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Scientists from 'Wellcome Trust' claim
to have identified for the first time what happens in our brain in
the face of an approaching fear. They measured activity in the
brain using fMRI while a subject played a game similar to Pac-Man
and received an electric-shocks when they were caught by the video
game predator.
They found that activity in the ventromedial prefrontal cortex
(behind the eyebrows) increased when the enemy was in the distance
- this part of the brain is active when one is planning how to
respond to a threat. As the video game enemy approached,
predominant activity shifted to the periaqueductal grey - the part
of the brain responsible for flight or fight and preparing for
reaction to pain.
The title of their study is 'Free Will Takes Flight', as it shows
that we act more on impulse when a threat increases.
Abstract can be found here
Article in Science Magazine can be found
here
From Science 24 August 2007: Vol. 317.
no. 5841, pp. 1043 - 1044
Neuroscience: The Threatened Brain
Stephen Maren
The world is a dangerous place. Every day
we face a variety of threats, from careening automobiles to stock
market downturns. Arguably, one of the most important functions of
the brain and nervous system is to evaluate threats in the
environment and then coordinate appropriate behavioral responses to
avoid or mitigate harm.
Imminent threats and remote threats
produce different behavioral responses, and many animal studies
suggest that the brain systems that organize defensive behaviors
differ accordingly (1). On page 1079 of this issue, Mobbs and
colleagues make an important advance by showing that different
neural circuits in the human brain are engaged by distal and
proximal threats, and that activation of these brain areas
correlates with the subjective experience of fear elicited by the
threat (2). By pinpointing these specific brain circuits, we may
gain a better understanding of the neural mechanisms underlying
pathological fear, such as chronic anxiety and panic disorders.
To assess responses to threat in humans,
Mobbs and colleagues developed a computerized virtual maze in which
subjects are chased and potentially captured by an "intelligent"
predator. During the task, which was conducted during
high-resolution functional magnetic resonance imaging (fMRI) of
cerebral blood flow (which reflects neuronal activity), subjects
manipulated a keyboard in an attempt to evade the predator.
Although the virtual predator appeared quite innocuous (it was a
small red circle), it could cause pain (low- or high-intensity
electric shock to the hand) if escape was unsuccessful. Brain
activation in response to the predatory threat was assessed
relative to yoked trials in which subjects mimicked the
trajectories of former chases, but without a predator or the threat
of an electric shock. Before each trial, subjects were warned of
the contingency (low, high, or no shock). Hence, neural responses
evoked by the anticipation of pain could be assessed at various
levels of threat imminence not only before the chase, but also
during the chase when the predator was either distant from or close
to the subject.
How does brain activity vary as a function
of the proximity of a virtual predator and the severity of pain it
inflicts? When subjects were warned that the chase was set to
commence, blood oxygenation level-dependent (BOLD) responses (as
determined by fMRI) increased in frontal cortical regions,
including the anterior cingulate cortex, orbitofrontal cortex, and
ventromedial prefrontal cortex. This may reflect threat detection
and subsequent action planning to navigate the forthcoming chase.
Once the chase commenced (independent of high- or low-shock
trials), BOLD signals increased in the cerebellum and
periaqueductal gray. Activation of the latter region is notable, as
it is implicated in organizing defensive responses in animals to
natural and artificial predators (3, 4). Surprisingly, this phase
of the session was associated with decreased activity in the
amygdala and ventromedial prefrontal cortex. The decrease in
amygdala activity is not expected, insofar as cues that predict
threat and unpredictable threats activate the amygdala (5, 6).
However, activity in these brain regions
varied considerably according to the proximity of the virtual
predator and the shock magnitude associated with the predator on a
given trial (see the figure). When the predator was remote, blood
flow increased in the ventromedial prefrontal cortex and lateral
amygdala. This effect was more robust when the predator predicted a
mild shock. In contrast, close proximity of a predator shifted the
BOLD signal from these areas to the central amygdala and
periaqueductal gray, and this was most pronounced when the predator
predicted an intense shock. Hence, the prefrontal cortex and
lateral amygdala were strongly activated when the level of threat
was low, and this activation shifted to the central amygdala and
periaqueductal gray when the threat level was high. |
