International Journal of Psychophysiology 91 (2014) 232–239

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Habituation in acoustic startle reflex: Individual differences in personality Angel Blanch a,b,⁎, Ferran Balada a,b,c, Anton Aluja a,b a b c

Department of Psychology, University of Lleida, Spain Institute of Biomedical Research (IRB Lleida), Spain Department of Psychobiology, Institute of Neurosciences, Autonomous University of Barcelona, Spain

a r t i c l e

i n f o

Article history: Received 21 June 2013 Received in revised form 20 December 2013 Accepted 2 January 2014 Available online 9 January 2014 Keywords: Habituation Startle reflex Personality

a b s t r a c t This study analyzed the relationship of individual differences in personality with habituation in the acoustic startle response (ASR). Data from nine trials in ASR to white noise bursts and a personality questionnaire based on the alternative big five personality approach were modelled with a latent growth curve (LCM) including intercept and slope habituation growth factors. There was a negative correlation between the intercept and slope, indicating that individuals with higher initial ASR levels had also a more pronounced and faster decrease in the ASR. Contrary to expectations, Extraversion and Sensation Seeking did not relate with habituation in ASR. Neuroticism and Aggressiveness related asymmetrically with the habituation rate in ASR. Higher levels of Neuroticism were related with faster habituation, whereas higher levels of Aggressiveness were related with slower habituation. Further studies with the LCM should be undertaken to clarify in a greater extent the association of personality with habituation in ASR. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Habituation is an intriguing and complex process. As an elementary form of learning, it takes place at the neural level, and implies a progressive decrement in a given response issued after a repeated stimulation. The dual-process theory is the most influential systematization of this phenomenon, with the key assumption of two intertwined processes in the central nervous system: habituation (decline of the response), and sensitization (intensification of the response). There are two important stimulus parameters, intensity and frequency, that influence both processes in different ways. Higher intensity levels tend to rule sensitization, whereas higher frequencies influence habituation in a greater extent. Within this framework, it is also useful to distinguish between shortterm habituation, a decay of the response in the same experimental session, and long-term habituation, a decay of the response between different experimental sessions (Groves and Thompson, 1970; Thompson and Spencer, 1966). Later refinements about habituation have suggested that the identification of influential pathways might be of use to clarify the cellular and neuronal mechanisms embedded within the habituation processes (Rankin et al., 2009; Thompson, 2009). On the other hand, a psychological constructionist approach to human emotion advocates for the fact that biological explanations of behaviour such as brain activity, chemical ⁎ Corresponding author at: Department of Pedagogy and Psychology, Faculty of Education Science, University of Lleida, Avda de l'Estudi General, 4, 25001 Lleida, Catalonia, Spain. Tel.: +34 973706529. E-mail address: [email protected] (A. Blanch). 0167-8760/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijpsycho.2014.01.001

circuitries, or synaptic connections should not replace the psychological accounts for the wide array of mental experiences. Albeit physiological attributes are certainly important elements in the configuration of emotion and mental states, they would not be specific for instance to emotions such as interest, happiness, sadness, anger, disgust, or fear (Barrett et al., 2007; Miller, 2010). Recent evidence from the neuroimaging literature supports this psychological constructionist scheme in contrast to a locationist approach, which defends the correspondence of emotion categories with different brain regions (Lindquist and Barrett, 2012; Lindquist et al., 2012). Thus, complex psychological descriptors might be outlined at a strongly wired network level, suggesting that emotion categories would emerge from non-specific general brain systems. Nevertheless, it has also been argued that neuroimaging studies tend to manage the effects of individual differences as noise that needs to be dealt with because of the emphasis on brain activation arrangements assumed as common across individuals (Hamann and Harenski, 2004; Murphy et al., 2012). Moreover, the association of specific brain activation with emotional and cognitive processing has been suggested to be strongly leveraged by individual differences such as personality, mood, dispositional affect, sex, and genotype, whereby these would be important modulators of the neurobiological basis of emotion and cognition (Canli et al., 2004; Hamann and Canli, 2004). Bearing this in mind, habituation in responses to emotional and cognitive processing is a neurobiological process influenced by individual differences in personality, despite the paucity of studies addressing this particular topic. In this work, we analyzed the association of habituation in acoustic startle reflex (ASR) and broad personality dimensions with a latent curve model (LCM). At present, there are only a few studies

