The right brain/left brain idea appears to be valid or useful as a broad generality only.
Often it is used as a metaphor to describe different approaches to problem-solving, or to other differences in attitude or personal style in behavior. People do something of the same kind in talking about on’es “feminine” or “masculine” side.
In Drawing on the Right Side of the Brain the author admitted in passing that the idea of differences between the two hemispheres may be overstated, and the specific location, if any, of different mental activities doesn’t really matter anyway. Central to the book–which is excellent, by the way–is the idea that many people when trying to draw resort to identifying specific objects and then try to render a symbol for them. The book attempts to teach one to avoid such behavior; rather than trying to draw “the nose” one observes that there is a specific curve or shadow here on the face one is trying to render, and another at a particular distance from it over here.
It is known that certain activities are localized within the brain. Starting in the mid-19th Century doctors, largely in Italy, became identifying such centers in the left hemisphere. For instance, there is a region there which governs the ability to speak in most people. An unfortunate offshoot of this research was the pseudo-science of Phrenology, which purported to measure people’s abilities and personality based on the size and contour of the skull, with all sorts of inclinations and skills being supposedly mapped out in great detail as though the brain were a rolltop desk filled with cubbyholes.
In the mid 20th Century researchers, mostly in the Soviet Union, began identifying specialized regions within the right hemisphere. There exists, for instance, parallel to the speech center of the left hemisphere a right hemisphere area which, when in a pathological state, produces “voices” only the person afflicted can hear.
The right hemisphere generally governs voluntary movement on the left side of the body and vice-versa, a phenomena which appears to have been first discovered by the ancient Egyptians, who did not otherwise believe that the brain was involved with thinking.
Growing knowledge about the disparities betwenecessarily en the right and left hemisphere has led to the myth that left handed people are less rigid and more creative. An observation used to shore up this idea is that many famous artists of the past, judging from their self portraits, were left handed. Painter and art historian David Hockney, for one, has pointed out that if Rembrandt showed himself holding his brush in his left hand, it may only be because he was looking at himself in a mirror as he painted.