Who Needs Emotions? The Brain Meets the Robot

organization of motivational–emotional systems 35
of motivation or the “manifestation of motivational potential,” which can
be expressed in different ways, such as autonomic activity, social or communicative
behavior, and subjective experience. Specific neurochemical
systems have evolved to enable these readouts and to render the organism’s
behavior and subjective state exquisitely sensitive to changes. When injected
into the brain, minute amounts of the neuropeptide angiotensin, a major
hormone involved in regulation of thirst and sodium appetite, induces immediate
and vigorous drinking in a nonthirsty rat (Schulkin, 1999). Gonadal
hormones such as estrogen and progesterone, acting on brain sites preserved
through evolution, trigger the potential for female sexual behavioral response
(Pfaff, 1980). A monkey has the ability to discriminate, via its choice behaviors,
drugs that specifically activate the dopamine system from drugs that
activate noradrenergic systems (Tidey & Bergman, 1998). Further, the latter
case is an example of how animals can choose to artificially amplify
emotions through psychoactive drugs, a point we will return to at the end
of the chapter. Organisms have the ability to sense ongoing interoceptive
changes associated with these different readouts. Thus, the notion that
neurochemically and genetically specified neural systems mediate particular
species-specific behaviors and behavioral states is a powerful model for
explaining the evolutionary development of emotional systems in the brain.
THE NATURAL HISTORY OF MOTIVATIONAL–
EMOTIONAL SYSTEMS
The simplest forms of motivational–emotional systems are termed taxes
(plural of taxis) and tropisms, or simple movements in response to a stimulus.
Motivational–emotional systems evolved from the movements of the
earliest organisms on earth at the beginning of life approximately 4 billion
years ago. Well before the advent of multicelled organisms and insects, bacteria
displayed what is known as chemotaxis (movement toward a beneficial
stimulus and away from a noxious stimulus). The work of Julius Adler (1966,
1969) with Escherichia coli has elegantly traced the genetic and molecular
origins of this behavior, and with a little imagination, one can observe the
ancient roots of motivation in the behavior of these organisms. These and
other motile bacteria will swim toward organic and inorganic attractants such
as oxygen, glucose, hydrophilic amino acids, and salt (in the right concentration)
and swim away from repellants such as ethanol, certain fatty acids,
and hydrophobic amino acids (Adler, Hazelbauer, & Dahl, 1973; Qi & Adler,
1989; Tso & Adler, 1974). Like primordial nervous systems, these cells possess
sensory reception, an integrating and transmitting mechanism, and an
excitation/effector pathway (see Fig. 3.2).