Who Needs Emotions? The Brain Meets the Robot

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as tree jumping (Doudet et al., 1995; Higley et al., 1996; Mehlman et al.,
1994). In another study of monkey social groups, treatment with drugs that
enhanced or reduced brain serotonin levels clearly revealed behavioral patterns
where animals with high serotonin showed lower levels of aggression
and enhanced social skills (Raleigh & McGuire, 1991).
In humans, there is convincing evidence for serotonin dysfunction in a
variety of disorders and disordered behavior. In psychiatric populations, there
is a well-established link between abnormally low central 5-HT levels and
increased aggressive or antisocial behavior, alcoholism, and impaired impulsive
control (Mann et al., 1996; Virkkunen and Linnoila, 1992). Patients with
reduced serotonin function have been shown to have higher rates of major
depression and suicide attempts or completed suicide (Coccaro et al., 1989;
Mann et al., 1996). Thus, research from studies on humans and other animals
clearly implicates an important and complex role for central serotonin
and its receptors in the control of behavioral state. In nonhuman animals,
this has been demonstrated in the realm of control of aggression and social
status or interactions; in humans, this involvement is expanded to regulation
of mood and emotions, particularly control of negative mood or affect.
Since serontin-containing neurons innervate nearly all regions of the neuraxis
in higher mammals, this role is also a particularly good example of the anatomical
and functional evolution of a neurochemical system: in crustaceans,
serotonin plays a specific role in social status and aggression; in primates,
with the system’s expansive development and innervation of the cerebral
cortex, serotonin has come to play a much broader role in cognitive and
emotional regulation.
Opioid Peptides: Pain and Pleasure
The opioid peptides and their receptors are a further example of neurochemical
modulation of affect. Since the discovery of endogenous opioid peptides
and their receptors nearly three decades ago (Lord, Waterfield, Hughes, &
Kosterlitz, 1977; Pert & Snyder, 1973), there has been enormous interest in
understanding the functional role of these compounds in the brain. Opioids,
which comprise multiple families of peptides such as the endorphins,
enkephalins, and dynorphins as well as their multiple receptor subtypes (mu,
delta, kappa), are found in various networks throughout the brain but particularly
within regions involved in emotional regulation, responses to pain
and stress, endocrine regulation, and food intake (LaMotte, Snowman, Pert,
& Snyder, 1978; Mansour et al., 1987). This distribution as well as extensive
empirical work has led to the notion that opioids play a major role in
diverse biological processes such as pain modulation, affect and emotion,