Hemispheric specialisation depends on brain state

The idea that the two hemispheres of the brain are specialised for different tasks was popularised in the 1940s by experiments with split-brain patients, in whom the nerve fibres connecting their cerebral hemispheres had been severed. But when healthy people with fully intact brains are tested, evidence for hemispheric specialisation is often not found. Now Christine Mohr and her colleagues have tested the idea that this is because hemispheric specialisation isn’t a permanent feature of the healthy brain, rather it depends on the brain’s state at any given moment. These different “functional brain states” last from “80 to 150ms”, and are characterised by different distributions of neuronal populations being active.

Over dozens of trials, 22 healthy participants (11 women) decided as quickly as possible whether either of two letter strings contained a word (e.g. dfdhk/ thing; located on either side of central fixation point). Meanwhile Mohr’s team recorded the surface electrical activity of their brains via 123 electrodes. Remember that because of the brain’s cross-wiring, stimuli presented on the left side of visual space are processed by the right hemisphere and vice versa. To measure hemispheric specialisation, the researchers exploited the fact that studies with brain-damaged patients have suggested a right-hemisphere advantage for processing emotion-related words.

Here, the female participants were quicker to detect the presence of a word if it was emotion-related (rather than neutral), regardless of which side of space it appeared on, and regardless of their current brain state. By contrast, for approximately half the presentations, when their brains were in a ‘left-anterior/right-posterior’ functional state, the male participants showed a clear advantage for emotion-related words, only if the word was processed by their right-hemisphere (after its presentation on the left side of space). So whereas the women showed no hemispheric specialisation for emotional words, the men did, but only when their brains were in a certain state of activation.

This then poses the question “whether functional brain states relate systematically to experimental and person variables or whether they are of random occurrence”, the authors said.

Mohr, C., Michel, C.M., Lantz, G., Ortigue, S., Viaud-Delmon, I. & Landis, T. (2005). Brain state-dependent functional hemispheric specialisation in men but not in women. Cerebral Cortex, 15, 1451-1458.
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