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Module 1.4b

Paul Bucy surgically removed a rhesus monkey’s amygdala, turning the normally ill-tempered
®
animal into the mellowest of creatures. So, too, with humans. People with amygdala lesions AP Science Practice
often display reduced arousal to fear- and anger-arousing stimuli (Berntson et al., 2011). Research
One woman with an amygdala lesion, patient S. M., has been called “the woman with no
fear,” even if being threatened with a gun (Feinstein et al., 2013). Notice the phrase, “other studies
link criminal behavior with amygdala
What, then, might happen if we electrically stimulated the amygdala of a normally dysfunction.” Because researchers
placid domestic animal, such as a cat? Do so in one spot and the cat prepares to attack, hiss- cannot ethically manipulate the
ing with its back arched, its pupils dilated, its hair on end. Move the electrode only slightly variable of amygdala dysfunction,
they cannot draw causal conclu-
within the amygdala, cage the cat with a small mouse, and now it cowers in terror. sions. This is an important point in
These and other experiments have confirmed the amygdala’s role in fear and rage. psychology: Correlation does not
Monkeys and humans with amygdala damage become less fearful of strangers (Harrison equal causation.
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
et al., 2015). Other studies link criminal behav-
ior with amygdala dysfunction (Dotterer et al.,
2017; Ermer et al., 2012a).
But we must be careful. The brain is not
neatly organized into structures that corre-
spond to our behavior categories. The amyg-
dala is engaged with other mental phenomena
as well. And when we feel afraid or act aggres-
sively, neural activity occurs in many areas
of our brain — not just the amygdala. If you GK Hart/Vikki Hart/Getty Images
destroy a car’s battery, the car won’t run. But
the battery is merely one link in an integrated
system.

The Hypothalamus Copyright © Bedford, Freeman & Worth Publishers.
Figure 1.4-10
Just below (hypo) the thalamus is the hypothal- The hypothalamus
amus (Figure 1.4-10), an important link in the
command chain governing bodily maintenance. This small but important
structure, colored yellow/orange
Some neural clusters in the hypothalamus influ- in this MRI scan, helps keep the
ence hunger; others regulate thirst, body tem- body’s internal environment in a
perature, and sexual behavior. Together, they help steady state.
maintain a steady (homeostatic) internal state.
To monitor your body state, the hypothal-
amus tunes into your blood chemistry and any
incoming orders from other brain parts. For Diomedia
example, if it picks up signals from your brain’s
cerebral cortex that you are thinking about sex,
your hypothalamus will secrete hormones. These hormones will, in turn, trigger the pitu-
itary, which controls your endocrine system (Figure 1.4-9) to influence your sex glands to
release their hormones. These hormones will intensify the thoughts of sex in your cerebral
cortex. (Note the interplay between the nervous and endocrine systems: The brain influ-
ences the endocrine system, which in turn influences the brain.)
A remarkable discovery about the hypothalamus illustrates how progress in science often hypothalamus [hi-po-THAL-
occurs — when curious, open-minded investigators make an unexpected observation. Two uh-muss] a limbic system neural
young McGill University neuropsychologists, James Olds and Peter Milner (1954), were trying structure lying below (hypo)
to implant an electrode in a rat’s reticular formation when they made a magnificent mistake: the thalamus; it directs several
They placed the electrode incorrectly (Olds, 1975). Strangely, as if seeking more stimulation, maintenance activities (eating,
the rat kept returning to the location where it had been stimulated by this misplaced electrode. drinking, body temperature), helps
govern the endocrine system, and
On discovering that they had actually placed the device in a region of the hypothalamus, Olds is linked to emotion and reward.
and Milner realized they had stumbled upon a brain center that provides pleasurable rewards.

The Brain: Brain Regions and Structures Module 1.4b 67

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