wertheim_1978_highwayhypnosis

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EXPLAINING HIGHWAY HYPNOSIS: EXPERIMENTALEVIDENCE FOR THE ROLE OF EYE MOVEME NTS

A. H. WERTHEIM?TrafficResearchCenter, Instituteof ExperimentalPsychology, University of Groningen, The Netherlands

(Received 4 April 1977; n reaised form 21 November 1977)Abstract-The frequent occurrence of highway hypnosis, a lowered state of alertness leading to thedevelopment of drowsiness and fadure to react adequately to changes in the road situation, is a danger wellrecognized among professional drivers. Current theories concerning highway hypnosis do not satisfactorilyexplain its nature or its origins and are very difficult, if not impossible, to validate in experimental research.In this paper a theory is presented which explains highway hypnosis as a complex syndrome of changes inthe human ability to perform several psychological functions. These changes result from the need toperform specrfic eye movements. This theory has been validated in the experimental research presented intlus paper. The functions investigated concerned the ability to react quickly to visual signals, to use priorwarning information in order to speed up such reactions, to detect movements in our visual surroundings, torecall information presented earlier and to maintain alertness (as indicated by alpha activity m the EEG).The experimental findings appear to explain many of phenomena of highway hypnosis. They are in line withthe theory that highway hypnosis develops with the need to make those specific eye movements which arerequired when we have to look at aspects of our visual surroundings which move (relative to the observer)in a very predictable pattern. Some suggestions for further research and some practical measures, whichmay be helpful in reducing the dangers of highway hypnosis, are proposed.

I. THEORETICAL FRAMEWORK1.1 Highway hypnosisAmong those human factors detrimental to traffic safety, the occurrence of drowsiness andlapses in attention are of high prominence. Especially on highways it often occurs that drivers,suffering from fits of drowsiness, drive off the road or fail to notice road curves or velocitychanges of other vehicles. The occurrence of hallucinations is a common observation amonglong distance truck drivers and many accidents happen due to the driver falling asleep.Although these phenomena are well known among professional drivers they have attractedrelatively little interest in scientific literature. References are generally of a contemplatorynature and no attempts have been made to study these phenomena in rigid scientific research.Only a few explanations have been suggested.McFairland and Moseley [1954] suggested that fatigue may be the most important factor. Itis however extremely difficult to measure fatigue (see for a discussion: Crawford [1971]). Inaddition it appears that drowsiness during car driving also occurs when there is no evidence ofexcessive fatigue [Roberts, 19711.Another suggestion was posed by Williams 119631, who introduced the term highwayhypnosis. He suggested that monotony of the surroundings and the necessity to attend only toa very small part of the visual field might induce some kind of hypnotic trance. Williamsshowed that subjects who have been hypnotized can drive without difficulty. Although thisexplanation sounds quite attractive it is very difficult to verify. One difficulty is that the conceptof monotony is not easily measured in such a way as to enable scientific experimentation. Asecond point is that it is very difficult, if not impossible, to define a hypnotic trance in terms ofbehavioral or physiological measures2Roberts [1971] has proposed another hypothesis. He suggested that the occurrence ofexcessive drowsiness might be due to what he termed functional hyperinsulinism, meaning acondition of oversensitivity to a certain concentration of sugar in the blood. A person sufferingfrom functional hyperinsulinism might experience sudden attacks of lowered consciousnesssimilar to those of diabetics, although less severe. The amount of sugar in the blood might be

tPresent address: Institute for Perception TNO,Kampweg 5, Soesterberg, The Netherlands.SAlthough Williams coined the term highway hypnosis to indicate a state of hypnotic trance, this paper is not concernedwith defining or validating hypnotic trance as an explanatory concept. Our preference for the term highway hypnosis stemsfrom its widely accepted use in the angle-saxon literature as a name for a well-known phenomenon.111


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112 A. H. WERTHEIMespecially dangerous to narcoleptics (persons who suffer from unexpected attacks of sleepi-ness), in which case a critical blood level of sugar could trigger a sleep attack. The maindifficulty with this hypothesis is that the common experience of drowsiness with most driverswould make almost everyone either a narcoleptic or a patient with functional hyperinsulinism.

In this paper a more specific theory concerning the occurrence of drowsiness and inattentionduring driving is presented. This theory holds that some of our mental abilities are related to theactivity of our oculomotor system (the neurological system responsible for the initiation of eyemovements). Accordingly, the need for eye movement patterns specifically required in drivingperformance has such a bearing on our oculomotor system that several other psychologicalfunctions are influenced.1.2 Attentive and intentive oculomotor control

The oculomotor system can be envisaged as part of a feedback control system whichenables positioning of our eyes toward those loci in the surroundings we want to see. Retinalinformation about the position of an image relative to the fovea (the retinal error signal) isgenerally thought to serve as the main feedback signal in this system. The role of propriocep-tive signals (stemming from the eye musculature) appears to be of little or no relevance (see fora discussion Brindley and Merton [1960]; Fender and Nye [1961]; Merton [1964]; Festinger andCanon [1965]; Robinson [1968]; Weisfeld [1972]). This does not mean that information fromother sources is not used in oculomotor control. Some kind of extra retinal information must beinvolved since otherwise eye movements would not be under voluntary control but follow therules of simple reflexes. The oculomotor neurons in the brain, which emit the neural signals tothe eye musculature, apparently receive both retinal and extra retina1 information. (For someelaborate discussions on the evidence of extra-retinal input on oculomotor control see Stein-bath [1969], Young [1971], Wertheim [1974], Festinger et al. [1976]).

Such an analysis of the oculomotor system has interesting implications: there is no logicalnecessity that retinal error information should always be fed back to the eye musculature. It isvery well conceivable that, under certain circumstances, eye movements made during normalvision become-to a relatively large extent-governed by extra retinal information.

When, for example, the eyes follow a visual signal moving in a highly predictable way, acertain degree of automatism will develop in the activity of the eye musculature. Some kind ofinternal motor program is likely to develop on the basis of an internal representation of theexternal movement. Such a motor program will reduce the need for retina1 error information asan input for the oculomotor neurons during oculomotor performance. At the same time visionwill remain unaffected.

We thus distinguish two components in oculomotor control. The attentive component refersto retina1 feedback and the intentive component refers to the intention to move our eyes.

Henceforward oculomotor activity will be termed attentive whenever retinal informationserves as the main input for the oculomotor neurons. Oculomotor activity will be calledintentive, whenever it is governed mainly by information stemming from other (internal) sources.

