Researchers link motor ability, from playing a physical game to a piano, as partly innate and partly the result of practice, which is just persistence. The innate part to achieve skilled performances is based on the brain's architecture, a trait genetically determined.

A team at the Salk Institute for Biological Studies investigated on mice how variation in the size of the cortex's areas (where the highest sensory and motor centers are located) correlates with motor performances.

The same cortical area can be twice larger in a human individual than in another, so that scientists were puzzled by how these variations influence behavioral performances.

The Salk team could see these by genetically manipulating area sizes in mice, using the Emx2 gene. Larger or smaller sensory or motor cortical areas were found to impair behavioral performances of the mice, like the ability to run an obstacle course, keep from falling off a rotating rod, and the mice presented lower balance and coordination skills. "It has been assumed that if a cortical area is larger, it would be more effective in processing information," said the senior author, Dr. Dennis O'Leary, professor in the Molecular Neurobiology Laboratory at the Salk Institute. "However our findings suggest that the area size that gives optimal performance is the one that is best tuned to the context of the neural system within which that area functions." said Dr. Axel Leingartner, one of the researchers. "If cortical areas are not properly sized, the information will not be processed effectively, resulting in diminished performance", said Dr. Sandrine Thuret, also involved in the research.

Too much expression of the Emx2, which reduces the sizes of cortical "sensorimotor" areas, and surprisingly, too little Emx2 - resulting in an increase of the sensorimotor areas - resulted both in significant impairment in tactile and motor behaviors.

When mice from the two groups were interbred, the resulting offspring presented normal levels of Emx2, and the area sizes, and behavioral performance were normal. "To us this rescue experiment was compelling, and even a bit shocking, because the offspring that performed normally were the progeny of the two lines of mice that performed poorly," said O'Leary. "Findings from the first two lines of mice tested show a correlation between area size and performance, but the genetic rescue proves the relationship between area size and performance."

This cortical size variability could explain at least partly the variability in human performance and behavior and could also be involved into genetic developmental cognitive disorders, like Williams syndrome. "Establishing such a correlation between area size in human cortex size and behavior is possible by combining, in the same individuals, tests of behavioral performance with functional MRI that can be used to measure the size of a cortical area based on neural activity." said O'Leary.

"There is no doubt that people vary considerably in performance in everything from hitting a baseball to playing a piano to even a simple measure such as visual acuity-indeed the full spectrum of sensory, motor, and cognitive function," he says.

"Just as the size of a cortical area can vary considerably between people, so can human behavioral performance. Our studies in mice lead us to conclude that in humans, variations in cortical area size figures prominently in explaining variations in behavioral performance."

However, the research has not assessed how intelligence varies linked to the size of the areas. "Neuroscientists have yet to develop an understanding of the biological underpinnings of intelligence. The behaviors we have studied are based on sensory and motor modalities. However, for most issues in biology, in the end, researchers conclude that both an environmental component and a genetic component contribute to the final outcome."