Attention Deficit Disorder and Brain Laterality

Here two topics that are very close to my heart: the puzzle of laterality and asymmetry and attention deficit disorder.

We live in an asymmetric Universe. Fundamental particles tend to rotate and wobble to the left rather than the right; most molecules show a lateral shift; and all the biologically active amino acids – the building blocks of proteins – are in the L-form. Meaning that in solution they bend light to the left. The R-forms, that bend light to the right, for the most part don’t work in biological systems. Some of the most extraordinary asymmetry is seen in the human brain. There’s a myth that about the hemispheres of the brain that I’ve talked about before, but bears repeating.

The idea that the left hemisphere of the brain is specialized for logic, analysis and language, while the right hemisphere is holistic, artistic and mystical has been circulating for over thirty years. It has even been suggested that the right hemisphere of the brain is the seat of intuition, which cannot be correct. It may be involved in instinct, but it is impossible to reduce a non-local psycho-spiritual experience to groups of brain cells. There may be correspondence – the right hemisphere becoming activated during intuition, but not causality: right hemisphere activation as the cause of intuition.

This notion of discrete functioning of the hemisphere has become so pervasive that it is commonplace to hear people describing themselves as being “right brained,” or accusing someone else of being “left brained.”

A simple concept that is also profoundly wrong. This is fine as a metaphor, but not as a fact. It is true that language is more highly represented in the left hemisphere of the brain in right-handed men. But language is bilaterally represented in women. Most men tend to use a small strip of the left hemisphere for language, women tend to use both hemispheres at once. There’s still a left-sided predominance, but it’s quite a different pattern from most men’s brain.

Boy’s brains mature more slowly than girls’ brains, and by adolescence are more lateralized than are the brains of girls and women. The truth is that we cannot neatly divide up the functions of our brains, and we need both sides of our brain if we are to function at our best.

Until very recently most experts said that handedness was a purely human attribute. Yet anyone who has every lived with a cat, dog or horse knows that that is wrong, and now there’s literature to prove it.

I tried to put that mistake to bed in a review article that included a detailed discussion about brain asymmetries in animals. I also reviewed the excellent evidence that certain regions of the brain have been becoming more lateralized over the last 100,000 years.

I’d like to show you why this talk of brain lateralization is not a sterile academic debate.

We do not know if left-handedness or mixed handedness is more common in ADD/ADHD. But the evidence is getting stronger and stronger that there is something different in people with ADD/ADHD in how they use the two sides of their brains.

Three conditions: autism, dyslexia and ADD/ADHD share one characteristic: they are all marked by what we call atypical cerebral asymmetry in that they don’t have the normal left hemisphere dominance for language. This is more of a problem for boy, since they normally make more use of their left hemispheres. But it’s not all bad: atypical cerebral asymmetry may also be associated with certain aspects of creativity.

Researchers in New Zealand have just reported something strange, but which is consistent with previous research. When healthy right-handed children are asked to bisect a line drawn on a page, when they use their right hand they tend to bisect the line toward the right, and with the left hand toward the left. But children with ADD/ADHD don’t do this. The researchers have now gone further and suggested that there are at least two types of ADD/ADHD, in one of which – ADHD-I – the primary problem is in the right hemisphere of the brain.

Using an entirely different methodology, researchers in Baltimore, Maryland, have found that children with ADHD use their brains differently when engaged in patterned motor activity.

In an earlier post I talked about the dorsolateral prefrontal cortex, and there is an important connection to it is called the cingulate cortex. A new study has shown that adults with ADD had significantly smaller overall amounts of cortical gray matter, and reduced volumes of their prefrontal and anterior cingulate regions. Both are highly lateralized in the human brain.

Compared with controls, unmedicated children with ADHD have a small right cingulate cortex, but it is normal in children who have been treated, indicating that the treatment is doing more than treating symptoms: it may actually be re-wiring the brain.

Research from the UCLA Neuropsychiatric Institute and David Geffen School of Medicine showed that in adults with ADHD, showed reduced left hemisphere specialization, were better at processing emotions, and worse at processing words compared to controls. They tend to use their right hemispheres more and their left hemispheres less.

Unaffected individuals constantly shift their attention toward the important or salient things in their environment. Children with ADHD have a problem with directed attention: the ability to allocate and direct attention toward a salient stimulus. New research has discovered that this is due to problems in a region of the brain called the parietal lobe that is known to play a significant role in shifting attention and detecting specific or salient targets in the environment. As a result the child’s brain does not know what is important.

These findings do not tell us whether the primary problem is a way in which specific parts of the brain talk to each other, or whether the scientists are actually measuring the way in which the brain reacts to deficits in one region. Perhaps other regions take over the functions of parts that are not working as they should.

The most important take home message is that we already have methods for stimulating and integrating the hemispheres of the brain. Few have so far been much used in ADD, but this information gives us a whole new way forward.