Figure 1 illustrates the difference between the attentive and the intentive component in

Intentwecomponent

programsetcFig. I. Schematic representation of the attentive and intentive components in oculomotor functioning.


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Explaining highway hypnosis: experimental evidence for the role of eye movements 113oculomotor control. Since the distinction between attentive and intentive oculomotor controlrefers to different inputs to the oculomotor neurons only, no relation with perception isnecessarily implied.

It is unlikely that a clear dichotomy in oculomotor control exists in such a way that it iseither attentive or intentive exclusively. The degree to which retinal information serves as amonitoring principle determines the extent to which eye movements are controlled attentivelyor intentively. Elsewhere [Wertheim, 19741 the distinction between these two types ofoculomotor control has been validated with evidence from the literature on visual systemfunctioning.

1.3 Oculomotor control and psychological functionsAlthough the distinction between attentive and intentive oculomotor control does not refer

to the sensory process of visual perception itself, it may have consequences for the cognitiveinterpretation of what we see. For example, when the projection of a visual signal shifts overthe retina, it may be perceived properly. However in order to identify it as due to a movementof something in our surroundings we have to compare mentally this retinal information withinformation on how we have moved our eyes. Otherwise we are unable to discriminate betweenretina1 image shifts which are due to external movements and those due to a movement of theeyes.

In other words, the attentive component (retinal information) and the intentive component(extra retinal information about eye movements) are to be related at cognitive levels. Whenoculomotor control is highly intentive, the fact that only a small attentive component is presentmay impair our ability to perform this mental comparison, impairing our ability to detect realmovement in our surroundings.

This is an example of how the difference between attentive and intentive oculomotor controlmay interfere with our ability to interpret what we perceive. Phenomena other than movementdetection may also be related to the distinction between attentive and intentive oculomotorcontrol.

Several characteristics of the task of car driving make it probable that, especially when theroad situation is highly predictable (as is the case with highways), the intentive component inoculomotor control will increase at the expense of the attentive component: drivers have todisregard most of the information present in the external visual field because of its irrelevanceto driving performance. The attentive component in oculomotor control may thus becomedependent solely on information derived from the perception of very few visual signals. Inaddition these signals may become highly predictable in their movement pattern, especiallyduring prolonged driving on long unchanging stretches of highway or during prolonged singlefile driving. A change in oculomotor system functioning from attentive to intentive controlcould influence several psychological functions which are crucial to the task of car driving.

This paper is a report on research concerning the differential influence of attentive vsintentive oculomotor control on our capacity to react swiftly to visual signals, on our ability touse warning signals for speeding up such reaction processes, on our capacity to detectmovements in the visual field, on our ability to recall what we perceived earlier and on ourgenera1 level of alertness as indicated by the amount of alpha activity in the EEG.

When taken together, our experimental findings point towards a new and rather analyticalexplanation of the phenomenon of highway hypnosis.

2. EXPERIMENTAL RESEARCH2.1 General method of experimentation

Measurement of the differential effect of the two types of oculomotor control necessitates amethod which enables manipulation of the intentive and attentive components. The beststrategy is to compare visual tracking conditions. This means that subjects (Ss) are required tofollow with their eyes a visual signal (the target) moving over a screen. When the movement-pattern of the target is very predictable, an internal motor program may develop and replaceretinal information as the main principle in oculomotor control. Thus the intentive componentwill be enhanced at the expense of the attentive component. On the other hand, when theAAPVol lO.No 2-C


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114 A. H. WEKTHEIMmovement pattern of the target is unpredictable no internal motor program can develop andoculomotor control will remain attentive.In the predictable condition (PC) the target was made to move in a circular path. In theunpredictable condition (UC) the target also moved in a circular pattern, but in such a way thatat unpredictable intervals (shorter than one cycle time) it switched between the paths oftangential, congruent circles (I Hz). Thus only directional predictability of target movementdiffered between the UC and the PC. Other movement characteristics, such as angular velocity,remained similar.In all but two of the experiments the target consisted of the disk-like projection of a lightbeam on a projection screen. The beam was moved by two mirrors, mounted on galvanometers,and was focussed by a lens. The coordinates of the target movements were provided by theoutput of a pure sine wave generator, synchronized with its inverse (Wo phase shifted). Thecoordinates in the UC were provided by the replay of an analog-7 tape, on which they had beenprerecorded with the help of a computer. In the other two experiments the target consisted of abright dot moving over an oscilloscope screen.Ss were positioned at a distance from the screen such that the maximum amplitude of atarget sweep over the screen provided a visual angle of 7.5 in the PC and IS in the UC (thelatter with a mean visual angle of 7.5). Sanders I19631showed that with visual angles between 0and 15 he occurrence of head movements is very unlikely while, in addition, there is little or nodifference in eye movement characteristics.Visual signals to which Ss were required to react or which they had to memorize, wereprojected in the moving target. For this purpose a transparent plastic disk, on which letters andsymbols were printed, was positioned between the lamp which originated the lightbeam, andthe mirrors moving it. The disk could revolve stepwise (8 msec per step) in such a way that eachsymbol couid be presented exactly at the point where the lightbeam passed through. Thussymbols could be presented at fast rates right in the center of the target, solving the problem ofhow to maintain focussed perception of signals when the eyes move.In the experiments on movement detection a visual background pattern of nine randomiypositioned bright dots (spanning a visual angle of 20) was used. It was projected on the screenwith a slide projector, a third mirror, mounted on another galvanometer, and a half silveredtransparent mirror. Movements of this background pattern, which Ss were required to detectand to which they had to respond, were accomplished by connecting the third galvanometer tothe output of another pure sine wave generator at a moment of maximum amplitude, for theperiod of one cycle (0.3 Hz). Thus the background pattern moved vertically, starting in upwarddirection (10 max ampi.)Signals presented in the target and background movements were triggered by the computer.Ss had to press a reaction-time push-button in response. Reaction-times (RTs) were measured inmsec and recorded on teletype.In some of the experiments a.c. measures of horizontal eye movements were registered (3set time constant) in order to measure eye tracking performance. For this purpose twoAg-AgC1 electrodes were positioned at the outer canthi of the eyes, their earth electrode beingplaced symmetrically to them at mid-frontal position. The signals derived from the eyes werecompared continu~ly with electrical signals produced by the hardware, associated withhorizontal target-position. The absolute difference (tracking error) was integrated, converted toa voltage, recorded on paper as a continuous tracing and detected on-line by a PDP-8E lab.computer, which monitored the whole experimental system. Calibration of eye movements wascarried out by increasing or decreasing the amplitude of the signals derived from the eyes insuch a way that error was minimal during a test-run in the PC, prior to the experiments.Eye tracking performance was operationalized as follows: Whenever tracking errorssurpassed a pre-set norm (error threshold) performance was defined as off-target. Otherwise itwas considered on-target. Thresholds could be set independency for the two conditions. Insome experiments Ss received feedback about their tracking performance: when performancewas off-target the light intensity of the target changed. With the help of a software clock thecomputer could measure the amount of time spent on- and off- target for each condition.In several experiments EEG alpha activity was measured with the help of bipolarmeasurement over the occipital-parietal areas of the scalp, using two Ag-AgCL EEG elec-


