‘Icrr~opliii~rrluc0104!. 1975. 14. 693-702. Pcrgamon
Press Printed in Ct. Brain
EYE MOVEMENT RESPONSES OF HEROIN ADDICTS AND CONTROLS DURING WORD AND OBJECT RECOGNITION* R. A. U.S. Army
Human
Engineering
MONTY~
Laboratory R.
University
Aberdeen
Proving
Ground,
Maryland
J. HALL
of Nevada,
Las Vegas
and MARJORIE EC & G, Inc., Special
A. ROSENBERGER Projects
(Accepted
Division
27 January
Las Vegas, Nevada 1975)
Summary-The eye movement responses of heroin addicts and matched controls were examined while they were engaged in word and object recognition tasks. Significant differences between the two groups were found which could be attributed to motivational or interest factors associated with the importance of the materials shown, and to basic differences in the physiological and central nervous system processes that regulate eye movements. Based on these findings together with earlier observations of differences in cutaneous sensitivity between addicts and controls, it was hypothesized that addiction may lead to an altered sensory capacity in the temporal domain which is concerned with gating and subsequent scanning of stimuli. The potential role of educational differences between the two groups was also discussed.
Recently, HALL, ROSENBERGER and MONTY (1974a) presented evidence which suggests that heroin addiction leads to fundamental changes in sensory capacity. Specifically, they found that the time taken to detect the direction of movement of a stylus drawn across the volar surface of the forearm is greater for heroin addicts than for non-addicts. Perception of non-temporal dimensions such as stylus pressures were not affected, suggesting that the effects of heroin addiction are highly specific and alter the central nervous system’s temporal processes which govern and regulate excitability cycles and cortical scanning. There is ample evidence that given the appropriate recording and data reduction techniques, the measurement of eye movements may be another valuable method for investigating certain properties of the central nervous system and changes in mental states (VINE, 1970; HALL and CUSACK, 1972; HUTT and OUNSTED, 1966). Thus, the purpose of the present experiments was to explore the possibility that heroin addiction, in addition to affecting cutaneous sensitivity, also brings about changes in the visual system which will be reflected in eye movements of subjects. The first experiment was designed specifically to elicit visual search, enabling comparisons to be made between the eye movements of heroin addicts and matched controls. To accomplish this, a word recognition test was utilized which required subjects to scan a variety of words to be recognized in a subsequent session. Eye movements were monitored continuously throughout this process. Because eye movements have been shown to reflect emotional states or needs, such as like or dislike (EXLINE,GRAY and SCHUETTE, 1965) personal preferences (STASS and WILLIS, 1967), and the need for social acceptance (SPENCE and FERNBERG,1967) three categories of words were employed (specifically neutral words, drug jargon and dirty words) in the hope that the “emotionally charged” words would reveal further differences between addict and control subjects. * This work was supported in part by the Advanced Research Projects Agency under ARPA Order No. 2128. This paper may be reproduced in full or in part for any purpose of the United States Government. t Request for reprints should be sent to Richard A. Monty, Behavioral Research Directorate. U.S. Army Human Engineering Laboratory, Aberdeen Proving Ground, Maryland 21005. 693
694
R. A. MONEY, R. J. HALLand
MAKJOKIE A. ROSHCB~KGEK
The purpose of the second experiment was to essentially replicate the first experiment, utilizing objects rather than words in order to minimize the operation of factors such as reading skills and acuity. METHODS Experiment
1
Subjects. The paid, volunteer subjects were 23 heroin addicts who were drawn from the addict population in the Las Vegas area and 23 non-addicted students from the University of Nevada, Las Vegas matched for age and IQ. Urine tests, interviews and drug histories were used to determine the subjects drug use and levels of toxication. The results of these tests are presented elsewhere (HALL, ROSENBERGER and MONTY, 1974b). Briefly, it was found that in addition to heroin, all addicted subjects used marijuana and that there was an occasional use of cocaine, amphetamines and barbiturates. All addicted subjects had urine assays which indicated morphine in medium to high concentrations, and an occasional subject had concentrations of morphine plus methadone, although none of the addicts were in the local methadone programme. An attempt was made to obtain addicts who were in a stable or adapted state. The drug histories indicated that all subjects had used heroin within the last 12 hours. Addicts who appeared to be suffering withdrawal symptoms or were heavily sedated were not used as subjects. Thus, the study should not be misconstrued as a study of the effects of heroin per se. Rather it is a study of the complex phenomenon of heroin addiction and all that it entails. Apparatus. The eyes were monitored at the rate of 60 frames per set with an oculometer that did not impose any artificial restraints on the subject (such as a bite board, glasses, or helmet) or interfere with his normal visual behaviour in any way. Further, the subject was not aware that his eyes were being monitored. The subject sat comfortably in an arm chair approximately 1.5 m in front of a polarized rear projection screen and viewed the material presented to him with a Carousel 35 mm slide projector. The camera for recording eye movement was concealed in what appeared to be a speaker box located directly beneath the viewing screen. A full description of the apparatus is presented by LAMBERT,MONTY and HALL (1974). Procedure. When the subject entered the studio, calibration data was obtained by having him view a Troxler calibration slide which projected a dark background with four dots, one in each corner of the viewing screen. This task provided excellent calibration data, since the disappearance and fading of the dots in the peripheral portion of the visual field is a very real phenomenon known as the Troxler effect (TROXLER, 1804). The subject was instructed to stare at one particular coloured dot and then to report the disappearance or change in any of the other three. This procedure was repeated for each of the four dots in the upper left, upper right, lower left and lower right corners of the screen. The word recognition test consisted of an observation session and a recognition session. Eye movements were tracked and recorded throughout both sessions. During the observation session, subjects were shown twenty-four slides, each containing various combinations of three word categories: neutral words taken from a common word association list, drug jargon taken from the drug jargon questionnaire developed by EARL and WOLTER (personal communication), and so-called “dirty words” which consisted of words having double meaning such as screw, etc. Eight of the slides contained three neutral words and one dirty word, eight contained three neutral words and one drug word and the remaining eight contained four neutral words. When projected, one of the four words appeared in the approximate centre of each of the four quadrants of a 1 m x 0.75 m viewing surface. The words themselves subtended an angle of approximately 1” x 3” at the eye. Each slide was displayed for 8 set and the interval between slides was that required to advance the projector, or approximately 2 sec. The subjects were told that immediately following presentation of the slides they would be tested to see how many words they could remember having seen before.
