Normal subjects are able to change voluntarily and continuously their head-eye latency together with their compensatory eye movement gain. A continuous spectrum of intent-latency modes of the subject's coordinated gaze through verbal feedback could be demonstrated. It was also demonstrated that the intent to counteract any perturbation of head-eye movement, i.e., the mental set, permitted the subjects to manipulate consciously their vestibular ocular reflex (VOR) gain. From the data, it is inferred that the VOR is always on. It may be, however, variably suppressed by higher cortical control. With appropriate training, head-mounted displays should permit an easy VOR presetting that leads to image stabilization, perhaps together with a decrease of possible misjudgements

Zangemeister, Wolfgang H.

English

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\N90-22960
VOLUNTARY PRESETTING OF THE VESTIBULAR OCULAR
REFLEX PERMITS GAZE STABILIZATION DESPITE
PERTURBATION OF FAST HEAD MOVEMENTS
Wolfgang H. Zangemeister
Hamburg University Neurological Clinic
Hamburg, West Germany
SUMMARY
Normal subjects are able to change voluntarily and continuously their head-eye latency
together with their compensatory eye movement gain. A continuous spectrum of intent-latency
modes of the subject's coordinated gaze through verbal feedback could be demonstrated. It was
also demonstrated that the intent to counteract any perturbation of head-eye movement, i.e., the
mental set, permitted the subjects to manipulate consciously their vestibular ocular reflex (VOR)
gain. From our data we infer that the VOR is always "on." It may be, however, variably sup-
pressed by higher cortical control. With appropriate training, head-mounted displays should per-
mit an easy VOR presetting that leads to image stabilization, perhaps together with a decrease of
possible misjudgments.
INTRODUCTION
For some time it has been known that visual and mental effort influence the vestibular ocular
reflex (VOR). Besides visual long- and short-term adaptation to reversing prisms (Melvill Jones
and Gonshor, 1982) and fixation suppression of the VOR (Takemod and Cohen, 1974; Dichgans
et al., 1978; Zangemeister and Hansen, 1986), the mental set of a subject can influence the VOR,
e.g., through an imagined target (Barr et al., 1976; Melvill Jones et al., 1984) or anticipatory
intent only (Zangemeister and Stark, 1981). In contrast to animals, human head and eye move-
ments are governed by a conscious will of the human performer that includes verbal communica-
tion. Thus in a given experimental setup, the synkinesis of active human gaze may be changed
according to instruction. The verbal feedback to the subject might permit a whole range of gaze
types, even with amplitude and prediction of a visual target being constant. The gaze types
(Zangemeister and Stark, 1982a) are defined by head minus eye latency differences (table 1). This
has been demonstrated particularly by looking at the timing of the neck elektromyogram as the head
movement control signal (Zangemeister et al., 1982b; Zangemeister and Stark, 1983; Stark et al.,
1986). In this study, we compared the voluntarily changeable human gaze types performed during
the same experiment with and without the addition of a randomly applied perturbation to the head-
eye movement system. We tried to answer three questions in particular:
1. Are we able to modulate continuously the types of coordinated gaze through conscious
intent during predictive active head movements?
2. What is the gaze (saccade and VOR/CEM (compensatory eye movement)) response to
passive random head rotation from zero head velocity with respect to the preset intent of a given
subject?
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3. Does random perturbation of the head during the early phase of gaze acceleration generate
responses that are the sum of responses to experiment (1) and (2)?
METHODS
Eye movements were recorded by monocular DC Electrooculography, head movements by
using a horizontal angular accelerometer (Schaevitz) and a high-resolution ceramic potentiometer
linked to the head through universal joints (Zangemeister and Stark, 1982c). Twelve normal sub-
jects (age 22-25) attended a semicircular screen sitting in a darkened room. While they actively
performed fast horizontal (saccadic) head rotations between two continuously lit targets at +30 °
amplitude with a frequency around 0.3 Hz, they were instructed to focus on the following tasks:
(1) "shift your eyes ahead of your head," (2) "shift your head ahead of your eyes." During (1)
they were instructed to shift eyes "long before" (i, type 1I), or "shortly before" (ii, type I) the head.
During (2) they were instructed to shift head "earlier" (i, type IRA), or "much earlier" (II, type
IIIB) than the eye, eventually "with the intent to suppress any eye movement" (type IIIB or IV).
Each task included 50 to 100 head movements.
Perturbations were done pseudorandomly, (1) from a zero P,V,A (position, velocity, accel-
eration) initial condition of the head-eye movement system, and (2) during the early phase of head
acceleration. They consisted of (1) fast passive head accelerations, of (2) short decelerating or
accelerating impulses during the early phase of active head acceleration and were recorded by the
head-mounted accelerometer. Perturbation impulses were generated through an apparatus that
permitted manual acceleration or deceleration of the head through cords that were tangentially
linked directly to the tightly set head helmet.
RESULTS
1. The subjects demonstrated their ability (fig. 1) to switch between gaze types in the
experimentally set predictive situation of constant and large-amplitude targets. The respective gains
(eye/head velocity) were: ty.II 0.9-1.1, ty.III 0.13, ty.IV 0.06-0.09. This result was expected
from our earlier studies (Zangemeister and Huefner, 1984; Zangemeister and Stark, 1982a,c). The
subjects showed differing amounts of success in performing the intended gaze type, with type IV
being the most difficult to perform, supposedly because of the high concentration necessary
(table 1).
2. Random perturbation of the head while in primary position, with head velocity and accel-
