Evolution of the amygdala: new insights from studies in amphibians.
The histology of amphibian brains gives an impression
of relative simplicity when compared with that of reptiles
or mammals. The amphibian telencephalon is small and
contains comparatively few and large neurons, which in
most parts constitute a dense periventricular cellular layer.
However, the view emerging from the last decade is
that the brains of all tetrapods, including amphibians,
share a general bauplan resulting from common ancestry
and the need to perform similar vital functions. To
what extent this common organization also applies to
higher brain functions is unknown due to a limited knowledge
of the neurobiology of early vertebrates. The amygdala
is widely recognized as a brain center critical for
basic forms of emotional learning (e.g., fear conditioning)
and its structure in amphibians could suggest how
this capacity evolved. A functional systems approach is
used here to synthesize the results of our anatomical investigations
of the amphibian amygdala. It is proposed
that the connectivity of the amphibian telencephalon
portends a capacity for multi-modal association in a limbic system largely similar to that of amniote vertebrates.
One remarkable exception is the presence of new sensory-
associative regions of the amygdala in amniotes:
the posterior dorsal ventricular ridge plus lateral nuclei
in reptiles and the basolateral complex in mammals.
These presumably homologous regions apparently are
capable of modulating the phylogenetically older central
amygdala and allow more complex forms of emotional
learning.