Running Head : EXTRAVERSION AND SENSORY DISCRIMINATION 1 Extraversion and Sensory Discrimination

The purpose of this study was to extend the electrocortical activity research in determining the biological bases of extraversion. Differences in auditory stimulation sensitivity have been previously found for extraverts and introverts, but there has been no focus on the potential differences in sensory discrimination abilities. Using event-related potential (ERP) techniques, this study’s focus is on the mismatch negativity wave (MMN)—an index of sensory discrimination—in determining possible the effects of personality on simple sensory tasks. Based on Eysenck’s arousal model, Brebner’s model of extraversion, and the adaptive model of the MMN, it was expected that extraverts would display significantly larger and less latent MMNs. ERPs were elicited from 64 female, undergraduate students, who were given the Eysenck Personality Questionnaire-Revised (EPQ-R), as well as the International Personality Item Pool version of Tellegen’s Multidimensional Personality Questionnaire (MPQ), to measure extraversion. A repeated measures mixed analysis of variance (ANOVA) was employed, and results contrary to expectations were obtained. Possible reasons as to why the hypothesis was not supported, the limitations of the study, and suggestions for future research are discussed. EXTRAVERSION AND SENSORY DISCRIMINATION 3 Extraversion and Sensory Discrimination Extraversion (E) is a personality dimension which has long been the subject of psychological research, is in many scales of personality—including the currently popular FiveFactor Theory (McCrae & Costa, 1999)— and whose definitions range from broad to narrow (Zuckerman, 2005). Eysenck viewed the typology of personality as essential to psychology as the development of the table of elements was for chemistry (Eysenck, 1973), and viewed personality types as clusters of several interrelated traits, and defined traits as significant intercorrelations between different habitual behaviours (Eysenck, 1990). In his factor analysis of personality, he found those high in E encompassing traits such as gregariousness, impulsivity, liveliness, dominance, and assertiveness, while those low in E encompasses the converse traits. These proposed traits of E echo early personality theorists, such as Jung, who defined E as the movement of all attention to the outside world— the social or physical environment (Jung, 1921). Exact definitions of E vary and evolve over time, and this is usually to complement ongoing research, making it clear the value of investigating biological bases of personality to further affirm, amend, and contribute to theoretical frameworks. Personality psychologist Gray (1973, as cited by Fowles 2006) noted that explanations for personality can be found in underlying psychological functions, and that the biological basis of these functions can be identified. Joireman and Kuhlman (2004) similarly state that a necessary criterion for theorizing a personality dimension is confirming its associated biological markers. Although imperative in the explanation of the individual differences in personality, and of paramount theoretical importance, a lucid understanding of the underlying biological bases of E have still not been obtained (Stelmack, 1990). EXTRAVERSION AND SENSORY DISCRIMINATION 4 Studying the biological bases of personality also has social relevance, as differences in personality, and accompanying traits, translate into differences in behaviour and preferences. An individual’s traits can be predictors of their life’s critical outcomes, such as physical health (Martin, Friedman, & Schwartz, 2007) and academic success (Chamorro-Premuzic & Furnham, 2003). People rating high in extraversion prefer and learn better with active discovery, while those low in E do better with passive learning (Eysenck, 1997). There is also a strong association between E and positive affect, although this is not a bidirectional relationship (Watson et al. 1999). These outward differences in behaviour have been attempted to be explained by many psychobiological theories. Psychobiological Theories of Extraversion By exploring the underlying biological bases, research could provide an explanation of trait behaviour— an enlightening etiology of personality, rather than a descriptive taxonomy (Clark & Watson, 2008). In understanding the evolution of research on the biological bases of personality, it is important to note the shift from top-down to bottom-up approaches. Eysenck started with personality theory and attempted to introduce constructs from biology in explaining individual differences. In contrast, Gray began with established behavioural and biological mechanisms (in animals) and attempted to explain which personality traits differences in these mechanisms would produce (Fowles, 2006). In either approach, understanding biological bases is essential for personality theory. Eysenck was one of the pioneering researchers to attempt to