ERPS AND EXTRAVERSION 1 RUNNING HEAD: ERPS AND EXTRAVERSION A New Look At Extraversion: An Event-Related Potential Analysis of Individual Differences Between Extraverts and Introverts. Kaylee Langille St.Thomas University April 2007
ERPS AND EXTRAVERSION 2 In his 1985 study, Brebner identified two key processes involved in stimulus response: stimulus analysis, and response organization. These processes are thought to represent differences between the personality dynamics of introversion and extroversion. While extroverts excel at the latter stage of organizing responses, they are over-stimulated by analysis. introverts on the other hand, have necessity for the excitement caused by stimulus analysis, and thus have an unconscious preference for it. In the present study, we will use an electrophysiological measure called the lateralized readiness potential (LRP) to study differences in stimulus-analysis and response-organization in highly extroverted and highly introverted participants. The LRP is found by subtracting negativities found over the (in this study) left and right index finger regions of the motor cortex. Increased negativity over central sites is related to motor preparation and execution by the contralateral index finger. The LRP will be used to identify the amount of time spent in both stimulus-analysis and response-organization by participants. It is hypothesized extroverts will show decreased latency of LRP onset in comparison to introverts, while introverts will show an earlier onset of N100, indicating more efficient stimulus-analysis.
ERPS AND EXTRAVERSION 3 Upon first encountering a new person, it is commonplace to make certain judgements about them and how they present themselves. These global judgements often consist of observations such as that of confidence or shyness, talkativeness, quietness, and whether or not they seem comfortable and open upon first meetings. While these are basic personality descriptors to most people, they actually represent a valuable and specific personality measure: that of extraversion. Without even knowing it, everyone makes hadaily judgements concerning the confidence, and socialability of people they encounter, reflecting their level of an invisible quality known as extraversion (Eaves & Eysenck, 1975). Extraversion, one of a larger group of personality measures, was perhaps most notably laid out by Hans Eysenck in his three-factor model of personality. Eysenck summarized the level of extraversion as the outgoing nature of the individual, the degree to which they are interactive with the people around them. Extraverts, people scoring highly on the scale, are gregarious, social, dominant, expressive, and excitement-seeking. Those at the lower end of the scale, called introverts, are more reserved, submissive, and less oriented towards meeting and interacting with new people. These personality differences lead extraverts towards large social gatherings, and what may be called thrill-seeking behavior. introverts, seemingly opposite, are felt to lend themselves to alone-time and low-key activities such as reading. While many theories exist to explain this difference, the most compelling and modernly accepted is Eysenck's own arousal theory, designating extraversion-introversion as related to physiological brain differences (Eaves & Eysenck, 1975). Arousal theory assumes that there is an optimum or ideal level of cortical arousal. When someone diverges away from this optimal level in either direction, they seek to move back towards it. Extraverts, according to Eysenck, are chronically below their optimum, constantly
ERPS AND EXTRAVERSION 4 feeling under-aroused. Because of this lack of arousal, extraverts seek external sources of stimulation to increase their arousal. This explains why extraverts thrive on meeting new people and engaging in new exciting behavior. introverts, however, are constantly over-aroused. Because of this excess of arousal, introverts seek calming and quiet activities to keep their cortical arousal levels lower. It is believed that at this optimum level of arousal, people are also at their optimal level of performance (Bullock & Gilliland, 1993). This constant quest for perfect arousal effectively explains numerous personality differences between the opposing ends of the extraversion scale. The need to increase arousal can account for a bold, people-loving, thrillseeking personality, while the need to calm down readily explains an introvert's quiet, solitary lifestyle. Alongside Eysenck's arousal theory, researchers such as John Brebner have further analysed individual differences between extraverts and introverts. Brebner proposed stages of processing known as stimulus-analysis (S-analysis) and response-organization (R-organization) which explain differences in the processing preferences of the two personality types. More withdrawn, quiet introverts are excited by stimulus analysis, a stage encorporated with thinking about something. They are inhibited, however, by organizing their actual response. Bolder extroverts are inhibited by the thinking process of stimulus analysis, and find excitement in response organization. This difference of presence finds its foundation in arousal theory, but expands upon it into more specific behavior differences (Brebner & Flavel, 1978). Brebner and Flavel predicted that because of their inclination towards response, they would make more accuracy errors when performing a task which demands speed, and would become slower and less accurate as the task became more difficult (thus demanding of more stimulus analysis). introverts, according to their study, would be affected less. The colleagues deduced most
