Who Needs Emotions? The Brain Meets the Robot

organization of motivational–emotional systems 57
In lobsters and crayfish, serotonin and octopamine direct or bias the readout
of stereotypical motor programs that cause dominant or subordinate
postures, respectively. Infusions of serotonin into the hemolymph induce
aggressive, dominant postures, even in formerly subordinate animals. In groups
of individuals, social status becomes established as a hierarchy develops. Thus,
social behavior can become conditioned and show plasticity; indeed, the social
status can determine the extent of the response to serotonin (Yeh, Fricke,
& Edwards, 1996), and infusion of serotonin into a subordinate animal actually
increases its willingness to fight (Huber et al., 1997).
In mammals, there is extensive evidence for involvement of serotonin in
modulation of aggression. As for other amines, a variety of methods have been
used to manipulate the serotonin system, including pharmacology, lesions,
microdialysis, and genetic knockout strategies. Also, it is very important to
note that the number and variety of serotonin receptors suggest that this
modulation is very complex (appraisal of the literature indicates that both too
little and too much tone in serotonergic neurons can disrupt aggression);
moreover, treatments that globally affect serotonin affect multiple receptor
systems and may not reveal any clear function. Early studies indicated that in
mice and rats depletion of serotonin with drugs induced a temporary increase
in aggression; for example, rats that normally ignore mice would engage in
mouse-killing behavior (Vergnes, Depaulis, & Boehrer, 1986). More recent
work with receptor-specific drugs indicated that treatment of rats with 5-HT1B
or 5-HT1A agonists tended to reduce aggression in a rat resident–intruder
model of aggressive behavior (Miczek, Mos, & Olivier, 1989). Moreover,
5-HT1B knockout mice show increased levels of aggression (Saudou et al.,
1994), and the 5-HT1A receptor is strongly implicated in expression of
anxiety-like behavior in animal tests (Gross et al., 2002).
In nonhuman primates, the link between aggression and serotonin is quite
compelling, although mainly based on an indirect measure of serotonin, cerebral
spinal fluid (CSF) measures of 5-hydroxyindoleacetic acid (5-HIAA), the
metabolite serotonin. Moreover, this work reveals an important relationship
between 5-HT, aggression, and social relationships among conspecifics, much
as in crustaceans. For example, Higley et al. (1996) and Mehlman et al. (1994)
examined the relationship between CSF 5-HIAA and behavior in free-ranging
monkeys in naturalistic environments. These studies show a clear correlation
between lowered CSF 5-HIAA levels and increased levels of impulsive
aggression. For example, among rhesus monkeys living in social colonies,
animals with the lowest quartile of CSF 5-HIAA had high levels of unprovoked,
escalated aggression and a higher risk of injury or death. These animals
would initiate aggression at inappropriate targets, such as high-ranking
males, and demonstrate impaired impulse control in other behaviors, such