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Pupillary activity while listening to verbal passages
JOURNAL
OF
RESEARCH
IN
PERSONALITY
12, 361-369 (1978)
Pupillary Activity While Listening GREGORY
L. WHITE
to Verbal Passages
AND IRVING
University of California,
MALTZMAN
Los Angeles
Twelve college students were presented with neutral, pleasant, and unpleasant verbal passages each of 120 set duration. Changes in pupil diameter before, during, and after each passage were continuously monitored. Dilation accompanied the pleasant and unpleasant passages relative to the neutral passage. No evidence for defensive pupillary constriction was found. Habituation occurred during periods of silence as well as during the stimulus passages. These results suggest that a study reported by Hess and Goodwin (1974) as demonstrating pupillary constriction to unpleasant visual material may be interpreted as reflecting differential habituation.
Pupillary activity has been used as an indicator of emotional arousal (Goldwater, 1972; Hess & Goodwin, 1974), social attitudes (Woodmansee, 1970), mental effort (Kahneman, 1973), the orienting reflex (Liberman, 1%5), as well as studied in its own right as a response to changes in illumination (Lowenstein & Loewenfeld, 1961). One particularly interesting hypothesis in relation to the pupillary response to affective stimuli is that it may be bidirectional in nature (Hess, 1972; Hess & Goodwin, 1974). The suggestion is that the pupil dilates to a pleasant stimulus and may constrict to a mildly aversive stimulus. If this were so, an objective physiological measure would be available that could differentiate between subjective psychological states. While Hess and his associates (Hess, 1965, 1972; Hess & Goodwin, 1974; Polt & Hess, 1968) have reported that pupillary constrictions have been obtained in a number of situations, attempts to obtain pupillary constriction by other investigators using affective stimuli have met with mixed success (e.g., Atwood & Howell, 1971; Barlow, 1969; Fredericks & Groves, 1971; Hutt & Anderson, 1967; Peavler & McLaughlin, 1967; Woodmansee, 1970). Many of the earlier studies purporting to obtain pupillary constriction to complex visual stimuli have been criticized on methodological grounds (Goldwater, 1972; Janisse, 1973). Use of photographic slides of complex Requests for reprints should be made to Gregory L. White, Department of Psychology. University of Maryland, College Park, Maryland 20742. 361 0092-6566178/0123-0361$02.0010 CopyrIght @ 1978 by Academic Press, Inc. All rights of reproduction in any form resel-ved
362
WHITE
AND
MALTZMAN
scenes induce possible contrast effects between slides and a complex photograph allows for eye movements to different segments of the photograph differing in brightness. Constrictions obtained under such conditions may be the result of fixating a brighter portion of a figure and sex differences in dilation and constriction to such complex stimuli and may be the consequence of men and women fixating different portions of the scene differing in brightness. More recently, several studies conducted in Hess’ laboratory have been described (Hess & Goodwin, 1974) that seem to avoid most of the earlier criticisms of studies purporting to obtain pupillary constriction to aversive stimuli. In one such study, a TV film was employed as the stimulus material with brightness levels controlled. The film had an initial violent scene followed by a situation that made a man appear as a fool. Both male and female viewers showed an initial marked rise in dilation above their base level. Women then showed a relatively gradual return to their preexposure base level. Men, in contrast, showed a relatively rapid decline in the size of their pupillary dilations following the initial episode. Most importantly, the magnitude of the pupillary response declined below the original base level. These results were interpreted as demonstrating that the men showed pupillary constrictions during the latter phases of the film relative to their initial base level. There are a number of methodological difficulties with the study reported by Hess and Goodwin which make the interpretation of its results ambiguous. One difficulty with the study and other studies purporting to show constriction to affective stimuli is that constriction is defined relative to the diameter of the pupil during a rest or control period prior to the onset of the stimulus materials. This interpretation of constriction presupposes that the base level pupil diameter is constant throughout the duration of the stimulus presentation. Such an assumption is unreasonable in view of the apparently ubiquitous occurrence of habituation of responses with repeated or continuous stimulation (Peake & Herz, 1973; Razran, 1971). An obvious alternative interpretation of Hess and Goodwin’s results is that while males showed smaller pupil diameters to the film than during the initial control period, a control period at the conclusion of the experiment would have shown even smaller pupil diameters. Males simply habituated at a faster rate than women. An obvious necessary control condition is to have a rest period following the presentation of stimulus materials during which time the diameter of the pupil could once more be determined. Another serious methodological shortcoming of the Hess and Goodwin study was the use of only one kind of stimulus material (i.e., one film). A control condition is needed in which a neutral stimulus sequence is presented in order to determine the rate of habituation independently of the
