Patients in vegetative and minimally conscious states still appear capable of acquiring certain forms of learning, reports a study published this week in Nature Neuroscience. This suggests that some of these patients could have partially preserved conscious processing that may not be apparent in behavior.
Tristan Bekinschtein and his colleagues used a conditioning technique, where subjects learnt to associate a tone with a subsequent puff of air to the eye. The association led to anticipatory muscle activity around the eye as soon as the tone was presented.
It was previously thought that learning this sort of association between a tone and an air puff requires explicit awareness of the relationship between the two stimuli. However, the vegetative patients also showed these anticipatory changes in muscle activity in response to the tone. This activity increased closer to the time that the air puff was expected, and the team found that patients who showed this type of conditioning were more likely to later show increased behavioral responsiveness.
The anticipatory learning did not happen in normal subjects who were given general anesthesia. This suggests that such learning cannot happen in unconscious states. Instead, patients in vegetative and minimally conscious states may have some preserved consciousness which is not visible via intentional movements or verbal responses, but which is still enough to support some types of learning.