Who Needs Emotions? The Brain Meets the Robot

102 brains
United States, pair up with sexual partners. Once they mate, they stick together
and raise their offspring as a family, even across generations. Given
that pair bonding is so rare, the monogamous prairie vole offers a possible
window into the biology of attachment.
Attachment (pair-bond formation) is a key part of love (Sternberg, 1988;
Hedlund & Sternberg, 2000; Bartholomew, Kwong, & Hart, 2001). There
can be attachment without love but not love without attachment (Carter,
1998). Perhaps the mechanisms that underlie attachment in voles are also
at work in humans. Vole researchers used a different strategy from the one
used to study fear. Rather than starting with the circuits and then trying to
figure out the chemistry, they started with chemical findings and attempted
to relate them to circuits (see also Chapter 3, Kelley).
Two features of prairie voles make them attractive for studying pair
bonding (Insel, 1997). The first is that monogamy also occurs in voles living
in laboratory settings. In the laboratory, bonding can be measured by putting
a vole in the middle chamber of a box with three compartments. In one
of these, it encounters its mate and, in the other, a stranger. Voles that have
mated spend time with their partner, whereas unbonded ones have no particular
preference. The second feature is that pair bonding is present only in
prairie voles and not in closely related montane voles, which are found in
the Rockies and live individually rather than in family groups. These animals
do not form mate preferences after having sex, so when put in the threechamber
box, they do not spend more time with a vole they mated with
than a novel one. Differences in the brains of these two kinds of vole might
provide important clues about the biology of pair bonding, family organization,
and perhaps love itself.
One of the main discoveries was that receptors for two hormones believed
to play an important role in reproductive behavior were located in
different circuits in prairie and montane voles (Insel, 1997): vasopressin and
oxytocin. They are found only in mammals and are related to ancestral hormones
that play a key role in behaviors like nest building in nonmammalian
species. In the mammalian brain, these chemicals function not just as hormones
but also as neurotransmitters and/or modulators.
The roles of these chemicals in the behavioral differences between the
voles have been determined by injecting drugs that either stimulate or inhibit
the action of vasopressin or oxytocin. The drugs have been injected
into the ventricles, cavities that contain cerebrospinal fluid, which flows from
the ventricles into the spaces surrounding neurons and, therefore, reach
widespread areas of the brain. When a drug that blocks the action of naturally
released oxytocin is put in the ventricles of a female prairie vole just
before mating, she mates but does not bond with the sex partner. The drug
disrupts attachment, not sex. This suggests that oxytocin released during