Principal Investigator: Todd M. Preuss
Evolutionary Specializations of Human Cerebral Cortex
We know almost nothing about what distinguishes the human brain from the brains of other animals, apart from its large size. The traditional research methods used by cognitive neuroscientists, such as in vivo imaging of functional activity in the human brain and the use of nonhuman primates as models of human brain organization, tell us something about what humans are like, especially insofar as we resemble other primates. They do not, however, provide the critical comparative information required to understand how humans differ from other primates. As a consequence, we have no theory of specifically human mind/brain systems, a theory that could, for instance, explain why humans think and behave like humans, rather than like rhesus monkeys.
New research paradigms are required to identify the distinctively human
elements of human mind/brain systems. The research proposed here represents
an especially promising approach to the problem. We will carry out comparative
studies of cortical organization in humans and a variety of nonhuman primates
including, most notably, chimpanzees, the closest relatives of humans.
Because the most rigorous test of whether a brain characteristic is a
human specialization is whether the characteristic is present in humans
but absent in our closest relatives, human-chimpanzee comparisons are
indispensable for understanding what is specifically human about the human
brain. Studies will be carried out using immunocytochernistry and related
methods, taking advantage of the wide array of antibodies and other markers
for particular molecular constituents of the nervous system. These techniques
make it possible to study cortical organization in great detail in autopsied
brain tissue, and therefore can be used to study species that are not
available for invasive research, including humans and chimpanzees. Because
the distribution of specific molecules varies across the cortical mantle,
histochemical techniques are very useful for mapping cortical areas. We
will develop detailed cortical maps in humans, chimpanzees, and other
primates, to determine whether human brain enlargement was accompanied
by the evolution of new cortical areas (as is widely believed) or by the
enlargement of existing areas. Histochemical methods also reveal species
differences in the cellular, laminar, and modular organization of the
cortex, differences that are far more common among mammals than generally
appreciated. By documenting variations at these levels of organization,
we will be able to evaluate an alternative interpretation of human brain
evolution, specifically, that human cognitive specializations resulted
from the modification of existing structures and systems rather than by
the addition of new areas. Our principal focus in these studies will be
on frontal cortex, which is critically involved in higher-order cognitive
functions and likely to have been extensively modified in human evolution.
