Brain-injured patients who show minimal to no signs of consciousness may still retain functioning brain networks that allow them to learn, researchers report in a study published online today (September 20) in Nature Neuroscience.

A brain scan Image: Wikipedia

"Determining cognitive processing capacity in behaviorally unresponsive patients has been a real dilemma for clinicians working in brain injury," said Joseph Giacino, the associate director of neuropsychology at the JFK Medical Center in Edison, NJ, who was not involved in the research. "This study provides strong evidence of an active, selective learning process in these patients."

Cognitive neuroscientist Tristan Bekinschtein from Cambridge University and his colleagues used a classical conditioning technique to determine whether cognitively impaired patients who had spent more than six months in a vegetative or partially conscious state were able to learn. This conditioning technique is the simplest way to study the neural structures that underlie learning and memory, said Bekinschtein, the paper's first author.

The researchers repeatedly paired two stimuli, a tone followed by a puff of air to the eye, to see whether the patients could learn to use the tone as a signal to anticipate the air puff. The study participants were divided into three groups --16 healthy individuals, 12 subjects given general anesthesia, and 22 vegetative or partially conscious patients -- and Bekinschtein measured activity in subjects' eye muscles in the half-second between playing the tone and spraying the air puff to test their response.

As expected, the healthy patients showed extensive eye muscle movement in the interval between the tone and the air puff, while the anesthetized patients showed no notable eye muscle activity between the two stimuli. Surprisingly, however, 15 of the vegetative, minimally conscious and severely impaired patients showed a significant spike in muscle activity after the hearing the tone, indicating at least some degree of learning.

Although the study authors did not measure brain activity specifically, Joseph Manns, an assistant professor of neuroscience and animal behavior at Emory University, speculated that for this conditioning to occur several regions including the medial temporal lobe (most notably the hippocampus), the prefrontal cortex, and the cerebellum have to be intact to some degree.

Giacino pointed out that while the cognitively impaired patients may have the neural circuitry that permits some learning, the study does not provide clear evidence that these patients are conscious of what they are learning. "It's possible that this learning process may be independent of awareness," said Giacino.

The authors did, however, find a strong correlation between cognitively damaged patients who seemed to exhibit such learning and those who showed some level of improvement months after their injury, as measured by a standard coma recovery scale.

"These findings suggest there is room for late recovery in vegetative patients and that classical conditioning could predict this type of recovery," said Bekinschtein. He and his colleagues now plan to incorporate electrophysiology, brain imaging, and behavioral measures into their assessments of the patients' cognition.


Related stories:

Manipulating Memory
[March 2009]

Revelations from the Unconscious
[12 September 2005]

Christof Koch's Ascent
[ 14 July 2003]