ERPS AND EXTRAVERSION 5 basically that extroverts, geared to response organization, perform better at simple timedemanding tasks. As the tasks get more difficult, or stimuli get more complex, introverts gain an upper hand because of their inclination towards analyzing the stimulus (Brebner & Flavel, 1978). While these theories may account for much of the differentiation between the poles of the extraversion personality measure, they do not represent any sense of causation. It cannot be said that one is born with these arousal differences, and thus born an extravert. Their brain differences are not known to precede or cause their personality differences, for example, just as it cannot be said that a variation in personality actually determines the brain's physiological difference. What can be noted, however, are things which seem to be concretely related to the fundamental differences between extraverts and introverts (Brebner & Flavel, 1978). David Myers, for example, has found a correlation between personal happiness and higher levels of extraversion (Myers, 2003). Scoring highly on extraversion scales has also been tied to substance abuse and experimentation (Lodhi & Thakur, 1993), and high self-esteem (Cheng & Furnham, 2003). Personality differences along the lines of extraversion-introversion and their corresponding characteristics have also been shown to effect things like parenting style (Russell, 1997), and how one copes with psychological disorders such as depression (Uehara, Sakado, Sakado, Sato, & Someya, 1999). One of the most analyzed implications of this personality dynamic is that of motor differences between extraverts and introverts. For example, it is suggested that extraverts express themselves with larger more frequent movement, as well as having longer lengths of stride At the cortical level, motor response differences are often thought of as the key to finding a biological basis for extraversion-introversion differences. In 1985 Robert Stelmack concluded that the two personality groups differ in their expression of motor behavior when it requires a
ERPS AND EXTRAVERSION 6 simple motor response. Later in his 2000 study alongside Cynthia Doucet, he found that the extraverts responded more quickly and frequently than introverts. To make this observation, Stelmack used electroencephalogram (EEG) recording to analyze electrical activity changes over the scalp which act as markers for processing. By considering some such markers, the P300 which represents the decision making process, and the N100 which represents the aknowledgement of a new stimulus, alongside observables like response time, Stelmack and his colleagues attempted to gain insight into cognition processing and motor response differences between extraverts and introverts. At the end of the study, the researchers found no difference in P300 scoring (as a representation of cognitive processing speed) when the demands of the task were straight forward. The extraverts did however, tend to have smaller N100 amplitudes, which further suggests their preference for the latter stages of processing rather than the earlier ones preferred by introverts (Stelmack & Michaud-Achorn, 1985). While Doucet and Stelmack did not find P300 differences, a recent study by Beauducel, Brocke, and Leue moved forward from Brebner's theories to find P300 amplitude relations to the processing of more difficult tasks. The researchers expected to find that those scoring highly in extroversion would be less aroused, and would make a larger investment in the task represented by a larger P300 amplitude. Using an auditory stimulus task, they found support for their hypothesis with significantly higher P300 amplitudes, as well as lower arousal levels, amongst their higher-scoring extroverted group (Beauducel, Brocke, & Leue, 2006). This experiment points to the P300's amplitude as another potential marker for individual differences, alongside the earlier-occurring (and thus seemingly favored by introverts) N100. While many studies hint at individual differences between extraverts and introverts at different levels of motor processing, the consideration of a new measure was necessary to gain
ERPS AND EXTRAVERSION 7 insight into the invisible in-between stages of processing. Only by differentiating the processes of stimulus analysis and response organization can researchers pinpoint their role in individual personality differences, and how motor responses reflect extraversion and introversion. Of increasing interest in the field of individual differences within personality is the lateralized readiness potential, or LRP. This event related potential, viewable in EEG recording, is closely related to the preperation leading up to, and the execution of, motor responses. The LRP, first noted in an 1965 study, is the result of increased negativity in the motor cortex over the side corresponding to a movement. When the right finger is prepared for movement, and moves, for example, a negative potential can be noted on the scalp above the left motor cortex region. The LRP is also visible on the opposite side, but causes a smaller, less dramatic potential. By subtracting these negativities the final LRP is calculated (Rammsayer & Stahl, 2004). Perhaps the first researchers to associate LRPs with the processes of stimulus analysis and response organization in extraverts and introverts were Rammsayer and Stahl. Using a speed based tone differentiation task, the colleagues electrophysiologically observed the processing of a chosen motor response. They calculated and compared both response and stimulus-locked lateralized readiness potentials for their participants, as well as observing other ERPs and general response time differences, and compared the extraverts and introverts (Rammsayer & Stahl, 2004). While they found a larger N100 ERP (a marker of aknowledgement of a stimulus) in introverts, the researchers failed to find significant results which would have lead to more concrete differences between extraverts and introverts. This lack of difference, they suggested, may have been because of the task simplicity. If more complexity were involved, as also suggested in the previously mentioned Stelmack study, the difference may be more detectable. This study, while inconclusive in many respects, served as further support for Brebner's concepts
