Consciousness, the Brain, and Behavior
241
Chemical Mediators
Dopamine is a major neurotransmitter in the pathway that
mediates the brain reward systems and motivation. For this
reason, drugs that increase synaptic activity in the dopamine
pathways increase self-stimulation rates—that is, they provide
positive reinforcement. Amphetamines are an example of such a
drug because they increase the presynaptic release of dopamine.
Conversely, drugs such as chlorpromazine, an antipsychotic
agent that blocks dopamine receptors and lowers activity in the
catecholamine pathways, are negatively reinforcing. The cat-
echolamines are also, as we will see, implicated in the pathways
involved in learning. This is not unexpected because rewards and
punishments are believed to constitute incentives for learning.
Emotion
Emotion can be considered in terms of a relation between an
individual and the environment based on the individual’s eval-
uation of the environment (is it pleasant or hostile, for exam-
ple), disposition toward the environment (am I happy and
attracted to the environment or fearful of it?), and the actual
physical response to it. While analyzing the physiological bases
of emotion, it is helpful to distinguish: (1) the anatomical sites
where the emotional value of a stimulus is determined; (2) the
hormonal, autonomic, and outward expressions and displays
of response to the stimulus (so-called
emotional behavior
);
and (3) the conscious experience, or
inner emotions,
such as
feelings of fear, love, anger, joy, anxiety, hope, and so on.
Prefrontal
cortex
Nucleus
accumbens
Midbrain nuclei
Brainstem
nuclei
Locus ceruleus
Figure 8–9
Schematic drawing of the mesolimbic dopamine pathway. Various
psychoactive substances are thought to work in these areas to
enhance brain reward.
Figure 8–10
Apparatus for self-stimulation experiments. Rats do not appear to be
bothered by the implanted electrode. In fact, they work hard to get
the electrical stimulation.
Adapted from Olds.
pressing rate above the level of random presses, the electric
stimulus is by defi nition rewarding. If the animal decreases the
press rate below the random level, the stimulus is punishing.
Thus, the rate of bar pressing with the electrode in dif-
ferent brain areas is taken to be a measure of the effective-
ness of the reward or punishment. Different pressing rates are
found for different brain regions.
Scientists expected the hypothalamus to play a role in
motivation because the neural centers for the regulation of
eating, drinking, temperature control, and sexual behavior
are there (Chapter 6). Indeed, they found that brain self-
stimulation of the lateral regions of the hypothalamus serves
as a positive reward. Animals with electrodes in these areas
have been known to bar press to stimulate their brains 2000
times per hour continuously for 24 h until they drop from
exhaustion! In fact, electrical stimulation of the lateral hypo-
thalamus is more rewarding than external rewards. Hungry
rats, for example, often ignore available food for the sake of
electrically stimulating their brains at that location.
Although the rewarding sites—particularly those for pri-
mary motivated behavior—are more densely packed in the
lateral hypothalamus than anywhere else in the brain, self-
stimulation can occur in a large number of brain areas. Thus,
while the hypothalamus coordinates the sequenced hormonal,
autonomic, and behavioral responses of motivated behaviors,
motivated behaviors based on learning involve additional
integrative centers, including the cortex and limbic system.
Motivated behaviors also use integrating centers in the brain-
stem and spinal cord—in other words, all levels of the nervous
system can be involved.
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