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Explaining highway hypnosis: experimental evidence for the role of eye movements 115trodes, with an earth electrode attached at the left mastoid bone or symmetrically in betweenand above the other two. The amount of alpha activity present in the EEG trace was measuredas follows: the EEG signals passed through an alpha band pass filter (8-13 Hz, 50dBloct.) online. The resulting signals were recorded as a paper-tracing, converted to absolute values, andintegrated on line with a condenser. Two such condensers were used. One measured theamount of alpha activity during the period of one condition, the other only during the periods ofon-target eye tracking performance within one condition. The condenser charges were con-veyed to the computer for continuous on-line measurement of the amount of alpha activity. Atthe end of each condition the condensers were reset at zero level.The amount of alpha activity was operationalized as mean alpha, indicating the computerreading of condenser charge, divided by the time over which measurements were taken. Bysubtracting the charge of the two condensers the amount of alpha during tracking errorperformance was measured. Thus the numerical values of mean alpha, presented in this report,refer to computer readings of alpha energy. The absolute values of alpha energy have nospecific meaning; they refer to the unit of measurement used by the computer when reading thecondenser charges on its analog input channels.Ss were seated in a dark, sound attenuated, experimental cabin, maintaining their head in afixed position resting on the headrest of a dentists chair. They faced the oscilloscope screen at30cm distance from the eyes or the large projection screen at 200 cm distance from the eyes.An intercom system provided communication with the experimenter. All projected visualsignals were presented through a small window in the back of the cabin, overhead of the Ss.Most Ss were students, both male and female, of the University of Groningen. Some werestudents at several vocational colleges. Their ages ranged between 18 and 32 years and all hadnormal or corrected vision. Care was taken to select only Ss who were not on medication inorder to eliminate the risk that this might somehow influence psychological performance orphysiological measurements. None of the Ss were familiar with the hypotheses. When inter-ested, they were given a detailed explanation after completion of the experiments. They werepaid for their cooperation. All data concerning alpha activity and reaction times were inter-preted statistically in terms of analyses of variances.2.2 Experiments on the influence of oculomotor control on RT and on the influence of a warningsignal on RT2.2.1 Experiment I

Introduction. This experiment was designed to investigate whether RT to a visual signal,presented in the moving target during attentive oculomotor control, differs from RT to the samesignal presented during intentive oculomotor control. A second purpose was to investigate thepossibility that oculomotor control interferes with the effect of warning signals on RT.When a warning signal precedes a critical signal (the stimulus to which a reaction isrequired) the duration of the interval between the warning signal and the critical signal, theforeperiod (FP), influences RT. When FPs are always of the same duration RTs are shorterthan when FP-duration varies (even when mean FP-duration is the same as with constant FPs).This so called foreperiod effect is usually ascribed to the greater ease with which the criticalsignal is anticipated with constant FPs. Presumably, some degree of mental or motor readinesscan be built up so as to reach an optima1 level at about the expected arrival of the critical signal.With variable FPs the arrival of the critical signal cannot be anticipated with such precision,which results in larger reaction times. These phenomena are related to timing and anticipationand thus are of great relevance to car driving. For example the brake lights of a car driving infront of us warn us to anticipate a change in driving speed of that car, to which we have toreact. Therefore it is important to investigate whether intentive or attentive oculomotor activityinfluences the foreperiod effect.

Method. A black asterisk, presented in the moving target, served as a warning signal andremained visible until displaced by the letter Z, which served as the critical signal. Theforeperiod effect was measured in both the PC and UC and in addition in a control condition, inwhich the target remained stationary.FPs were either constant (CFP) or variable (VFP). CFP duration was 1.5 set VFPs were of


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116 A. H. WERTHEIM0.5, 1, 1.5, 2 and 2.5 set duration, presented in random order. Thus mean VFP duration was also1.5 sec.

The signals were presented in three series of ten blocks. A block consisted of about 20stimuli. Blocks with CFPs alternated with blocks in which VFPs were presented.

Twelve Ss were assigned to two experimental groups of equal size. One group received thePC during the first ten blocks and the UC during the last ten blocks; the other group vice versa.The middle 10 blocks consisted of the control condition, in which the target did not move.Between blocks short pauses were given. Ss were trained prior to the experiment (on thecontrol condition) with both CFPs and VFPs until the variability of their reaction times wassatisfactorily reduced (to the level where the standard deviation reached a level between 10 and20% of median RT).

Results. As illustrated in Fig. 2, RTs were shorter in the PC than in the UC. In addition, theforeperiod effect was clearly observed: RTs with VFPs were longer than those with CFPs.Interestingly, the magnitude of this difference was larger in the UC than in the PC or in thecontrol condition. These findings were all statistically significant (P cO.01). Although Fig. 2suggests that RTs in the control condition are smaller than RTs in the PC this difference has nostatistical significance.

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Fig. 2. RT and the foreperiod effect as a function of eye-trackmg condition (Exp. I). 04, Constantforeperiods; O---O, variable foreperiods; C, control condition; PC, predictable eye-tracking; UC, un-predictable eye-tracking.Discussion. The data from this experiment indicate that the type of oculomotor control

influences both the speed with which we are able to react to a visual signal and our capacity touse warning signals as an aid to speed up such reactions. As compared to attentive oculomotorcontrol we gain from intentive oculomotor control in terms of the general level of our reactiontimes, but we lose in terms of the effective use we can make of warning signals. Reactions arefaster when the eyes move intentively but consistent warning periods are more effective whenoculomotor control is attentive.

An alternative explanation for these findings could be that it is not the type of oculomotorcontrol, but the position of the projection of the visual signals on the retina, which influencesRT. When we look straight at a signal we perceive it foveally and RT might be shorter thanwhen we do not focus the signals directly. Signals which have been presented during off-targeteye tracking performance may be associated with longer RTs. Therefore the experiment wasreplicated with the additional measurement of eye tracking performance.


