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Extraversion-Related Differences in Stimulus Analysis: Effectiveness of the Lateralized
Extraversion-Related Differences in Stimulus Analysis: Effectiveness of the Lateralized
Several theories of personality have been developed by experimental psychologists in an
attempt to define how humans think, feel, and behave. Hans Eysenck, Robert McCrae, and Paul
Costa have attributed to the development of possible theories of personality. Together, they
claim that personality as a whole is a combination of various character dimensions, and one that
is persistent in their theories of personality is extraversion. This dimension consists of primary
traits that describe the outward characteristics of the individual noticed by surrounding people
(Eysenck, 1967; McCrae & Costa, 2003). According to Eysenck (1967), extraversion
encompasses traits of sociability, impulsiveness, activity, liveliness, and excitability. Every
individual possess a combination of varying levels of these traits, which shape their phenotypic
or outward form of personality. Extraversion related differences in personality are also
examined from a genotypic level, which is based on Eysenck’s (1967) theory of excitation-
inhibition balance. This theory describes how individuals differ in their level of excitation and
inhibition from a biological basis. These differences can be defined by using laboratory
measures of behavioural processes including those of vigilance, speed and accuracy, and sensory
thresholds (Eysenck, 1967). Measurement of these outward behavioural processes has allowed
researchers to examine extraverted related differences in information processing.
A theory has been devised in attempt to explain the relationship between extraversion and
individual differences in information processing that governs behaviour or movement.
Movement occurs after an individual analyzes a stimulus and organizes a response, two
processes in which extroverts and introverts differ. John Brebner (1985) suggests that extraverts
are more likely to demonstrate motor responses that are quicker and more frequent compared to
introverts who tend to stay engaged in the analysis of a physical stimulus. This theory is based
on Eysenck’s (1967) earlier theory of excitation and inhibition that suggest that extroverts obtain
more arousal from generating a response rather than analyzing a physical stimulus. The reverse
is true for introverts who find stimulus analysis arousing rather than response organization.
Therefore, introverts are more efficient in stimulus analysis and extroverts generate quicker
Several studies have examined how extraverts and introverts differ in aspects of stimulus
analysis, by breaking the physical response down into two main components, those being
reaction time (RT) and movement time (MT). Reaction time is comprised of the time needed to
evaluate a stimulus and select a correct response, where movement time is the duration of
movement execution (Doucet & Stelmack 1997; Doucet & Stelmack, 2000). Research studies
have employed a response panel system to assess individual differences in RT and MT in
extraverts and introverts. Reaction time is calculated from stimulus onset to the lift-off of a
home button and movement time is the time from lift-off to the pressing of the response button.
Reaction time can also represent the time required to press a response button (Doucet &
Stelmack 1997; Doucet & Stelmack, 2000). Significant results demonstrate that extraverts
generate quicker movement times than introverts, however, no results have been found to
support any group differences in reaction time (Doucet & Stelmack, 1997; Doucet & Stelmack,
To further support the discovery of extraverted related differences in reaction time,
researchers have also applied electroencephalography (EEG). EEG is a psychophysiological
measure of brain activity. This technology is effective for measuring event related potentials
(ERPs), electric brain potentials that are exhibited in reaction to an incoming stimulus.
Researchers argue as to whether certain ERPs provide accurate measures of aspects of
information processing used in generating responses. The P300 evoked-response potential, a
positive potential that occurs 300 ms after the onset of a stimulus, has been suggested to be an
accurate measure of the time of decision related to a task-relevant stimulus (Doucet & Stelmack,
2000; Rammsayer & Stahl, 2004; Stahl & Rammsayer, 2004). There are inconsistencies as to
whether P300 latency is a valid marker of speed of stimulus analysis in extroverts and introverts.
Doucet and Stelmack (2000) have found longer P300 latencies in extroverts, while Rammsayer
& Stahl (2004) have found shorter P300 latencies for introverts. However, the P300 is generally
seen as a valid indicator of speed of central processing.
