ABOVE: Patterns of neural activity known as alpha waves, often recorded via electroencephalography, may stem from the visual cortex. 
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The paper
R.D. Traub et al., “Layer 4 pyramidal neuron dendritic bursting underlies a post-stimulus visual cortical alpha rhythm,” Commun Biol, 3:230, 2020.

Groups of neurons firing in sync produce predictable and measurable brainwave patterns, including the alpha rhythm, which dominates when we’re relaxed and our eyes are closed. While researchers have long suspected the alphas originate in a brain region called the thalamus, the waves’ definitive source and function remain elusive, says Roger Traub, a mathematical neurologist with IBM.

Experimenting with slices of rat brain tissue, Traub’s colleagues inserted electrodes into a piece of visual cortex and used drugs to chemically induce a stable alpha rhythm. This rhythm, the team found, emanated from pyramidal neurons in the fourth layer of the visual cortex. People think that because there aren’t many pyramidal neurons in that fourth layer, they aren’t important, but those neurons appear to generate alpha waves, Traub says.

Comparing a model of neurons’ electrical firing to the team’s experimental data, he confirmed the cortex as the source of the alpha waves. The waves form when pyramidal neurons fire in sync, and the model suggested that the period of the resulting oscillation (the time it takes to complete one cycle) was determined by synaptic excitation, as opposed to the synaptic inhibition common in other brain waves. When excited, pyramidal cell activity oscillates at a different frequency (the number of waves per second) than nearby sensory neurons, possibly interrupting the flow of information throughout the cortex.

The findings are “very convincing,” says University of Salzburg neuroscientist Wolfgang Klimesch, who was not involved in the study. He questions, however, whether the result will translate into humans. The alpha frequency in a rodent, for example, may be different than in a human. “It’s a very questionable assumption” that the frequencies in different animals are the same, Klimesch says.