PASADENA-Clusters of large neurons found exclusively in the brains of humans and other primates closely related to humans may provide these species with enhanced capacities for solving hard problems, as well as for self-control and self-awareness.
In the April 27 issue of Proceedings of the National Academy of Science, neurobiologists Patrick Hof from Mount Sinai and John Allman from Caltech and their colleagues have found an unusual type of neuron, that is likely to be a recent evolutionary acquisition.
The neurons in question are spindle-shaped cells, which are almost large enough to be seen with the naked eye. Their location in the brain is in the frontal lobe near the corpus callosum, which connects the two halves of the brain.
Allman, the Hixon Professor of Psychobiology and professor of biology; Hof; and their team studied 28 different species of primates and found the spindle neurons only in humans and very closely related apes. The concentration of spindle neurons was greatest in humans, somewhat less in chimpanzees, still less in gorillas, and rare in orangutans.
According to Allman, "This declining concentration matches the degree of relatedness of these apes to humans." There were no spindle cells in gibbons, which are small apes, or in of any of the other 22 species of monkey or prosimian primates they examined. The spindle cells were also absent in 20 nonprimate species examined including various marsupials, bats, carnivores and whales.
The cells in question are found in an area of the brain already linked to psychiatric diseases. According to Allman, "In brain imaging studies of depressed patients, there is less neuronal activity in the region and the volume of the area is smaller. The activity of the area is increased in obsessive compulsive patients."
The activity of the area has been shown to increase with the difficulty of the cognitive task being performed. This suggests that the area enhances the capacity to do hard thinking. Activity is also increased when a subject withholds a response or focuses its attention, suggesting the area is involved in self-control.
Furthermore, the spindle neurons themselves are especially vulnerable to degeneration in Alzheimer's disease, which is characterized by diminished self-awareness. From this Allman suggests, "Part of the neuronal susceptibility that occurs in the brain in the course of age-related dementing illnesses may have appeared only recently during primate evolution."