Who Needs Emotions? The Brain Meets the Robot

organization of motivational–emotional systems 55
Walker, Brooks, & Holden-Dye, 1996). One study showed a 70% homology
between cloned Drosophila D 1/D 5 receptors and their human counterpart
as well as stimulation of cAMP production by DA (Gotzes, Balfanz, &
Baumann, 1994). Although the functional role of DA has not been examined
in lower species as extensively as in vertebrates, there is certainly some
evidence that it may influence cellular function, adaptive behaviors, and
possibly plasticity in many animals. In Drosophila, the DA system appears
to regulate development, feeding, sexual behavior, and possibly learning
(Neckameyer, 1996, 1998; Tempel, Livingstone, & Quinn, 1984; Yellman,
Tao, He, & Hirsh, 1997). In honeybees, DA receptors have been well characterized
and proposed to play a role in motor behavior (Blenau & Baumann,
2001; Kokay & Mercer, 1996). Menzel and colleagues (1999) have used classical
conditioning in the honeybee to demonstrate a potential analogue for a
DA role in reward learning. Appetitive conditioning to sucrose (olfactory
conditioning to the proboscis extension reflex) is impaired with depletion
of biogenic amines and restored by DA. Further studies of reward learning
have shown that a neuron, termed VUMmx1, shows similar “reward prediction”
properties to the mammalian homologue (Menzel, 2001). In Aplysia,
DA appears to be a transmitter in a central pattern generator important for
feeding (Kabotyanski, Baxter, Cushman, & Byrne, 2000; Diaz-Rios, Oyola,
& Miller, 2002), and recent investigations show that dopaminergic synapses
mediate neuronal changes during operant conditioning of the buccal reflex
(Nargeot, Baxter, Patterson, & Byrne, 1999; Brembs et al., 2002). These
examples suggest that throughout evolutionary development of species DA
has retained a role of reward–motor coupling. Its expanded capacity to
modulate and modify the activity of cortical networks involved in cognition,
motor planning, and reward expectation is apparent in mammalian species.
Serotonin: Aggression and Depression
A further example of monoamine modulation of motivated behaviors and
affective processing is the serotonergic system. Serotonin (5-HT), an indoleamine
synthesized from the amino acid tryptophan, has been widely implicated
in many behavioral functions, including behavioral state regulation and
arousal, motor pattern generation, sleep, learning and plasticity, food intake,
mood, and social behavior. In terms of anatomy in the mammalian brain,
serotonergic systems are widespread; their cell bodies reside in midbrain
and pontine regions, and there are extensive descending and ascending projections.
Descending projections reach brain-stem and spinal motor and
sensory regions, while the ascending inputs project to widespread regions in
the cortex, limbic system, basal ganglia, and hypothalamus—indeed, the