Eye movements of heroin addicts
695
In the recognition session, 36 two-word slides were presented one at a time in a self-paced fashion. For each two-word slide, the subject was asked to press a response button and to say the word aloud if he had seen one of the words before or to press the button and say “no” if he had not seen either word previously. One word, the “critical” word consisted of either a drug, neutral or dirty word and the other word, a “dummy” word, was drawn from a population of neutral words which the subjects had not seen during the observation session. The position of the critical word (left or right) was determined at random. Of the 36 critical words, six in each category had been shown during the observation session and six had not. The eye movements of the subjects were monitored continuously throughout both the experimenter-paced observation session and the subject-paced recognition session. Experiment
2
Subjects. The subjects were the same as those used in the previous experiment. Apparatus and Procedure. The apparatus and general procedures were the same as
in the previous experiment except that during the observation portion of the experiment, subjects were shown 24 slides at the rate of one every 8 sec. Each slide contained four objects, one centred in each of the four quadrants of the slide. Twenty of the slides contained a drug object such as apparatus for inserting heroin, packages of heroin, tie-off or tournequets for injection, etc., in one of the four quadrants and neutral (nondrug related) objects such as wallets, ashtrays, pencils, eye glasses, etc. in the remaining quadrants. The remaining four slides contained neutral objects only. During the recognition session, drug and neutral items which had either been seen or not seen during the observation session were presented paired with a neutral or dummy object which had not been seen and the subject was asked to report verbally “yes” or “no” and press the response button as soon as he was sure that he had or had not seen the object before. As in the word recognition session, this session was self-paced. As in the previous experiment, the subjects eye movements were monitored throughout both the observation and recognition session.
RESULTS
Experiment Observation
1
session. The mean number of fixations with durations of six frames (100 msec or longer) falling within each quadrant were subjected to an analysis of variance (BUTLER, KAMLET and MONTY, 1969) with groups as a between effect and position of the word on a slide as a within effect. The main effect for groups, F(1, 44) = 8G31, P < 0001, and position, F(3, 132) = 48.41, P < OWl, both reached significance while the interaction failed to reach significance at the 0.05 level of confidence. The data underlying these effects are shown in Figure 1. It can be seen that the number of fixations are somewhat higher for the upper right and upper left than for those at the lower left and lower right positions for both groups, and that controls had consistently more fixations than addicts. These differences may reflect position preferences of the subjects or possible differences in system accuracy at various portions of the field. However, they do not affect the groups differentially. The average duration of these fixations was subjected to an identical analysis of variance and again groups, F( 1, 44) = 9.30, P < 0.01, and position, F(3, 132) = 5.62, P -=c0.01, both reached significance while their interaction did not. It can be seen by comparing Figure 1A and 1B that while the number of fixations per position was greater for the controls than for the addicts, the average duration of a fixation was greater for the addicts than for controls. In other words, during the observation session, both groups were attending to the words, but the control subjects tended to scan more frequently between words. To determine if there were any differences between addicted and non-addicted subjects in terms of their reaction to emotionally loaded words (i.e., drug and dirty words)
R. A. MONTY, R. J. HALL and
696
MARJORIE A. ROSENBERGER
CONTROLS ADDICTS
t
z Yi
I
I
UPPER LEFT POSITION
8 5 & c-
80
number
LOWER LEFT
LOWER RIGHT
OF WORD ON 4 WORD
CONTROLS
POSITION
1. Mean
,
I
UPPER RIGHT
SLIDE
-(B)
70-
UPPER LEFT
Fig.
-
of fixations
UPPER RIGHT
w
LOWER LEFT
LOWER RIGHT
OF WORD ON 4 WORD
and average duration session of Experiment
SLIDE
of fixations I.
during
the observation
each subject’s fixation duration for either a drug or dirty word was subtracted from his own average fixation duration for neutral words occurring in the same position. This was done to avoid confounding with possible subject preferences or apparatus artifacts associated with position. These scores were subjected to an analysis of variance with groups as a between effect, and words (drugs versus dirty) and position as within effects (BUTLER et al., 1969). The main effect for groups reached significance, F(1, 44) = 16.79, P < 0.001, indicating that the addicts’ fixations for the emotionally loaded words relative to neutral words were substantially longer than those of the controls, while the difference between drug and dirty words and the interaction of words with groups, both failed to reach significance at the 0.05 level of confidence. Each of these effects is shown in Figure 2. The only other effects to reach significance were position, F(3, 132) = 3.81, P < 0.05, and the word x position interaction, F(3, 132) = 3.82, P < 0.05. The data underlying the interaction indicated that in general, the average duration of fixation for both dirty and drug words relative to neutral words increased with movement from upper left to upper right to lower left, but then in the lower right position decreased substantially for the dirty words, so that the average duration of fixation on dirty words was actually shorter than for the neutral words. This effect ‘is of relatively little interest because of its failure to interact with groups. In short, during the observation session during which subjects were engaged in a form of visual search, while the mean number of fixations of the control subjects tended to be larger than that of the addicts, the average duration of fixations was larger for addicts than for controls. Further, addicts spent substantially more time looking at drug and dirty words relative to neutral words than did controls. However, differences between addicts and controls exist even for the neutral word category. t(44) = 1.8, P < 0.05 (mean duration of fixations per trial of 38.52 versus 33.13 frames, respectively).
Eye movements
of heroin
addicts
697
5
z
MEAN
CONTROLS
-
ADDICTS
D
FIXATION
u
FOR
NE!LR4L_WO!RZ-
TYPE
Fig. 2. Average
_
OF
WORD
duration of fixations in frames on drug words and dirty words neutral items during the observation session of Experiment 1.
relative
to
This suggests that there may be two types of differences between addicts and controls: differences produced by the motivational aspect of the drug and dirty words, and differences caused by some more basic phenomenon stemming from the rate at which information is processed. Although acuity and IQ tests did not indicate what these factors might be, it is possible that some basic differences in sensory processes or reading skills could account for the differences in fixation durations on neutral words. Recognition session. Although of secondary interest, the recognition scores were examined in the following manner. The number of recognition errors, defined as the number of critical words either incorrectly identified or not identified at all, were subjected to an analysis of variance with groups as a between effect and type of word and familiarity (i.e., whether a word had been seen (old) or not seen (new) during the observation session) as within effects. The main effect for type of word reached significance, F(2, 88) = 42.10, P < 0.001, as did the groups x words interaction, F(2, 88) = 3.36, P < 0.05. The data underlying this interaction are shown in Table 1. It can be seen that while both groups made the same number of errors with the dirty words, the addicts made more errors than the controls with the neutral words, and the controls made more errors with the drug jargon than the addicts. This last observation is not surprising since much of the drug jargon was probably unknown to many of the controls. The familiarity x groups interaction reached significance, F(1, 44) = 4.46, P < 0.05, indicating that while the control group made more errors for the old than for the new words (average number of errors of 1.64 versus 1.03 for old and new, respectively) the addicts’ error distribution was exactly the opposite (average number of errors of 1.14 versus 1.74 for old and new, respectively). In other words, the control subjects were not recognizing old words while the addicts were not catching new words. Finally, the words x familiarity interaction, F(2, 88) = 10.90, P < OG)l, also reached significance, indicating Table
Word Recognition (Expt. 1) Controls Addicts Object Recognition (Expt. 2) Controls Addicts
1. Average
number
of recognition
errors
in both experiments
Neutral
Drug
Dirty
X
2.20 2.94
1.17 0.76
0.63 0.63
1-33 144
1.13 1.15
2.39 1.83
Test
Test 1.76 1.49
698
R. A. MONTY, R. J. HALL and MARJORIEA. ROSENBERGER