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addressing the interrelationship of habituation in ASR, and to the best of our knowledge, only one study has employed the LCM. More specifically, there are at least three main interrelated substantial reasons why this is a meaningful topic of study. First, habituation might be a core mechanism underlying the biological background in personality insofar as there are in fact noticeable individual differences in habituation processes. Thus, studying its relationship with personality may contribute to a better understanding of the biology and psychophysiological aspects of personality. Second, startle habituation has been considered as a basis for judgement and consideration of impairments in several functions such as motor or cognitive responses. Including personality measures in both, basic and applied studies about ASR and its habituation phenomenon may be more informative and particularly useful in the study of mental disorders because of their links with extreme scores in personality variables. Finally, there is a wealth of studies about the ASR, although little consideration has been given to the startle habituation blocks usually presented before the experimental part, due in part to the diversity of methods used to calibrate the habituation process. The proposed method to study the habituation portion in ASR studies, the LCM, has been previously applied to study gender differences (Lane et al., 2013). The present study addresses however the role of broad personality factors in this neurobiological process. 1.1. Characterization of habituation There is an extensive amount of literature using different methods to portray habituation responses. However, not all of them allow for an easy accommodation of personality traits to determine whether they relate or not with habituation. A common approach consists in adding up single habituation trials into a fewer amount of sections or blocks, and then compare the habituation means amongst the different blocks (Blumenthal, 2001; Blumenthal et al., 1995; Bradley et al., 1993). Furthermore, regression-based methods attempt to define some sort of habituation function and then correlating the habituation parameters of interest with personality measures, apply the particular model to subgroups of individuals, or contrast the model outcomes with real habituation data (Alonso et al., 2005; Gilmore and Thomas, 2002; LaRowe et al., 2006). The calibration of habituation has also been addressed from computational models mainly based in neural networks. Within this approach, habituation is assumed as a synaptic-like scheme aimed to explain some type of subsequent behaviour (Marsland, 2009; Sirois and Mareschal, 2002; Wang, 1994; West and Grigolini, 2010). The latent curve model (LCM) is a more novel approach that has been recently recommended to tap habituation (Lane et al., 2013). The variance– covariance matrix and mean vector of the habituation trials in a given experiment are used to estimate a latent underlying trajectory. The intercept and slope factors define the change in this trajectory by representing the initial level and habituation rate of the studied process, respectively. Hence, apart from allowing estimating the magnitude and direction of habituation initial level and growth rate, it is possible to compare these parameters amongst different groups of individuals, or predict the habituation process with exogenous variables. 1.2. Habituation and individual differences in personality The pathways by which different personality dimensions might relate with habituation remain largely unclear with only a small amount of research works addressing this topic. Some studies have explored the association of different habituation scenarios with personality traits in open-field investigations with non-human subjects (Ellenberg et al., 2009; Martin and Réale, 2008; Rodríguez-Prieto et al., 2011). Moreover, research with humans such as patients with schizophrenia (Akdag et al., 2003; Mason et al., 1997), the general healthy population (MartinSoelch et al., 2006), or undergraduate students (Anderson et al., 2011; Blumenthal, 2001; LaRowe et al., 2006) suggest that habituation could be associated with individual differences in personality traits.

