Who Needs Emotions? The Brain Meets the Robot

56 brains
serotonergic system is proposed to be “the most expansive neurochemical
network in the vertebrate CNS” (Jacobs & Azmitia, 1992). It is a system that
is highly conserved across phyla; serotonin is an important neurotransmitter
in many invertebrates, and over 15 subtypes of 5-HT receptors have arisen
through molecular evolution, most of which interact with G proteins
(Peroutka & Howell, 1994; Saudou & Hen, 1994). This extensive development
of receptor subtypes suggests a great diversity of signaling within
serotonin systems across phyla. Serotonin is a particularly interesting and
appropriate chemical signal to examine in the context of the evolution of
motivated behavior and emotion. Over the past 25 years or so, its pharmacology,
physiology, and molecular biology have been extensively studied in
crustaceans, insects, mollusks, worms, and mammals. Moreover, dysfunction
of serotonin in humans has been implicated in many psychiatric disorders,
such as depression, anxiety, obsessive–compulsive behavior, and
alcoholism (Nemeroff, 1998). Serotonin-selective reuptake inhibitors (SSRIs)
are among the most commonly prescribed psychiatric drugs. Thus, understanding
the functions of 5-HT through analysis of its role in behavior and
brain function may provide important insights into the neurochemical basis
of human emotion and its disorders.
While serotonin clearly has roles of a very diverse nature, there is an interesting
common thread that weaves through much of the research on this
substance. Work from a variety of approaches leads to the general consensus
that 5-HT plays a critical role in the modulation of aggression and agonistic
social interactions in many animals and possibly regulation of aggressive
behavior and mood in nonhuman primates and humans (Insel & Winslow,
1998). Experiments on crustaceans such as the lobster and crayfish clearly
implicate serotonin, as well as octopamine (a phenol analogue of norepinephrine),
in the control of behaviors concerned with the maintenance of social
hierarchies. The fighting behavior of these phylogenetically ancient animals,
highly successful predators and scavengers, has been extensively studied in
artificial aquatic environments.
In a typical scenario, intensity of fighting increases in a step-wise
fashion beginning with threat displays upon first contact, followed
by phases of ritualized aggression, restrained use of claws, and in
rare instances ending in periods of unbridled combat. The presence
of such a structured behavioral system, combined with an opportunity
to bring the analysis to the level of individual neurons, thus offers
unique opportunities for exploring fundamental issues of interactions
between aggression, dominance, and amine neurochemistry.
(Huber et al., 2001, p. 272)