that for the old words the average number of errors were 1.89, 1.37 and 0.91 for the neutral, drug and dirty words, respectively, while for the new words the corresponding errors were 3.24, 056 and 0.35. In short, all subjects performed better with the so called emotionally loaded words than with the neutral words. They were especially adept at detecting new emotional words and were especially poor in failing to detect old neutral words. Taken together, these results would seem to argue in support of the contention that the drug and dirty words are in fact different from the neutral words. Further, the failure of the main effect for groups to reach significance is especially important because it indicates that both groups were performing at about the same level (average number of errors of 1.33 for the controls versus 1.44 for the addicts) implying that the groups were equally motivated but that there may be differences in where they directed their attention. Finally, an analysis of the times taken to view each two word slide during the recognition session also failed to show any significant difference between groups. Eye movement data. The mean number of fixations were subjected to an analysis of variance (BUTLER et al., 1969) with groups as a main effect and type of words (neutral, drug or dirty) and familiarity (old versus new) as within effects. Only the critical words were utilized because the eye movement data surrounding the dummy words was not analyzed. It is important to remember that during the recognition session, observation was self-paced. Only the words x familiarity interaction, F(2, 88) = 3.62, P < 0.05, reached significance. The data underlying this interaction simply indicated that while there was little difference in the mean number of fixations on new versus old neutral or drug words there were less fixations on old dirty words than any other category and more fixations on new dirty words than any other category. In short, there was no one-to-one correspondence between recognition scores and the average number of fixations per word, An identical analysis was applied to the average duration of fixations. The main effect for groups reached significance, F(1, 44) = 4.66, P < 0.05, indicating that the average fixation for controls was only 37.87 frames while that for addicts was 48.02 frames. Similarly, the main effect for familiarity was significant, F(1, 44) = 12.86, P < 0.01, indicating that the mean duration of fixation for old words was greater than for new words. Finally, the words x familiarity interaction also reached significance, F(2, 88) = 403, P < 0.05. The data underlying this interaction indicated that while the mean duration of fixations on the neutral words was nearly identical irrespective of whether they were old or new, it increased substantially for the old drug and dirty words and decreased slightly for the new drug words. In other words, all subjects tended to look longer at old drug or dirty words, but there were no differences with respect to neutral words. This behaviour may reflect the differences in attitude toward the various categories of words noted in the recognition data. Experiment 2 Observation session. The mean number of fixations falling in each quadrant were subjected to an analysis of variance with groups as a between effect and position of the word on a slide as a within effect. The main effects for groups, F(1, 44) = 6.21, P < 0.05, and position, F(3, 132) = 59.66, P < OXrOl,both reached significance indicating that the control subjects made significantly more fixations than the addicts (mean number of fixations of 1.70 versus 1.49, respectively) and there were more fixations on the upper quadrants than the lower quadrants (mean number of fixations of 2.00, 1.92, 1.21 and 1.25, reading from left to right). The groups x position interaction failed to reach significance at the 0.05 level of confidence. In short, these results are identical to those found in the word recognition test. The average duration of these fixations was subjected to an identical analysis of variance. In contrast to Experiment 1, only the main effect for position reached significance, F(3, 132) = 16.49, P < OQOl. The data underlying this effect indicated that the average duration of fixations (measured in number of frames) revealed the same pattern
Eye movements
of heroin
addicts
699
as the number of fixations (average duration of fixations in frames of 63.69, 57.88, 4444 and 48.89, respectively). Finally, to determine if there were any differences between addicted and non-addicted subjects in terms of their reactions to neutral and drug objects, each subject’s fixation duration for a drug object was subtracted from his own average fixation duration for neutral objects. These scores were subjected to an analysis of variance with groups as a between effect and position as a within effect. Again, both the main effects for groups, F(1, 44) = 12.81, P < 0.01, and position. F(3. 132) = 5.75, P < 0.01, reached significance, indicating that while both groups spent more time viewing drug objects than neutral objects, looking time for these items was substantially longer for the addicts than for the controls (average number of frames longer than for neutral items of 30.04 versus 15.35, respectively). With respect to position, the average duration of a fixation on the drug objects relative to the neutral objects increased with movement from left to right and then decreased in position four (13.24, 2.533, 32.70, 17.52, respectively) in a manner similar to that noted in Experiment 1. Again this could reflect position preferences, system artifact, or both. As in Experiment 1, during the observation session, the control subjects made significantly more fixations than did the addicts, while addicts spent substantially more time looking at drug objects relative to neutral objects than did controls. In contrast to Experiment 1, however, there were no differences between groups in the time spent looking at neutral objects, t < 1.0. RecognitioFz session. As in the previous experiment, the recognition errors were subjected to an analysis of variance with groups as a between effect and objects (neutral versus drug) and familiarity (old versus new) as within effects. The main effect for objects reached significance, F(1,44) = 59.68, P < 0.001, as did the objects x groups interaction, F(1, 44) = 5.49, P < 0.05, indicating that in contrast to Experiment 1, both groups made fewer errors in recognizing neutral objects than drug objects (see Table 1). This may simply reflect the complexity of the drug objects relative to the neutral objects. Further, the control subjects made substantially more errors on the drug objects than did the addicts. The data underlying the familiarity x objects interaction, F( 1, 44) = 71.39, P < OGll, indicated that for the old objects there were 1.91 errors on the neutral objects versus 1.48 errors on the drug objects, while for the new objects there were 0.37 errors on the neutral objects and 2.74 errors for the drug objects. In contrast to Experiment 1, there was no interaction of groups with familiarity. The mean number of fixations were subjected to an analysis of variance with groups as a between effect and type of object (neutral versus drugs) and familiarity (old versus new) as within effects. As in the previous experiment, these analyses were performed only on the critical items, i.e., fixations on the dummy objects were not examined. The data underlying the significant main effect for objects, F(1, 44) = 13.77, P < 0401, indicated that there were more fixations on the drug objects than on the neutral objects (1.95 versus 1.78). The significant object x familiarity interaction, F(1, 44) = 9.28, P < 0.01, revealed that while there was very little difference in the average number of fixations on old neutral and drug objects (average number of fixations of 1.84 versus 1.87, respectively), for new objects, the average number of fixations on drug objects increased relative to old objects while the average number of fixations on neutral objects decreased (average number of fixations of 2.03 versus 1.71, respectively). The average duration of fixations were subjected to an identical analysis. The main effect for groups, F(l., 44) = 7.76, P < 0.01, reached significance indicating the average duration of fixations for addicts was somewhat longer than controls. Similarly, the main effect for familiarity, F(1, 44) = 20.31, P < 04lO1, indicated that the average duration of fixations was greater for old than for new objects while the main effect for type of objects, F(1, 44) = 22.46, P < 0001, indicated that fixation durations were generally greater for drug objects than for neutral objects. More important, however, are the familiarity x groups, F(1, 44) = 5.84, P < 0.05, familiarity x objects, F(1, 44) = 12.41, P < 0.001, and familiarity x objects x groups, F(1, 44) = 4.85, P < 0.05, interactions. The data underlying this latter interaction are shown in Figure 3. It can be seen that
700
R. A. MONTY.R. J.
HALL
MARJORIE A. ROSENBERGER
and
’
CONTROL OLD
NEUTRAL TYPE
m
DRUG OF OBJECT
Fig. 3. Average duration of fixations in frames during the recognition session of Experiment 2.
the interaction can be accounted for primarily by the fact that there was little difference in the average duration of fixations for addicts for old drug and neutral objects, while in all other cases drug objects led to more fixations than neutral objects. Stated differently, the average duration of fixations on old objects were substantially longer than all other fixations irrespective of whether the objects were drug or neutral objects. Eye movement data. As in Experiment 1, there were no differences in the number of fixations of the addicts and controls although both groups made more fixations on the drug objects than the neutral objects, especially if they had not been seen before. The average duration of fixations did differ for groups indicating, as in Experiment 1, that addicts fixate longer than controls. In contrast to Experiment 1, however, these differences were differentially affected by neutral and drug items and by familiarity. Specifically, the addicts’ fixation on old items were disproportionately long.
