Synchronizing Brain's Neural Activity

The rhythmic electric fields generated by the brain during deep sleep and other periods of intensely coordinated neural activity could amplify and synchronize actions along the same neural networks that initially created those fields, according to a new study. The finding indicates that the brain's electric fields are not just passive by-products of neural activity—they might provide feedback that regulates how the brain functions, especially during deep, or slow-wave, sleep. Although similar ideas have been considered for decades, this is the first direct evidence that the electric fields generated by the cerebral cortex change the behavior of the neurons that engender them. The brain is an intricate network of individual nerve cells, or neurons, that use electrical and chemical signals to communicate with one another. Every time an electrical impulse, or action potential, races down the branch of a neuron, a tiny electric field surrounds that cell. Researchers created an experimental model that mimicked what might happen in the intact brain of a living animal. First, they suspended a slice of brain tissue from the visual cortex of a ferret in artificial cerebrospinal fluid. The living cortical tissue behaved as though the ferret brain were in slow-wave (non–rapid eye movement), sleep, during which the brain produces sluggish but highly synchronous waves of electrical activity. The team's next step was to find out what would happen to the neural activity in the brain slice when it was subjected to a weak electric field.

They surrounded the cortical sample with an electric field that approximated the size and polarity of the fields produced by an intact ferret brain during slow-wave sleep to create an exaggerated version of the exact feedback loop they were investigating. Essentially, they enveloped the brain slice in an echo of itself. When the team applied this electric field echo, they found it amplified and synchronized the neural activity in the brain slice. The field didn't create disorder—it increased harmony. The roar of the brain slice became louder and more regular. Not only did the researchers show that this positive feedback facilitated the synchronous slow waves of electrical activity in the slice of ferret brain, they also showed that an electric field of the same strength, but opposite polarity, disrupted its synchronous neural activity. The new study faces a couple methodological imperfections: First, the simple and uniform electric field created by electrodes in the laboratory does not perfectly mimic the complexity of electric fields generated by a living brain. Second, the experimental model relied on an incredibly thin slice of neural tissue—hardly the same as an intact brain. Researchers say these flaws are unlikely to change the general conclusions of the study, however, because the underlying mechanisms of electrical activity remain consistent enough between the lab model and a living organism.

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http://www.scientificamerican.com/article.cfm?id=brain-electric-field