A defining difference between man and non-human primates has been found in the circuitry of brain cells involved in language, according to researchers at the Medical College of Georgia.
Their findings belie the notion that the primary difference between man, monkey and chimpanzee is the size of the brain and opens up a new area of study that may explain man's capacity for complex communication, they say.
They also say this circuitry may be what goes awry in still unexplained conditions that affect language, such as schizophrenia, autism and even epilepsy.
"Language is something that makes us different from other species; now we have found a structural correlation," said Dr. Manuel F. Casanova, psychiatrist and neurologist at MCG and the Department of Veterans Affairs Medical Center in Augusta.
"We are trying to understand what makes the human brain different from non-human primates," said Dr. Daniel Buxhoeveden, physical anthropologist at MCG. "We also are trying to understand the pathology of diseases of the brain that are not understandable by classical methods. We think the difference is going to be in the way the brain is organized."
To study that organization, they have completed detailed computer analyses of a basic, functional unit of the human and non-human brain known as a minicolumn, a group of 80 to 100 cells and the wiring that connects them. Millions of these minicolumns are found throughout the brain; their studies focused on the minicolumns found in an area of the brain involved in language called the planum temporale.
There they found distinct, microscopic differences in the minicolumns of humans and non-human primates. They also found differences in the minicolumns in the right and left side of the brain in humans that weren't present in non-humans; previous studies found distinctive lateralization – or left-right differences – in humans and chimps.
"These minicolumns are different in their structure in the human brain and also different in that they are lateralized, larger on the left side than on the right side of the brain," Dr. Buxhoeveden said. "We didn't find this in the chimpanzee or the monkey." In most humans, language function is housed on the left side of the brain, which would make the left-right differences found all the more pertinent, Dr. Casanova said.
These differences may explain man's capacity for the subtleties of communication – such as understanding concepts and formulating and expressing responses – compared to the non-human primates' more fundamental communication system.
"Other species can communicate but they really don't have a language," Dr. Casanova said. "Like the bumble bee. They can't compare the pollen catch of that day to a month ago or maybe what they expect to find a day or a year from now. So, where should we look for something that makes us different from other species? Within the language regions of the brain."
Numerous previous studies have compared the individual brain cells and the brain in toto in man and animals and found that cells are essentially identical and that the brains looked – from the outside at least – very similar other than size differences.
For example, the chimpanzee is man's closest evolutionary relative; the two have about 99 percent of their DNA in common, but the human brain is about three and one-half times larger. "The question has been, what else changed besides size," Dr. Casanova said.
After examining brain tissue from humans, rhesus monkeys and chimpanzees obtained from the Yakovlev-Haleem Brain Collection at the Armed Forces Institute of Pathology in Washington, D.C., they believe that how brain cells are organized to work together is a key difference.
"We found evidence that the brain is organized differently in humans in this area of the brain, even though the outside looks the same," Dr. Buxhoeveden. "This provides an anatomical substrate, a hint that the brain is wired differently in humans in the language area than in the chimpanzee or the monkey."
"Now we will be looking at this to describe pathologies that are subtle, that don't have classical symptoms that you see in diseases such as Alzheimer's, where there are obvious, major things wrong with brain tissue," he said. "In diseases such as schizophrenia and autism, those obvious symptoms aren't there. But we believe that you will find them in the most subtle wiring of the brain."
The researchers are expanding their studies by including other species, including non-primates such as dolphins, mammals which have large, highly developed brains.
###The researchers' findings are published in the August issue of the American Journal of Physical Anthropology and scheduled for publication this fall in Brain Behavior and Evolution. Support for the research was provided by the Maryland-based Stanley Foundation, a major supporter of psychiatric research worldwide; Dr. Casanova is a Stanley Scholar, which means the foundation also provides ongoing support for his mentoring of future psychiatrists.
Other co-authors on the studies include Andrew E. Switala, computer programmer and mathematician; and Dr. Emil Roy, linguistics and computer applications expert.
Materials provided by Medical College Of Georgia. Note: Content may be edited for style and length.
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