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yield equivalent empirical findings. Several studies have failed to confirm this asser- tion. This study investigated whether the reliable differences in Wave V of ...
Journal of Research in Personality 35, 117–126 (2001) doi:10.1006/jrpe.2000.2299, available online at http://www.idealibrary.com on

Congruency of the Relationship between Extraversion and the Brainstem Auditory Evoked Response Based on the EPI versus the EPQ Luz-Eugenia Cox-Fuenzalida and Kirby Gilliland University of Oklahoma

and Rhonda J. Swickert College of Charleston

According to Eysenck and Eysenck (1985), the Eysenck Personality Inventory (EPI) and the Eysenck Personality Questionnaire (EPQ) extraversion scales should yield equivalent empirical findings. Several studies have failed to confirm this assertion. This study investigated whether the reliable differences in Wave V of the brainstem auditory evoked response (BAER) reported between introverts and extraverts based on the EPI could be replicated with the EPQ. Significant correlations between BAER Wave V and EPQ extraversion scores were found. Results for BAER Wave V also supported Eysenck and Eysenck’s view that the EPI and EPQ ought to provide comparable findings. However, this study also highlighted the important manner in which different classes of dependent measures may mediate the degree of functional comparability between the EPI and EPQ.  2001 Academic Press Key Words: BAER; EPI; EPQ; extraversion.

Eysenck (1967) presented a biological theory of extraversion that proposed physiological arousal, mediated by the ascending reticular activating system (ARAS), as the underlying mechanism that leads to much of the observed behavioral differences between introverts and extraverts. The authors express their gratitude to Dr. Judy Lauter for the loan of her equipment and for her consultation on BAER recording techniques. The authors also thank Marni Hill, Shawn Scarsdale, and Amanda Snyder for their work as research assistants. Address correspondence and reprint requests to Luz-Eugenia Cox-Fuenzalida, Department of Psychology, 455 West Lindsey Street, #705, University of Oklahoma, Norman, OK 73019. Fax: (405) 325-4737. E-mail: [email protected]. 117 0092-6566/01 $35.00

Copyright  2001 by Academic Press All rights of reproduction in any form reserved.

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Eysenck’s hypothesized relationship between phasic arousal and extraversion has led to many behavioral and physiological studies that have generally lent support to his theory (Eysenck & Eysenck, 1985; Gale, 1973; Matthews & Gilliland, 1999; Stelmack & Wilson, 1982). Since 1963, the Eysenck Personality Inventory (EPI) has often been used to assess the trait of extraversion. The EPI consists of 24 items selected to measure extraversion, including the subscales of sociability and impulsivity that contain 13 and 9 items, respectively (Eysenck & Eysenck, 1968). In 1975, Eysenck and Eysenck introduced the Eysenck Personality Questionnaire (EPQ), which included a significant revision of the extraversion scale. Specifically, the impulsivity items were largely replaced with additional sociability items resulting in a scale much more unidimensional in nature. However, Eysenck and Eysenck (1975, p. 3) expressed confidence in the comparability of the two extraversion scales, assuring that ‘‘whatever has been discovered about correlates of extraversion and neuroticism with the use of the older [EPI] scales must be assumed to apply with equal force to the new [EPQ] scales.’’ Thus, they suggested that the EPI and EPQ provide functionally equivalent scales of extraversion. Subsequent research has raised important doubts about this assurance. Clearly, the factor structure of the two scales is decidedly different (Helmes, 1980; Rocklin & Revelle, 1981; Roger & Morris, 1991). Furthermore, because the two scales have different subscale structures and because other researchers have suggested that sociability and impulsivity may differ as a measure of the purported biological basis of the trait, the use of one scale or the other may lead to different research outcomes. For example, some research has supported the view that the subscale of impulsivity is largely responsible for the behavioral or cognitive correlates of extraversion (Gilliland, 1976; Matthews, 1987; Neubauer, 1992; Revelle, Humphreys, Simon, & Gilliland, 1980; Smith, Rypma, & Wilson, 1981). Other research has supported the view that sociability is primarily responsible for these relationships (Larsen, 1985; Swickert & Gilliland, 1998; Thackray, Jones, & Touchstone, 1974; Wilson, 1990). Some researchers have even suggested that it is perhaps more advantageous to consider the extraversion subscales as distinct constructs (Carrigan, 1960; Guilford, 1975). One recent study provided an intriguing insight into the possible comparability of the EPI and EPQ extraversion scales. Swickert and Gilliland (1998) examined the relationship between extraversion and the brainstem auditory evoked response (BAER). The BAER is a noninvasive EEG technique used for investigating the auditory neural pathway. The BAER provides a stable series of seven positive waves that represents the activity of various neural structures stimulated as neural impulses travel from the outer ear to the lowest auditory projection areas of the cortex. This highly stable evoked re-