PUPILLARY
ACTIVITY
363
content of the stimulus materials. Such a neutral stimulus sequence would also permit a more adequate interpretation of the initial increase in magnitude of pupillary dilations. The initial increase in dilation followed by a fall which Hess and Goodwin ascribe to the content of the film is commonly found in habituation studies of other measures of the orienting reflex (Thompson, Groves, Taylor, & Roemer, 1973). The present study was designed to assess the possibility of the occurrence of pupillary constrictions to aversive stimuli after correcting for potential methodological shortcomings in the study described by Hess and Goodwin (1974) purporting to demonstrate a bidirectional pupillary response. Accordingly, our subjects were presented with verbal stimuli, listening to a pleasant, unpleasant, and a neutral passage. Pupillary activity was recorded and measured before, during, and after each passage presentation. The present study does not, however, investigate the plausible hypothesis that Hess’ and Goodwin’s results may be due to contrast effects. METHOD Twelve volunteer male undergraduate students were individually tested. Participation in the experiment fulfilled a course requirement. Apparutus. A Whittaker model 1081s TV pupillometer system was employed to record pupillary activity. This system provides a chart record linearly related to actual vertical diameter of the pupil. All experimental instructions and stimuli were presented binaurally using audio tape. Procedure. Following adjustment and calibration of the apparatus the participants were instructed to fixate a 10 cm diameter cross located on a wall screen I .2 m distant. A series of 10-1000 Hz tones at approximately 90 db (A) each of IO set duration were delivered with a mean interstimulus interval of 60 sec. This phase was designed to habituate the participants to the experimental situation. After a 5 min rest period the participants were returned to the apparatus, instructed to fixate the cross, and were presented with the verbal passages. Each 120 set passage was preceded and followed by 30 set of silence in order to record base level pupillary activity. The intensity of the passages measured at the earphones averaged approximately 65 db (A). A 2-min rating period separated the test periods and their preceding and following periods of silent rest. During each rating period the participants evaluated the preceding passage on four 2l-point scales ranging from -10 to +lO. The scales were designed to obtain ratings on the degree of pleasantness, arousal, concentration, and interest of each passage. The three passages, presented in counterbalanced order, consisted of a sexually arousing erotic (E) passage taken from a contemporary novel (Robbins, 1972, p. 266). a neutral passage (N) taken from the same novel (Robbins, 1972, p. 327), and an adaptation of a description of a lynching mutilation (M) (Miller & Dollard, 1941). The erotic and mutilation passages were selected because participants in an unrelated study (Cantor. Bryant, & Zillman, 1974) had rated them as very arousing and quite pleasant (E) and unpleasant (M). Participants.
RESULTS
Participants’ for correlated
mean ratings of the three passages and the results oft tests measures are presented in Table I. It is apparent that the
364
WHITE
AND MALTZMAN TABLE
MEAN
1
RATINGS OF EACH PASSAGE, NEUTRAL (N), EROTIC (E), (M), AVERAGED OVER A 21-POINT SCALE RANGING FROM
AND MUTILATION -10 TO +lO
Mean rating N Scale item 1. 2. 3. 4.
How pleasant? How arousing? Degree of concentration How interesting?
M
E
A
SD
.42 -3.08 2.08 -2.33
2.94 5.73 5.33 4.68
x 3.83 5.67 6.25 5.33
SD
x
SD
4.55 3.23 4.41 3.75
-6.47 2.25 6.83 4.25
3.75 7.21 3.79 4.81
Correlated t for each comparison N vs M N vs E E vs M 3.07* 2.41* 2.11* 2.55*
1.86* 2.36* 1.9s* 2.59*
2.80* 1.41 .57 .83
* p < .05, one-tailed probability.
erotic and mutilation passages were felt to be arousing and that the erotic passage was considered more pleasant than the neutral passage, which in turn was rated as more pleasant than the mutilation passage. Ratings of degree of concentration and interest were also in the direction predicted. Examination of the ratings of individual participants revealed no case in which a participant rated the mutilation passage as pleasant or the erotic passage as unpleasant. Results from the rating scales therefore indicate that we were successful in selecting passages that were arousing (M and E), as compared to a neutral passage, and pleasant (E) and unpleasant (M) in the effect induced. Measurements in millimeters of pupil diameter were made from the chart record every 5 set during the 30 set periods of silence preceding and following each passage and during each 120 set passage. These values were then averaged over 15 set intervals. The averaged values collapsed across order of presentation, which was not a significant effect, are shown in Fig. 1. Repeated measures analyses of variance were employed to assess the effects of time period and verbal passage on pupil diameter. The analyses reported below were performed both on the points sampled every 5 set and on the 15 set averaged points. It should be mentioned that the usual F test for repeated measures rests on the assumption of homogeneity of the covariance matrix of the repeated measures. Departures from homogeneity increase the probability
PUPILLARY
365
ACTIVITY
I 2 3 4 5 6 7 8 9 IO II 12 Blocks of 15 sec. FIG. 1. as pleasant.
