Hidden Brain Activity Distinguishes Between Vegetative Patients
by Stephen Luntz
Photo credit: Srivas Chennu. Both the left and center patients are in a vegetative state, but the one able to imagine activity in response to instructions has connectivity that resembles a healthy individual (right)
Some patients in apparently vegetative states later report that they were aware of what was happening around them. For the first time, brain scans may be able to identify whether someone falls into this category, or whether they really are out of reach of communication.
Unlike those in a coma, patients in a vegetative state can swallow and will sometimes open their eyes and show signs of cycling between sleep and something resembling wakefulness. Moreover, some show brain responses to instructions, even to the point of being able to imagine playing tennis.
Nevertheless, the lack of response to traditional tests can be interpreted as an absence of awareness. Consequently, many people classified as being in a persistent vegetative state may actually be minimally conscious or more. Unfortunately, identifying these cases is very difficult.
Dr Srivas Chennu of University of Cambridge tested 32 patients who had been diagnosed as vegetative and minimally conscious with high-density electroencephalography (EEG). Publishing in PLoS Computational Biology, they reveal a wide variation in the connected networks identifiable in the brains they studied, from some that resembled those of healthy individuals to a near complete absence of networking.
The key difference, Chennu and his co-authors conclude, lies in the long-distance interactions between widely separated parts of the brain. “Intriguingly, some patients in behaviourally unresponsive vegetative states who demonstrated evidence of covert awareness with functional neuroimaging stood out from this trend: they had alpha networks that were remarkably well preserved and similar to those observed in the controls,” the authors write.
Identifying aware patients using brain scans is more challenging than outsiders may expect, the authors note, because, these technologies can give us too much information. “Modern neuroimaging methods for assaying such connectivity, including Magnetic Resonance Imaging (MRI) and high-density electroencephalography (EEG), provide a surfeit of data that need to be reduced in dimensionality and coalesced into patterns to provide an overarching understanding of connectivity networks in the brain.”
The authors used a technique known as Graph-theoretical analysis to reduce ten minutes of data to a test that correlated with much more time-consuming or expensive studies of patient response. The EEG data Chennu has used may not be as definitive as that which can be obtained from MRIs, but being quicker and cheaper could be used as a screen before deciding whether to conduct further tests.
Besides offering clinicians the opportunity to establish patients’ level of awareness and prospects for recovery, Chennu’s says his work throws light on questions of what consciousness is, and how it emerges from brain behavior.