DISCUSSION
Experiment
1
The self-paced recognition session revealed that the variation in word type did in fact elicit differences in recognition scores which may reflect differences in the interest of the subjects in the categories of words shown. However, the eye movement data did not reflect differences between addicts and controls on either this dimension or the familiarity dimension. This may be due to the fact that the situation was self-paced and this enabled the addicts to increase their fixations to a level compatible with the controls. Nevertheless, the average duration of a fixation for addicts was still substantially longer than that for control subjects, suggesting that there is probably some basic difference in the rate at which information is processed, paralleling the changes that were found in cutaneous sensitivity (HALL et al., 1974a). Experimeht
2
While the first experiment suggests that there is some physiological or structural mechanism operating which accounts for differences in the eye movement behaviour of addicts and controls, it is also possible to account for the observed differences by postulating some basic differences in sensory ability or reading behaviour although acuity tests and IQ tests did not indicate that such differences existed. The second experiment was performed to eliminate this possibility. The self-paced recognition session revealed that the variation in object type did in fact elicit differences in recognition scores, just as the words elicited differences in Experiment 1. However, on the word recognition tests both groups made fewer errors with drug items than with neutral items, while on the object recognition test just the reverse
Eye movements of heroin addicts
701
was true. This may indicate that the drug objects were more complex than the neutral objects and somewhat more difficult to recognize. This possibility is also supported by the fact that addicts made somewhat fewer errors on drug objects with which they are more likely to be familiar than did the controls. Gerwral discussiorz
The two experiments revealed some interesting differences between addicts and controls. Specifically, with respect to the observation session, both experiments revealed that while controls tend to make more fixations than addicts, the number of fixations was not differentially affected by the type of material shown. By contrast, in Experiment 1, the average duration of a fixation was longer for addicts than for controls on all categories of words, but especially on the drug and dirty words. In Experiment 2, however, while there were no differences in fixation durations of the two groups on neutral objects, addicts spent substantially more time looking at drug objects relative to neutral objects than did the controls. During the recognition session, the number of fixations of addicts and controls on critical items were nearly identical while the fixation durations of the addicts were longer than those of the controls especially with respect to the old items. Finally, the recognition scores revealed that in both experiments, addicts and controls made about the same number of errors, but they were differentially affected by types of words or objects. Taken together then, these results would seem to indicate that the following factors are operating: motivational or interest factors associated with the nature of the stimulus material, e.g., drug versus neutral items and old versus new items, and possibly differences in reading skill and the ability to manipulate printed material as indicated by the differences noted between words and objects. Nevertheless, the observation that fixation durations are consistently and substantially longer for addicts than for controls suggest that there is a depression in the physiological and central nervous system processes that regulate eye movements. An alternate explanation could be that the observed differences in eye movements are not the result of addiction at all, but rather stem from differences in reading skills and motivational factors attributable to educational background. Specifically, all of the control subjects were college students while only 24% of the addicts had college backgrounds. This possibility is seriously weakened by the findings of FISHER (1976) which demonstrate that as reading experience increases both the average number of fixations and the duration of those fixations steadily decrease. In the present study, while the fixation durations of addicts were longer than that of controls, fewer fixations were made. Nevertheless, in subsequent studies it would be of interest to covary level and age. In conclusion, it is interesting to note that during the standard acuity check prior to experimentation, many addicts volunteered a variety of different complaints about their vision even though they tested 20-20. When examining the results of the difference in eye movement between addicts and controls, it would seem well to be aware of the fact that we may be dealing with changes in the central nervous system which are specific and long-lasting but not easily observable as a general effect on complex tasks. When the results of the visual data taken here and the cutaneous data of HALL et al. (1974a) are combined, there is reason for speculation that the addicts demonstrate an altered sensory capacity in the temporal domain which is involved with the gating and subsequent scanning of stimuli. Aclirlowledgements-The authors gratefully acknowledge the comments and contributions of DENNISF. FISHER, ROI~ERTKARSHand ROBERTH. LAMBERT.
REFERENCES BUTLER, D. H., KAMLET,A. S. and MONTY,R. A. (1969). A multi-purpose analysis of variance FORTRAN IV computer program. Psychonomic Monograph Supplements, 2: 301-319.
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R. A. MONEY, R. J. HALL and MARJORIE A. ROSENBERGER
EXLINE, R. V., GRAY, D. and SCHUETTE, D. (1965). Visual behavior in a dyad as affected by interview content and sex of respondent. J. Personality Social Psycho!. 1: 201-209. FISHER, D. F. (1976). Spatial factors in reading and search: The case for space. In: Eye Mooements and Psychological Procrsses (MONTY. R. A. and SENDERS, W., Eds.), Lawrence Erlbaum Associates, Hillsdale N.J. (In press). HALL. R. J. and CUSACK. B. L. (1972). The measurement of eye behavior: critical and selected review of voluntary eye movement and blinking. Tech. Memo 1X-72. U.S. Army Human Engineering Laboratory, Aberdeen Proving Ground. MarylandHALL. R. J.. ROSENBERGER.M. A. and MONTY. R. A. (1974a). Cutaneous nercention of heroin addicts: evidence of an altered temporal process. Bull. Psychworn. sot. 3:‘352-354. L I HALL, R. J., ROSENBERGER, M. A. and MONTY, R. A. (1974b). An experimental investigation of the visual behavior of young heroin addicts and matched controls. Jourrlal Supplement Abstract Service 4: 7. HUTT, C. and OWNSED, C. (1966). The biological significance of gaze aversion with particular reference to the syndrome of infantile autism. &haul Sci. 11: 346356. LAMBERT, R. H., MONTY, R. A. and HALL, R. J. (1974). High speed data processing and unobtrusive monitoring of eye movements. B&au. Res. Meth. Instrumentutiorr 6: 525-530. SPENCE.D. P. and FEIIVBEKG,C. (1967). Forms of defensive looking: A naturalistic experiment. J. nera. ment. Dis. 145: 261-271. STASS,J. W. and WILLIS. JR., F. N. (1967). Eye-contact, pupil dilation and personal preference. Psychonorn. Sci. I: 375-376. TROXLER, D. (1804). Uber das Verschwinden gegebener, gegenstande innerhalb unsers gesichtskreises. In: Ophthalmologischr Bibliothrk. Vol. 2. Pt. 2 (HIMLY, K. and SCHMIDT, J. A., Eds.). pp. l-53. VINE, I. (1970) Communication by facial-visual signals. In: Social Behavior in Birds and Mammals: Essays OIZthr Social Ethology of Animals and Man (CROOK, J. H., Ed.), pp. 279-354, Academic Press, New York.