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Explaining highway hypnosis: experimental evidence for the role of eye movements2.2.2 Experiment II

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Introduction. If the effects found in the former experiments were due to the position ofsignal projections on the retina relative to the fovea, we should categorize RTs according totracking performance present at the moment of stimulus presentation. There are two types oftracking performance: on- and off-target, corresponding to fovea1 and parafoveal perception.There are also two types of signals: warning signals and critical signals. Thus four types of RTscan be defined. If the interaction between foreperiod effect and oculomotor control is due tofovea1 or parafoveal perception of one or both of the signals, it is expected that with all Ss atleast one of the four types of RT will consistently fail to show the interaction.

Method. Six Ss participated. The general method was the same as in the former experiment,with the exception that the order of presentation of the eye tracking conditions was balancedamong Ss. Eye movements were measured. The computer labelled each RT according to theretinal position of the warning- and critical signals associated with it.

Results. Figure 3 shows that the same configuration of data as in the former experiment wasobtained. During the UC RTs were longer than during the other two conditions. Again theforeperiod effect was larger in the UC as compared to the PC and control condition. As can beseen from Fig. 4 none of the four types of RT showed any consistent absence of thisphenomenon. Statistically the effects were independent of type of RT.

Discussion. The data from this experiment indicate that it is not retinal position of theprojection of a signal on the retina which determines RT, but type of oculomotor control.

We have assumed that oculomotor control is more intentive when driving on highways ascompared to secondary roads where the visual scene is less predictable. The data from these

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JLC UC c PC IRg. 4Fig. 3. RT and the foreperiod effect as a function of eye-tracking condition (Exp. II). O-_-O, Constantforeperiods; O---O, variable foreperiods; C, control condition; PC, predictable eye-tracking condition;UV,unpredictable eye-tracking condition.Fig. 4. RT and the foreperiod effect as a function of eye-tracking condition and retinal position of thestimulus image (Exp. II). O--O, Constant foreperiods; O---O, variable foreperiods; C, control condo-tion; PC, predictable eye-tracking condition; UC, unpredictable eye-tracking condition; W, warning signal;

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experiments indicate, that on highways we may be able to react faster than on secondary roads.On the other hand warning signals associated with short consistent warning periods (such as thebrakelights of the car in front) may have less effect on our reactions to a critical signal (such asa decrease in velocity of the car in front) on highways than on secondary roads with lesspredictable visual surroundings.2.3 An experiment on oculomotor control, RT and movement detection2.3.1 Experiment III

Introduction. The detection of motion in our visual surroundings depends on our ability todistinguish between shifts of retinal images which are due to movements of the eyes and shiftswhich result from real movements in the visual field. Somehow retinal information andinformation about oculomotor activity must be compared. The outcome of this mental opera-tion is crucial to a valid interpretation of motion is visual perception (e.g. Haber [1970],Dichganz and Bizzi [1972]: Haber and Hersherson [1973], Carette and Friedman [1975]).

As stated earlier, it is likely that during intentive oculomotor control, when retinal in-formation is not fed back to the eye musculature, the ability to perform this mental operation isimpaired. Under such circumstances the ability to perceive movement is probably impaired,compared to circumstances in which oculomotor control is predominantly attentive. Thishypothesis can be verified by comparing movement detection in the PC with the UC.

Method. Impaired movement detection will only occur when the retinal shifts induced byexternal movements do not differ much from those which can be expected to result from eyemovements. A sudden retinal shift will easily be recognized as due to external movement whenthe eyes are moving smoothly (as is the case in our eye-tracking tasks). This is why a smoothsinusoidal movement of a visual background pattern (as described in Section 2.1) was used.

The degree to which intentive oculomotor control impairs movement detection can bemeasured by comparing movement detection time (MDT) in the PC with the UC. In order toverify whether this additional task requirement interferes with other aspects of the task, Sswere again required to react also to visual signals presented in the moving target. This RT taskalso insured the maintainance of eye-tracking performance. Thus it was expected that althoughMDT would be longer in the PC than in the UC, RT in the PC would again be shorter than inthe UC.

Twelve Ss participated. They faced a very large screen in order to decrease chances that thescreen borders were too close to the background dot pattern as to provide extra cues on itsmovement. Ss were instructed to press the reaction button upon noticing an asterisk and alsowhenever they thought the background pattern moved in vertical direction.

Obviously it was to be insured that Ss would not divert their gaze from the target towardsthe background in order to verify whether the dots moved or not. Such a diversion implies eyemovements directed by perception and thus attentive oculomotor control. Therefore a pay-offsystem was used which highly motivated Ss to maintain their eyes focussed on the movingtarget. Short RTs (faster than mean RT during training) were financially rewarded. Those over500 msec were punished by a financial loss. Ss were told that MDTs were of little importanceand were only required in order to increase task load.

Only one PC and one UC were given, each of which lasted for eight minutes, during whichapproximately 75 RTs and 30 MDTs were measured. Background movements and signals in thetarget to which a reaction was required were never presented in concurrence. After eachexperiment a control condition was included in which Ss were instructed to react to backgroundmovements in the absence of a target. Thus a measure was obtained of MDT when attentionwas focussed on the background. In addition, eye movements were recorded. If the Ss were tochange their gaze from the target towards the background, this would become visible in the eyemovement record as a very fast saccadic eye movement. Thus it was possible to verify, duringthe experiment, whether the eye-tracking instructions were followed properly.

Results. As can be seen from Fig. 5 once again RT was shorter in the PC than in the UC.MDT was, as expected, shorter in the UC than in the PC. These effects were statisticallysignificant (p < 0.01 and p < 0.05 respectively).