The N100 evoked-response potential, a negative potential that occurs 100 ms after the
onset of a stimulus, indicates attention to a presented stimulus. Both the N100 amplitude and
latency have been useful in determining differences in attention to varying degrees of auditory
intensity (Doucet & Stelmack, 2000; Rammsayer & Stahl, 2004) and speed of auditory
perception (Miller, Ulrich & Rinkenauer, 1999). Significant differences in N100 amplitude to
higher auditory tones has been discovered in introverts (Doucet & Stelmack, 2000; Rammsayer
& Stahl, 2004) indicating that they elicit greater sensitivity to auditory stimuli. Rammsayer and
Stahl (2004) support this notion as they failed to find extraversion related differences in N100
amplitude when applying visual stimuli rather than auditory stimuli. Auditory intensity has been
suggested to influence perceptual processing speed as Miller, Ulrich and Rinkenauer (1999)
found increased N100 latencies to high intensity auditory stimuli. Both the N100 and the P300
potentials are significant measures of information processing, however, they alone have failed to
provide enough significant evidence for extraverted related differences in stimulus analysis.
To analyze differences in central nervous system activity in extroverts and introverts,
current research has also applied the psychophysiological measure known as the lateralized
readiness potential (Coles, 1988; Rammsayer & Stahl, 2004; Stahl & Rammsayer, 2004). This
measure has been used to assess differences in preparation and activation of physical responses
(Coles, 1988). The lateralized readiness potential is a negative potential that occurs prior to
selection of specific hand movement. When a response is required from the left hand, this
negative deflection is seen over contralateral areas of the central motor cortex, and vice versa for
right-handed responses (Coles, 1988). The LRP serves as a marker for the central motor
processes that take place before and after the selection of specific hand movement. The period
between the onset of the stimulus and the LRP is referred to as the S-LRP interval, which
represents the time needed to perform stimulus analysis and response organization. The time
following the LRP is signified as the R-LRP interval and represents the duration of time before
execution of a response (Leuthold, Sommer, & Ulrich, 2004).
As of present, very few studies have applied the LRP to measure extraversion related
differences in response preparation and execution (Rammsayer & Stahl, 2004; Stahl &
Rammsayer, 2004). There have been significant findings that suggest that extroverts have
shorter R-LRP latencies than introverts do (Rammsayer & Stahl, 2004). Significant extraversion
related differences in duration of S-LRP latencies have not been found. Rammsayer and Stahl
(2004) failed to find any difference in S-LRP latency and suggest that this is due to low
Shucard, Abara, McCabe, Benedict, and Shucard (2004) examined the effects that motor
response and increased stimulus complexity had on P300 amplitude and latency while
completing an auditory continuous performance task. Participants were placed in one of two
groups that were required to listen to degraded and undegraded syllables. The motor attention
group was asked to respond to target syllables by pressing a response button, while the covert
attention group was listened to target syllables without responding (Shucard et al.). Shucard et
al. discovered that stimulus complexity affected P300 latency when motor responses were and
were not required. P300 latency was longer for more complex stimuli (degraded syllables) than
for simple stimuli (undegraded syllables), which suggests longer durations of stimulus analysis
and response times (Shucard et al.). P300 latency was greater for complex stimuli when
participants were required to make a motor response over (Pz) and (Cz) when no response was
required (Shucard et al.). If an increase in stimulus complexity affects P300 latency, this may
support Rammsayer and Stahl’s (2004) suggestion to increase stimulus complexity in order to
find extraversion related differences in S-LRP latencies.
The present study will seek to analyze extraversion related differences in stimulus
analysis and response preparation processes using various electrophysiological measures.
Measures of response time, N100 and P300 amplitude and latency, as well as lateralized
readiness potentials will be recorded and analyzed for participants’ responses to complex
auditory stimuli. It is predicted that extroverts will generate quicker overall response times,
while introverts will generate enhanced P300 and N100 event related potentials compared to
extroverts. In addition, introverts will elicit shorter S-LRP latencies than extroverts will, as they
perform more efficiently in stimulus analysis and response preparation than response execution.
Participants included 20 male and female (5 male, 14 female and 1 undetermined due to
faulty records) Saint Thomas University undergraduates. Three of the participants’ data could
not be included in the data analysis due to technical reasons. To be included in the study, all
participants completed the Eysenck Personality Questionnaire-Revised (EPQ-R; Eysenck &
Eysenck, 1991). This is a 100-item questionnaire containing four scales: Extraversion,
Neuroticism, Psychoticism, and Lie. According to results from this test, participants were
divided into two groups of those who scored extremely high (>18) and low (<10) on the
extraversion scale. Bonus points were given as compensation for participation.
