Monday, May 7, 2012
Brains: Right v. Left, Top v. Bottom
“Everybody knows” that the brain is divided into right and left halves, clearly demarcated by a fissure down the middle, much like a walnut. And “everybody knows” that that division is a visual reminder of a division of functions-- that those two halves do different jobs, and whichever of them is more active determines not only functioning but personality. People proudly declare themselves to be “right-brain” types (i.e., creative; unbound by all that linear restrictiveness of the left hemisphere) or (sometimes apologetically) “left-brain” types-- linear, detail-oriented, thinking mainly inside the box. Left and right are such important categories, for handedness, driving, even politics, that we’re ready to accept the idea that the brain works that way too.
The right/left brain emphasis was strengthened by popular books like Drawing on the right side of the brain--- but this did not make the idea a better description of human functioning. When you “draw on the right side of the brain”, you may be using methods that will create better art, but you are not actually putting a separate brain function to work. The book Drawing on the right side of the brain offered some excellent exercises for improving drawing skills, but the exercises would have worked just as well if the word “brain” had never been mentioned at all. (Although no doubt fewer copies of the book would have been sold.)
However, although I suppose it’s not too good to be true, the right-left brain distinction is too simple to be true. As Borst, Thompson, and Kosslyn (writing in the American Psychologist in 2011) have put it, “one could argue that for the most part, the left and right halves of the brain may be more like the kidneys or the lungs: There are two of them, but they are redundant. In some respects the hemispheres are different, but most of those differences appear to be quantitative (e.g., affecting the amount of time required to perform a task), not qualitative. “ If this were not true, people deprived of speech by a stroke on the left side would not be able to regain their ability.
Like other researchers on the topic of brain functions and anatomy, Borst and his colleagues described a different brain division—one that’s not so obvious, or so reminiscent of the everyday walnut. That division is into top and bottom-- dorsal and ventral parts of the cortex.
Basing their statements on analyses of many brain studies, Borst et al proposed that the dorsal and ventral areas are different in terms of their ways of processing information. The dorsal system, they suggested, is expectation driven. This means that the dorsal system functions in specific ways and uses them to make predictions about what happens next, or about what happened in the past. It processes sensory information in an orderly fashion, paying attention to time of events. It remembers information in terms of relationships between places or between times. It pays attention to movement, and the differences in place and time that go along with it. It encodes changes in spatial relations between two objects as they occur over time, and so has an important role in control over body movements.
Borst et al also proposed that the ventral system is classification driven. It works to identify what is seen, heard, or felt, and does this by concentrating on individual objects rather than looking for relationships between objects. It examines characteristics like color, texture, sound frequency, and loudness, but not movements.
When people embrace the right hemisphere/left hemisphere distinction, it’s easy for them to imagine each hemisphere operating independently of the other. The obvious metaphor is the two hands, which can co-operate but which can also make independent movements. But in fact, unless you’re one of those unfortunate split-brain patients (who already had serious damage to one hemisphere), the two sides of your brain always work together. For that matter, so do your hands, which is why patting your head and rubbing your tummy is so difficult.
It’s a lot more difficult to imagine dorsal and ventral functions occurring independently, and we don’t have any common metaphors that seem relevant, other than perhaps that “top” is better than “bottom”. It’s obvious that the posited dorsal and ventral functions work together under normal circumstances. Dorsal functions guide your movements as you prepare to catch a ball-- but you wouldn’t be trying to catch it if you didn’t know it was a ball. You wouldn’t want to hurt a bird flying wildly toward you, or to be hurt by catching a hand grenade.