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Most of these studies have relied on the biological foundations of personality (Eysenck, 1994; Gray, 1987; Zuckerman, 2005). In accordance with the Eysenck's theory, Extraversion and Neuroticism relate with the cortical and limbic brain systems, respectively. The cortical system deals with the arousal tone elicited by input stimuli, the limbic system with the response and/or regulation of emotional states. Higher scorers in Extraversion would have a lower level of arousal in the ascending reticular activation system (ARAS) in contrast with lower scorers in Extraversion who would display a higher level of arousal. Besides, higher scorers in Neuroticism would be more sensitive to emotions and more prone to anxiety and agitation, in contrast with lower scorers in Neuroticism who would be less arousing and more prone to steadiness under stressful conditions. Gray's theory in turn suggests impulsivity and anxiety as juxtaposed to Eysenck's Extraversion and Neuroticism, and associated with the behavioural activation and inhibition systems (BAS and BIS). The BAS and BIS are thought to be sensitive to signals of either reward or punishment, respectively, and relate with different brain structures connected with either approaching or inhibiting behaviours. Finally, the alternative five model postulated by Zuckerman focusses on the explanation of personality from evolutionary psychology. This model subsumes four major personality dimensions currently found across different personality frameworks plus an activity factor: Extraversion/sociability, Neuroticism/anxiety, constraint vs impulsive Sensation Seeking, and aggression/hostility vs agreeableness. Extraversion and Neuroticism are the two central personality dimensions addressed in greater depth when analyzing habituation in psychophysiological responses. Overall, it has been suggested that the lower cortical activation level in extroverts would lead them to faster habituation rates than introverts, whereas the higher sensitivity to emotions of higher scorers compared with lower scorers in Neuroticism might relate with abnormally slower habituation rates (Cook et al., 1991; Eysenck, 1994; LaRowe et al., 2006; Larsen and Ketelaar, 1991). Nevertheless, slower habituation rates in electrodermal activity for high sensation seekers (De Pascalis et al., 2007), and low trait anxious subjects (Wilken et al., 2000) have also been reported. Pioneering research about personality and habituation was mostly focussed on electroencephalography (EEG) and electrodermal response (EDR), and critically scrutinized theoretical and methodological grounds (O'Gorman, 1977; Smith et al., 1990). Habituation in the acoustic startle response (ASR) and its potential relationship with personality have been studied in a much lesser extent.

1.3. Habituation in acoustic startle response (ASR) and personality The ASR refers to a quick contraction of the orbicularis oculi muscle which closes the eye and happens at about 30–50 ms after the onset of an auditory stimulus. The ASR has been broadly used to study the neuronal, emotional, and cognitive basis of sensor motor responses and information processing (Blumenthal et al., 2005; Filion et al., 1998; Lang et al., 1990). Nevertheless, although habituation might have the capability to modulate ASR (Bradley et al., 1993; Koch, 1999), it has also been argued that the eye blink component might not habituate likewise the rest of the startle response (Carlsen et al., 2011). As far as we know, there is no research about habituation and personality with the LCM approach. The present study relies on this method because it provides a useful framework to study patterns of change in ASR habituation, and more importantly, the exploration of the potential links of habituation in ASR with individual differences in personality. Moreover, no research works have analyzed the association of personality as measured with the broad dimensions derived from the Zuckerman's alternative big five model (Zuckerman, 2005) with habituation in ASR. The aim of this study was therefore, to evaluate whether these personality broad dimensions were related with growth trajectories in ASR as gauged by a LCM.

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Evidence regarding the association of personality with habituation in the ASR is scarce and focussed mostly in Extraversion and Neuroticism. For instance, high extraverts and high sensation seekers habituated faster to startle probes than lower scorers in Extraversion and Sensation Seeking (LaRowe et al., 2006). Arguably, this could not be the case when introducing experimental attention tasks at different stimulus intensities (Blumenthal, 2001). We hypothesized that extraverts and sensation seekers should show steeper decreases than introverts and nonsensation seekers in ASR as indicated by their association with the intercept and slope factors within the LCM. On the other hand, higher scorers in Neuroticism who appear to be highly sensitive to the appraisal of shocking and stressful stimulation, tend to show higher response levels than stable individuals to negative affect, and might display a slower habituation rate in ASR (Cook et al., 1991; Gray, 1987; Larsen and Ketelaar, 1991). Clinical studies with schizophrenia patients who typically score higher in Neuroticism, indicate higher delays in habituation than normal controls (Akdag et al., 2003; Meincke et al., 2004). Thus, we hypothesized that higher scorers in Neuroticism would show a slower habituation rate than lower scorers in Neuroticism. Moreover, there are some conceivable links between aggressive personality and habituation. Some traits that might be entangled within this personality dimension have received some support for its association with habituation in ASR. Antisocial psychopathic traits related with slower habituation rates in ASR. This was adduced as a consistent link between psychopathic antisocial traits with deficits in habituation (Anderson et al., 2011). However, higher scorers in a Constraint personality factor from the MPQ (Tellegen and Waller, 1992) that tapped