ERPS AND EXTRAVERSION 8 of stimulus analysis and response organization, especially in their observation of longer movement times (in the final motor reaction process) within their introverted group (Rammsayer & Stahl, 2004). Even more recently, Stahl and Rammsayer built upon their previous study, again looking at the differences in processing speeds of introverts and extraverts and their relationship to the different stages of processing. Using a response panel with a home button, and a complex visual stimulus (as opposed to their previous simple auditory stimulus), the colleagues found that central processing is indeed faster in introverts. There was no detected different in response time between the two groups. Their research suggests that LRPs, calculated in two different ways, are an effective way to look at very differences between introverts and extraverts in their processing of stimuli into motor actions. They also reccomended future research involve highly complex stimuli to maximize results (Rammsayer & Stahl, 2004). The effective differentiation skills of the LRP based on the works of Stahl and Rammsayer lies in the division of SLRPs, or stimulus-locked lateralized readiness potentials, and RLRPs, response-locked lateralized readiness potentials. The SLRP, calculated from the stimulus forward, is thought to represent the earlier stages of processing, stimulus analysis. The RLRP, on the other hand, moves from the act of response, backwards, and is tied to later processes, or response-organization. Because of their ties to these stages, these two LRPs are believed to further distinguish the differences between the processing of extraverts and introverts (Rammsayer & Stahl, 2004). Building on previous works by Rammsayer, Stahl, and their colleagues, as well as the foundations laid by Brebner, the study at hand will attempt to verify several hypotheses by considering electroencephalogram (EEG) recordings from both extraverts and introverts. By
ERPS AND EXTRAVERSION 9 comparing specific event related potential (ERP) landmarks, it is possible to detect processing differences in the motor responses of the two personality groups at different stages, such as those of stimulus analysis and response organization. It is hypothesized that 1. introverts will show an earlier onset of N100, indicating more efficient stimulus-analysis, 2. extraverts will have a larger P300 amplitude because the tone is complex, 3. extraverts will have shorter RLRP latencies (or a quicker onset) because of their tendency towards response-organization, and lastly 4. that introverts will have shorter SLRP latencies because of their own tendency towards the stimulus analysis stages it represents. Combined these hypotheses represent individual differences between the motor response processing of extraverts and introverts. Methods Participants Participants were chosen after completing a revised version of the Eysenck Personality Questionnaire (EPQ-R) scoring survey to be submitted over the internet. The questionnaire was available to students of St.Thomas University, the majority being enrolled in introductory psychology classes. The survey consisted of 100 yes or no questions, aimed at pinpointing Eysenck's different aspects of personality. Of interest in this study were questions pertaining to the student's level of extraversion which was judged using questions such as are you a talkative person? and are you rather lively?. Students who scored over 18 on the questionnaire were considered to be among the extraverted group, scoring high in extraversion, while those scoring below 10 served as the introverted group, scoring low in extraversion. Students were contacted regarding their scores and students considered part of either extreme group (introverts or extraverts) were invited to follow up with an in-lab experiment. Those who voluntarily
ERPS AND EXTRAVERSION 10 responded were able to schedule appointments online for the experiment at hand. The resulting participants consisted of 20 students, 10 female and 5 mile, but due to technical reasons the final analysis of only 17 of the participants were considered. Participants completed a consent form before taking part in the experiment and were not taking any medication at the time of the study. Stimuli/Procedure Participants were sat in a monitored room in a comfortable chair, about 1m from a computer screen, seperate from the room in which recording took place. They were instructed to listen to a series of complex high and low tones through a set of headphones, and respond according to promptings on the screen. Because left- and right-finger trials were counterbalanced within the subject, the instructions expressed which index finger the participant should use when responding to which tone. The screen, for example, may say high - right, low - left. The participants were asked to respond as quickly and accuractely as possible, using their left and right index fingers on a keypad. They were each given practice trials with both positive and negative feedback until their timing and accuracy were sufficient for the task. The participants then completed the task which consisted of six blocks of 200 test trials per block. Each block began with practice trials to ensure the participants' continued understanding. After these practice trials, the blocks consisted of only negative feedback, otherwise no feedback was given on the screen. Participants were given the option of a break between the presentation of each block. The tones which served as the test trials per block were of a complex nature, each lasting 300 ms with a rise and fall time of 10 ms. The tones, according to high or low standings, had a frequency of either 500 or 700 Hz.