It should be mentioned that very few omissions occurred and no significant differences in their


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Explaining highway hypnosis: experimental evidence for the role of eye movements 119means

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J IC uiFig. 5. RT towards signals presented in the moving target (RT) and towards movements of the background(MDT) as a function of eye-tracking condition (Exp. III). O---O, RT; O---O, MDT; PC, predictableeye-tracking condition; UC, unpredictable eye-tracking condition.

number between the UC and the PC were observed for either RT or MDT. In addition Ssappeared not to have diverted their attention from the target towards the random dot pattern.This assumption is supported by the fact that MDT in the control condition was much faster(k750msec) than in the experimental conditions and that only very few saccades wereobserved in the eye movement recordings.Discussion. The data from this experiment rather strongly support the notion that the typeof oculomotor control influences our ability to detect movement in our visual surroundings.This has an important bearing on the task of car driving. Obviously, when driving in very

predictable surroundings, such as on highways, we have more difficulty in detecting anunexpected movement than when the road situation is less predictable. This may implyimpaired preception of movement characteristics (such as velocity changes of other traIIicvehicles). It could also explain why sometimes a road curve is not noticed and why driverssometimes fail to correct a slowly developing steering bias. In these instances aspects of thevisual field move relative to the drivers visual gaze, but these movements fail to be noticed.2.4 Experiments on the relation between oculomotor cont rol and some aspects of the memorysystem2.4.1 Experiment IV

Introduction. The investigation of differential influences of attentive and intentive oculomo-tor control on information processing should include measures indicating memory processes. Intrafhc situations, the ability to recall information presented shortly before it is needed, iscrucial. For example, when approaching changes in a road situation (such as crossings ordangerous curves) we have to remember all related road signs we may have passed justpreviously. This short term aspect of our memory system can be measured by presenting anumber of symbols (letters, words) sequentially, which then have to be recalled.Man has only a limited capacity for recall. The total number of items recalled correctly froma list usually lies between five and nine. When the number of items recalled is plotted againstthe serial position of each item in its list, the resulting graph, known as the serial position curve(SPC), has the form of an U. The first and last items of a list are usually recalled better than


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120 A. H. WERTHEIMthose presented in the middle. The following experiments were carried out in order to verifywhether aspects of the SPC are influenced by type of oculomotor control.

Method. Twenty-four Ss participated. They were presented with twelve lists each. A listconsisted of twelve five-letter words (Dutch nouns). The letters composing a word wereprojected sequentially in the moving target. Each letter was presented for one second, afterwhich during half a second no symbol was projected. In between words a small asterisk wasshown. In order to guard against the danger that this method of sequential letter presentation-and not oculomotor control-would influence aspects of the SPC, a control condition wasincluded in which the target remained stationary. Thus effects exclusively due to oculomotorcontrol must appear when comparing the SPC in both the PC and the UC with the controlcondition. During each of the three eye-tracking conditions four lists were presented. When alist was completed the target was colored red, which indicated to the Ss that he had to reportverbally all items he recalled. During recall eye-tracking was not required.

Results. All words recalled appeared to have been read correctly. The procedure ofsequential letter presentation thus did not influence word perception. The SPC in the controlcondition does not differ from those described in the literature (e.g. Klings and Riggs [1972]). Ascan be seen in Fig. 6, no clear differences occurred between the three SPCs.

;..2 3 L 5 6 7 8 9 10 11 12serbal posltlon

Fig. 6. Serial position curve of recall as a function of eye-tracking condition (Exp. IV). O-_-O,Predictable eye-tracking condition; O---O. unpredictable eye-tracking condition; O--O, control condi-

tion.

Discussion. The results show that there seem to be no differential effects of attentive andintentive oculomotor control on recall. The data should however not be considered conclusive.In each eye-tracking condition only four lists were presented. Thus when a Ss misses one itemthe result is a 20% loss of recall in that serial position. Therefore a second experiment wascarried out using more lists in each eye-tracking condition.2.42 Experiment V

Method. Twelve Ss participated, none of which had participated in the previous experiment.They were presented with ten lists in each eye-tracking condition. The rest of the procedurewas the same as in experiment IV.

Results. As illustrated by Fig. 7, the data showed the same structure as in experiment IV. Nodifferences between the three SPCs were observed.

Discussion. Although at first glance the results of the two memory experiments show thatthe type of oculomotor control is not related to the SPC, this finding should be interpreted withsome reservation. The task of memorization has two distinct phases: the acquisition ofinformation into our memory system and its recall at a later stage. The effects of eyemovements on the mechanism of recall have, strictly speaking, not been measured, since Sswere only required to track the target with their eyes during the phase of information


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Explaining highway hypnosis: experimental evidence or the role of eye movements

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TJalZZ0 -x& 60-0z2;ca O-

. yt I I I I 1 1 t t t 1 I2o 1 2 3 1 5 6 7 8 9 10 It 12

ser~ol posf tlonFig. 7. Serial position curve of recall as a function of eye-tracking condition (Exp. V). o--O, Predictableeye-tracking condition; O---O, unpredictable eye-trackmg condition; O----O, control condition.

acquisition. Thus the only conclusion we may draw is that oculomotor control is not reiated tothe phase of information acquisition in memory. Whether recall itself is related to ocuIomotorcontrol remains to be investigated.2.5 Experiments on the relation between oculomotor cont rol and occipital alpha activ ity2.5.1 Experiment VIintroduction. One of the methods to fight highway hypnosis is to deliberately maintainalertness [Williams, 19631. Obviously lowered alertness is one of the phenomena constitutingthe syndrome of highway hypnosis. Therefore it should be investigated whether oculomotorcontrol also relates to alertness.One of the methods to investigate whether oculomotor control influences alertness is tocorrelate oculomotor control with physiologic~ measures which are generally understood to beassociated with alertness or arousal and which, in addition, are known to be influenced byactivities of the visual system. The most prominent among these measures is the alpha activityof the EEG (8-12 Hz, 30-80 pV) which can be measured over the visual centers of the brain(occipital-parietal). Alpha activity is characteristically present during cfosure of the eyes.Opening of the eyes usually attenuates alpha activity, which means that its amplitude isgenerally lower than with closed eyes, while sometimes it disappears completely. It has beenproposed, that during (visual) attention open-eye alpha activity is even more attenuated but thatduring a state of mental relaxation, physiological inactivity, or during loss of vigilance itsamplitude is somewhat higher (e.g. Walter [1959]; Morel1 (19661; Matousek et al. [1969];Editorial [1971]; Shagass [19723;Gale et al. [1971]; Mackie [19773).In the folIowing experiments it will be investigated whether the type of ocuIomotor controlhas any relation to the amount of alpha activity in the EEG.

Method. During eye-tracking performance in the UC it is not possible that oculomotoractivity is monitored by other than retinal information as long as performance is on-target. Onthe other hand during on-target performance in the PC some degree of automatism is likely todevelop in oculomotor control. Thus oculomotor control during on-target performance in theUC wit1be more attentive, while in the PC it may become more intentive. In this experiment wetherefore compared the amount of alpha activity between the UC and the PC during periods ofon-target eye-tracking performance.The influence of feedback about eye-tracking performance was also measured in thisexperiment. It was reasoned that when Ss are alerted to off-target performance (by an increaseof target brightness) the detection of tracking errors would be facilitated. Thus the differencebetween the UC and the PC (in terms of attentive versus intentive ocuIomotor control) might beenhanced.