behavioural inhibition and low impulsivity, related also with a slower habituation rate in ASR (LaRowe et al., 2006). Hence, we hypothesized that individuals scoring high in the aggressive personality dimension would display a slower habituation rate in ASR. 2. Method 2.1. Participants Participants were 102 female undergraduate psychology students, with an average age of 20.77 years old (SD: 2.05). The students participated in the experimental sessions during the non-menstrual days of their cycle. Participation was voluntary and economically encouraged with 15€. All students were advised to refrain from smoking and/or having alcoholic or stimulant drinks for at least 12 h prior to the experiment. The ethical committee of our University approved this study, and all participants provided a written informed consent. 2.2. Instruments and procedure Personality was assessed with the Zuckerman–Kuhlman–Aluja Personality Questionnaire (Aluja et al., 2010). The ZKA-PQ is a 200-item personality questionnaire with a 4-point Likert-type response format (1, Disagree Strongly; 2, Disagree Somewhat; 3, Agree Somewhat; 4, Agree Strongly). The instrument encompasses twenty narrow facets. There are four facets per each five broader personality factors: Extraversion (positive emotions, social warmth, exhibitionism, and sociability),

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Fig. 1. ASR magnitudes (microvolts) in nine startle reflex probes for low (light triangles) and high scorers (dark triangles) in five personality dimensions. Low and high scorers are defined below and above the 30th and 70th percentiles, respectively.

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Sensation Seeking (thrill and adventure seeking, experience seeking, disinhibition, and boredom susceptibility/impulsivity), Neuroticism (anxiety, depression, dependency, and low self-esteem), Aggressiveness (physical aggression, verbal aggression, anger, and hostility), and Activity (work compulsion, general activity, restlessness, and work energy). Alpha reliabilities for each broad personality dimension in this study were .92, .86, .92, .91 and .86, respectively. The participants completed the personality questionnaire after being informed about the experiment. Habituation was evaluated with the electromyographic (EMG) recordings to nine startle probes: 50 ms bursts of broadband noise calibrated at 105 dB (A) and delivered through headphones. The probes were randomly spaced within a time window of 0–164 s (2.73 min) with random inter-trial intervals from 15 to 35 s. Blink responses to the startle probes were measured with two 6 mm silver chloride (AgCl) cup skin surface electrodes filled with high conductive recording paste, placed below the lower left eyelid in line with the pupil and

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separated by 15–20 mm (Blumenthal et al., 2005). EMG signals were recorded with Biopac MP100 at a 1000 Hz sampling rate. Recordings took place inside an electromagnetically isolated room (Faraday cage). The participants sat in a chair and were attached by the electrodes to the recording equipment. EMG responses were processed with AckqKnowledge software and a simple Java programming language routine (Blanch et al., 2013). Habituation magnitudes in ASR's were quantified as differences between baseline mean EMG responses at 50 ms preceding the onset of startle probes and the maximum value of the EMG response between 20 and 120 ms after the startle response onset. 2.3. Data analyses The LCM was specified with the nine habituation responses as observed indicators, and an intercept (π0) and slope (π1) latent growth factors characterizing the initial status and the growth rate in habituation,

Fig. 2. Top panel: Vectors of means (M), and variances (Var) of study variables. Bottom panel: Correlation matrix of nine habituation trials (h1–h9), and five personality variables (ext = Extraversion; sse = Sensation Seeking; act = Activity; neu = Neuroticism; agg = Aggressiveness). Correlations equal or higher than ±0.19 were significant at p b 0.05. Bivariate correlation scatter plots below the diagonal, Pearson correlations above the diagonal, histograms on the diagonal.

