Of the eight patients who were classified as unable to respond to language in the bedside examination, evidence of covert consciousness based on the hand-squeeze exercise was found in four, including the three originally classified as vegetative. In two additional patients, higher-order cortex activity was seen in response to either language or music. While higher-order cortical activity doesn't prove that a patient is conscious, Edlow notes, finding a response in those structures could have implications for a patient's eventual recovery.

He also stresses that negative responses to these tests should not be taken as predicting a low likelihood of recovery. Not only did about 25 percent of the healthy controls have no detectable brain response during the hand squeeze imagery test, but one of the comatose patients who had no responses to language, music or motor imagery during the early fMRI and EEG tests went on to have an excellent recovery 6 months later. In fact, no associations were found between early brain responses and long-term outcomes, which could relate to the small size of the study or the fact that several patients were sedated during the fMRI and EEG tests.


"It is also difficult to measure the false-positive rate for stimulus-based fMRI and EEG tests in these patients, since there is no definitive, gold-standard test to diagnose their level of consciousness," explains Edlow, who is an assistant professor of Neurology at Harvard Medical School. "Much more work needs to be done to determine the utility of these techniques for detecting consciousness in patients with severe traumatic brain injuries. Based on these results, our team at the MGH Laboratory for NeuroImaging of Coma and Consciousness is working on improving the accuracy of these tests, and we are planning a larger follow-up study in the near future."

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