To Peter Rosenberger, performing long division is like playing tennis.

Some people are born with special talents that enable them to learn the skills for math or tennis more quickly than average, says Rosenberger, an assistant professor of neurology at Harvard Medical School.

Others lack these gifts. It's not that they can never learn to calculate or serve, Rosenberger says. It's just that they must invest more time and effort into developing these skills than most people.

"You don't acquire the talent," says Rosenberger, director of the Learning Disorders Unit at Massachusetts General Hospital in Boston. "You acquire the skill to make up for the lack of talent. A learning disability isn't an absolute block to learning."

Having no head for figures is a learning disability called developmental dyscalculia. For a century, doctors have recognized that injuries to certain parts of the brain, particularly the parietal lobes, seem to affect mathematical ability. This problem is simply called dyscalculia.

But little has been written about developmental dyscalculia, which is present at birth and not linked to an obvious brain injury. Developmental dyscalculia is not as well known as dyslexia -- the inability to process words and letters in an organized way -- but may be even more common, says Rosenberger. While dyslexia is thought to affect 3 to 5 percent of the population, he says, dyscalculia might affect as much as 10 percent.

"Trouble with numeric language is much more common than trouble with alphabetic language," says Warren Weinberg, an associate professor of neurology at the University of Texas Southwestern Medical Center at Dallas.

"It is very rare to see dyscalculia as an isolated phenomenon," says Weinberg, who has written extensively about learning disabilities. "All dyslexics have some dyscalculia."

However, developmental dyscalculia often goes unrecognized because the ability to calculate isn't as vital in school as the ability to read and write, Rosenberger says.

"In life and in school, it was okay if you weren't good in math, but it wasn't okay if you weren't good in reading," agrees Sandi O'Connor of Richardson, Tex., whose dyslexia spurred her to teach math to children with learning disabilities.

"Our whole system in the United States is based on your ability to read," O'Connor says. "If you can't add and subtract, you can always get somebody to do them for you."

Yet poor math ability is more likely to be considered a sign of low intelligence than poor reading ability is, Rosenberger says: "Until fairly recently, inability to calculate has been considered a bellwether of intelligence."

Actually, Rosenberger has found that dyslexic children and dyscalculic children score about the same on IQ tests. In a paper published last year in the "Annals of Neurology," however, Rosenberger did note several differences between the two groups:

The dyscalculic children had more difficulty copying complex figures and making spatial calculations, such as dividing a circle into three parts. Their poor spatial orientation makes them unable to line up numbers correctly on paper for adding, subtracting or long division.

The dyscalculic children were more likely to prefer their right hand and right foot but left eye, a trait called mixed laterality. Normally, people prefer the same hand and foot as eye. This characteristic may reflect a difference in how dyscalculic children's brains are organized, says Rosenberger.

Dyscalculic children had more attention span deficits (also linked to mixed laterality). "It's been known for some time that calculations are sensitive to attention span deficits," says Rosenberger. "If attention is the problem, math is the skill you're going to have problems with."

Most dyscalculic children learn to count and add and subtract single numbers without difficulty, Rosenberger says. They encounter problems when they begin working with calculations that require "borrowing" or "carrying," he says.

Many dyscalculic children can do higher math because it uses skills other than calculation, Rosenberger says. "Algebra is clearly language."

Dyscalculia seems to place children at a greater risk for depression, partly out of frustration and feelings of failure, says Weinberg. He recommends providing a calculator to dyscalculic children instead of forcing them to struggle with arithmetic on paper.

"It's not a clinical problem," Weinberg says. "It's an educational problem."