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respectively (Curran and Hussong, 2002; Willet and Sayer, 1994). There were two kinds of models. A Level 1 unconditional model represented the change in habituation of each person, indicating individual differences in the intercept and slope growth factors. We fixed to unity the corresponding intercept's parameters for each habituation measure. Time was coded by fixing the slope's parameters to 0, 19, 42, 67, 82, 102, 118, 145, and 164 s, which represented the unequal spacing of the presentation of each auditory burst (Biesanz et al., 2004; Stoel and van den Wittenboer, 2003). A Level 2 conditional model represented the influence of the five personality factors as predictors of change on the habituation growth factors. The five personality dimensions were represented as latent variables measured by their respective four facets. There were five models assessed in five steps. At each step, the model where the personality variable had the weakest effects on the individual growth factors was removed and the model was re-estimated again. The first model evaluated the concomitant effects of the five personality variables whilst the fifth model evaluated the effect of a single personality dimension onto the growth factors.

3. Results The top-left panel in Fig. 1 shows the habituation block across 164 s for the entire sample, which depicts a decreasing trend in the ASR responses. A multivariate analysis of variance (MANOVA) for repeated measures indicated that this linear trend was significant (F = 51.350, p b .001). The other panels in Fig. 1 show a similar pattern for extreme scoring groups in each personality dimension, defined below the 30th and above the 70th percentile in the ASR. All panels show the sharp drop in the response from the fourth to the fifth habituation probes, probably due to the shorter time interval between both of them. There were different habituation decrements that were clearer for Extraversion and Neuroticism than for the rest of personality factors. This might suggest that habituation in the ASR, could be particularly influential for extravert vs introvert, and emotionally stable vs emotionally unstable individuals. However, there were no significant effects for the interaction of the decreasing ASR trend with any personality variable with close to one Wilks-lambdas (λ, for 8 df): Extraversion (λ = .897, F = .745), Sensation Seeking (λ = .904, F = .651), Neuroticism (λ = .880, F = .866), Aggressiveness (λ = .827, F = .1.359), and activity (λ = .784, F = 1.796). The first panel in Fig. 2 shows the means and standard deviations of the study variables, whereas the bottom panel shows the Pearson correlation coefficients. All correlations amongst the habituation probes were positive and highly significant. Correlations amongst the personality variables were only significant for Extraversion–Neuroticism (−.41, p b 0.001), Extraversion–Activity (.32, p b 0.001), and Neuroticism–Aggressiveness (.41, p b 0.001). There were two significant correlations between Aggressiveness with the first and seventh habituation probes (−.19, and −.22, p b 0.05).

Table 1 shows the outcomes of an initial linear LCM, with a significant χ2 value for 40 degrees of freedom (χ2[40] = 133.001, p b .001; TLI = .930, CFI = .938, RMSEA = .152, and SRMR = .064) significant mean intercept and slope, intercept variance, intercept and slope correlation, and non-significant slope variance. In this linear model, the high modification index (MI) for the slope loading in the fifth habituation trial (MI = 37.368, parameter change = 99.685), suggested to free this parameter and estimate the model again (Bollen, 1989; Rosseel, 2012). Model fit improved with a lower χ2 value for 39 degrees of freedom (χ2[39] = 87.667; TLI = .962, CFI = .967, RMSEA = .111, and SRMR = .056), and a significant χ2 difference test between the linear and non-linear models (Δχ2[1] = 133.001 − 87.667 = 45.334, p b .001). This model corresponded to a non-linear trajectory that better fitted the startle habituation as already reported (Lane et al., 2013). The mean intercept and slope were significant (μπ0 = 40.270, μπ1 = − .078; p b .001) suggesting that the mean ASR magnitude to the first trial was 40.270 μV, with a decreasing rate of − .078 μV for a one unit change in the time score. The intercept and slope variances were also significant (σ2π0 = 715.776, σ2π1 = .006, p b .01) indicating between individual differences in the initial response and habituation rate. There was a negative correlation between the intercept and slope (r = −.766; p b .01). Fig. 3 depicts this relationship showing that individuals with higher initial reactivity had sharper decrements in the ASR across the nine trials (i.e., faster habituation), whereas individuals with lower initial reactivity had more modest decrements across the nine trials (slower habituation). Table 2 shows the evaluation of the relationship of personality with habituation growth factors. For each personality dimension, the first

Table 2 Relationships of five personality variables on habituation intercept and slope across five models.