ERPS AND EXTRAVERSION 11 ERP Recording The event-related potentials were recorded from the electrocap with electrodes arranged in the international 10-20 system. The cap featured additional electrodes (C3 and C4 ) over the index finger region of the motor cortex on each side. These electrodes were specifically added because of their proximity to valuable areas. The data for this study was recorded and scored from all electrodes for the P300 and N100 responses to the tones. Two clip-on electrodes were placed on the ears as reference, and EOG activity was recorded using bipolar electrodes place above and below the left eye for vertical movement, as well as on the outside edge of both eyes to record horizontal movement. All EEG activity was recorded after passing through an amplifier. It was visually edited for flawed sections, which were eliminated. The data was then subject to artificat rejection, a seperate rejection process for excessive EOG acivity, and epoched before averaging. Data Analysis Averages were obtained for the N100 ERPs of all the tones for each participant, scoring the largest negative peak between 50 and 200 ms. ERP averages were also recorded for the P300 of each participant in response to all tones, scoring the largest positive peak between 250 and 600 ms. Both ERPs were obtained through band-pass filtering with a zero phase shift low pass of 20 hz at 48 db/oct, and a high pass of 0.25 at 48 db/oct. The lateralized readiness potential (LRP) was also scored for each participant in both. The LRP is found by subtracting negativities found over, in this particular study, the left and right index finger regions of the motor cortex. To differentiate between stimulus analysis and response organization, the the response-locked (RLRP) and stimulus-locked (SLRP) LRPs are scored. The response-locked lateralized readiness potential, or RLRP, was calculated by indexing the time from the onset of the LRP to the actual
ERPS AND EXTRAVERSION 12 motor response (the button press). The onset of the LRP is scored by simply taking half of the peak height, and recording it and the time at which it occurs. The stimulus-locked lateralized readiness potential, or SLRP, indexes the time between the stimulus onset and the LRP onset. The index for the SLRP was recorded in the same manner as the RLRP. Results Using a between-groups t-test, reaction time was analyzed for introverts and extraverts. Reaction time was found to be longer for the introvert group (M = 386) than the extrovert group (M = 335), t(15) = 1.77, p =.09. Using a repeated measure analysis of variance (ANOVA), the P300 event related potential amplitude was analysed. The P300 amplitude from the Pz and CP4 electrodes served as repeated measures, and extraversion as a between subjects factor. It was found that there was a significant interaction between the electrode position, and extraversion, F (2,30) = 2.7, p =.08. This interaction was primarily caused by the larger amplitude of the P300 ERP for extraverts (M = 7.2 µv) as compared to introverts (M = 3.6 µv) at the P4 electrode. No other tests considered reached the generous significance level of.10. Discussion The present results differed dramatically from the researcher expectations, as well as the outcome of previous research pursuits. While it was predicted that LRP and N100 differences would exist, none were significantly found. The only results beyond a basic difference in reaction time which reached noteable significance were that of the P300 at a single electrode. Because of its detection only at a single electrode, the significance of P300 amplitude differences
ERPS AND EXTRAVERSION 13 are considered weak, though supportive of the current hypothesis. A larger P300 for extraverts may represent their tendency towards later stages of processing (the decision making/response organizing, rather than the earlier stages of stimulus analysis represented partially by the N100). P300 differences are in line with previous research which both detects P300 differences, and provides support for extraversion-introversion differences when it comes to alternating stages of stimulus processing (Beauducel, Brocke, & Leue, 2006). Combined with previous research results, P300 analysis appears to be a valuable enquiry into individual differences between extraverts and introverts. None of the other measures hypothesized, however, served to support the direction of past research. This could be due to a number of aspects of the study at hand. One consideration is that of complexity and simplicity. While the auditory stimuli employed in this study were considered complex, the ammount of complexity may not have been adequate to provide the intended response. It has been shown in previous research that those scoring high in extraversion have a decreasing level of performance, and a higher investment (shown by P300 amplitudes), as stimuli get more complex. This complexity almost levels the playing field between introverts and extraverts, as introverts excel at this analysis but fall behind when it comes to the later stages of response. In the task at hand, it appears as if neither group is able to excel past one another. This suggests that the stimulus was not complex enough to stimulate the introverts beyond the abilities of the extraverts, but also that it was not simple enough to merely let the extraverts advance (Rammsayer & Stahl, 2004). Future research could attempt to avoid this by placing their stimulus in the extremes of complexity or simplicity, as previously suggested by Stahl and Rammsayer. Complexity and simplicity may also be considered when it comes to the response. A response which features a single button for each finger is a simple response, a more complex