Mean pupil size while listening to sexual, neutral, and unpleasant, respectively.
neutral,
and mutilation
passages
rated
of Type 1 error. Greenhouse and Geisser (Winer, 1971, p. 523) present a formula for calculation of E, a measure of departure from homogeneity. E is used to adjust the degrees of freedom, making the F test more conservative than it otherwise would have been. In this report, the degrees of freedom for each F test have been adjusted based on calculation of the appropriate E. The rest periods prior to the onset of a passage showed no significant differences due to time period, verbal passage, or their interaction. When three 5-set periods immediately prior to the onset of a passage are compared to the three periods immediately after onset, there is a highly significant effect of Time [F(2,20) = 13.13, MSe = 6.88,~ < .OOl] and a significant Time x Verbal Passage interaction [F(4,40) = 5.37, MSe = 4.72, p < .005]. This initial increase in pupil diameter is associated with a large linear trend component [F(1,28) = 38.03, MSe = 6.88, p < .OOl]. These results indicate that the onset of a passage is accompanied by a relatively rapid increase in pupil diameter, dilation. The effect occurs in all three passages, but is greater in the arousing passages, the sexual or erotic and the mutilation as compared to the neutral passage. This initial dilation is presumably a reflection of the orienting reflex to novel stimulation which is influenced by the interests of the participants and their affective reaction as well as the physical parameters of the stimulus. Analyses of the 120 set of each verbal passage averaged over 15 set intervals yielded a significant main effect for Time [F(3,29) = 3.01, MSe = 1335.06, p < .05], Passage [F(2,22) = 5.49, MSe = 13244.37,~ < .025], and their interaction [F(5,59) = 4.85, MSe = 431.17,~ < .OOl]. A withinpassage trend analysis revealed a significant linear trend for the neutral
366
WHITE
AND
MALTZMAN
passage [F(1,29) = 4.28, p < .05], quadratic trends for the erotic passage [F( 1,29) = 7.02, p < .025], and for the mutilation passage [F( 1,29) = 4.91, p < .05]. These results confirm the impression gained from Fig. 1 that the neutral passage showed a continuous decrease in pupil diameter following a brief and relatively small increase. In contrast, the mutilation and sexual passages induced an extended increase followed by a decreasing pupil diameter. Figure 1 suggests that maximal dilations occurred to the mutilation and sexual passages after approximately 60 set followed by a decline, habituation, with the continued presentation of the passage. Results from the last 60 set and the 30 set postpassage baseline, averaged over 15 set intervals, were analyzed for all three passages in order to confirm this impression and the implications of the quadratic trend obtained in the previous analysis. There was a main effect of Time [F(2,18) = 12.96, MSe = 1588.36, p < .OOl] and Passage [F(2,22) = 6.09, it4Se = 11373.82, p <: .Ol], with no interaction between the two factors. There was also a significant linear trend to the decrease [F(1,18) = 59.07, p < .OOl]. The absence of interactions and the linear trend further indicate that pupil diameter was habituating to the three passages after 60 set and that the rate of habituation of the response to the passages was similar. The decline in pupillary diameter was essentially parallel for all passages once the decline commenced. A somewhat similar pattern of results was obtained when the data for the 30 set postpassage baseline was analyzed at 5 set intervals. There was a significant main effect for Time [F(2,22) = 5.83, MSe = 7.55, p < .Ol] and an effect for Passage [F(2,22) = 2.68, A4Se = 156.19, p < .I01 that approached significance. There was no interaction between Passage and Time. These results indicate that pupil diameter continued to decrease during the period of silence following a passage and this occurred in a similar fashion regardless of the nature of the preceding passage. Finally, the prepassage and postpassage baselines were compared at 15-set intervals. The main effect of Time approached significance [F(2,21) = 2.82, MSe = 5547.15, p < .lO] while there was a significant Time x Passage interaction [F(4,42) = 4.73, A4Se = 1223.88, p < .005]. The interaction suggests that the decline in pupil size from prepassage to postpassage rest periods was greater after the neutral passage than after the two arousing passages. All passages, however, tended to show a decline between periods of rest. Inspection of the individual records of participants in response to the different passages indicated that the group data as represented in Fig. 1 reflect rather accurately the results for the individual. That is, there were no participants who exhibited larger pupil diameters in response to the neutral passage than to either of the arousing passages or who showed
PUPILLARY
ACTIVITY
367
smaller pupil diameters during the arousing passages than during the rest period that followed that particular passage. DISCUSSION
Participants’ ratings of the three passages indicated that we were successful in our attempt to obtain an essentially neutral passage and two arousing passages which differed in the direction of affect, one pleasant and the other unpleasant or aversive. No evidence of pupillary constriction was obtained when constriction is defined as a pupil diameter induced by aversive stimulus material which is smaller than the diameter induced during a rest period following as well as preceding the stimulus and as compared to a neutral condition. The unpleasant mutilation passage consistently evoked larger pupil diameters than the initial rest period and the neutral passage. Although there was a decline in the pupil diameter starting approximately midway through the arousing passages we would describe this decrement as habituation with continued stimulation. This decline characterized the response to all three passages, and is a result to be expected if pupillary dilation to nonadequate stimuli is interpreted as a manifestation of an orienting