Thoughts on tests for vegetative and minimal consciousness

http://www.scientificamerican.com/article.cfm?id=learning-in-vegetative-state

http://www.newscientist.com/article/dn17813-minimally-conscious-patients-can-learn.html?DCMP=OTC-rss&nsref=online-news

The following are just some thoughts on these two articles regarding this newly invigorated investigation into distinguishing between people who are in a vegetative state and those who are in a minimally conscious state.

The articles point out that some patients who were thought to be in a vegetative state are capable of some simple associative learning a la Pavlonian or classical conditioning. But what does this tell us about consciousness in these patients? Is the ability to learn these very simple associations sufficient for us to say something is conscious?

For one, I think many people will agree that there are degrees of consciousness, such that humans are somehow more conscious than our primate cousins or our beloved pet dogs. Certainly, dogs are capable of this very simple type of learning as evidenced by the famous Pavlonian trials. But when we consider other organisms capable of classical conditioning, the concept of consciousness begins to become a bit hazy.

It has been demonstrated that sea slugs are capable of this sort of learning. (Bear with me here, as I'm just recalling this from memory and may not be completely accurate). Sea slugs are naturally hard-wired to retract its gills in the presence of certain stimuli, no different than humans are to blinking when something is in their eyes. It has been shown that sea slugs are able to associate a conditioned stimulus to an unconditioned stimulus (which elicits a response--retracting of the gills) after several trials--or the learning period. While scientists are able to give a much more definitive explanation for the phenomena in a sea slug, the implications are that other, more complex organism operate under similar learning mechanisms.

But is this alone enough to say that the sea slug is conscious? There are some philosophical issues that need to be addressed in answering this question, and regardless of which side you choose, the issue of minimally conscious patients complicate the matter further.

If you believe that a sea slug is not conscious, then I would think that you are likely to say that a person in a seemingly vegetative state that is capable of conditional learning is not conscious either. After all, the input-output function is similar and, by choosing this answer, you are more or less taking for granted that the function alone determines consciousness.

For those who believe that the sea slug is conscious, the burden is now on you to differentiate the character or level of consciousness between a minimally conscious patient and a fully conscious one. For example, is a minimally conscious patient that is capable of learning to associate a sound with blinking substantially more conscious than, say, the sea slug, or has this person been reduced to the likes of these gastropods? I think a complete answer to this question will need to address the philosophical and scientific aspects of consciousness, which in turn will need to be reconcilable with today's ethical stance on consciousness or destroy it altogether.

On this sole metric for consciousness, we face difficult questions on whether someone who is in a minimally conscious state should be considered as anything more than a sea slug trapped in a human body. However, as stated earlier, degrees of consciousness are a popular notion, and there is no reason to believe that a diminished consciousness in humans can't also be subject to a continuum.

It has already been shown that one seemingly vegetative patient showed a lot of highly organized and specialized neural activity when verbally asked to think about playing tennis (I think there's a link in the SciAm article somewhere about this). Moreover, stories of seemingly vegetative patients somehow responding to touch or words of a loved one is not uncommon. But is this anything more than a slightly more complex form of classical conditioning? While I haven't read the actual journal on these studies (and I trust that they have taken into consideration the following comments I am about to make), could there be tests for varying degrees of consciousness? For example, instead of asking the patient to visualize playing tennis, ask them to visualize hitting a fuzzy green ball over the net inside the paint to the opponent on the other side with a racquet, then compare the two neural activity patterns to find any significant similarities. Another example would be to condition the number "6" to blinking (via a puff of air), then asking them to calculate simple equations resulting in the number 6.

While this test has many problems in it of itself (e.g., damage to the Broca's area such that they can't understand any words), it could possibly shed a lot of new light on the matter. If they are able to respond to the question "what is two plus four" with a blink, but is unresponsive to the question "what is one plus one," it could offer insights into the degree of the patients consciousness. For example, such results would indicate that the association is not made at the gray-matter level (i.e., the neural activity in the auditory cortex induced by uttering the word "six" into the patients ear) but a "higher," more abstract level of consciousness, such that it is not the word "six" that is associated with blinking but the concept of six. Cases like this are probably extremely rare due to the chances of the appropriate parts of the brain remaining intact after severe overall damage, if not impossible, but I thought it was just something fun to think about.