When you're standing on a bus and the driver suddenly brakes, your brain manages to tense your muscles in a fraction of a second so you don't fall. This automatic response is born deep in the cortex, in an area that plans complex movements, the supplementary motor cortex. Scientists set out to check what happens in this area a second before the jolt in people with Parkinson's disease and in healthy older adults.
The participants stood on a platform that suddenly shifted forward or backward. An electroencephalogram recorded brain activity, and then an algorithm isolated from it the source that responded most strongly to the jolt. It turned out that in people with Parkinson's disease, the louder the "stuck record" of the beta rhythm sounded before the jolt, the worse they performed on a clinical balance test. In healthy older adults, there was no such link.
Previously it was thought that the beta rhythm was simply a marker of Parkinson's disease, but now it's clear: it actively prevents the brain from updating information about body position. When the record gets stuck, the brain doesn't hear new signals from muscles and joints, so the reaction to the jolt lags and the person loses balance.
For now, this knowledge hasn't turned into a drug or a device, but it changes the way we look at the problem. Instead of measuring the average loudness of the rhythm, it's worth catching short episodes when the record gets stuck especially badly. These episodes could become an early sign of future falls in people with Parkinson's disease.
The supplementary motor area (SMA), the region from which the signal was extracted, is involved in movement planning and was first described in monkeys in the 1950s. Damage to it in humans causes alien hand syndrome.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.