Lesions of Consciousness
We’ll probably never know whether Terry Wallis also had some awareness before his awakening. But a type of brain imaging known as diffusion tensor imaging (DTI) has given researchers hints about how his brain has changed since.
DTI is a variation of MRI that offers an unprecedented view of the brain’s wiring system–the long, thin tails of neurons that carry electrical signals between different regions. Wallis’s first DTI scan, recorded eight months after his first word, revealed that he had profound brain damage. But scientists also discovered possible signs that new neural connections had sprouted between brain structures. In particular, a large area in the back of his brain appeared to have more neural fibers than normal, all oriented in the same direction. The area encompassed by these new fibers included a part of the brain known as the precuneus, which is highly active during conscious wakefulness but less active during sleep or anesthesia.
Eighteen months after that scan, Wallis was doing even better. He could move his previously paralyzed legs, an improvement “as unexpected as him recovering speech,” says Schiff. When the researchers imaged his brain a second time, they found that the unusual area in the back had normalized, while a region involved in regulating movement seemed to have grown more connected. The findings were published last year in the Journal of Clinical Investigation.
The researchers can’t yet be sure that the changes they saw in the brain images really do indicate the growth of new neuronal connections, nor that those changes sparked Wallis’s recovery. “Why did he emerge? None of us can answer this,” says Hirsch. “But it suggests a biological underpinning to recovery.”
Brain imaging might eventually be used as a diagnostic tool to help spot those who are most likely to recover. “We need to develop better ways to model and measure the emergence to consciousness and collect enough data so that we can make statistical predictions for recovery,” says Schiff. However, identifying the telltale changes that predict awakening promises to be difficult. Schiff and Hirsch have scanned more than a dozen other patients in addition to Wallis, including several who have awakened, and they have yet to find specific patterns or changes in brain activity that might signal that a patient is improving. But they’re still looking–at whether the network hubs of the brain are active, for example, or whether activity in different brain areas is in sync. “We think of patients with traumatic brain injury as patients with lesions of consciousness,” says Hirsch. She hypothesizes that consciousness arises from a network of connections rather than in a specific location in the brain.
The brain is constantly processing information: sights and sounds are recorded and synthesized in different parts of the brain, then fused together in other areas, creating a cohesive picture of the outside world. And early evidence indicates a link between consciousness and the ability to integrate information. In a study of 60 patients in the vegetative state, Laureys found that the seven patients who later awakened recovered brain metabolism in regions that connect the cortex with the thalamus, a relay center in the brain.