Press Printed in Ct. Brain
EYE MOVEMENT RESPONSES OF HEROIN ADDICTS AND CONTROLS DURING WORD AND OBJECT RECOGNITION* R. A. U.S. Army
Human
Engineering
MONTY~
Laboratory R.
University
Aberdeen
Proving
Ground,
Maryland
J. HALL
of Nevada,
Las Vegas
and MARJORIE EC & G, Inc., Special
A. ROSENBERGER Projects
(Accepted
Division
27 January
Las Vegas, Nevada 1975)
Summary-The eye movement responses of heroin addicts and matched controls were examined while they were engaged in word and object recognition tasks. Significant differences between the two groups were found which could be attributed to motivational or interest factors associated with the importance of the materials shown, and to basic differences in the physiological and central nervous system processes that regulate eye movements. Based on these findings together with earlier observations of differences in cutaneous sensitivity between addicts and controls, it was hypothesized that addiction may lead to an altered sensory capacity in the temporal domain which is concerned with gating and subsequent scanning of stimuli. The potential role of educational differences between the two groups was also discussed.
Recently, HALL, ROSENBERGER and MONTY (1974a) presented evidence which suggests that heroin addiction leads to fundamental changes in sensory capacity. Specifically, they found that the time taken to detect the direction of movement of a stylus drawn across the volar surface of the forearm is greater for heroin addicts than for non-addicts. Perception of non-temporal dimensions such as stylus pressures were not affected, suggesting that the effects of heroin addiction are highly specific and alter the central nervous system’s temporal processes which govern and regulate excitability cycles and cortical scanning. There is ample evidence that given the appropriate recording and data reduction techniques, the measurement of eye movements may be another valuable method for investigating certain properties of the central nervous system and changes in mental states (VINE, 1970; HALL and CUSACK, 1972; HUTT and OUNSTED, 1966). Thus, the purpose of the present experiments was to explore the possibility that heroin addiction, in addition to affecting cutaneous sensitivity, also brings about changes in the visual system which will be reflected in eye movements of subjects. The first experiment was designed specifically to elicit visual search, enabling comparisons to be made between the eye movements of heroin addicts and matched controls. To accomplish this, a word recognition test was utilized which required subjects to scan a variety of words to be recognized in a subsequent session. Eye movements were monitored continuously throughout this process. Because eye movements have been shown to reflect emotional states or needs, such as like or dislike (EXLINE,GRAY and SCHUETTE, 1965) personal preferences (STASS and WILLIS, 1967), and the need for social acceptance (SPENCE and FERNBERG,1967) three categories of words were employed (specifically neutral words, drug jargon and dirty words) in the hope that the “emotionally charged” words would reveal further differences between addict and control subjects. * This work was supported in part by the Advanced Research Projects Agency under ARPA Order No. 2128. This paper may be reproduced in full or in part for any purpose of the United States Government. t Request for reprints should be sent to Richard A. Monty, Behavioral Research Directorate. U.S. Army Human Engineering Laboratory, Aberdeen Proving Ground, Maryland 21005. 693
694
R. A. MONEY, R. J. HALLand
MAKJOKIE A. ROSHCB~KGEK
The purpose of the second experiment was to essentially replicate the first experiment, utilizing objects rather than words in order to minimize the operation of factors such as reading skills and acuity. METHODS Experiment
1
Subjects. The paid, volunteer subjects were 23 heroin addicts who were drawn from the addict population in the Las Vegas area and 23 non-addicted students from the University of Nevada, Las Vegas matched for age and IQ. Urine tests, interviews and drug histories were used to determine the subjects drug use and levels of toxication. The results of these tests are presented elsewhere (HALL, ROSENBERGER and MONTY, 1974b). Briefly, it was found that in addition to heroin, all addicted subjects used marijuana and that there was an occasional use of cocaine, amphetamines and barbiturates. All addicted subjects had urine assays which indicated morphine in medium to high concentrations, and an occasional subject had concentrations of morphine plus methadone, although none of the addicts were in the local methadone programme. An attempt was made to obtain addicts who were in a stable or adapted state. The drug histories indicated that all subjects had used heroin within the last 12 hours. Addicts who appeared to be suffering withdrawal symptoms or were heavily sedated were not used as subjects. Thus, the study should not be misconstrued as a study of the effects of heroin per se. Rather it is a study of the complex phenomenon of heroin addiction and all that it entails. Apparatus. The eyes were monitored at the rate of 60 frames per set with an oculometer that did not impose any artificial restraints on the subject (such as a bite board, glasses, or helmet) or interfere with his normal visual behaviour in any way. Further, the subject was not aware that his eyes were being monitored. The subject sat comfortably in an arm chair approximately 1.5 m in front of a polarized rear projection screen and viewed the material presented to him with a Carousel 35 mm slide projector. The camera for recording eye movement was concealed in what appeared to be a speaker box located directly beneath the viewing screen. A full description of the apparatus is presented by LAMBERT,MONTY and HALL (1974). Procedure. When the subject entered the studio, calibration data was obtained by having him view a Troxler calibration slide which projected a dark background with four dots, one in each corner of the viewing screen. This task provided excellent calibration data, since the disappearance and fading of the dots in the peripheral portion of the visual field is a very real phenomenon known as the Troxler effect (TROXLER, 1804). The subject was instructed to stare at one particular coloured dot and then to report the disappearance or change in any of the other three. This procedure was repeated for each of the four dots in the upper left, upper right, lower left and lower right corners of the screen. The word recognition test consisted of an observation session and a recognition session. Eye movements were tracked and recorded throughout both sessions. During the observation session, subjects were shown twenty-four slides, each containing various combinations of three word categories: neutral words taken from a common word association list, drug jargon taken from the drug jargon questionnaire developed by EARL and WOLTER (personal communication), and so-called “dirty words” which consisted of words having double meaning such as screw, etc. Eight of the slides contained three neutral words and one dirty word, eight contained three neutral words and one drug word and the remaining eight contained four neutral words. When projected, one of the four words appeared in the approximate centre of each of the four quadrants of a 1 m x 0.75 m viewing surface. The words themselves subtended an angle of approximately 1” x 3” at the eye. Each slide was displayed for 8 set and the interval between slides was that required to advance the projector, or approximately 2 sec. The subjects were told that immediately following presentation of the slides they would be tested to see how many words they could remember having seen before.