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122 A. H. WERTHEIMTwenty Ss were assigned to two experimental groups of equal size. Each group receivedtwo experimental runs, one with and one without feedback, in different order of presentation.The target was a bright spot moving over an oscilloscope screen, 30 cm in front of the eyes. Anexperimental run consisted of twenty alternating one minute PCs and UCs (trials). In between

runs a rest period was allowed. This procedure was chosen to ensure that any changes in alphaactivity which might result from the length of the experimental sessions (as a resutt of fatigueswoutd affect the PC and the UC equally, thus making comparisons between them independentof such gradual baseline changes.Eye movements were measured, as was the amount of alpha activity in the EEG duringon-target eye-tracking performance. In each trial mean alpha was defined as the amount ofalpha activity divided by the corresponding time on-target.Jesuits. Figure 8 illustrates that there was a (statistically significant, p < 0.01) difference in

the amount of alpha activity during on-target performance between the PC and the UC.Although there was a slight effect of feedback in the expected direction (the difference beinglarger with than without feedback), this was statistically not significant. Most alpha activity wasobserved in the PC. 50:

Fig. 8. Occipital alpha energy in the EEG as a function of eye-tracking condition during on-targeteye-tracking performance (Exp. VI). c----O, Predictable eye-tracking condition; O--O, unpredict-able eye-tracking condition.Discussion. The data from this experiment indicate that the amount of alpha activity in theEEG is related to the type of oculomotor control: during intentive oculomotor control morealpha activity is present than during attentive oculomotor control. If alpha activity is related toa state of mental relaxation then obviously intentive oculomotor control implies a more relaxedstate of mind than attentive oculomotor control. This is a very important suggestion because itmeans that our general level of alertness may depend on the way we move our eyes.

2.5.2 Experiment VIIinr~o~uctio~. A further test on whether attentive oculomotor control is associated with an

attenuation of alpha activity, can be provided when off-target alpha is compared to on-targetalpha. The point is that during the correction of eye-tracking errors retinal information has to beused. Thus, when Ss are made aware of eye-tracking errors and are instructed to immediatelyreduce them, the attentive component in oculomotor control is enhanced when corrections aremade. This implies more attentive oculomotor control during off-target as compared toon-target performance, especially in the PC (in the UC during on-target performance oculomo-tor control is already largely attentive). The following experiment was carried out in order toverify whether the amount of alpha activity also decreases during off-target as compared toon-target eye-tracking performance.Method. Eight Ss participated. They received only one run of twenty trials and were given


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Explaining highway hypnosis: experimental evidence for the role of eye movements 123feedback about their performance on all trials. Apart from the fact that alpha activity was alsomeasured during off-target performance the rest of the procedure was the same as in the formerexperiment.

Results. As illustrated by Fig. 9, mean amount of alpha activity was lower duringoff-target as compared to on-target tracking (statistically significant at p < 0.02). Again it wasfound that during on-target tracking the amount of alpha activity in the PC was higher than inthe UC.

Fig. 9. Occipital alpha energy in the EEG as a function of eye-tracking performance (Exp. VII).O--_-O, Predictable eye-tracking conditron on-target; t--O, predictable eye-tracking condttionoff-target; O---Z, unpredictable eye-tracking condition on-target; O---O, unpredictable eye-trackingcondition off-target.

Discussion. The data from the last two experiments indicate that during intentive oculomo-tar control more alpha activity is present in the EEG than when oculomotor control is attentive.It is however possible that our method of measurement has influenced the findings. Feedbackconsisted of a change in brightness of the target. It was effected by a small noisy electromotorattached to the brightness control of the oscilloscope. This auditory stimulation may haveattenuated alpha activity. In addition increased brightness of the target-during off-targetperformance-could have attenuated alpha activity too [Barry and Beh, 19721. The effort tomaintain eye accomodation during the whole period of the experimental task may also haveattenuated alpha activity [Mulholland and Peper, 19711.This is most likely to have occurred inthe UC; it is less imperative to focus on the target all the time when, as in the PC, the eyesmove more or less automatically.For these reasons it was necessary to repeat the latter experiment to see whether the samedata can be observed using a method without these methodological difficulties.2.5.3 Experiment VIIIMethod. In order to replicate experiment VII without its methodological flaws the mirrorsystem was used again. Thus the distance between the target and the eyes was increased to200 cm, which implies no serious accomodative effort of the eyes when focussing on the target.Error feedback was provided by reducing the brightness of the lamp in the mirror system.which itself was placed outside the experimental cabin. If the small amount of alpha activityduring off-target performance was due to the increase of target brightness in experiment VII,now the results should be reversed.t The experimental procedure was the same as inexperiment VII, apart from the fact that ten Ss participated.

tBrightness fluctuations as such, which occur during error performance, could also account for attenuation of alphaactivity. However there is no way to check this possibility. In the PC errors may be due to some degree of variability in themtemai motor program which provides the input for intentive oculomotor activity. If so, errors may well appear anddisappear during continuous intentive oculomotor control. Thus we can only insure attentive oculomotor control duringerror performance by providing feedback.


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124 A. H. WERTHFIM

150-

Fig IO. Occipital alpha energy in the EEG as a functi on of eye-tracking performance (Exp VIII).V-0. Predictable eye-tracking condit ion on-target; t--O. predictable eye-trackmg condit ionoff-target: O---i), unpredictable eye-tracking conditi on on-target; O---O, unpredictable eye-tracking

condition off-target.

Results. As shown in Fig. 10, the data from this experiment are very much in line with thoseof the former two experiments. Alpha activity during on-target performance is less attenuatedthan alpha activity during off-target performance. This difference is larger in the PC than in theUC. These findings were statistically significant @ < 0.01) and thus indicate that the resultsfrom the prior two experiments cannot be considered artefactual.Discussion. The results from the three experiments on alpha activity and oculomotor controlhave a rather important bearing on our understanding of the concept of highway hypnosis.When intentive oculomotor control increases, the amount of alpha activity in the EEG alsoincreases. This may indicate that the general level of alertness is also reduced. If the processcontinues long enough, a driver might thus slowly become drowsy and even doze off into shortlapses of a sleeplike state. This explains why it is difficult to fight fits of drowsiness whenthey occur. The point is, that we apparently influence our level of alertness at least to someextent by the way we move our eyes, and in an involuntary manner.