Table 1 Between individual differences in change in habituation responses. Parameters and fit indices

Linear

Non-linear

μπ0 μπ1 σ2π0 σ2π1 ρπ0π1 χ2 df TLI CFI RMSEA SRMR

38.946⁎⁎⁎ −.076⁎⁎⁎ 619.018⁎⁎⁎ .004 −.709⁎⁎ 133.001⁎⁎⁎

40.270⁎⁎⁎ −.078⁎⁎⁎ 715.776⁎⁎⁎ .006⁎⁎ −.766⁎⁎ 87.667⁎⁎

40.000 .930 .938 .152 .064

Fig. 3. Scatterplot of estimated intercept and slope values in the unconditional model.

39.000 .962 .967 .111 .056

Note. Intercept mean (μπ0), slope mean (μπ1), intercept variance (σ2π0), slope variance (σ2π1), intercept-slope correlation (ρπ0π1). **p b 0.01; ***p b 0.001.

Variable

M1

M2

M3

M4

M5

SS

.00 .19 −.15 .29 .28 −.35 .22 −.39 −.24 .36* .09 .27

— — −.13 .25 .26 −.28 .22 −.42* −.24 .36* .09 .23

— — — —

— — — — — —

— — — — — — — —

AC EX NE AG R2π0 R2π1

.22 −.20 .24 −.46* −.27* .41* .07 .18

.13 −.35* −.20 .35* .04 .15

.15 −.19 .02 .04

Note. Intercept coefficient of determination (R2π0); Slope coefficient of determination (R2π1). *p b 0.05 .

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entry indicates the effect of personality on the intercept, whilst the second entry indicates the effect on the slope. The two bottom rows are the coefficients of determination for the intercept and slope factors, respectively. Model 4 which included Neuroticism and Aggressiveness showed the best balance between fit and explanation of growth factors amongst the five evaluated models. The fit indices for this model indicated a fair representation of observed data (χ2[126] = 241.546, TLI = .969, CFI = .975, RMSEA = .095, SRMR = .082), explaining the 4 and 15% of variance in the intercept and slope factors, respectively. Neuroticism was significantly associated with the slope factor (−.35, p b .05), suggesting that participants who scored higher in Neuroticism had a faster habituation, whereas lower scorers had a slower habituation in ASR. Aggressiveness was also significantly associated with the slope factor (.35, p b .05). These outcomes indicate that participants who scored higher in Aggressiveness had a slower habituation, whereas lower scorers had faster decrements in the habituation curve. 4. Discussion To the best of our knowledge, this is the first study addressing the association of broad personality dimensions with habituation in ASR with the LCM. The conclusions that may be derived from the present findings, therefore, should be taken with a degree of caution. The personality factors considered in this work were those contemplated from the alternative big five model, measured with a facet-based self-report instrument (Aluja et al., 2010; Zuckerman, 2005): Extraversion, Sensation Seeking, Neuroticism, Aggressiveness and Activity. The main findings suggest that Extraversion and Sensation Seeking had no effects on either the initial habituation level or the habituation rate in ASR, contrary with our expectations and previous outcomes (LaRowe et al., 2006). On the other hand, Neuroticism and Aggressiveness had a significant impact on the habituation slopes in ASR. These outcomes were in disagreement with our hypothesis concerning Neuroticism (Cook et al., 1991; Larsen and Ketelaar, 1991), although were consistent with the hypothesized relationship of habituation with the Aggressiveness dimension (Anderson et al., 2011). When interpreting these findings, it should be kept in mind the approximation of the present work when defining the personality dimensions as calibrated by narrow facets that are more specific to its respective broader dimension. 4.1. Extraversion and Sensation Seeking The lack of an association of Extraversion with habituation latent growth factors in ASR was contrary to the findings reported in past research. However, it should also be noticed that in the LaRowe's study (2006) a Positive Emotionality factor was not related with habituation in ASR, and that this factor has been consistently associated with Extraversion. Apparently, faster ASR habituation would relate with Extraversion through its common attributes with disinhibited personality traits such as Sensation Seeking (LaRowe et al., 2006). However, the present data indicate that similarly to Extraversion, Sensation Seeking did not associate with habituation in ASR. Both personality dimensions enclosed narrow facets that might not share common traits but by the exhibitionism Extraversion facet. This fact was also taken into consideration concerning habituation in EDR in terms of the moderating effects of Neuroticism (LaRowe et al., 2006). Hence, Neuroticism might indeed have played a moderating role in the Extraversion and Sensation Seeking's lack of relationship with the ASR found here, even though habituation in ASR would be less sensitive than habituation in EDR to the influence of Neuroticism related traits (LaRowe et al., 2006). 4.2. Neuroticism and Aggressiveness The negative relationship of the Neuroticism dimension with the habituation slope growth factor suggests that higher scorers in Neuroticism had a faster habituation than emotionally stable individuals did,