reflex (Liberman, 1965). The significant increase in pupil diameter that occurred following the onset of a passage and the greater increase in the arousing passages as compared to the neutral passage is also in accord with an orienting reflex interpretation of the obtained pupillary changes. These results suggest that the interpretation offered by Hess and Goodwin (1974) for the initial increase in pupillary dilation that they obtained to their TV film (ascribing it to the special content of the film) is unwarranted. Further, though the present results do not show a decline in pupillary diameter below baseline for the two arousing passages, the trend analyses strongly suggest that if the postpassage observation period had been of longer duration, then there would have been habituation below prepassage baseline levels. Responsivity to the neutral passage clearly habituated below the level of the initial rest period. If Hess and Goodwin’s interpretations were applied literally, then we would be forced to conclude that this neutral passage induced pupillary constrictions. A more reasonable interpretation is that the decline in pupil diameter is a consequence of habituation, a characteristic of the orienting reflex and all autonomic response systems (Razran, 1971). The present results provide no evidence to support an interpretation of the pupillary response as a bidirectional response which may dilate to pleasant stimuli and constrict to mildly aversive stimuli. It might be argued, however, that the bidirectional response is a U-shaped function of stimulation. Pleasant and very unpleasant stimuli evoke dilation while mildly unpleasant stimuli evoke constriction, and that the mutilation
WHITE
368
AND MALTZMAN
passage was too aversive to induce constriction. The difficulty with this theory is that it is not falsifiable. Since there is no standardized independent measure of degree of aversiveness that is generally accepted, it is always possible to argue that a particular unpleasant stimulus such as the mutilation passage in the present experiment is either too unpleasant or pleasant to produce the desired effect. But a theory which is not falsifiable is of little scientific value. It is apparent that despite the results of the present experiment it may still be possible to find evidence of pupillary constriction to aversive stimuli which is obtained under artifact free conditions, and the constriction is clearly not a consequence of habituation. The present experiment, however, does demonstrate that previous results taken to represent constriction (Hess & Goodwin, 1974) are amenable to alternative reasonable interpretations. The present experiment further demonstrates that control conditions utilizing essentially neutral stimulus materials are essential for the proper interpretation of experiments investigating the bidirectional response of the pupil to affective stimuli. Rest periods of nonstimulation after as well as before stimulation are also critical in the assessment of the degree of habituation or assumed constriction. REFERENCES Atwood, R. W., & Howell, R. J. Pupillometric and personality test score differences of pedophiliacs and normals. Psychonomic Science, 1971, 22, 115- 116. Barlow, J. D. Pupillary size as an index of preference in political candidates. Perceptual and Motor
Skills,
1969, 28, 587-590.
Cantor, J. R., Bryant, J., & Zillman, D. Enhancement of humor appreciation by transferred excitation. Journal of Personality and Social Psychology, 1974, 30, 812-821. Fredericks, R. S., & Groves, M. H. Pupil changes and stimulus pleasantness. Proceedings of the Annual Convention of the American Psychological Association, 1971,6,371-372. Goldwater, B. C. Psychological significance of pupillary movements. Psychological BuNetin,
1972, 77, 340-355.
Hess, E. H. Attitude and pupil size. Scientijc American, 1965, 212, 46-54. Hess, E. H. Pupillometrics. In N. S. Greenfield & R. A. Sternbach (Eds.), Handbook of psychophysiology. New York: Holt, Rinehart, & Winston, 1972. Hess, E. H., & Goodwin, E. The present state of pupillometrics. In M. P. Janisse (Ed.), Pupillary dynamics and behavior. New York: Plenum, 1974. Hutt, L. D., & Anderson, J. P. The relationship between pupil size and recognition threshold. Psychonomic Science, 1967, 9, 477-478. Janisse, M. P. Pupil size and affect: A critical review of the literature since 1960. The Canadian Psychologist, 1973, 14, 31 l-329. Kahneman, D. Attention and effort. Englewood Cliffs, NJ: Prentice-Hall, 1973. Liberman, A. E. Some new data on the pupillary component in man. In L. G. Voronin, A. N. Leontiev, A. R. Luria, E. N. Sokolov, & 0. S. Vinogradova (Eds.), Orienting reflex and exploratory behavior. Washington, DC: American Institute of Biological Sciences, 1965. Lowenstein, 0.. & Loewenfeld, I. Influence of retinal adaptation upon the pupillary reflex to light in normal man: Part II. Effect of adaptation to dim illumination upon pupillary reflexes elicited by bright light. American Journal of Ophthalmology, 1%1,5,644-654.
PUPILLARY
ACTIVITY
369
Miller, N. E., & Dollard, .I. Social /earning and imitation. New Haven: Yale University Press, 1941. Peake, H. V. S.. & Herz. M. J. (Eds.), Habituation. Vol. I. Behavioral studies. New York: Academic Press, 1973. Peavler, W. S., & McLaughlin, J. P. The question of stimulus content and pupil size. Psychonomic
Science,
1%7,
8, 505-506.
Pelt. J. M., & Hess, E. H. Changes in pupil size to visually presented words. Psychonomic Science, 1%8, 12, 389-399. Razran. G. Mind in evolution. New York: Houghton Mifflin, 1971. Robbins. H. The Betsy. New York: Simon & Schuster, 1972. Thompson, R. F., Groves, P. M., Taylor, T. J., & Roemer, R. A. A dual-process theory of habituation: Theory and behavior. In H. V. S. Peake & M. J. Herz (Eds.). Habituation. Vol. I. Behavioral studies. New York: Academic Press, 1973. Winer, B. J. Statistical principles in experimental design. New York: McGraw-Hill, 1971. Woodmansee. J. J. The pupil response as a measure of social attitudes. In C. Sumers (Ed.). Attitude measurement. Chicago: Rand-McNally, 1970.
