Transcript of Audio Lecture
Traits pertaining to approach-versus-avoidance (such as
the sensation seeking traits that we assessed in your homework
assignment) have received some of the most attention from researchers
interested in the biological bases of personality. One such trait is
introversion and extroversion. In the following lesson, we will review
two major theories linking differences in brain functioning to
differences in extroversion. First we will examine a model proposed Hans
Eysenck in the 1960s and then we will examine a more recent model
proposed by Jeffrey Gray.
Slide two. Eysenck's model of
introversion/extroversion starts with the fact that for every individual
there is an optimal level of cortical arousal in the brain. That is, as
cortical arousal varies from low to medium to high, we see an
inverted-U-shaped function between arousal level and levels of
subjective well-being. At relatively low levels of cortical arousal, the
person is under-stimulated, and may feel sluggish, bored, and restless.
At very high levels of arousal, the person is over-stimulated, and may
feel hyper-vigilant, panicky, and overwhelmed. Thus, at both ends,
well-being is low. At the middle level of arousal though, well-being is
fairly high and as is experienced as states of interest, challenge,
excitement, and enthusiasm. Thus, people experience the highest levels
of well-being when the level of arousal or stimulation of their cortex
is at some optimal middle level.
Slide three. The second thing we need to consider are
the sources of stimulation that contribute to arousal of the cerebral of
cortex. Two sources of stimulation are stimulation from sensory input
(such as stimulation from the eyes, the ears, from touch and taste) and
stimulation from inside (in particular from the part of the brain stem
called the ascending reticular activating system, a set of neurons that
seem responsible for modulating the level of awakeness or alertness in
the cortex). At any point, then, your cortical arousal is the sum of
external stimulation and stimulation due to ARAS activity.
Slide four. Well assuming that there’s an optimal
level of arousal for every individual, then what happens if you have a
more active ascending reticular activating system or ARAS? This means
that as soon as you wake up in the morning, your ARAS is already
providing lots of internal stimulation. You feel relatively alert, maybe
close to your optimal level of arousal and you are not needing much more
to reach your optimum level. Simply doing quiet or solitary
activities—reading a book, chatting with one close friend—might be
sufficient for your to reach your optimal level. On the other hand,
meeting many new people or entering a noisy situation are likely to
cause you to become over-aroused. Therefore, you generally seek to
decrease the level of external stimulation. You limit your number of
social contacts and seek quieter activities. Thus, as a result of having
a more active ARAS and having therefore more internally generated
cortical arousal, you seek less external stimulation and therefore
appear introverted.
Now what would happen if you have a less active ARAS
and thus less internally generated cortical arousal? When you wake up in
the morning, your ARAS is NOT giving you enough stimulation, you are
under-aroused, and so prone to feeling restless and bored. Consequently,
you seek to increase your arousal by seeking external sources of
stimulation—new people, crowds, dynamic situations, and so on. Thus, as
a result of having a less active ARAS and having therefore less
internally generated cortical arousal, you seek more external sources of
stimulation and therefore appear extraverted.
Slide 5. There are several sources of evidence for
this theory. One is the effects of environmental stimulation. One
interesting program of research involves situations in which there is
very little external stimulation, such as when a person is doing a
vigilance task. A classic vigilance task would be looking at a radar
screen in the middle of the night on a submarine. Most of the time there
is little external stimulation and nothing is happening on that hypnotic
little radar screen. So, if your ARAS is not providing much internal
stimulation, you may quickly feel restless, and your attention may
wander, especially when it is so much more stimulating to fantasize what
you’ll do when you finally get off of this dang submarine. On the other
hand, if your ARAS is providing lots of internal cortical arousal, then
watching the screen may be a perfectly pleasant way to spend your time,
and consequently you can sustain attention for extended periods of time.
And in fact the research shows that introverts are better than
extraverts at sustained attention during vigilance tasks. Conversely, in
a very noisy environment we find the opposite to be the case. Noisy
situations boost extroverts up to their optimal level of stimulation,
causing them to feel better and perform better. The introverts, on the
other hand, find the noisy situations over-stimulating and
anxiety-producing, making it difficult for them to concentrate.
A second source of evidence involves reflexes. For
example, when you enter a dark room, your pupils dilate in order to take
in more photons. When you enter a bright room, your pupils constrict in
order to limit the number of photons coming in. Since these are
automatic reflexes you might not expect them to have any connection to a
personality trait like introversion and extroversion, but they do. So
whose pupils do you think will constrict faster in the bright room? It’s
the introverts—they are quicker at restricting the amount of stimulation
coming in, as if their central nervous system is saying: Quick, I’m
easily overwhelmed and need to restrict incoming stimulation!