ERPS AND EXTRAVERSION 14 response may have caused further differentiation within the results, and between those scoring at the high or low end of the extraversion scale. Past research encorporating variations such as holding down a home button, may have impacted their results, causing them to diverge from the results of the study at hand (Rammsayer & Stahl, 2004). It appears that past research has hinted at varation in results, according to different natures and styles of response demands. Two other considerations must be made when considering the insignificant results. It has been commonly documented that the tiny, difficult-to-detect LRP forms require large sample sizes to be readily valuable. The small sample size implemented in this study does not lend itself well to detecting small differences between individuals, and finding significant differences between the average scores of the extravert and intravert groups. Besides the LRP scoring, the small sample size may have also effected the significance of the other ERP measures considered. Also effecting this may have been a final consideration, the scoring cut offs used in the intial personality questionnaire. It is possible that using more extreme groups of extraverts and introverts, with scores further from the middle, may have maximized the potential of finding results. If the personality types people have comes to associate with extraversion are expressions of their arousal and processing differences, then those who score highest on extraversion scales would also most extremely exemplify the low arousal and inclination towards the latter, response-organization, stages of processing. This kind of maximization may have pushed the potential results of this study into the realm of significance. Limitations such as these could quite easily be improved upon in future research, and the lack of significance here should not serve as a deterrence to the line of investigation. Past research has indicated promising leads into physiological enquiries into individual differences, especially those considering the lateralized readiness potential. If this measure was further
ERPS AND EXTRAVERSION 15 validated in the future it would open numerous doors in both research, and application. Finding a concrete biological measure is valuable in many respects. When it comes to personality types and individual differences, most specifically that of extraversion-introversion, it allows for movement beyond the limitations of pencil and paper questionnaires. While these attributes are currently defined by self-report answers to questions, these answers and even the questions and any interviewer themlseves, may bring various biases along with them. It may, for example, be more desirable for participants to showcase themself as a fun-loving and popular extravert. These biases could be avoided if a biological tool could be used as a more precise diagnostic tool. In the future, perhaps, a trained individual could take a look at analysed EEG recordings, and deduce someones exact personality profile. On a larger level, this type of biological tool could prove to be valuable beyond simple typology. Insight into the processing differences of extraverts and introverts could lead to a number of considerations, such as the health concerns which may differ between these two groups. Such fundamental differences could predispose one group to different types of stress, for example. The response to these problems could also be different. Responding to the mental and physical health needs of extraverts and introverts may be decidedly different, as well as teaching techniques and other interactions. Along with pinpointing key differences between the two groups and how they interact with the world, there are also benefits to verifying the initial suggestions of Brebner: the existence of differing processing stages he calls stimulus analysis and response organization. If these stages are further supported they will prove useful in furthering research not only into individual differences, but also into the broader areas of cognition and other psychological considerations. The footprint of a study such as this could be huge if properly initiated.
ERPS AND EXTRAVERSION 16 References Beauducel, A., Brocke, B., & Leue, A. (2006). Energetical bases of extraversion: Effort, arousal, EEG, and performance. International Journal of Psychophysiology, 62(2), 212-223. Brebner, J. "Personality Theory and Movement." Individual Differences in Movement. Ed. B. Kirkcaldy. Lancaster, UK: MTP, 1985 Brebner, J. & Flavel, R. (1978). The effect of catch-trails on speed and accuracy among introverts and extraversion a simple RT task. Br. J. Psychology. 69, 9-15. Bullock, W., & Gilliland, K. (1993). Eysenck's arousal theory of introversion-extraversion: a converging measures investigation. Journal of Personality and Social Psychology, 64(1), 113-123.
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