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which was contrary to our expectation. It could be argued that higher scorers in Neuroticism might follow a similar impaired habituation pattern such as that shown by schizophrenic patients who also score typically high in Neuroticism. However, it should be considered that the study of habituation in ASR with schizophrenic subjects is helpful to assess sensorimotor gating and information processing shortcomings. Moreover, although schizophrenic patients tend to score high in Neuroticism, this personality dimension does not appear to impinge an effect in their abnormal habituation patterns, which have been rather attributed to the intensive use of medication, and also to functional anomalies in information processing (Akdag et al., 2003; Meincke et al., 2004). The positive relationship of the Aggressiveness dimension with the habituation slope factor was in accordance with the hypothesized association. This finding indicates that individuals who scored low in Aggressiveness had a faster habituation rate than individuals scoring high in this personality dimension. In contrast, higher scorers in Aggressiveness displayed a lower habituation rate in the ASR. Put in another way, individuals with lower scores in Aggressiveness had steeper decreases in the habituation curve. These findings substantiate the association of psychopathic traits (Anderson et al., 2011) rather than the association of the Constraint personality factor (LaRowe et al., 2006) with a slower habituation rate in ASR. The Aggressiveness and Neuroticism dimensions correlated strongly in the present study (0.41, p b 0.001), hence, it is somehow surprising that both had an equivalent magnitude effect but of opposite sign when associated with their respective habituation decrementing trajectories. This could be attributable to the way of tapping each broad personality dimension with four narrower personality facets. The Aggressiveness facets of anger and hostility have a robust relationship with the Neuroticism dimension. On the other hand, the Aggressiveness facets of physical and verbal aggression are rather embedded with aggression and impulsivity. Thus, these two latter facets are likely to be the main contributors to the observed positive association with the slope growth factor. 4.3. Study limitations This study has some limitations to bear in mind. The main design of the study was not specifically addressed to study habituation. The recording of the nine acoustic startle probes corresponded to the habituation block of a wider experiment about individual differences and personality (Blanch et al., 2013). For instance, there were no different intensities of the acoustic stimulation. Increasing the complexity of the experimental design could additionally contribute to analyze different courses of habituation in ASR concerning personality variables (Smith, 1983). Moreover, these nine habituation trials differ somewhat from the optimal number of trials to calibrate habituation within the LCM (Lane et al., 2013), even though this figure was advised for stimulus intensities of 100 dB, whereas the present study used a stimulus intensity of 105 dB. In addition, the present sample was bounded only to females, who tend to score significantly higher in Neuroticism than males (Schmitt et al., 2008), whereas there are sex differences that might influence the magnitude of the ASR. Women are more predisposed than men to display enhanced levels of physiological reactivity, particularly with opposite-sex experimenters as in the present study (Kofler et al., 2001; LaRowe et al., 2004). A more diverse sample with similar proportions of males and females might have supplied a different set of outcomes than those reported here, perhaps more in the line with the faster habituation of extraverted and Sensation Seeking people hypotheses. Furthermore, each broad personality dimension was tapped with four narrower personality facets. This approach hinders somehow the uncovering of the effects that each facet might exert in the habituation growth factors. However, studying the role of narrower facets was out of the scope of this research because the study aim was precisely to assess the influence of broad personality dimensions in the habituation in