The generation of auditory stimuli and recording of participants’ responses to them were
regulated by computer. Auditory tones were created and presented through headphones
(Etymotic ER3A). The auditory signals were a combination of three separate tones that can be
described as: (0.3*sin(2*pi*t*f)+0.3*sin(2*pi*t*f*1.2599)+0.3*sin(2*pi*t*f*1.4983). They consisted of 500 Hz sine
waves and 700 Hz sine waves with a rise and fall time of 10 ms and duration of 300 ms.
Participants were seated in a comfortable chair 100 cm from a computer screen that
displayed visual feedback, in a sound-attenuated room separate from recording. Participants
were given a response panel that consisted of a series of buttons and were instructed to respond
to target stimuli by pressing the outermost left and right buttons using either their left or right
Instructions were displayed on the computer monitor for participants to respond to low
tones (500 Hz) and high tones (700 Hz) by pressing the uppermost and lowermost buttons on the
response panel with their left or right index finger. Assignment of response condition of left and
right finger was counterbalanced within subjects. Participants were instructed to respond as
The experiment was approximately 40 minutes. It consisted of a practise session that
provided participants with positive and negative feedback before each of six blocks that
consisted of 200 test trials. Participants received only negative feedback during the test trials.
The purpose of the practise sessions was to ensure that participants understood the directions and
to familiarize them with the auditory tones.
The electroencephalogram (EEG) was recoded using a Nuamps Digital EEG Amplifier
and an electro-cap of the international 10-20 system (Jaspers 15) using tin electrodes recoding
from 31 sites, with the addition of electrodes C3’ and C4’ that were located 1 cm anterior to C3
and C4. These positions were chosen as they cover areas specific for hand movement.
Reference electrodes were placed on both ears and a ground electrode was placed in the cap 1 cm
anterior to the Fz electrode. EOG was recorded using references from the infra- and supra-
orbital ridges of the left eye and from references lateral to each eye and then corrected using
Raw EEG data was band-pass filtered at zero phase shift with a low-pass filter of 20 Hz
at 48 db/oct and a high-pass of 0.25 Hz at 48 db/oct, and then underwent blink noise reduction.
Data was then epoched 100 ms before the onset of the stimuli and for 900 ms after, and then
baseline corrected with respect to the 100 ms prestimulus interval. S-LRP data was filtered and
blink noise reduced. It was then epoched 100 ms before the onset of the stimuli and for the
following 800 ms after and baseline corrected according to the 100 ms prestimulus interval. R-
LRP underwent the same procedure but was epoched 800 ms before the response and for 300 ms
after the response, then baseline corrected according to the 700 ms interval before completion of
the response. All data was artefact rejected at a +/- 50 uV threshold.
N1 and P3 amplitudes were gathered according to the 100 ms prestimulus baseline. The
N1 amplitude was measured as the maximum peak occurring between 50 and 200 ms at
electrodes FC3, FCZ, FC4, C3, C4, and CZ. The maximum peak occurring between 250 and 600
ms at electrodes P3, PZ, P4, CPZ, CP3, and CP4 was gathered for the P3 amplitude. All N1 and
P3 latencies were gathered at these maximum peaks over the stated electrodes.
The LRP was calculated according to Coles (1988) method for derivation of the
lateralized readiness potential. The first step in this calculation is to take potentials recorded
over both the left and right motor cortex at C3’ and C4’ and perform subtractions for left and
right hand movements. Therefore, for a right-handed movement, potentials from the right side
would be subtracted from the left and vice versa for left-handed movements. Then data from
these subtractions are averaged over both left and right hemispheres to obtain a measure of
averaged lateralized activity. In order to calculate the onset latency of the LRP for both S-LRP
and R-LRP intervals, the LRP peak amplitude was found and 50% of this amplitude was taken.
Reaction time was analyzed using between groups t-test. It was found that response time
was longer for introverts (M = 386 ms) and extroverts (M = 335 ms) t (15) = 1.77, p = .09.
P3 amplitude was analyzed using a repeated measures analysis of variance with
amplitudes at P3, Pz, and CP4 and extraversion as a between subjects factor. There was a
significant interaction between electrode position and extraversion F (2, 30) = 2.7, p = .08. This
was primarily due to the larger amplitude for extroverts (M = 7.2 µV) than introverts (M = 3.6
No other tests reached the liberal significance level of .10.
Several studies have failed to find significant extraverted related differences in response
time (Doucet & Stelmack, 1997; Rammsayer & Stahl, 2004; Stahl & Rammsayer, 2004).
