February 2011
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Changing dominant hemispheres

Being left-handed, I have had a special interest in the differences between the brain’s hemispheres. However, over the years I have come to suspect much that is said about the differences. All this left-brain right-brain nonsense is just that, nonsense. Aside from language and spatial processing, there has been little evidence for hemisphere specialization. Now a study has been published by Chi and Snyder that shows a very large effect but also with a good deal of vagueness about exactly what the effect actually is.

Establishing a direct current through the brain from a cathode to an anode on the scalp (transcranial direct current stimulation) will inhibit neural activity at the cathode end and increase it at the anode end. Here is the abstact:

Our experiences can blind us. Once we have learned to solve problems by one method, we often have difficulties in generating solutions involving a different kind of insight. Yet there is evidence that people with brain lesions are sometimes more resistant to this so-called mental set effect. This inspired us to investigate whether the mental set effect can be reduced by non-invasive brain stimulation. 60 healthy right-handed participants were asked to take an insight problem solving task while receiving transcranial direct current stimulation (tDCS) to the anterior temporal lobes (ATL). Only 20% of participants solved an insight problem with sham stimulation (control), whereas 3 times as many participants did so (p = 0.011) with cathodal stimulation (decreased excitability) of the left ATL together with anodal stimulation (increased excitability) of the right ATL. We found hemispheric differences in that a stimulation montage involving the opposite polarities did not facilitate performance. Our findings are consistent with the theory that inhibition to the left ATL can lead to a cognitive style that is less influenced by mental templates and that the right ATL may be associated with insight or novel meaning. Further studies including neurophysiological imaging are needed to elucidate the specific mechanisms leading to the enhancement.

A threefold improvement is certainly a clear effect! As a tool tDCS is not a very precise and so it is difficult to know what is really happening in the brain – how large an area is affected, how localized and so on. The authors discuss these problems.

Although I find much that is written about lateralization very simplistic, there is something here to uncover. Even in the most symmetrical animals there seems to be some asymmetry. There are traces of hemispheric dominance in many groups of vertebrates. It is likely that there is was long-standing evolutionary logic in the pronounced handedness of humans. Chi and Snyder’s experiment is more interesting for showing that there are large effects there to be investigated then in the conclusions that can be drawn from this particular setup.

Chi, R., & Snyder, A. (2011). Facilitate Insight by Non-Invasive Brain Stimulation PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0016655

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