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In the recognition session, 36 two-word slides were presented one at a time in a self-paced fashion. For each two-word slide, the subject was asked to press a response button and to say the word aloud if he had seen one of the words before or to press the button and say “no” if he had not seen either word previously. One word, the “critical” word consisted of either a drug, neutral or dirty word and the other word, a “dummy” word, was drawn from a population of neutral words which the subjects had not seen during the observation session. The position of the critical word (left or right) was determined at random. Of the 36 critical words, six in each category had been shown during the observation session and six had not. The eye movements of the subjects were monitored continuously throughout both the experimenter-paced observation session and the subject-paced recognition session. Experiment
2
Subjects. The subjects were the same as those used in the previous experiment. Apparatus and Procedure. The apparatus and general procedures were the same as
in the previous experiment except that during the observation portion of the experiment, subjects were shown 24 slides at the rate of one every 8 sec. Each slide contained four objects, one centred in each of the four quadrants of the slide. Twenty of the slides contained a drug object such as apparatus for inserting heroin, packages of heroin, tie-off or tournequets for injection, etc., in one of the four quadrants and neutral (nondrug related) objects such as wallets, ashtrays, pencils, eye glasses, etc. in the remaining quadrants. The remaining four slides contained neutral objects only. During the recognition session, drug and neutral items which had either been seen or not seen during the observation session were presented paired with a neutral or dummy object which had not been seen and the subject was asked to report verbally “yes” or “no” and press the response button as soon as he was sure that he had or had not seen the object before. As in the word recognition session, this session was self-paced. As in the previous experiment, the subjects eye movements were monitored throughout both the observation and recognition session.
RESULTS
Experiment Observation
1
session. The mean number of fixations with durations of six frames (100 msec or longer) falling within each quadrant were subjected to an analysis of variance (BUTLER, KAMLET and MONTY, 1969) with groups as a between effect and position of the word on a slide as a within effect. The main effect for groups, F(1, 44) = 8G31, P < 0001, and position, F(3, 132) = 48.41, P < OWl, both reached significance while the interaction failed to reach significance at the 0.05 level of confidence. The data underlying these effects are shown in Figure 1. It can be seen that the number of fixations are somewhat higher for the upper right and upper left than for those at the lower left and lower right positions for both groups, and that controls had consistently more fixations than addicts. These differences may reflect position preferences of the subjects or possible differences in system accuracy at various portions of the field. However, they do not affect the groups differentially. The average duration of these fixations was subjected to an identical analysis of variance and again groups, F( 1, 44) = 9.30, P < 0.01, and position, F(3, 132) = 5.62, P -=c0.01, both reached significance while their interaction did not. It can be seen by comparing Figure 1A and 1B that while the number of fixations per position was greater for the controls than for the addicts, the average duration of a fixation was greater for the addicts than for controls. In other words, during the observation session, both groups were attending to the words, but the control subjects tended to scan more frequently between words. To determine if there were any differences between addicted and non-addicted subjects in terms of their reaction to emotionally loaded words (i.e., drug and dirty words)
R. A. MONTY, R. J. HALL and
696
MARJORIE A. ROSENBERGER
CONTROLS ADDICTS
t
z Yi
I
I
UPPER LEFT POSITION
8 5 & c-
80
number
LOWER LEFT
LOWER RIGHT
OF WORD ON 4 WORD
CONTROLS
POSITION
1. Mean
,
I
UPPER RIGHT
SLIDE
-(B)
70-
UPPER LEFT
Fig.
-
of fixations
UPPER RIGHT
w
LOWER LEFT
LOWER RIGHT
OF WORD ON 4 WORD
and average duration session of Experiment
SLIDE
of fixations I.
during
the observation
each subject’s fixation duration for either a drug or dirty word was subtracted from his own average fixation duration for neutral words occurring in the same position. This was done to avoid confounding with possible subject preferences or apparatus artifacts associated with position. These scores were subjected to an analysis of variance with groups as a between effect, and words (drugs versus dirty) and position as within effects (BUTLER et al., 1969). The main effect for groups reached significance, F(1, 44) = 16.79, P < 0.001, indicating that the addicts’ fixations for the emotionally loaded words relative to neutral words were substantially longer than those of the controls, while the difference between drug and dirty words and the interaction of words with groups, both failed to reach significance at the 0.05 level of confidence. Each of these effects is shown in Figure 2. The only other effects to reach significance were position, F(3, 132) = 3.81, P < 0.05, and the word x position interaction, F(3, 132) = 3.82, P < 0.05. The data underlying the interaction indicated that in general, the average duration of fixation for both dirty and drug words relative to neutral words increased with movement from upper left to upper right to lower left, but then in the lower right position decreased substantially for the dirty words, so that the average duration of fixation on dirty words was actually shorter than for the neutral words. This effect ‘is of relatively little interest because of its failure to interact with groups. In short, during the observation session during which subjects were engaged in a form of visual search, while the mean number of fixations of the control subjects tended to be larger than that of the addicts, the average duration of fixations was larger for addicts than for controls. Further, addicts spent substantially more time looking at drug and dirty words relative to neutral words than did controls. However, differences between addicts and controls exist even for the neutral word category. t(44) = 1.8, P < 0.05 (mean duration of fixations per trial of 38.52 versus 33.13 frames, respectively).
Eye movements
of heroin
addicts
697
5
z
MEAN
CONTROLS
-
ADDICTS
D
FIXATION
u
FOR
NE!LR4L_WO!RZ-
TYPE
Fig. 2. Average
_
OF
WORD
duration of fixations in frames on drug words and dirty words neutral items during the observation session of Experiment 1.
relative
to
This suggests that there may be two types of differences between addicts and controls: differences produced by the motivational aspect of the drug and dirty words, and differences caused by some more basic phenomenon stemming from the rate at which information is processed. Although acuity and IQ tests did not indicate what these factors might be, it is possible that some basic differences in sensory processes or reading skills could account for the differences in fixation durations on neutral words. Recognition session. Although of secondary interest, the recognition scores were examined in the following manner. The number of recognition errors, defined as the number of critical words either incorrectly identified or not identified at all, were subjected to an analysis of variance with groups as a between effect and type of word and familiarity (i.e., whether a word had been seen (old) or not seen (new) during the observation session) as within effects. The main effect for type of word reached significance, F(2, 88) = 42.10, P < 0.001, as did the groups x words interaction, F(2, 88) = 3.36, P < 0.05. The data underlying this interaction are shown in Table 1. It can be seen that while both groups made the same number of errors with the dirty words, the addicts made more errors than the controls with the neutral words, and the controls made more errors with the drug jargon than the addicts. This last observation is not surprising since much of the drug jargon was probably unknown to many of the controls. The familiarity x groups interaction reached significance, F(1, 44) = 4.46, P < 0.05, indicating that while the control group made more errors for the old than for the new words (average number of errors of 1.64 versus 1.03 for old and new, respectively) the addicts’ error distribution was exactly the opposite (average number of errors of 1.14 versus 1.74 for old and new, respectively). In other words, the control subjects were not recognizing old words while the addicts were not catching new words. Finally, the words x familiarity interaction, F(2, 88) = 10.90, P < OG)l, also reached significance, indicating Table
Word Recognition (Expt. 1) Controls Addicts Object Recognition (Expt. 2) Controls Addicts
1. Average
number
of recognition
errors
in both experiments
Neutral
Drug
Dirty
X
2.20 2.94
1.17 0.76
0.63 0.63
1-33 144
1.13 1.15
2.39 1.83
Test
Test 1.76 1.49
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R. A. MONTY, R. J. HALL and MARJORIEA. ROSENBERGER