OF
RESEARCH
IN
PERSONALITY
12, 361-369 (1978)
Pupillary Activity While Listening GREGORY
L. WHITE
to Verbal Passages
AND IRVING
University of California,
MALTZMAN
Los Angeles
Twelve college students were presented with neutral, pleasant, and unpleasant verbal passages each of 120 set duration. Changes in pupil diameter before, during, and after each passage were continuously monitored. Dilation accompanied the pleasant and unpleasant passages relative to the neutral passage. No evidence for defensive pupillary constriction was found. Habituation occurred during periods of silence as well as during the stimulus passages. These results suggest that a study reported by Hess and Goodwin (1974) as demonstrating pupillary constriction to unpleasant visual material may be interpreted as reflecting differential habituation.
Pupillary activity has been used as an indicator of emotional arousal (Goldwater, 1972; Hess & Goodwin, 1974), social attitudes (Woodmansee, 1970), mental effort (Kahneman, 1973), the orienting reflex (Liberman, 1%5), as well as studied in its own right as a response to changes in illumination (Lowenstein & Loewenfeld, 1961). One particularly interesting hypothesis in relation to the pupillary response to affective stimuli is that it may be bidirectional in nature (Hess, 1972; Hess & Goodwin, 1974). The suggestion is that the pupil dilates to a pleasant stimulus and may constrict to a mildly aversive stimulus. If this were so, an objective physiological measure would be available that could differentiate between subjective psychological states. While Hess and his associates (Hess, 1965, 1972; Hess & Goodwin, 1974; Polt & Hess, 1968) have reported that pupillary constrictions have been obtained in a number of situations, attempts to obtain pupillary constriction by other investigators using affective stimuli have met with mixed success (e.g., Atwood & Howell, 1971; Barlow, 1969; Fredericks & Groves, 1971; Hutt & Anderson, 1967; Peavler & McLaughlin, 1967; Woodmansee, 1970). Many of the earlier studies purporting to obtain pupillary constriction to complex visual stimuli have been criticized on methodological grounds (Goldwater, 1972; Janisse, 1973). Use of photographic slides of complex Requests for reprints should be made to Gregory L. White, Department of Psychology. University of Maryland, College Park, Maryland 20742. 361 0092-6566178/0123-0361$02.0010 CopyrIght @ 1978 by Academic Press, Inc. All rights of reproduction in any form resel-ved
362
WHITE
AND
MALTZMAN
scenes induce possible contrast effects between slides and a complex photograph allows for eye movements to different segments of the photograph differing in brightness. Constrictions obtained under such conditions may be the result of fixating a brighter portion of a figure and sex differences in dilation and constriction to such complex stimuli and may be the consequence of men and women fixating different portions of the scene differing in brightness. More recently, several studies conducted in Hess’ laboratory have been described (Hess & Goodwin, 1974) that seem to avoid most of the earlier criticisms of studies purporting to obtain pupillary constriction to aversive stimuli. In one such study, a TV film was employed as the stimulus material with brightness levels controlled. The film had an initial violent scene followed by a situation that made a man appear as a fool. Both male and female viewers showed an initial marked rise in dilation above their base level. Women then showed a relatively gradual return to their preexposure base level. Men, in contrast, showed a relatively rapid decline in the size of their pupillary dilations following the initial episode. Most importantly, the magnitude of the pupillary response declined below the original base level. These results were interpreted as demonstrating that the men showed pupillary constrictions during the latter phases of the film relative to their initial base level. There are a number of methodological difficulties with the study reported by Hess and Goodwin which make the interpretation of its results ambiguous. One difficulty with the study and other studies purporting to show constriction to affective stimuli is that constriction is defined relative to the diameter of the pupil during a rest or control period prior to the onset of the stimulus materials. This interpretation of constriction presupposes that the base level pupil diameter is constant throughout the duration of the stimulus presentation. Such an assumption is unreasonable in view of the apparently ubiquitous occurrence of habituation of responses with repeated or continuous stimulation (Peake & Herz, 1973; Razran, 1971). An obvious alternative interpretation of Hess and Goodwin’s results is that while males showed smaller pupil diameters to the film than during the initial control period, a control period at the conclusion of the experiment would have shown even smaller pupil diameters. Males simply habituated at a faster rate than women. An obvious necessary control condition is to have a rest period following the presentation of stimulus materials during which time the diameter of the pupil could once more be determined. Another serious methodological shortcoming of the Hess and Goodwin study was the use of only one kind of stimulus material (i.e., one film). A control condition is needed in which a neutral stimulus sequence is presented in order to determine the rate of habituation independently of the
