Mar 23 2007

Zeroing in on Dyscalculia

As Steve Martin once quipped, “Some people have a way with words, and other people….Oh,… not have way, I guess.” Likewise, some people have a facility with numbers and others can’t add with a calculator. The fault lies not in the stars, but in our brains.

A new study by Dr Roi Cohen Kadosh, of the UCL Institute of Cognitive Neuroscience, uses a novel approach to locate that part of the cerebral cortex which is responsible for carrying out calculations – or more specifically that part which is not working in individuals with dyscalculia (a disorder characterized by difficulty carrying out calculations). They used transcranial magnetic stimulation (TMS) to temporarily disrupt function in the right intraparietal sulcus of normal subjects. This disruption resulted in the same deficits as subjects who have congenital dyscalculia.

What is cool about this kind of research is what it tells us about how brain function is organized. In the late 18th and early 19th centuries neuroscientists argued about whether or not cognitive function is diffusely represented in the brain or if each type of function localizes to a specific brain region. It is an interesting fact of history that phrenologists (the pseudoscience of determining personality by reading the bumps on the skull) were on the correct side of this scientific debate – the brain is exquisitely compartmentalized.

In the last 150 years neuroscientists have developed an increasingly sophisticated model of the different parts of the brain, what specifically they do, and how they interact together. In fact, our model of what the brain does has evolved alongside our model of where the brain does it. This type of modeling also forces us to think very carefully and precisely about how different distinct neurological cognitive functions work together to create the end result of some specific behavior.

Our knowledge of what the brain does has largely been driven by studying the effects of damage to particular brain regions. Classically – if someone has a stroke in their left temporal lobe and then could not speak, this taught neuroscientists that the left temporal lobe is responsible for speech. In more recent years the ability to image brains in great detail with magnetic resonance imaging has greatly enhanced our ability to study the brain by pathology. MRI has also given us a completely different tool – function MRI (fMRI), with which we can see which parts of the brain become active during specific tasks. This is a positive, rather than negative, way of examining brain function.

Now, Dr. Roi has combined these two techniques – using TMS to create a temporary dysfunction in a particular region of the brain and then observing what deficits result. Very cool. His results also conform to prior studies that localize calculation to the right parietal lobe.

It also demonstrates how neuroscientists have to be careful and thoughtful in designing and interpreting such studies. For example, if you ask someone to tell you what is 5×6 and they give you the answer “30,” did they actually perform a calculation? Perhaps they just memorized their multiplication table. If someone has difficulty answering math questions, do they have a math deficit, or a language deficit, or a global deficit in cognition (i.e. perhaps they are just not smart in general)?

So cognitive researchers have conducted numerous studies to parse out these various possibilities. It turns out, we do use language to understand and articulate math problems. And we do memorize the answers to common calculations (like multiplication tables). But by examining a variety of tasks you can separate out the various brain regions at work, and when this is done it is clear that actual calculations take place in the brain in the right intraparietal sulcus. The best task to determine this is subtraction – which is nice because that is the test clinical neurologists use at the bed side to test calculation.

Studies also show that different brain regions vary in their ability independently of each other. So someone can be very articulate, but mathematically inept, or vice versa. One can also be generally intelligent but lack math ability, or even cognitively impaired but a mathematical “savant.”

About 5% of the population suffers from diagnosable dyscalculia, which interestingly is the same percentage of the population that suffers from dyslexia (poor reading ability) and attention deficit disorder (ADD), now recognized to be a disorder of executive function which localizes to the frontal lobes. It is perhaps not a coincidence, as we may simply be defining these disorders as the bottom 5% of ability in these areas. Since ability in each area is likely to vary on a normal curve, ability is a continuum, and any cutoff will be somewhat arbitrary.

Some have used this argument to say that therefore they should not be considered disorders at all, but I think such arguments are just the “false continuum” logical fallacy. There are plenty of arbitrary cutoffs of biological factors that we use to define disorders. Blood pressure, for example, or even the level of sodium in the blood. For cognitive abilities, disorders are defined as the relative lack of a skill or ability that is generally had by humans and which results in demonstrable harm. Most people have the ability to learn how to read and do basic calculations, and the lack of such ability is detrimental in modern society. So in that context these are disorders.

At present testing for such disorders is accomplished by functional tests of ability. Dr. Roi’s research perhaps give us a glimpse of the future, when we will be able to make more precise and accurate diagnoses by quantitatively measuring neurological activity in specific brain regions.

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