Self and Emotion 187“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityself-stimulate as often as 700 times per hour. The reward could be so
strong that the animal would even forego food and sex. These areas
were interpreted as producing pleasure, as their stimulation apparently
gave the animal great satisfaction. On the other hand, there were areas
(aversive centers) that the animal would avoid stimulating, and there
were other areas that were motivationally neutral.^10
Since the initial observations of Kluver and Olds, a number of
emotion-related brain centers (both subcortical and cortical) have been
identified. Some of these are described below. Most of these structures
are included in the so-called limbic system, roughly equivalent to the
emotional part of the brain. (For anatomical locations, refer to Figs. 9.2
and 9.3 of this chapter, and Figs. 7.10 and 7.11 of Chapter 7.)
9.5.1 The amygdala
Among the emotional centers the amygdala is probably the most studied
(Figs. 9.2 and 9.3). Named in Latin for its almond shape, the amygdala
is a large group of nuclei deep in the medial temporal lobe.^11 Because
of its size and complex organization, the amygdala plays a pivotal role in
many emotional events, the most prominent of which is fear and anger.
In retrospect, many of the behavioral changes observed in the Klu-
ver-Bucy experiment can be attributed to the removal of the amygdala.
The amygdala receives multiple inputs and projects to the hypothalamus
for autonomic emotional expression.^12 It facilitates stereotyped emo-
tional movements such as freezing, fleeing, and biting. Synaptic changes
have been detected in the amygdala following classical fear conditioning
training.^13 The reciprocal connection between amygdala and the ventro-
medial prefrontal cortex explains the interaction between emotion and
decision-making. Its input to the hippocampus allows for storage and
retrieval of emotionally charged experience. Psychiatric disorders such
as anxiety, depression, post-traumatic stress disorder, phobias, and even
autism and schizophrenia, have been ascribed in some way to amyg-
dala dysfunction. Interestingly, the size of amygdala has been positively
correlated with the degree of aggressiveness across animal species.