Who Needs Emotions? The Brain Meets the Robot

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motivation arousal necessary for ingestive behavior (Marshall, Richardson,
& Teitelbaum, 1974; Ungerstedt, 1971).
In addition to mediating the processing of ongoing incentive stimuli in an
organism’s environment, DA signals appear to be an integral part of learning
and plasticity in the many forebrain regions that they influence (Di Chiara,
1998; Cardinal, Parkinson, Hall, & Everitt, 2002; Sutton & Beninger, 1999).
Indeed, several major hypotheses concerning the functional role of DA primarily
emphasize its role in associative or incentive learning or the ability of
the organism to learn about beneficial or potentially beneficial stimuli in its
environment and react appropriately. For example, Robinson and Berridge have
proposed that DA is important for attributing incentive salience to neural
representations of rewards, through a process that enables an environmental
stimulus to be attractive, or “wanted,” and to elicit voluntary approach behavior
(Berridge & Robinson, 1998; Robinson & Berridge, 1993). The work
of Schultz (2000), utilizing single-cell recording in the awake, behaving monkey,
suggests that DA neurons fire to predicted rewards and track expected
and unexpected environmental events, thereby encoding “prediction errors,”
that is, information about future events of potential salience or value to the
animal. Studies show that neurons in prefrontal–striatal networks are sensitive
to reward expectations and activated in association with motor responses
to specific learned cues or events (Pratt & Mizumori, 2001; Schoenbaum,
Chiba, & Gallagher, 1998; Schultz, Tremblay, & Hollerman, 2000). Work in
the prefrontal cortex is particularly interesting in this regard. Prefrontal networks
are equipped with the ability to hold neural representations in memory
and use them to guide adaptive behavior; DA and particularly D 1 receptors
are essential for this ability (Williams & Goldman-Rakic, 1995). In rats, D 1
receptor activation in the prefrontal cortex is necessary for active retention of
information that guides future behavior in a foraging task and modulates hippocampal
inputs to the prefrontal cortex (Seamans, Floresco, & Phillips, 1998).
Thus, DA may “prime” and ultimately reinforce motor strategies that result
in adaptive, beneficial behavior. Electrophysiological work indicates that DA
is able to hold or gate neurons in a primed “up state” and to facilitate the potential
for the network to learn new information and initiate plasticity (Lewis
& O’Donnell, 2000; Wang & O’Donnell, 2001). Our own work has shown
that activation of D 1 receptors in corticostriatal networks is essential for hungry
rats’ ability to learn an instrumental response for food (Baldwin, Sadeghian,
& Kelley, 2002; Smith-Roe & Kelley, 2000).
As mentioned earlier, monoamines are abundant throughout phylogenetic
development. Dopamine, DA receptors, and associated proteins such
as transporters and synthetic and phosphorylating enzymes have been found
in all species thus far examined, including nematodes, mollusks, crustaceans,
insects, and vertebrates (Cardinaud et al., 1998; Kapsimali et al., 2000;