Discount brains

An international group of researchers has induced dyscalculia (difficulty in comprehending numbers) in people with otherwise unimpaired mathematical abilities by applying a small magnetic field to a specific region of the brain.

The group, led by Roi Cohen Kadosh, of the Institute of Cognitive Neuroscience at UCL, used transcranial magnetic stimulation (TMS) to disrupt electrical activity in the parietal lobe. The system used in the study consists of a TMS coil and several miniature ultrasound transmitters, which send continuous pulses to a sensor device, attached to participants’ heads. By combining the system with functional magnetic resonance imaging and the BrainVoyager neuronavigator, Cohen Kadosh’s group applied focal magnetic pulses to the intraparietal sulcus (IPS).

Participants were presented with sets of two numbers, one written in a larger font than the other. In one task they were asked to determine which was numerically larger, and, in another, which was physically larger. The TMS device was used to apply a magnetic field to the IPS for several hundred milliseconds just as the tasks were about to be performed. It was found that magnetic stimulation applied to the right, but not the left, IPS, led to significantly increased reaction times when the participants performed both tasks. The performance of the healthy participants treated with TMS was comparable to that of untreated dyscalculics.

Disruption of neural activity in the right IPS led not to dyscalculia per se, but to impaired visuospatial processing which underlies the ability to evaluate numbers. The right parietal lobe has previously been implicated in visuospatial processing, and has it been theorized that this region is involved in evaluating the magnitude of not just quantities but also of space and time. For example, neuroimaging studies show that people with damage to the right parietal lobe have impaired visuspatial processing leading to an difficulty with numerical processing.

In this study, determining which number is numerically larger is a quantitative task, while determining which number is physically larger is a spatial task. Both were impaired when TMS was applied to the right parietal lobe. Although it cannot be ruled out that the TMS led to a disruption of brain regions adjacent to or connected with the IPS, the study provides the first direct evidence of the essential role of the parietal lobe in automatic magnitude processing. This ability to compare the relative sizes of two objects is impaired in dyscalculics but is performed automatically in non-dyscalculics.

The findings are also in line with the idea that the brain contains degenerate neural systems, i.e. that the same function can be performed by different parts of the brain. The left parietal lobe – which is more closely connected than the right parietal lobe to the speech centres in the left hemisphere – has also been implicated in processing of numerical data. But disruption of the left IPS did not induce dyscalculia in the study. The authors suggest that the right IPS is necessary for numerical processing when the verbal processing of numbers – carried out in the speech centres of the left hemisphere – is impaired. The findings may also lead to a method for the early diagnosis of dyscalculia, which affects about 5% of the population.

Reference:

Cohen Kadosh, R. et al. (2007). Virtual dyscalculia induced by parietal lobe TMS impairs automatic magnitude processing. Curr. Biol. doi: 10.1016/j.cub.2007.02.056 [Full text]