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sponse technique has been well documented along with methodological and participant variables that are most likely to influence it (Hall, 1992; Hughes, 1985; Hughes, Helgason, & Wilbur, 1988; Jacobson, 1985; Møller, Jannetta, & Jho, 1994; Møller, Janetta, & Sekhar, 1988; Stockard & Rossiter, 1977). Research has suggested that Waves I and II reflect activity of the eighth acoustic nerve (Hall, 1992), while Waves III through VII are generated in various brainstem nuclei (Markand, 1994; Møller et al., 1994). A number of personality researchers have established a fairly reliable BAER difference between introverts and extraverts (Andress & Church, 1981; Bullock & Gilliland, 1993; Stelmack & Wilson, 1982; Swickert & Gilliland, 1998), although some have reported marginally significant results (Stelmack, Campbell, & Bell, 1993), but in this latter study methodological control factors along with small participant size could have accounted for the lack of significance. Specifically, introverts appear to have shorter Wave V latency as compared with extraverts. Wave V of the BAER has been linked to the inferior colliculus, the area of the auditory pathway that transects the ARAS. In addition, several researchers (Brodal, 1957, 1981; Guyton, 1981; Scheibel & Scheibel, 1958) have suggested links between the ARAS and the auditory pathway. These studies support the view that the BAER appears to provide a useful measure that differentiates between introverts and extraverts at the psychophysiological level. The Swickert and Gilliland (1998) study offered three important contributions. First, they affirmed the Wave V latency differences found by previous researchers. Second, their study used a large random sample of participants across the full range of extraversion, rather than a more circumscribed extreme group or small participant sample method often adopted in psychophysiological studies. And third, these researchers explored the relationship between the impulsivity and sociability subscales of the extraversion dimension. They reported that the sociability subscale yielded significant correlations with Wave V latency, while impulsivity did not. If sociability is a better predictor of BAER Wave V activity, and the EPQ extraversion scale is weighted almost exclusively on sociability, then the EPQ extraversion scale ought to yield significant correlations with BAER Wave V latency. In addition, the assurance offered by Eysenck and Eysenck (1975) regarding the comparability of the EPI and EPQ extraversion scales could be tested directly by comparing the relationship between BAER Wave V latency and the EPI extraversion scale with the relationship between BAER Wave V latency to the EPQ extraversion scale. The purpose of this study was to determine whether introverts and extraverts, as classified by the EPQ, differ in BAER Wave V latency in much the same manner as reported for participants classified by the EPI. In addition, this study was also designed to test whether either the EPI or EPQ

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extraversion scale has a significantly stronger relationship to BAER Wave V activity. Like the Swickert and Gilliland (1998) study, a large sample that is more representative of the full range of extraversion was used. Based on Eysenck’s assertion, it was predicted that both the EPI and EPQ would be significantly related to BAER Wave V latency. In addition, because Swickert and Gilliland (1998) reported that the EPI extraversion subscale of sociability is more closely related to BAER Wave V latency than impulsivity and because the EPQ extraversion scale is weighted more heavily on sociability (Rocklin & Revelle, 1981; Roger & Morris, 1991), it seems very plausible that the EPQ extraversion scale may be a better predictor of Wave V BAER activity than the EPI extraversion scale. METHOD Participants Participants (N ⫽ 78) consisted of undergraduate introductory psychology students from a university located in the Midwest. The participants in this study were randomly selected from a larger investigation of EEG measures and personality (N ⫽ 149). The first 74 participants in this larger sample formed the participant pool for a study reported previously (Swickert & Gilliland, 1998). Seventy-eight participants were randomly selected for this study, which meant that this study and the Swickert and Gilliland (1998) investigation had an overlap in participant samples of approximately 50%. A number of factors such as click rate, filtering, and participant characteristics are known to influence the BAER (Hall, 1992; Moore, 1983; Working Group on Auditory Evoked Potential Measurements, 1988). To address these issues a number of methodological and control procedures were implemented in this study. First, participants were screened on a number of criteria believed to influence CNS activity. Individuals ingesting large quantities of alcohol (more than 12 drinks per week), caffeine (more than 4 cups of coffee per day), nicotine (more than 5 cigarettes a day), or any CNS active medication were not included in the sample. Second, because the dependent variable of the study relies upon auditory pathway functioning, individuals reporting any hearing deficit were excluded from the sample. Third, research indicates that BAER latencies are unreliable for females in the midcycle of their menstrual cycle and for females reporting severe PMS symptomatology (Elkind-Hirsch, Wallace, Malinak, & Jerger, 1994; Howard, Mason, Taghavi, & Spears, 1992). Therefore, female participants were screened with selected items from the Menstrual Distress Questionnaire (Moos, 1968) to assess menstrual cycle and severe premenstrual symptomatology. Consequently, sessions were scheduled to avoid testing during a female participant’s midcycle. Females reporting severe PMS symptomatology were not included in the study. Males and females were equally represented in the sample of participants in this study.

