would beserved by a parametric reduction or totalcancellation of the VOR during head-freepursuit by a central suppression mechanism.In fact, it would be advantageous to have afully functional VOR during head-free pursuitto help compensate for gaze disturbance pro-duced by perturbations of the head,33 such asoccur during walking or running.

In the two patients with combined cerebellarand vestibular failure there was almost totalabsence of any CEM response during active

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head rotations. This is consistent with thepreviously reported finding of the absence ofany discernible COR response in two subjectswith combined cerebellar and vestibular failurefrom our institution.34 It was therefore sug-gested that the cerebellum is responsible forenhancement of the normal COR in patientswith vestibular failure.34 One of the subjectsfrom the latter study (case 1) was also includedin the present series, whereas the other hadadditional extrapyramidal and pyramidal fea-tures, suggesting a diagnosis ofmultiple systematrophy. As there was no reason to suspect anymajor cervical spine, soft tissue or spinal cordabnormality in the two patients from thepresent study, it could be reasonably assumedthat cervical input to the vestibular nucleishould have been intact. Likewise there was noevidence of other brainstem abnormalities, aconclusion supported by the normal saccadicvelocities and absence of other central neuro-logical involvement. Connections betweenneck afferents and cerebellar flocculus havebeen demonstrated in the cat,35 and thechanges that we have noted with respect tocerebellar influence on the COR may beanalogous to the impaired ability to modulatethe VOR seen in animals36 37 and inpatients38-41 with cerebellar lesions. Studies inrats have also demonstrated the importance ofintact olivocerebellar pathways in the com-pensation of ocular position following uni-lateral labyrinthectomy.42 An interestingfinding from an earlier study of the COR in LDsubjects, where no enhancement of the CORwas found, was that a number of the subjectshad evidence of central neurological distur-bances, such as broken smooth pursuit, whichcould possibly have been the result of cer-ebellar or brain stem disease.23 These featuresmay have explained the absence of any CORenhancement in this patients.The cerebellum is known to be essential for

optimal pursuit performance,40 `-` and it is ofsome interest that CEMs are not present abovefrequencies at which the pursuit responsebegins to break down,2 although we did notmeasure the higher frequency responses in ourpatients. It is possible that this frequencylimitation may simply reflect the fact thatvisual feedback is required for adaptation ofthe COR in labyrinthine-deficient patients.What is more difficult to explain is the findingthat two patients with isolated cerebellardegeneration have been shown to have anabnormally elevated COR gain.34 Assumingthat the features of cerebellar degeneration areof a similar nature, why should vestibularfailure in addition, result in virtual absence ofthe CEM response? One possibility is that theprimary pathology may be located in thevestibular nuclei, where cervical and pursuitinputs are known to converge in animal stud-ies.47 This pathological pattern would beunusual, however, as there was no clinicalevidence of involvement of other brain stemnuclei in our patients. Further studies areneeded to determine whether the COR gain isin fact elevated in most patients with cerebellardisease before any definite conclusions can be

made regarding cerebellar influences on thegain of the CEM responses.

Dr J Waterston was supported by the British PostgraduateMedical Foundation.

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4 Huygen PLM,VerhagenWIM,Theunissen EJJM, NicolasenMGM. Compensation of total loss of vestibulo-ocularreflex by enhanced optokinetic response. Acta OtolaryngolSuppl (Stockh) 1989;468:359-64.

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A note on Heterochromia iridis

"Her eyes are so adorable, but one of them is blue"....so runs an old Arthur Daley/Terence Christmas jingle.Sector pigmentation of one eye is common and isascribed to persistence of the pupillary membrane.Irides of totally different colour are well known,harmless but rare. Aristotle named it heteroglaucous (Gkglaukos, sea-green). The Emperor Anastasios I probablyhad this condition and was generally called Dicorus.Alexander the Great was similarly affected.'

The association with deafness constitutes Waarden-burg's syndrome and heterochromia iridis may be seenin congenital Horner's syndrome, the affected sidebeing blue or depigmented in comparison with thenormal side.

JMS PEARCE

1 Gladstone RM. Development and significance of hetero-chromia on the iris. Arch Neurol 1969;32:184-92.

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