The Brain Makes Us Feel
Some of the same structures that you just saw that are
used in memory are also involved in emotions. Do you know what motivates
you? How about:
- The desire for pleasurable sensations and
experiences
- The desire to avoid unpleasant sensations and
experiences
It was first discovered in animals that there
is a "reward" system in the brain. If you implant metal wires
into parts of this system and deliver small amounts of electrical
current, animals seem to like it (when surgeons did this with people,
the patients confirmed that the stimulation was pleasurable). In the
animal studies, experimenters showed that animals would self-stimulate.
That is, they would press a lever again and again if the lever were
connected so that electrical current would be delivered with each lever
press.
If electrodes were put in other parts of the brain,
the animals would not work for stimulation, and in some brain areas,
animals would work hard to prevent delivering current. It is as if
certain other brain areas are unpleasantness centers.
What Makes Humans Different?
Laughter is one clear difference from animals ("laughing"
hyenas are not really laughing). For more on the brain's role
in laughing, click
here.
Likewise, emotion-based crying seems unique to humans.
All animals produce tears in response to eye irritants.
Emotions
Primitive animals have mostly emotional brains. Those
parts of the brain are present also in humans, but humans have a new
structure on the surface of the brain, called "neocortex"
(cortex means outer surface). It is this neocortex that gives us the
ability to think, to make choices, and to reflect on our emotions rather
than unthinkingly act on them.
The brain's "reward
system" is driven partly by the neurotransmitter, dopamine. If neurons in the reward system
are getting bathed in dopamine, you feel good. If not, you don't feel
good. There is growing evidence that the dopamine system is deficient in
people who persist in excessive pleasure seeking, such as drug
addiction, compulsive gambling, or over-eating.
Brain scans, averaged over 5 obese
people and 5 people of normal weight, with the scan tuned to
detected receptor molecules for dopamine. The bright areas are
where the dopamine receptors are. The obese people had
fewer dopamine receptors. That is, their reward system was not
getting the normal amount of stimulation. Similar
dopamine-receptor deficiencies have been seen in drug addicts.
Source: National Institute of Drug Abuse.
The obvious interpretation is that obese people don't
get enough "reward" from life, and that they over-eat to
compensate. Another possibility is that their over-eating
over-stimulates the dopamine system and causes the dopamine-receptor
system to "down-regulate." That is, the brain quits making as
many receptors because there is a super-abundance of dopamine. Receptor
down-regulation from over-stimulation has been well documented in
numerous other kinds of situations. Can
you think how to test that possibility?
Drug addictions
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Craving for any sort of
drug (alcohol, tobacco, cocaine, etc.) seems to involve this same
dopamine reward system found in obesity. Normal rewarding things, like eating ice
cream or a good steak, trigger the release of dopamine in the
reward system. This dopamine is soon destroyed or taken back up.
But taking addictive drugs tend to promote sustained high levels of
dopamine - a chemical "flood." |
This creates a problem. The neurons that make dopamine
shut down for a while ("down regulate" as we mentioned above).
To get the same happy, feel-good experience, ordinary rewards, like ice
cream or steak, no longer make us happy. The addict has to take the drug
to experience that same intense feeling of reward. The addict is also driven by the desire to avoid the suffering
experience the he feels without the drug.
 
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