PUPILLARY
ACTIVITY
363
content of the stimulus materials. Such a neutral stimulus sequence would also permit a more adequate interpretation of the initial increase in magnitude of pupillary dilations. The initial increase in dilation followed by a fall which Hess and Goodwin ascribe to the content of the film is commonly found in habituation studies of other measures of the orienting reflex (Thompson, Groves, Taylor, & Roemer, 1973). The present study was designed to assess the possibility of the occurrence of pupillary constrictions to aversive stimuli after correcting for potential methodological shortcomings in the study described by Hess and Goodwin (1974) purporting to demonstrate a bidirectional pupillary response. Accordingly, our subjects were presented with verbal stimuli, listening to a pleasant, unpleasant, and a neutral passage. Pupillary activity was recorded and measured before, during, and after each passage presentation. The present study does not, however, investigate the plausible hypothesis that Hess’ and Goodwin’s results may be due to contrast effects. METHOD Twelve volunteer male undergraduate students were individually tested. Participation in the experiment fulfilled a course requirement. Apparutus. A Whittaker model 1081s TV pupillometer system was employed to record pupillary activity. This system provides a chart record linearly related to actual vertical diameter of the pupil. All experimental instructions and stimuli were presented binaurally using audio tape. Procedure. Following adjustment and calibration of the apparatus the participants were instructed to fixate a 10 cm diameter cross located on a wall screen I .2 m distant. A series of 10-1000 Hz tones at approximately 90 db (A) each of IO set duration were delivered with a mean interstimulus interval of 60 sec. This phase was designed to habituate the participants to the experimental situation. After a 5 min rest period the participants were returned to the apparatus, instructed to fixate the cross, and were presented with the verbal passages. Each 120 set passage was preceded and followed by 30 set of silence in order to record base level pupillary activity. The intensity of the passages measured at the earphones averaged approximately 65 db (A). A 2-min rating period separated the test periods and their preceding and following periods of silent rest. During each rating period the participants evaluated the preceding passage on four 2l-point scales ranging from -10 to +lO. The scales were designed to obtain ratings on the degree of pleasantness, arousal, concentration, and interest of each passage. The three passages, presented in counterbalanced order, consisted of a sexually arousing erotic (E) passage taken from a contemporary novel (Robbins, 1972, p. 266). a neutral passage (N) taken from the same novel (Robbins, 1972, p. 327), and an adaptation of a description of a lynching mutilation (M) (Miller & Dollard, 1941). The erotic and mutilation passages were selected because participants in an unrelated study (Cantor. Bryant, & Zillman, 1974) had rated them as very arousing and quite pleasant (E) and unpleasant (M). Participants.
RESULTS
Participants’ for correlated
mean ratings of the three passages and the results oft tests measures are presented in Table I. It is apparent that the
364
WHITE
AND MALTZMAN TABLE
MEAN
1
RATINGS OF EACH PASSAGE, NEUTRAL (N), EROTIC (E), (M), AVERAGED OVER A 21-POINT SCALE RANGING FROM
AND MUTILATION -10 TO +lO
Mean rating N Scale item 1. 2. 3. 4.
How pleasant? How arousing? Degree of concentration How interesting?
M
E
A
SD
.42 -3.08 2.08 -2.33
2.94 5.73 5.33 4.68
x 3.83 5.67 6.25 5.33
SD
x
SD
4.55 3.23 4.41 3.75
-6.47 2.25 6.83 4.25
3.75 7.21 3.79 4.81
Correlated t for each comparison N vs M N vs E E vs M 3.07* 2.41* 2.11* 2.55*
1.86* 2.36* 1.9s* 2.59*
2.80* 1.41 .57 .83
* p < .05, one-tailed probability.
erotic and mutilation passages were felt to be arousing and that the erotic passage was considered more pleasant than the neutral passage, which in turn was rated as more pleasant than the mutilation passage. Ratings of degree of concentration and interest were also in the direction predicted. Examination of the ratings of individual participants revealed no case in which a participant rated the mutilation passage as pleasant or the erotic passage as unpleasant. Results from the rating scales therefore indicate that we were successful in selecting passages that were arousing (M and E), as compared to a neutral passage, and pleasant (E) and unpleasant (M) in the effect induced. Measurements in millimeters of pupil diameter were made from the chart record every 5 set during the 30 set periods of silence preceding and following each passage and during each 120 set passage. These values were then averaged over 15 set intervals. The averaged values collapsed across order of presentation, which was not a significant effect, are shown in Fig. 1. Repeated measures analyses of variance were employed to assess the effects of time period and verbal passage on pupil diameter. The analyses reported below were performed both on the points sampled every 5 set and on the 15 set averaged points. It should be mentioned that the usual F test for repeated measures rests on the assumption of homogeneity of the covariance matrix of the repeated measures. Departures from homogeneity increase the probability
PUPILLARY
365
ACTIVITY
I 2 3 4 5 6 7 8 9 IO II 12 Blocks of 15 sec. FIG. 1. as pleasant.