that for the old words the average number of errors were 1.89, 1.37 and 0.91 for the neutral, drug and dirty words, respectively, while for the new words the corresponding errors were 3.24, 056 and 0.35. In short, all subjects performed better with the so called emotionally loaded words than with the neutral words. They were especially adept at detecting new emotional words and were especially poor in failing to detect old neutral words. Taken together, these results would seem to argue in support of the contention that the drug and dirty words are in fact different from the neutral words. Further, the failure of the main effect for groups to reach significance is especially important because it indicates that both groups were performing at about the same level (average number of errors of 1.33 for the controls versus 1.44 for the addicts) implying that the groups were equally motivated but that there may be differences in where they directed their attention. Finally, an analysis of the times taken to view each two word slide during the recognition session also failed to show any significant difference between groups. Eye movement data. The mean number of fixations were subjected to an analysis of variance (BUTLER et al., 1969) with groups as a main effect and type of words (neutral, drug or dirty) and familiarity (old versus new) as within effects. Only the critical words were utilized because the eye movement data surrounding the dummy words was not analyzed. It is important to remember that during the recognition session, observation was self-paced. Only the words x familiarity interaction, F(2, 88) = 3.62, P < 0.05, reached significance. The data underlying this interaction simply indicated that while there was little difference in the mean number of fixations on new versus old neutral or drug words there were less fixations on old dirty words than any other category and more fixations on new dirty words than any other category. In short, there was no one-to-one correspondence between recognition scores and the average number of fixations per word, An identical analysis was applied to the average duration of fixations. The main effect for groups reached significance, F(1, 44) = 4.66, P < 0.05, indicating that the average fixation for controls was only 37.87 frames while that for addicts was 48.02 frames. Similarly, the main effect for familiarity was significant, F(1, 44) = 12.86, P < 0.01, indicating that the mean duration of fixation for old words was greater than for new words. Finally, the words x familiarity interaction also reached significance, F(2, 88) = 403, P < 0.05. The data underlying this interaction indicated that while the mean duration of fixations on the neutral words was nearly identical irrespective of whether they were old or new, it increased substantially for the old drug and dirty words and decreased slightly for the new drug words. In other words, all subjects tended to look longer at old drug or dirty words, but there were no differences with respect to neutral words. This behaviour may reflect the differences in attitude toward the various categories of words noted in the recognition data. Experiment 2 Observation session. The mean number of fixations falling in each quadrant were subjected to an analysis of variance with groups as a between effect and position of the word on a slide as a within effect. The main effects for groups, F(1, 44) = 6.21, P < 0.05, and position, F(3, 132) = 59.66, P < OXrOl,both reached significance indicating that the control subjects made significantly more fixations than the addicts (mean number of fixations of 1.70 versus 1.49, respectively) and there were more fixations on the upper quadrants than the lower quadrants (mean number of fixations of 2.00, 1.92, 1.21 and 1.25, reading from left to right). The groups x position interaction failed to reach significance at the 0.05 level of confidence. In short, these results are identical to those found in the word recognition test. The average duration of these fixations was subjected to an identical analysis of variance. In contrast to Experiment 1, only the main effect for position reached significance, F(3, 132) = 16.49, P < OQOl. The data underlying this effect indicated that the average duration of fixations (measured in number of frames) revealed the same pattern
Eye movements
of heroin
addicts
699
as the number of fixations (average duration of fixations in frames of 63.69, 57.88, 4444 and 48.89, respectively). Finally, to determine if there were any differences between addicted and non-addicted subjects in terms of their reactions to neutral and drug objects, each subject’s fixation duration for a drug object was subtracted from his own average fixation duration for neutral objects. These scores were subjected to an analysis of variance with groups as a between effect and position as a within effect. Again, both the main effects for groups, F(1, 44) = 12.81, P < 0.01, and position. F(3. 132) = 5.75, P < 0.01, reached significance, indicating that while both groups spent more time viewing drug objects than neutral objects, looking time for these items was substantially longer for the addicts than for the controls (average number of frames longer than for neutral items of 30.04 versus 15.35, respectively). With respect to position, the average duration of a fixation on the drug objects relative to the neutral objects increased with movement from left to right and then decreased in position four (13.24, 2.533, 32.70, 17.52, respectively) in a manner similar to that noted in Experiment 1. Again this could reflect position preferences, system artifact, or both. As in Experiment 1, during the observation session, the control subjects made significantly more fixations than did the addicts, while addicts spent substantially more time looking at drug objects relative to neutral objects than did controls. In contrast to Experiment 1, however, there were no differences between groups in the time spent looking at neutral objects, t < 1.0. RecognitioFz session. As in the previous experiment, the recognition errors were subjected to an analysis of variance with groups as a between effect and objects (neutral versus drug) and familiarity (old versus new) as within effects. The main effect for objects reached significance, F(1,44) = 59.68, P < 0.001, as did the objects x groups interaction, F(1, 44) = 5.49, P < 0.05, indicating that in contrast to Experiment 1, both groups made fewer errors in recognizing neutral objects than drug objects (see Table 1). This may simply reflect the complexity of the drug objects relative to the neutral objects. Further, the control subjects made substantially more errors on the drug objects than did the addicts. The data underlying the familiarity x objects interaction, F( 1, 44) = 71.39, P < OGll, indicated that for the old objects there were 1.91 errors on the neutral objects versus 1.48 errors on the drug objects, while for the new objects there were 0.37 errors on the neutral objects and 2.74 errors for the drug objects. In contrast to Experiment 1, there was no interaction of groups with familiarity. The mean number of fixations were subjected to an analysis of variance with groups as a between effect and type of object (neutral versus drugs) and familiarity (old versus new) as within effects. As in the previous experiment, these analyses were performed only on the critical items, i.e., fixations on the dummy objects were not examined. The data underlying the significant main effect for objects, F(1, 44) = 13.77, P < 0401, indicated that there were more fixations on the drug objects than on the neutral objects (1.95 versus 1.78). The significant object x familiarity interaction, F(1, 44) = 9.28, P < 0.01, revealed that while there was very little difference in the average number of fixations on old neutral and drug objects (average number of fixations of 1.84 versus 1.87, respectively), for new objects, the average number of fixations on drug objects increased relative to old objects while the average number of fixations on neutral objects decreased (average number of fixations of 2.03 versus 1.71, respectively). The average duration of fixations were subjected to an identical analysis. The main effect for groups, F(l., 44) = 7.76, P < 0.01, reached significance indicating the average duration of fixations for addicts was somewhat longer than controls. Similarly, the main effect for familiarity, F(1, 44) = 20.31, P < 04lO1, indicated that the average duration of fixations was greater for old than for new objects while the main effect for type of objects, F(1, 44) = 22.46, P < 0001, indicated that fixation durations were generally greater for drug objects than for neutral objects. More important, however, are the familiarity x groups, F(1, 44) = 5.84, P < 0.05, familiarity x objects, F(1, 44) = 12.41, P < 0.001, and familiarity x objects x groups, F(1, 44) = 4.85, P < 0.05, interactions. The data underlying this latter interaction are shown in Figure 3. It can be seen that
700
R. A. MONTY.R. J.
HALL
MARJORIE A. ROSENBERGER
and
’
CONTROL OLD
NEUTRAL TYPE
m
DRUG OF OBJECT
Fig. 3. Average duration of fixations in frames during the recognition session of Experiment 2.
the interaction can be accounted for primarily by the fact that there was little difference in the average duration of fixations for addicts for old drug and neutral objects, while in all other cases drug objects led to more fixations than neutral objects. Stated differently, the average duration of fixations on old objects were substantially longer than all other fixations irrespective of whether the objects were drug or neutral objects. Eye movement data. As in Experiment 1, there were no differences in the number of fixations of the addicts and controls although both groups made more fixations on the drug objects than the neutral objects, especially if they had not been seen before. The average duration of fixations did differ for groups indicating, as in Experiment 1, that addicts fixate longer than controls. In contrast to Experiment 1, however, these differences were differentially affected by neutral and drug items and by familiarity. Specifically, the addicts’ fixation on old items were disproportionately long.