However, it is interesting that the present study did find longer response time for both introverts
and extroverts. A possible explanation of this effect may be a result of extraversion related
differences in attention on monotonous vigilance tasks. Schmidt, Beauducel, Brocke and Strobel
(2004) claim that response time may be a sensitive measure of performance on monotonous
vigilance tasks. Schmidt et al. found that mean response times of extraverts and introverts
increased with time spent on a 40-minute task. Schmide et al. suggest that this explanation may
relate to Eysenck’s arousal-activation theory. Eysenck (1967) proposed that levels of
extraversion could be tied to the reticular formation arousal system. This theory proposes that
introverts generally have a high arousal level, where extroverts tend to have low arousal levels or
high inhibition. Therefore, introverts may perform better at sustained attention tasks because
they do not require high amounts of stimulation to keep them engaged. Extroverts do require
greater stimulation and as a result may loose attention if a maximal level is not present. Longer
reaction times in both introverts and extroverts may be explained by the level of stimulation that
the task in the present study elicited. The type of task may have been at a median arousal level so
that introverts were generally investing more attention than extroverts were which is noted in the
difference between group mean latency. However, both groups may have found the task to be
monotonous and neither group invested their attention because participants may have been
fatigued or board coming into the experiment.
Several studied have applied the P3 measure to analyse extraversion related differences in
response selection and failed to find significant differences (Doucet & Stelmack, 2000; Stahl &
Rammsayer, 2004; Rammsayer & Stahl, 2004), especially in regards to P3 amplitude. However,
the present study did demonstrate a significant difference in P3 amplitude at electrode P4 in
extroverts. One possible explanation for this effect may be the use of extreme groups. Polich
and Martin (1992) suggest that effects of personality on P3 measures are only seen with the use
of extreme groups, which the study at hand applied. Another possible explanation may be
related to the uncontrolled individual difference of gender. The study at hand did not control for
females and males in the study, and as a result, there were more females than males. Polich and
Martin (1992) controlled for gender in their study of P3 and personality and found that females
tended to have larger P3 amplitudes than males. Therefore, the effects of larger P3 amplitude
may also be due the lack of control for gender.
No significant differences were found for the remaining measures applied in the present
study. Minimal research has been done examining the use of lateralized readiness potential to
determine extraversion related differences in stimulus analysis and response organization.
Results have varied in their ability to demonstrate these affects. Rammsayer and Stahl (2004)
found no significant difference in the S-LRP interval between introverts and extroverts, but did
find that response organization, represented by the R-LRP interval was longer for introverts than
extroverts. Stahl and Rammsayer (2004) found shorter S-LRP latencies for introverts,
demonstrating that stimulus analysis processes were faster in introverts. They failed to find
significant differences in R-LRP latencies.
A possible explanation for the present study’s failure to find any difference in S-LRP
latency or stimulus analysis may result from inadequate complexity of stimuli. Stahl and
Rammsayer (2004) suggested that future research should increase the level of complexity of
stimuli in order to increase the cognitive demands of the task. This was thought to evoke
extraversion related differences in stimulus analysis. The study at hand did increase the
complexity of the stimuli, but the level may not have been sufficient to create group differences
in stimulus analysis. Another explanation may be related to the use go-trails rather than go-nogo
trials. Stahl and Rammsayer (2004) suggest that nogo-trails evoke adequate levels of response-
excitation required for differentiation in R-LRP latencies. The study at hand applied a simple go-
trial task, where participants were responding to all tones and not withholding any response.
Therefore, participants may not have received sufficient response excitation. Furthermore, other
issues that may relate to insignificant result may be due to sampling. The study at hand included
only university students in the sample, which may not be representative of the true population.
As well, data was lost due to technical difficulties for the calculation of the R-LRP, which may
have affected the results. In addition, the sample size was relatively small in comparison to past
studies (Stahl & Rammsayer, 2004; Rammsayer & Stahl, 2004).
In conclusion, the present study failed to demonstrate that the lateralized readiness
potential is an adequate measure of extraversion related differences in stimulus analysis and
response organization. However, it does support the use of response time and P3 as adequate
measures of stimulus analysis. Further research should expand its sample range, rather than limit
it to university students, as well as ensure a larger sample. Increasing the complexity of auditory
stimuli may also evoke desired differences.
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