Personality Scales The Eysenck Personality Inventory (Form A; Eysenck & Eysenck, 1968) and the Eysenck Personality Questionnaire (Eysenck & Eysenck, 1975) were used to assess the dimensions of extraversion and neuroticism. Reliability estimates for the EPI range from r ⫽ .80 to .97 (Eysenck & Eysenck, 1975). Extraversion on the EPI is composed of the subscales of impulsivity and sociability. Reliability estimates for the EPQ range from r ⫽ .80 to .90 (Eysenck & Eysenck, 1975). Extraversion on the EPQ is almost exclusively a measure of sociability or social extraversion.

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Apparatus Standard, 10-mm gold EEG electrodes were placed at Cz, A2, and Fpz (refer to the 10– 20 International System of electrode placement; Jasper, 1958). The Cz lead was referenced to A2, while Fpz served as ground. Electrode sites were cleaned with alcohol and mildly abraded with Omniprep ECG and EKG skin prepping paste. Electrodes were held in place with Grass EC 2 electrode cream and adhesive tape. Electrode impedance was kept below 5 kΩ. The administration of auditory stimuli and the recording of the brainstem auditory evoked responses were conducted with a Biologic clinical averager. The artifact rejection standard was set at 20 µV, and filter settings of 150 to 3000 Hz were employed. Condensation clicks of 0.1 ms were introduced monaurally to the right ear at 70 dB normal hearing level (nHL) at a rate of 11.1 cps. White noise at 40 dB nHL was used to mask the unstimulated left ear. Auditory evoked responses were averaged over 2048 click trials with an analysis time of 10 ms.

Procedure Participants were contacted prior to the experiment and asked to refrain from consuming substances known to influence CNS activity (e.g., alcohol, caffeine, and nicotine) for 2 h prior to their participation in the study. This 2-h interval was used to minimize the effect of these drugs on CNS activity while avoiding inducement of withdrawal effects in the participants. Further, it has been suggested in the literature that participants’ preexperimental activities may adversely impact precise physiological measurement (Gale & Edwards, 1983). In an effort to control pretest behavior, participants were asked to arrive 1 h prior to their BAER recording session. At that time, participants completed informed consent forms, the EPI, and the EPQ and performed a standard version of the Bakan Vigilance Task (1959). Following the 1-h pretesting period, participants were instrumented with electrodes. Participants were then seated in a supine position in a sound-attenuated, electrically shielded room. They relaxed for approximately 10 min in this position and then BAER recording commenced. In an additional effort to reduce muscle artifact, participants were instructed to close their eyes and be as still as possible, but to remain awake. To control for time-of-day effects, all testing was performed between 9:00 AM and 4:00 PM (Revelle, Humphreys, Simon, & Gilliland, 1980). Two consecutive trials of auditory evoked responses were recorded. Following BAER recording sessions, participants were debriefed.

RESULTS Both gender and neuroticism were partialed out in an effort to control for gender effects identified in BAERs (Markand, 1994; Thivierge & Cote, 1987) and to minimize the potential arousing influence of the limbic system (Eysenck, 1967), respectively. Because the most reliable differences between introverts and extraverts have been reported in Wave V of the BAER, the assessment and analyses of absolute latency was limited to Wave V. To test the first hypothesis, partial correlational analyses were used to assess the relationship between BAER Wave V and extraversion as measured by the EPQ. Wave V absolute latency was significantly related to the EPQ extraversion scale for both trial 1 [r (76) ⫽ .23, p ⫽ .046] and trial 2 [r(76) ⫽ .26, p ⫽ .026]. To confirm past research findings and to assure the participant sample used in this study corresponded well to past samples reported in the literature, the partial correlation between the EPI extraversion scale and the