DISCUSSION
Experiment
1
The self-paced recognition session revealed that the variation in word type did in fact elicit differences in recognition scores which may reflect differences in the interest of the subjects in the categories of words shown. However, the eye movement data did not reflect differences between addicts and controls on either this dimension or the familiarity dimension. This may be due to the fact that the situation was self-paced and this enabled the addicts to increase their fixations to a level compatible with the controls. Nevertheless, the average duration of a fixation for addicts was still substantially longer than that for control subjects, suggesting that there is probably some basic difference in the rate at which information is processed, paralleling the changes that were found in cutaneous sensitivity (HALL et al., 1974a). Experimeht
2
While the first experiment suggests that there is some physiological or structural mechanism operating which accounts for differences in the eye movement behaviour of addicts and controls, it is also possible to account for the observed differences by postulating some basic differences in sensory ability or reading behaviour although acuity tests and IQ tests did not indicate that such differences existed. The second experiment was performed to eliminate this possibility. The self-paced recognition session revealed that the variation in object type did in fact elicit differences in recognition scores, just as the words elicited differences in Experiment 1. However, on the word recognition tests both groups made fewer errors with drug items than with neutral items, while on the object recognition test just the reverse
Eye movements of heroin addicts
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was true. This may indicate that the drug objects were more complex than the neutral objects and somewhat more difficult to recognize. This possibility is also supported by the fact that addicts made somewhat fewer errors on drug objects with which they are more likely to be familiar than did the controls. Gerwral discussiorz
The two experiments revealed some interesting differences between addicts and controls. Specifically, with respect to the observation session, both experiments revealed that while controls tend to make more fixations than addicts, the number of fixations was not differentially affected by the type of material shown. By contrast, in Experiment 1, the average duration of a fixation was longer for addicts than for controls on all categories of words, but especially on the drug and dirty words. In Experiment 2, however, while there were no differences in fixation durations of the two groups on neutral objects, addicts spent substantially more time looking at drug objects relative to neutral objects than did the controls. During the recognition session, the number of fixations of addicts and controls on critical items were nearly identical while the fixation durations of the addicts were longer than those of the controls especially with respect to the old items. Finally, the recognition scores revealed that in both experiments, addicts and controls made about the same number of errors, but they were differentially affected by types of words or objects. Taken together then, these results would seem to indicate that the following factors are operating: motivational or interest factors associated with the nature of the stimulus material, e.g., drug versus neutral items and old versus new items, and possibly differences in reading skill and the ability to manipulate printed material as indicated by the differences noted between words and objects. Nevertheless, the observation that fixation durations are consistently and substantially longer for addicts than for controls suggest that there is a depression in the physiological and central nervous system processes that regulate eye movements. An alternate explanation could be that the observed differences in eye movements are not the result of addiction at all, but rather stem from differences in reading skills and motivational factors attributable to educational background. Specifically, all of the control subjects were college students while only 24% of the addicts had college backgrounds. This possibility is seriously weakened by the findings of FISHER (1976) which demonstrate that as reading experience increases both the average number of fixations and the duration of those fixations steadily decrease. In the present study, while the fixation durations of addicts were longer than that of controls, fewer fixations were made. Nevertheless, in subsequent studies it would be of interest to covary level and age. In conclusion, it is interesting to note that during the standard acuity check prior to experimentation, many addicts volunteered a variety of different complaints about their vision even though they tested 20-20. When examining the results of the difference in eye movement between addicts and controls, it would seem well to be aware of the fact that we may be dealing with changes in the central nervous system which are specific and long-lasting but not easily observable as a general effect on complex tasks. When the results of the visual data taken here and the cutaneous data of HALL et al. (1974a) are combined, there is reason for speculation that the addicts demonstrate an altered sensory capacity in the temporal domain which is involved with the gating and subsequent scanning of stimuli. Aclirlowledgements-The authors gratefully acknowledge the comments and contributions of DENNISF. FISHER, ROI~ERTKARSHand ROBERTH. LAMBERT.
REFERENCES BUTLER, D. H., KAMLET,A. S. and MONTY,R. A. (1969). A multi-purpose analysis of variance FORTRAN IV computer program. Psychonomic Monograph Supplements, 2: 301-319.
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EXLINE, R. V., GRAY, D. and SCHUETTE, D. (1965). Visual behavior in a dyad as affected by interview content and sex of respondent. J. Personality Social Psycho!. 1: 201-209. FISHER, D. F. (1976). Spatial factors in reading and search: The case for space. In: Eye Mooements and Psychological Procrsses (MONTY. R. A. and SENDERS, W., Eds.), Lawrence Erlbaum Associates, Hillsdale N.J. (In press). HALL. R. J. and CUSACK. B. L. (1972). The measurement of eye behavior: critical and selected review of voluntary eye movement and blinking. Tech. Memo 1X-72. U.S. Army Human Engineering Laboratory, Aberdeen Proving Ground. MarylandHALL. R. J.. ROSENBERGER.M. A. and MONTY. R. A. (1974a). Cutaneous nercention of heroin addicts: evidence of an altered temporal process. Bull. Psychworn. sot. 3:‘352-354. L I HALL, R. J., ROSENBERGER, M. A. and MONTY, R. A. (1974b). An experimental investigation of the visual behavior of young heroin addicts and matched controls. Jourrlal Supplement Abstract Service 4: 7. HUTT, C. and OWNSED, C. (1966). The biological significance of gaze aversion with particular reference to the syndrome of infantile autism. &haul Sci. 11: 346356. LAMBERT, R. H., MONTY, R. A. and HALL, R. J. (1974). High speed data processing and unobtrusive monitoring of eye movements. B&au. Res. Meth. Instrumentutiorr 6: 525-530. SPENCE.D. P. and FEIIVBEKG,C. (1967). Forms of defensive looking: A naturalistic experiment. J. nera. ment. Dis. 145: 261-271. STASS,J. W. and WILLIS. JR., F. N. (1967). Eye-contact, pupil dilation and personal preference. Psychonorn. Sci. I: 375-376. TROXLER, D. (1804). Uber das Verschwinden gegebener, gegenstande innerhalb unsers gesichtskreises. In: Ophthalmologischr Bibliothrk. Vol. 2. Pt. 2 (HIMLY, K. and SCHMIDT, J. A., Eds.). pp. l-53. VINE, I. (1970) Communication by facial-visual signals. In: Social Behavior in Birds and Mammals: Essays OIZthr Social Ethology of Animals and Man (CROOK, J. H., Ed.), pp. 279-354, Academic Press, New York.