There is one curious aspect in our data so far: the data on alpha activity indicate that duringintentive oculomotor control the general level of alertness decreases, but in some of theexperiments mentioned earlier it was shown that, at the same time, reactions to visual signalsbecome faster. It is possible that when Ss have to perform the reaction time task, they arealerted to such an extent that the difference in alpha activity between the PC and the UC iseliminated. In other words, it is possible that intentive oculomotor control is only associatedwith a relaxed state of mind when no additional task requirements exist. The followingexperiment was designed to investigate into this question.

2.5.4 Experiment IXIntroduction. It is possible that alertness, as measured by alpha energy, does not interfere

with our ability to react quickly to visual signals. However it is very difficult to validate thissuggestion because from the psychophysiological literature it has become clear that phy-siological measures of states of awareness or arousal do not correlate with performancemeasures. The current view is that the behaviorally and physiologically defined concepts ofalertness attention and arousal are not entirely congruent (see for some discussions Posner[1975]. Kahneman 119731, Gale [1977]). Most investigators have failed to observe a clearrelationship between alpha and RT. This does not necessarily invalidate alpha as a measure ofalertness [Gale, 19771. A few studies have shown small but significant correlations between RTand measures of alpha activity other than energy (see for some discussions Surwillo [1969,19751, Wertheim [1974], Gaillard [1977], Gale [1977]).

We have shown in different experiments that the oculomotor control system influences both


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Explaining highway hypnosis: experimental evidence for the role of eye movements 125alpha energy in the EEG and RT to visual signals. Thus replicating these findings in one experiment,in which both alpha energy and RT are measured concurrently, may help clarify the existingtheoretical confusion.There are three possible outcomes for such an experiment. First, oculomotor control mayprove to have an independent influence on alpha and RT. In this case we should observe acorrelation between alpha and RT between the PC and the UC but not within each of theconditions. Secondly, the oculomotor control system may be related to one system of alertnesswhich relates to alpha energy and also influences RT. If so we should observe a correlationbetween alpha energy and RT both within and between the two tracking conditions. Finally, theadditional effort required to perform the RT task may alert the Ss to such an extent that thedifferences in alpha energy are eliminated. In that case we should observe a replication of theRT differences between the PC and the UC but no evidence of differences in alpha energy (ifany alpha at all is to be observed).

Method. Since the additional effort required by the RT task is likely to reduce somewhat thegeneral amount of alpha activity in the EEG, little alpha activity was expected and probablyonly during on-target performance. Therefore alpha activity had to be measured with greaterprecision than before. In addition, alpha, measured during on-target performance, cannot bemeaningfully related to RTs following signals presented during off-target performance. Thussignals were presented only during on-target eye-tracking performance.Stimuli consisted of black asterisks, presented in the moving target at variable intervalsbetween 4 and 6 set, but only when performance was on-target. Care was taken to prevent toolarge a difference between the number of signals presented during the PC as compared to theUC, since in the PC much more time was spent on-target. Therefore in the PC intersignalintervals were slightly longer than in the UC. This method provided roughly equal numbers ofRTs in both conditions (about 200 during each experiment). An experiment consisted of twentyalternating PC and UC trials of 1.5 min duration, separated by 45 set pauses.In order not to contaminate the EEG trace with motor artefacts associated with the manualreaction, no alpha was measured from the moment of signal presentation to one second afterthe manual response. In order to determine whether alpha activity was present or not at amoment of signal presentation, an alpha amplitude filter was introduced, which filtered out allEEG activity-leaving the alpha band pass filter-of amplitudes under 20% of closed-eye alphaactivity (determined beforehand). By connecting a relay to the output of this amplitude filter, itwas possible for the computer to label RTs according to whether alpha activity was present orabsent at the moment of signal presentation.Since only very little alpha activity was expected, the EEG was amplified rather strongly.However, band pass filters do not actually filter out irrelevant frequencies but depress theiramplitudes. Strong amplification of the EEG thus increases the risk that irrelevant frequencieswill pass the band pass filter. For this reason visual inspection of the unfiltered EEG trace wasadded to the hard-ware identification of alpha activity. After each experiment a number of staffmembers of the laboratory judged whether alpha activity, present in the polygraph tracing ofthe filtered EEG, was parallelled by visually recognizable alpha activity in the unfiltered EEGtracing. Thus two groups, of five Ss each, were obtained. Characteristic sample tracings of bothgroups are provided in Figs. 11 and 12. Ss in group I showed clear evidence of alpha activity;those in group II showed little or no alpha activity. It was reasoned that any relationshipbetween oculomotor control and alpha activity and between oculomotor control and RT, andany possible relation between alpha activity and RT should be most prominent in group I.

Results. AS illustrated by Fig. 13, in group I alpha activity measured during on-targetperformance was much more present during the PC than during the UC. In group II thisdifference was more or less absent. We can thus conclude that the additional visual inspectionof the EEG traces has added to the identification of alpha activity. The effects were statisticallysignificant at p < 0.01. As indicated in Fig. 14, RTs (to signals concurrent with alpha activity)were shorter in the PC than the UC. Figure 15 shows in addition, that within conditions there isno difference between RTs following signals associated with alpha activity and those followingsignals presented in the absence of alpha activity (the differences which appear in Fig. 15 in thegraph of group II are statistically not significant).Discussion. The data from this experiment are rather convincing replications of those fromthe other experiments in which EEG and RT measurements were taken independently. Once


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126 A. H. WERTHEIM

A A

F F~Fig. 11. Fig.12

Fig. I I. Sample tracing of a subject from group I (much alpha). A, Unfiltered EEG; B, alpha activityfiltered from the EEG; C, stimulus marker (downward deflection= stimulus presentation; upwarddeflection = response): D, horizontal eye-movements; E, tracking error; F, time marker (Sec.).Fig. 12. Sample tracing of a subject from group II (little alpha). For interpretation of the tracings seeFig.11.

Fig. 13. Occipital alpha energy in the EEG during on-target eye-tracking performance, as a function ofeye-tracking condition (Exp. IX). o----O, Predictable eye-tracking condition Group I; O---O,unpredictable eye-tracking condition Group I; O--O, predictable eye-tracking condition Group II.O---O, unpredictable eye-tracking condition Group II.again it was shown that intentive oculomotor activity is associated with more alpha activitythan attentive oculomotor control. In addition reaction times are shown again to be shortestduring intentive oculomotor control. However, since no evidence of any relation between theabsence or presence of alpha activity and RT was observed within conditions, we feel justifiedto conclude that the type of oculomotor control influences alpha activity and RT independently.This finding is important to our understanding of the phenomenon of highway hypnosis:intentive oculomotor control may reduce our general level of alertness but enhances at thesame time our capacity to react swiftly to visual signals. This paradoxical finding explains whyintrospectively we may have great difficulties in detecting our own state of drowsiness whendriving on highways.