eration being zero (fig. 2), resulted in large saccades/quick phases of long duration, and a large
and delayed VOR/CEM, if the subject had low preset intent to withstand the perturbation; in this
case head acceleration showed a long-lasting damped oscillation. Respective gains were: figure 2
(upper): 0.35 (upper) 0.45 (lower); figure 2 (lower left): 0.5 (upper), 0.17 (lower). With
increasing intent of the subject (fig. 2 left, middle, and lower), head acceleration finally became
highly overdamped, but still with comparable initial acceleration values, and eye movements
showed increasingly smaller and shorter quick phases as well as an early short VOR response. In
addition, with the highest intent a late anticompensatory eye movement was obtained.
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3a. Random perturbations of the accelerating head, i.e., sudden acceleration or deceleration
of gaze in flight (fig. 3), were characterized by small VOR responses after the perturbation in case
of high intent of the subject as in gaze type IIIB, or much higher VOR/CEM gain in case of low
intent comparable to gaze type I. Respective gains were: figure 3 (left) ty.I 0.55, ty.3 0.06,
ty.IV 0.08 (left), 0.09 (right); figure 3 (right): 0.13 (upper), 0.90 (lower).
3b. Random perturbations were also applied during coordinated head-eye movements in
pursuit of a sinusoidaUy moving target (maximum velocity 50°/sec) with the VOR being sup-
pressed through constant fixation of the pursuit target. Figure 4 (left) demonstrates the different
amount of VOR fixation suppression as a function of changing intent during fixation of a sinu-
soidal target of the same frequency. With perturbation (fig. 4, right) a response was obtained that
was comparable to the result of experiment (2). That is, depending on the subject's intent and
concentration, the VOR response was low for high intent and vice versa (gain fig. 4, right:
0.044).
Therefore, the three initial questions could be answered as follows:
1. In nonrandom situations subjects can intentionally and continuously change their gaze
types.
2. Gaze responses to passive random head accelerations depend on the subject's preset
intent.
3. Perturbation of predictive gaze saccades in midflight results in the sum of tasks one and
two.
DISCUSSION
The input-output characteristics of the VOR are subject to major moment-to-moment fluctua-
tions depending on nonvisual factors, such as state of "arousal" (Melvill Jones and Sugie, 1972)
and mental set (Collins, 1962). More recently, it has been found that the influence of "mental set"
depends explicitly upon the subject's conscious choice of intended visual goal (Barr et al., 1976;
Sharpe et al., 19081; Baloh et al., 1984; Fuller et al., 1983), i.e., following earth-fixed or head-
fixed targets during head rotation. Consistent alteration of the mentally chosen goal can alone pro-
duce adaptive alteration of internal parameters controlling VOR gain (Berthoz and Melvill Jones,
1985). Obviously, comparison of afferent retinal slip detectors with concurrent vestibular afferents
can be substituted by a "working" comparison made between the vestibular input and an efferent
feedback copy of either the concurrent, or the imagined or anticipated concurrent, oculomotor out-
put, as proposed by Miles and Eighmy (1980).
Our results here demonstrate the ability of the subjects to perform short-term adaptation dur-
ing verbal feedback instructing for eye-head latency changes that changed the types of active gaze.
These results are comparable to the data from Barr et al. (1976), in that an almost immediate
change between different VOR gains with constant visual input could be generated. In addition,
our perturbation experiments expanded these data, demonstrating the task- (or gaze-type) depen-
dent attenuation of the VOR. This is in contrast to results in animals, where perturbation of
44-3 •
visually triggered eye-head saccades resulted in an acceleration of the eye (Guitton et al., 1984;
Fuller et al., 1983), because a conscious task-influence of the VOR is impossible. Therefore not
only can a representation of the target's percept (Barr et al., 1976) be created, but also an internal
image of the anticipated VOR response in conjunction with the appropriate saccade.
We hypothesize that through the cortico-cerebeUar loop a given subject is able to continu-
ously eliminate the VOR response during predictive gaze movements. This is done internally by
generating an image of the anticipated VOR response in conjunction with the appropriate saccade,
and then subtracting it from the actual reflex response. This internal image can be manipulated
intentionally and continuously WITHOUT a VOR on/off switch. In this way a flexible adaptation
of the conscious subject to anticipated tasks is performed.
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Zangemeister,W. H., andStark,L. (1982a)Gazetypes: interactionsof eyeandheadmovements
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44--6
Table 1.- Gazetypesdefinedbylatency:eyeminusheadlatency.TypeII: earlypredictionof
eye,lateheadmovement;eyemovementdominatesgaze.I: headfollows eyeshortlybefore
eyehasreachedtarget;classicalgazetype. III: headandeyemovementstartaboutsimulta-
neously.Predictivegazetype. IV: earlypredictionof head,lateeyesaccade;headmove-
mentdominatesgaze. Suppressionof VOR/CEMin III andIV. Seealsofigure lb.
Type Eyelatency-headlatency,msec Averagerateof successin generatingintentionally
differentgazetypesthroughverbalfeedback,%
I +50 76
II <50 56
HIa >50-200 69
I_b >200-550
IV >550 16
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b) Explanatory scheme for the continual change of gaze types (lower).
44-8
ORIGINAL PAGE IS
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MAX IMRL INTENT
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A _
Figure 2.- Random perturbation from primary position. (a) Low and (b) very high intent.
Random perturbation from primary position, (c) low and (d) very high intent. Random
perturbation from primary position, explanatory scheme (e).
44-9
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Voluntary presetting of the vestibular ocular reflex permits gaze stabilization despite perturbation of fast head movements

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