Mean pupil size while listening to sexual, neutral, and unpleasant, respectively.
neutral,
and mutilation
passages
rated
of Type 1 error. Greenhouse and Geisser (Winer, 1971, p. 523) present a formula for calculation of E, a measure of departure from homogeneity. E is used to adjust the degrees of freedom, making the F test more conservative than it otherwise would have been. In this report, the degrees of freedom for each F test have been adjusted based on calculation of the appropriate E. The rest periods prior to the onset of a passage showed no significant differences due to time period, verbal passage, or their interaction. When three 5-set periods immediately prior to the onset of a passage are compared to the three periods immediately after onset, there is a highly significant effect of Time [F(2,20) = 13.13, MSe = 6.88,~ < .OOl] and a significant Time x Verbal Passage interaction [F(4,40) = 5.37, MSe = 4.72, p < .005]. This initial increase in pupil diameter is associated with a large linear trend component [F(1,28) = 38.03, MSe = 6.88, p < .OOl]. These results indicate that the onset of a passage is accompanied by a relatively rapid increase in pupil diameter, dilation. The effect occurs in all three passages, but is greater in the arousing passages, the sexual or erotic and the mutilation as compared to the neutral passage. This initial dilation is presumably a reflection of the orienting reflex to novel stimulation which is influenced by the interests of the participants and their affective reaction as well as the physical parameters of the stimulus. Analyses of the 120 set of each verbal passage averaged over 15 set intervals yielded a significant main effect for Time [F(3,29) = 3.01, MSe = 1335.06, p < .05], Passage [F(2,22) = 5.49, MSe = 13244.37,~ < .025], and their interaction [F(5,59) = 4.85, MSe = 431.17,~ < .OOl]. A withinpassage trend analysis revealed a significant linear trend for the neutral
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passage [F(1,29) = 4.28, p < .05], quadratic trends for the erotic passage [F( 1,29) = 7.02, p < .025], and for the mutilation passage [F( 1,29) = 4.91, p < .05]. These results confirm the impression gained from Fig. 1 that the neutral passage showed a continuous decrease in pupil diameter following a brief and relatively small increase. In contrast, the mutilation and sexual passages induced an extended increase followed by a decreasing pupil diameter. Figure 1 suggests that maximal dilations occurred to the mutilation and sexual passages after approximately 60 set followed by a decline, habituation, with the continued presentation of the passage. Results from the last 60 set and the 30 set postpassage baseline, averaged over 15 set intervals, were analyzed for all three passages in order to confirm this impression and the implications of the quadratic trend obtained in the previous analysis. There was a main effect of Time [F(2,18) = 12.96, MSe = 1588.36, p < .OOl] and Passage [F(2,22) = 6.09, it4Se = 11373.82, p <: .Ol], with no interaction between the two factors. There was also a significant linear trend to the decrease [F(1,18) = 59.07, p < .OOl]. The absence of interactions and the linear trend further indicate that pupil diameter was habituating to the three passages after 60 set and that the rate of habituation of the response to the passages was similar. The decline in pupillary diameter was essentially parallel for all passages once the decline commenced. A somewhat similar pattern of results was obtained when the data for the 30 set postpassage baseline was analyzed at 5 set intervals. There was a significant main effect for Time [F(2,22) = 5.83, MSe = 7.55, p < .Ol] and an effect for Passage [F(2,22) = 2.68, A4Se = 156.19, p < .I01 that approached significance. There was no interaction between Passage and Time. These results indicate that pupil diameter continued to decrease during the period of silence following a passage and this occurred in a similar fashion regardless of the nature of the preceding passage. Finally, the prepassage and postpassage baselines were compared at 15-set intervals. The main effect of Time approached significance [F(2,21) = 2.82, MSe = 5547.15, p < .lO] while there was a significant Time x Passage interaction [F(4,42) = 4.73, A4Se = 1223.88, p < .005]. The interaction suggests that the decline in pupil size from prepassage to postpassage rest periods was greater after the neutral passage than after the two arousing passages. All passages, however, tended to show a decline between periods of rest. Inspection of the individual records of participants in response to the different passages indicated that the group data as represented in Fig. 1 reflect rather accurately the results for the individual. That is, there were no participants who exhibited larger pupil diameters in response to the neutral passage than to either of the arousing passages or who showed
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smaller pupil diameters during the arousing passages than during the rest period that followed that particular passage. DISCUSSION
Participants’ ratings of the three passages indicated that we were successful in our attempt to obtain an essentially neutral passage and two arousing passages which differed in the direction of affect, one pleasant and the other unpleasant or aversive. No evidence of pupillary constriction was obtained when constriction is defined as a pupil diameter induced by aversive stimulus material which is smaller than the diameter induced during a rest period following as well as preceding the stimulus and as compared to a neutral condition. The unpleasant mutilation passage consistently evoked larger pupil diameters than the initial rest period and the neutral passage. Although there was a decline in the pupil diameter starting approximately midway through the arousing passages we would describe this decrement as habituation with continued stimulation. This decline characterized the response to all three passages, and is a result to be expected if pupillary dilation to nonadequate stimuli is interpreted as a manifestation of an orienting reflex (Liberman, 1965). The significant