56 Biological Psychology
learning were developing. In 1953, Krech and Rosenzweig
began a collaborative study of relationships between brain
chemistry and behavior. Krech did classic early work in animal
learning (under his earlier name, Kreshevsky) and was a col-
league of and collaborator with Tolman. Mark Rosenzweig re-
ceived his PhD in physiological psychology at Harvard in
1949 and joined the psychology department at the University
of California, Berkeley, in 1951. Soon after they began their
joint work in 1953 they were joined by E. L. Bennett and later
by M. C. Diamond. Their initial studies concerned brain levels
of AChE in relation to the hypothesis behavior and included
analysis of strain differences (see Krech, Rosenzweig, &
Bennett, 1960). More recently, they discovered the striking
differences in the brains of rats raised in “rich” versus “poor”
environments. William Greenough (1984), at the University of
Illinois, replicated and extended this work to demonstrate dra-
matic morphological changes in the structures of synapses and
neurons as a result of experience.
The use of model biological systems has been an impor-
tant tradition in the study of neural mechanisms of learning.
This approach has been particularly successful in the analysis
of habituation, itself a very simple form or model of learning.
Sherrington did important work on flexion reflex “fatigue” in
the spinal animal at the turn of the century. In 1936, Prosser
and Hunter completed a pioneering study comparing habitu-
ation of startle response in intact rats and habituation of
hindlimb flexion reflex in spinal rats. They established, for
habituation, the basic approach of Sherrington, namely that
spinal reflexes can serve as models of neural-behavioral
processes in intact animals. Sharpless and Jasper (1956) es-
tablished habituation as an important process in EEG activity.
Modern Russian influences have been important in this
field—the key studies of Evgeny Sokolov (1963), first on
habituation of the orienting response in humans and more re-
cently on mechanisms of habituation of responses in the sim-
plified nervous system of the snail.
The defining properties of habituation were clearly estab-
lished by Thompson and Spencer in 1966, and the analysis
of mechanisms began. Several laboratories using different
preparations—Aplysia withdrawal reflex; Kandel and his
many associates (see Kandel, 1976); vertebrate spinal re-
flexes; Thompson, Spencer, Farel; crayfish tail flip escape;
Krasne (1969), Kennedy—all arrived at the same underlying
synaptic mechanism—a decrease in the probability of trans-
mitter release from presynaptic terminals of the habituating
pathway. Habituation is thus a very satisfying field; agree-
ment ranges from defining behavioral properties to synaptic
mechanisms. In a sense, the problem has been solved.
Habituation also provides a most successful example of the
use of the model biological systems approach to analysis of
neural mechanisms of behavioral plasticity (see Groves &
Thompson, 1970).
Special mention must be made of the elegant and detailed
studies by Eric Kandel and his many associates on long-
lasting neuronal plasticity in the Aplysiagill-withdrawal
circuit (Kandel, 1976; Hawkins, Kandel, & Siegelbaum,
1993). This simplified model system (together with work on
the hippocampus) made it possible to elucidate putative
processes that result in long-lasting synaptic plasticity, for
example, biochemical models of memory formation and stor-
age. Eric was awarded the Nobel Prize for Physiology and
Medicine in 2000 in part for this work.MOTIVATION AND EMOTIONPhysiological and neural mechanisms of motivation and
emotion have been a particular province of biological psy-
chology and physiology in the twentieth century. In more re-
cent years, the fields of motivation and emotion have tended
to go separate ways (see Brown, 1961, 1979). However mo-
tivation and emotion have common historical origins. In the
seventeenth and eighteenth centuries, instinct doctrine served
as the explanation for why organisms were driven to behave
(at least infrahuman organisms without souls). Darwin’s
emphasis on the role of adaptive behavior in evolutionary
survival resulted in the extension of instinct doctrine to
human behavior. Major sources of impetus for this were
Freud’s and McDougall’s notions of instinctive human moti-
vation. Watson rebelled violently against the notion of in-
stinct and rejected it out of hand, together with all biological
mechanisms of motivation. As Lashley (1938) put it, he
“threw out the baby with the bath.”EmotionThe dominant theory of emotion in the first two decades of
the century was that of James and Lange—“We feel afraid
because we run” (see James, 1884). Actually, James focused
more on the subjective experience of emotion, and Lange, a
Danish anatomist, focused on the physiological phenomena,
believing that subjective experience is not a proper topic for
science. But between them they developed a comprehensive
theory of emotion. The basic idea is that we first perceive an
emotionally arousing situation or stimulus (“a bear in the
woods” is a favorite example), which leads to bodily (physi-
ological) changes and activities, which result in the experi-
enced emotion.
This general view was challenged by the American physi-
ologist Walter B. Cannon in the 1920s and 1930s. He actually