assimilate personality theory with a biological explanation. Eysenck (1967) explained the behavioural characteristics of extraversion (E) with his theory of arousal, which suggests extraverts have a lower level of cortical activity and a higher threshold of arousal than introverts. From this assumption, he EXTRAVERSION AND SENSORY DISCRIMINATION 5 suggested that extraverts have higher arousal thresholds than introverts, and therefore higher optimal levels of arousal by stimulation. Although the assumption of differing tonic levels of cortical activity was eventually not strongly supported by any empirical data, the concept more so served heuristic benefits to future research questions and findings. Eysenck's arousal theory played a seminal part in the development of Zuckerman’s theory of sensation seeking and optimal levels of arousal. In this theory, high levels of sensation seeking is associated with E, meaning that extraverts are seeking out environments high in stimulation while introverts are content with those low in stimulation (Zuckerman, 2005). Depue and Collins (1999) view agency (i.e. assertiveness and dominance) and affiliation (i.e. sociability) as part of a single characteristic they term ‘interpersonal engagement’, and impulsivity as the other central characteristic of E. Depue (2006) holds the view that E is an index of human variation in incentive-facilitated behaviour, with those high in E being high in such behaviour. He proposes that agentic E reflects the activity of a behavioural approach system based on positive incentive motivation. Rammsayer (2003) points out that as it focuses entirely on the mesolimbic DA system, it does not explain individual differences in sensory and motor functions in extraverts and introverts, which are thought to be mediated by another dopamine pathway: the mesostriatal DA system. Brebner (1983) proposed a model of E explaining differences in sensory information processing and motor functions, suggesting that extraverts experience stronger excitation from response organization, meaning they prefer fast responding and receive satisfaction from outward actions, while introverts experience stronger excitation from stimulus analysis. The former suggestion (faster motor movement for extraverts) has been supported by research using auditory stimulation (Rammsayer & Stahl, 2004). Differences found in sensory discrimination EXTRAVERSION AND SENSORY DISCRIMINATION 6 may help to clarify the reasons for the sensory information processing differences in this model. Mismatch Negativity Wave (MMN) Important research on extraversion using basic auditory stimuli has shown extraverts are less sensitive to sensory stimulation than introverts, thus showing smaller stimuli-responses on measures of electrocortical activity (Stelmack, Achorn, & Michaud, 1977; Stelmack, 1990). While there are studies looking at the differences in simple sensory detection and sensitivity of extraverts and introverts, there is little evidence showing their sensory discrimination abilities. Finding basic differences in these abilities would further elucidate the biological bases of extraversion. This issue was explored by Sasaki, Campbell, Bazana, and Stelmack (2000) by comparing extraverts with introverts, and possible differences in an event-related potential (ERP) associated with sensory discrimination, known as the mismatch negativity wave (MMN). ERPs are the electric potentials, or EEGs, that are time-locked to specific events (such as a sound). They are much smaller in voltage than EEGs, and are regarded as marker of underlying psychological processes occurring either in preparation or in result of the event. To discriminate ERPs from non-event-related background EEGs, all activity that is time-locked to an event (slight before and after the event) is averaged. The logic is that background noise (nonrelated EEGs) are not time-locked to the event and occur randomly, effectively reducing them and leaving genuine event-related potentials visible (Coles, 1990; Handy, 2005). The MMN wave is an ERP component that is elicited by any discriminable change in ongoing auditory stimuli. For example, when a participant is presented with a series of repetitive high-pitched tones (standard stimuli) with an infrequently occurring low-pitch tone (deviant stimuli), each time the deviant stimulus occurs an MMN will be elicited. It is obtained by subtracting the ERP waves to the standard stimuli from the ERP waves to the deviant stimuli, resulting in a difference EXTRAVERSION AND SENSORY DISCRIMINATION 7 wave. The MMN usually peaks in amplitude 150-250ms after the onset of a deviant stimulus and is most often reported at fronto-central and central scalp electrodes (Naatanen, 2007a). Its amplitude increases and latency decreases as the magnitude of sound change between the standard and deviant stimuli increases (Pakarinen et al., 2007). The MMN depends on the presence of sensory memory traces in the auditory cortex Naatanen, 2007a). When identical auditory stimuli are