How Brains React to Sound Can Separate Conscious From Vegetative Patients

By Valerie Ross | May 13, 2011 2:42 pm

What’s the News: A non-invasive test that measures brain waves could help doctors better diagnose whether a patient is truly in a vegetative state, according to a preliminary study published today in Science. What’s more, the results suggest that a particular pathway of communication in the brain is disrupted in vegetative patients but not patients with somewhat less severe brain damage—which could not only improve diagnosis, but help researchers better understand these tragic conditions.

How the Heck:

  • The researchers recorded the brain activity of 21 people with severe brain injuries and 22 healthy controls using an EEG, a set of electrodes placed on the scalp that pick up electrical signals generated by the brain’s neurons firing. Of the patients with brain damage, eight had been previously diagnosed as being in a vegetative state, while the other thirteen were in a minimally conscious state, meaning they retained some level of conscious awareness and ability to interact.
  • While recording the subjects’ brainwaves, the researchers played each of them a series of sounds with unexpected changes of tone (a one-time warble in the note). When healthy people and patients in a minimally conscious state heard the blip, their brain waves showed a spike that lasted around 170 milliseconds. For patients in a vegetative state, the spike lasted less than 100 milliseconds. That may not sound like much, but 70 milliseconds is a significant chunk of time when you’re looking at brainwaves.
  • The researchers made a mathematical model, combining their data with information about how different areas of the brain are connected to see what neurological processes might have led to this pattern of brain waves. Communication from the frontal cortex—a high-level brain region that’s important in decision making and rational thought—to other parts of the brain seemed to be disrupted in vegetative patients, they found.
  • In other patients and healthy controls, the surprising sound triggered auditory brain regions to send signals to the frontal cortex, which then responded by sending another set of signals. In vegetative patients, the auditory regions still sent signals to the frontal cortex—but the frontal cortex didn’t send any back.

What’s the Context:

  • Current methods of testing whether someone is truly in a vegetative state are time-consuming and often depend on judgment calls by clinicians. Up to 40% of patients in a minimally conscious state may be incorrectly diagnosed as vegetative, earlier work has found. Using objective recordings of brain activity rather than subjective assessments could make diagnosis easier and vastly reduce the number of mistakes.
  • Other researchers have also investigated ways to use brain scans to determine whether a patient is in a vegetative state, such as using an MRI scanner to pick up brain activity suggesting responsiveness in patients who can’t communicate.

Not So Fast:

  • The study was preliminary, and involved only a small number of patients. To suss out whether this test will make an effective diagnostic tool, the researchers say, more patients must be tested.

Reference: Melanie Boly et al. “Preserved Feedforward But Impaired Top-Down Processes in the Vegetative State.” Science, May 13, 2011. DOI: 10.1126/science.1202043

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