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ASR. A latent variable scheme assuming each personality dimension as a broader construct provides a more complete and robust method to study the potential relationship of comprehensive personality dimensions with habituation latent growth factors in ASR. 4.4. Future research directions From our short but intense experience in the study of ASR, we feel that three intertwined main directions could guide future research works addressing the relationship of personality with habituation in ASR. First, further theoretical elaboration is strongly needed to assess the link of individual differences in personality with habituation rates. Well-grounded hypotheses would help to respond more theoretically driven questions. There are approaches to habituation and startle reflex research that have focussed on two folded processes such as habituation and sensitization (Groves and Thompson, 1970), or the assumption of excitatory and inhibitory mechanisms (Hoffman and Ison, 1980; Stein, 1966). This background may fit well to Gray's theory of personality that considers different brain structures in activation and inhibition systems (Gray, 1987). For instance, two interesting hypotheses that could be advanced in future research relate precisely with the BAS and BIS. The BAS could be more influential regarding increments in the habituation rate (sensitization), whereas the BIS could be more influential in decrements in the habituation rate (habituation). Second, the eye blink component of the ASR might not habituate as the rest of the human startle response when characterized from the orbicularis oculi muscle (Carlsen et al., 2007, 2011). There are two separated segments in the form of two peaks in a single EMG response to an acoustic stimulus, the first is related with the auditory blink response, and the second one with the startle reflex (Meincke et al., 2002). Both components articulate through different neuronal circuitries elicited from the auditory nerve, the auditory blink response through the nuclei of the inferior colliculus, and the startle response through the pontomedullary reticular formation (Valls-Solé et al., 2008). Therefore, future studies could perhaps compare the relative habituation rates of these two components of the ASR and their respective relationships with personality. Third, individual differences in personality derived from narrower facets may relate with the performance of different biological systems, particularly concerning the low serotonin levels that have been linked with aggression, impulsiveness and neuroticism traits (Carver and Miller, 2006). There is in turn some evidence suggesting potential links between serotonin receptor polymorphisms with prepulse inhibition of the ASR (Bräuer et al., 2009). Thus, future studies within more complex research designs could address the unravelling of complex relationships amongst personality narrower traits, habituation growth factors as captured by LCM, and serotonin levels. 4.5. Conclusion Taken together, the outcomes in this research suggest that there are substantial links between individual differences in personality and habituation processes in ASR. However, further research is needed to disentangle more precisely the specific nature of these associations. As far as we know, this is the first study that specifically addresses the association of comprehensive personality dimensions with habituation to ASR by using the LCM approach. Therefore, it should be kept in mind that further replications are needed to contrast the present and other hypotheses as suggested above, or addressing other potential topics that may be of interest in the ASR field. The findings in the present study suggest that the LCM is a useful methodology to study the links between individual differences in personality and habituation in ASR. Nevertheless, a more in depth description of the paths that link the biological bases of human personality with habituation processes would be of great aid in the interpretation of the results from studies in this line of research.

Acknowledgements This research was supported by a grant from the “Plan Nacional” number: PSI2011-24789, Ministerio de Ciencia e Innovación, Spain. This research was performed within the Catalonian Consolidated Research Group 2009 SGR 809. The authors express their appreciation to the referees for their helpful comments on an earlier version of this manuscript.

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