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BAER Wave V latency was calculated. This partial correlation was significant for trial 1 [r(76) ⫽ .27, p ⫽ .019] and for trial 2 [r (76) ⫽ .24, p ⫽ .041]. These results confirmed that EPI extraversion was significantly related to Wave V absolute latency. In addition, analyses of the subscales of the EPI (impulsivity and sociability) yielded findings comparable to past studies as well. Specifically, while sociability was significantly related to Wave V latency for trial 1 [r(76) ⫽ .28, p ⫽ .013] and trial 2 [r(76) ⫽ .25, p ⫽ .029], impulsivity was not for either trial 1 [r(76) ⫽ .06, p ⫽ .622] or 2 [r(76) ⫽ .01, p ⫽ .910]. To test the second hypothesis, that EPQ extraversion scores may be more significantly related to BAER Wave V latency than EPI extraversion scores, a t test designed to measure the significance of the difference between dependent correlation coefficients was calculated. This analysis revealed no significant difference for trail 1 [t (75) ⫽ .4991, p ⬎ .20] or trial 2 [t (75) ⫽ .2492, p ⬎ .20]. Thus, the correlation coefficients of EPI extraversion and Wave V were not significantly different from those of the EPQ extraversion and Wave V for trials 1 or 2. DISCUSSION This study was the first to investigate the relationship between BAER and extraversion as measured by the EPQ extraversion scale. It also allowed comparison across the full range of the extraversion dimension. The findings of Swickert and Gilliland (1998) indicated that the EPI subscale of sociability was a significantly better predictor of BAER than either impulsivity or extraversion. Thus, it was predicted that there would be a significant relationship between EPQ extraversion, which is primarily composed of sociability items, and BAER Wave V activity. Indeed, based on the EPQ, introverts had shorter Wave V latency compared with extraverts, and therefore this hypothesis was clearly supported. The findings of this study also confirmed the relationship between BAER Wave V and EPI extraversion demonstrated in previous studies (Andress & Church, 1981; Bullock & Gilliland, 1993; Stelmack & Wilson, 1982; Swickert & Gilliland, 1998). Correlational analyses revealed significantly shorter Wave V latency for introverts classified by the EPI as compared with extraverts. Also, this study confirmed past findings (Swickert & Gilliland, 1998) that the sociability subscale of extraversion is a better predictor of Wave V activity than the subscale of impulsivity. Furthermore, this study provides a unique comparison of the relationship between BAER activity and extraversion as measured by both the EPI and EPQ. Because of the increased weighting on sociability in the EPQ extraversion scale, it was predicted that the EPQ extraversion scale may be related to BAER Wave V latency to a more significant degree than the EPI extraversion scale. A subsequent test investigating the difference between the correla-

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tion coefficients of EPI extraversion and Wave V and EPQ extraversion and Wave V yielded no significant findings. Therefore, the hypothesized stronger relationship between EPQ extraversion and Wave V latency, inferred from the findings of Swickert and Gilliland (1998), was not supported. Both extraversion scales appear about equally related and apparently provide core content of sociability that is related to BAER Wave V activity. In fact, the absence of a significantly stronger relationship between the EPQ extraversion scale and BAER Wave V latency may simply suggest that the EPQ does not provide a more refined or more accurate measure of sociability than the EPI. Consequently, the EPQ and the EPI may provide the same measure of extraversion when the criterion measure is not influenced by impulsivity. However, an important point to consider is that when a criterion measure is influenced by impulsivity, as it apparently may be in some behavioral studies (see Rocklin & Revelle, 1981), the EPI and the EPQ extraversion scales may fail to provide comparable predictive ability. In summary, this investigation answered important questions regarding the relationship between brainstem psychophysiological measures and extraversion. This study established the relationship between the EPQ extraversion scale and Wave V of the BAER. In addition, significant relationships were found between the EPI and EPQ and Wave V absolute latency for two BAER trials, and neither measure of extraversion (EPI nor EPQ) was found to have a significantly stronger relationship than the other. These findings suggest that both the EPI extraversion scale and the EPQ extraversion scale apparently predict Wave V BAER activity equally well. This finding adds to the growing psychophysiological evidence supporting the biologically based theory of introversion–extraversion. In addition, at least in terms of BAER Wave V latency, these results are supportive of the assertion made by Eysenck and Eysenck (1975) concerning the practical comparability of the extraversion scale of the two questionnaires. However, while this study may appear to support the comparability of these two extraversion scales for Wave V of the BAER, this degree of comparability may not be true for all dependent measures. One important point highlighted by the comparison of the results of this investigation with past research is that the degree to which the criterion measure is differentially sensitive to the influence of sociability or impulsivity may be critical in determining whether the EPI and EPQ extraversion scale are in fact functionally equivalent. REFERENCES Andress, D., & Church, M. (1981). Differences in brainstem auditory evoked responses between introverts and extraverts as a function of stimulus intensity. Psychophysiology, 18, 156–157. Bakan, P. (1959). Extraversion-introversion and improvement in an auditory vigilance task. British Journal of Psychology, 50, 325–332.

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