Conversely, who’s pupils dilate faster in the dark room? It’s the
extroverts—as if their central nervous system is saying: Quick, I’m
stimulation hungry and need to let in as much external stimulation as
possible!
A third source of evidence involves the effects of drugs that
depress or stimulate the central nervous system. In the case of
depressants, we find that those people who naturally have relatively low
levels of central nervous system activation, low levels of ARAS
activity, the extroverts, get intoxicated more quickly by depressants
such as alcohol. So, for example, if we had a drinking contest and lined
up shot glasses in front of an extrovert and an introvert, while the
extrovert may be more likely to boast how they can hold their liquor, in
fact they're more likely to fall off their chair before the introvert
does because alcohol is a central nervous system depressant and the
alcohol needs to do less depressing of the central nervous system to
knock out the extravert. But the reverse in the case of the stimulants.
If we remove the shot glasses and instead line up tabs of amphetamines
in front of the introvert and the extrovert, it will take fewer
amphetamines to make the introvert feel overwhelmed and panicky because
they started out at higher level of arousal.
So there is compelling
evidence in support of Eysenck's theory. Yet, while Eysenck's was
probably on the right track, the specifics of his physiological
model—namely, that it’s actually the ascending reticular activating
system that is causing the differences—have been criticized. Let’s look
at a more recent model.
Slide 6. Gray’s model of approach and inhibition
offers an alternative model that is more compatible with recent neurophysiological findings. Gray suggests that there are two systems—a
behavioral approach system and a behavioral inhibition system and that
the essential neurophysiological components reside in the cortex rather
than in the brain stem. In particular, brain activity in the left
frontal cortex is associated with behavioral approach tendencies,
whereas brain activity in the right frontal cortex is associated with
behavioral inhibition tendencies. Specifically, when the left frontal
region activated, we find people are more sensitive to actual or
potential rewards—what they could gain. When there is right frontal
activity, they are more sensitive to the actual or potential costs—what
they could lose. When the approach system is active, people think about
opportunities. When the inhibition system is active, people think about
dangers. When the approach system is active, and people are focused on
rewards and opportunities, they feel confident and happy. When the
inhibition system is active, and people focused on dangers and costs,
they feel uncertain and anxious. And of course behaviors are follow from
these thoughts and feelings. If you’re thinking about opportunities and
feeling confident, you’re gonna go for it; so behavioral approach system
activation is associated with going for your goals. If you’re thinking
about dangers and feeling anxious, you’re going to be cautious; so
behavioral inhibition system activation is associated with stopping and
checking. People with dispositional tendencies towards more behavioral
approach system activation tend to be impulsive people because they’re
so focused on opportunities. People with dispositional tendencies
towards more behavioral inhibition system activity tend to be anxious
people because they’re so focused dangers.
Slide 7. One of the most
compelling features of Gray’s model is how it explains some of the basic
dimensions of the five factor model and of Eysenck’s model as
combinations of these two underlying neuro-physiological systems. In
this graph, on the x axis is the degree to which a person shows
behavioral approach system activity, and on the y axis is the degree to
which a person shows behavioral inhibition system activity. This model
conceptualizes introversion and extraversion as simply the balance
between the behavioral approach and behavioral inhibition systems. When
a person has relatively low levels of BAS activity and relatively high
levels of BIS activity, they appear introverted because they are more
focused on the potential costs than on the potential rewards of taking
action. In extroverts, the balance is flipped the other way. There’s
more behavioral approach activity than behavioral inhibition activity,
and as a result, they’re more focused on the rewards than the costs of
taking action.
When the behavioral approach and behavioral inhibition
systems are in balance, people are not clearly introverted or
extraverted; however, these people may differ as a result of differences
in their overall levels of BAS and BIS activity. People who have low
levels of both behavioral approach and behavioral inhibition tendencies
will be neither impulsive nor anxious, and so may appear relatively
emotionally stable, calm, sanguine. On the other hand, People who have
high levels of both behavioral approach and behavioral inhibition
tendencies will have both compelling wishes and compelling fears. So,
this is the sort of individual who when deciding whether to ask someone
out on a date might think: "Oh, I’d love to go out with that person, I
really like them, and if they said yes it would be fantastic!", and then
also think "Whoa, hey, but what if I get rejected. If she does I would
feel terrible, I could not stand it!" So they have a strong conflict--an
approach avoidance conflict--because they feel a strong approach and a
strong inhibition towards the same goal. So in this model, highly
emotional (i.e., neurotic) individuals are that way because they have
this continual conflict between very much wanting to do things and
feeling very worried about doing those same things.
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