increase in pupil diameter that occurred following the onset of a passage and the greater increase in the arousing passages as compared to the neutral passage is also in accord with an orienting reflex interpretation of the obtained pupillary changes. These results suggest that the interpretation offered by Hess and Goodwin (1974) for the initial increase in pupillary dilation that they obtained to their TV film (ascribing it to the special content of the film) is unwarranted. Further, though the present results do not show a decline in pupillary diameter below baseline for the two arousing passages, the trend analyses strongly suggest that if the postpassage observation period had been of longer duration, then there would have been habituation below prepassage baseline levels. Responsivity to the neutral passage clearly habituated below the level of the initial rest period. If Hess and Goodwin’s interpretations were applied literally, then we would be forced to conclude that this neutral passage induced pupillary constrictions. A more reasonable interpretation is that the decline in pupil diameter is a consequence of habituation, a characteristic of the orienting reflex and all autonomic response systems (Razran, 1971). The present results provide no evidence to support an interpretation of the pupillary response as a bidirectional response which may dilate to pleasant stimuli and constrict to mildly aversive stimuli. It might be argued, however, that the bidirectional response is a U-shaped function of stimulation. Pleasant and very unpleasant stimuli evoke dilation while mildly unpleasant stimuli evoke constriction, and that the mutilation
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passage was too aversive to induce constriction. The difficulty with this theory is that it is not falsifiable. Since there is no standardized independent measure of degree of aversiveness that is generally accepted, it is always possible to argue that a particular unpleasant stimulus such as the mutilation passage in the present experiment is either too unpleasant or pleasant to produce the desired effect. But a theory which is not falsifiable is of little scientific value. It is apparent that despite the results of the present experiment it may still be possible to find evidence of pupillary constriction to aversive stimuli which is obtained under artifact free conditions, and the constriction is clearly not a consequence of habituation. The present experiment, however, does demonstrate that previous results taken to represent constriction (Hess & Goodwin, 1974) are amenable to alternative reasonable interpretations. The present experiment further demonstrates that control conditions utilizing essentially neutral stimulus materials are essential for the proper interpretation of experiments investigating the bidirectional response of the pupil to affective stimuli. Rest periods of nonstimulation after as well as before stimulation are also critical in the assessment of the degree of habituation or assumed constriction. REFERENCES Atwood, R. W., & Howell, R. J. Pupillometric and personality test score differences of pedophiliacs and normals. Psychonomic Science, 1971, 22, 115- 116. Barlow, J. D. Pupillary size as an index of preference in political candidates. Perceptual and Motor
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Cantor, J. R., Bryant, J., & Zillman, D. Enhancement of humor appreciation by transferred excitation. Journal of Personality and Social Psychology, 1974, 30, 812-821. Fredericks, R. S., & Groves, M. H. Pupil changes and stimulus pleasantness. Proceedings of the Annual Convention of the American Psychological Association, 1971,6,371-372. Goldwater, B. C. Psychological significance of pupillary movements. Psychological BuNetin,
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Hess, E. H. Attitude and pupil size. Scientijc American, 1965, 212, 46-54. Hess, E. H. Pupillometrics. In N. S. Greenfield & R. A. Sternbach (Eds.), Handbook of psychophysiology. New York: Holt, Rinehart, & Winston, 1972. Hess, E. H., & Goodwin, E. The present state of pupillometrics. In M. P. Janisse (Ed.), Pupillary dynamics and behavior. New York: Plenum, 1974. Hutt, L. D., & Anderson, J. P. The relationship between pupil size and recognition threshold. Psychonomic Science, 1967, 9, 477-478. Janisse, M. P. Pupil size and affect: A critical review of the literature since 1960. The Canadian Psychologist, 1973, 14, 31 l-329. Kahneman, D. Attention and effort. Englewood Cliffs, NJ: Prentice-Hall, 1973. Liberman, A. E. Some new data on the pupillary component in man. In L. G. Voronin, A. N. Leontiev, A. R. Luria, E. N. Sokolov, & 0. S. Vinogradova (Eds.), Orienting reflex and exploratory behavior. Washington, DC: American Institute of Biological Sciences, 1965. Lowenstein, 0.. & Loewenfeld, I. Influence of retinal adaptation upon the pupillary reflex to light in normal man: Part II. Effect of adaptation to dim illumination upon pupillary reflexes elicited by bright light. American Journal of Ophthalmology, 1%1,5,644-654.
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Miller, N. E., & Dollard, .I. Social /earning and imitation. New Haven: Yale University Press, 1941. Peake, H. V. S.. & Herz. M. J. (Eds.), Habituation. Vol. I. Behavioral studies. New York: Academic Press, 1973. Peavler, W. S., & McLaughlin, J. P. The question of stimulus content and pupil size. Psychonomic
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Pelt. J. M., & Hess, E. H. Changes in pupil size to visually presented words. Psychonomic Science, 1%8, 12, 389-399. Razran. G. Mind in evolution. New York: Houghton Mifflin, 1971. Robbins. H. The Betsy. New York: Simon & Schuster, 1972. Thompson, R. F., Groves, P. M., Taylor, T. J., & Roemer, R. A. A dual-process theory of habituation: Theory and behavior. In H. V. S. Peake & M. J. Herz (Eds.). Habituation. Vol. I. Behavioral studies. New York: Academic Press, 1973. Winer, B. J. Statistical principles in experimental design. New York: McGraw-Hill, 1971. Woodmansee. J. J. The pupil response as a measure of social attitudes. In C. Sumers (Ed.). Attitude measurement. Chicago: Rand-McNally, 1970.