3. CONCLUSIONS: A NEW LOOK AT HIGH WAY HYPN OSISTh e experimental findings from the research mentioned in this paper justify a new inter-pretation of the phenomenon of highway hypnosis. The distinction between attentive and


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Explaining highway hypnos is: experimental evidence for the role of eye movements 127

LOO l6 -..; 300 -. .\2 . .g0

200;100::I I 1 r300Lk-h--JLC UC group I group IIFig. 14. Fig. 15.

Fig. 14. Occipital alpha energy in the EEG (during on-target eye-tracking performance) and RT (to stimul ipresented when alpha was present m the EEG) as a function of eye-tracking condrt ion. O---O, Group I:

C--O. Group II; PC. predictable eye-tracking condit ion; UC, unpredictable eye-tracking condi tion.Fig. 15. RT to signals presented in the presence or absence of occipi tal alpha in the EEG as a functron ofeye-tracking condi tion. O--O, Alpha present; O--O, no alpha present; PC, predictable eye-tracking

condition; UC, unpredictable eye-tracking condition.

intentive oculomotor control is central to this new understanding. When a certain degree ofautomatism develops in the activity of the eye musculature and oculomotor control becomesintentive, this has implications for several psychological faculties. In other words, a number ofour psychological functions are affected by the way we move our eyes. In any task wherespecific eye movements are required to follow very predictable movements in the visualenvironment, these psychological phenomena are likely to develop during the performance.One such a task is car driving, in which case we have to monitor visually all movements ofother traffic vehicles as well as changes in the road situation itself. The more predictable thesemovements and changes, the more oculomotor control becomes intentive. Thus it is obviousthat on highly predictable stretches of roads and on highways, these dangers exist to a greaterextent than in the characteristically less predictable visual conditions of secondary roads.In our research some psychological functions have been investigated under conditionsfavoring intentive or attentive oculomotor control. It was shown that intentive oculomotorcontrol enables fast reactions but impairs our ability to use warning signals or to detectmovements. In addition it appears that intentive oculomotor control is associated with a certaindegree of mental relaxation or lowered alertness. Taken together, these phenomena draw apicture of what actually constitutes highway hypnosis: a rigid cognitive set develops creating asubjectiv e notion of unchanging visual surroundings. This notion cannot be validated by ourperceptual system, because changes in the movement pattern of the visual field-such as occurduring road curves, slowly developing steering bias or velocity changes of other cars-remainundetected. In addition a general decrease of alertness occurs.

Interestingly, additional support for this view of highway hypnosis becomes from a numberof informal interviews with professional drivers, conducted by the author. It appeared thathighway hypnosis is widely recognized and often accepted as a professional risk. Most of thedrivers said that they fight drowsiness by changing their head position relative to the visualfield. A common method is to look over the rim of their spectacles or to maintain for some timea tilted head position. Such methods fit surprisingly well with our explanation of highwayhypnosis. When head position is changed the movement pattern of the eye musculature mustchange too, and old internal motor programs cannot any more serve as input for the oculomotorneurons. The result is enhancement of the attentive component in oculomotor control. Anothermethod often heard is to increase or to change continuously the velocity of the vehicle. Thismethod also fits our views, because changing or increasing velocity decreases the degree of


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128 A. H. WERTHEIMpredictability of the road : situation, which in turn enhances the attentive component in oculomo-tor control.

The main difference between this understanding of highway hypnosis and the moretraditional views is, that not monotony but predictability of the road: situation is crucial to itsdevelopment. Monotony is a concept which refers to the complexity, to the amount ofinformation present in our surroundings. It refers to the degree of environmental stimulation. Inour view, highway hypnosis is not induced by the degree of monotony but by the degree to whichthose aspects in the visual field, which have to be looked at, move (relative to the observer) in apredictable pattern. A very monotonous road situation does not necessarily imply a verypredictable one, as for example when driving in heavy fog. In that situation it is most unlikelythat highway hypnosis develops.

Our concept of highway hypnosis explains why the general level of fatigue is not necessarilyrelated to it, even though it may sometimes have a facilitatory effect on its development. Thepoint here is that there are some indicationsl that specific brain mechanisms cannot beactivated continuously. After long periods of activity some rest is needed for the recovery ofthe specific mechanisms. This could be the case with the neural mechanisms responsible forrelaying retinal information to the oculomotor neurons in the brain; for attentive oculomotorcontrol. If so, it is likely that the attentive component in oculomotor control cannot bemaintained continuously over long periods of time, especially not when its neurological systemhas been activated for some time prior to the driving situation. This is why fatigue mayfacilitate the development of intentive oculomotor control, and thus of highway hypnosis.

Obviously the human brain is not adequately equipped to meet all the specific deminds ofdriving on highways. Therefore it is difficult to suggest remedies apart from reducing the degreeof predictability of road situations. Such action would however increase other kinds of riskssuch as mechanical break downs. side-slippings, etc.

The only possibility to reduce the dangers of highway hypnosis seems to lie in drivereducation. Drivers should learn to recognize the signs of highway hypnosis, such as misjudge-ment of velocities, crossing of road border-lines, late responses to the brake lights of a car infront, and probably also yawning. Under such circumstances it may be beneficial to rest for ashort period and close the eyes for a while, in order to enable the neuro-logical systemresponsible for attentive oculomotor control to relax and recover. It is a common experience ofprofessional drivers, that a short nap often stops the build up of drowsiness for long periodsafterwards.

Although the research presented in this paper indicates that oculomotor activity is related tosome psychological functions, it is by no means exhaustive. Much more research is needed todetermine what other functions are influenced and why. In addition the interfering effects ofalcohol, drugs and much-used medicines on oculomotor control (if any) should be investigated.Such a research effort is important to our understanding of the risks involved in many tasks inwhich specific eye movements are required. In this context it is an urgent necessity to carryresearch out of the laboratory into more complex and more realistic (simulated) field conditions.Acknomledgements--This paper IS part of a Ph.D. chssertation [Wertheim, 19771, grant for which was provided by thePraeventiefonds. The stimulating criticism of .I. A. Michon. the technical assistance of J. Clots. who designed anddeveloped the apparatus and the enthusiastic assistance of K. Brookhuis and T. Gaasbeek during experimentation, aregratefully acknowledged.

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