Evidently, good answers to this question are not confined to the wilds of science fiction and thought experiment. In fact, I think there are actually people with a pretty good idea: patients of the Wada test. These are people who have had their brain hemispheres anesthetized one at a time so that the rest of the brain—the complementary other hemisphere and the subcortical regions—remains functioning in some ways, and by all accounts conscious. Some of these patients report on their conscious experiences afterward, for example, describing what it is like to have one’s language dominant hemisphere temporarily shut down, finding oneself at a complete—albeit only temporary—loss of words.

The medical fact that there are conscious experiences not only of the loss of certain regions (as with stroke and other brain injuries) but of alternately losing and regaining one, then the other, hemisphere has gone virtually unacknowledged by those who study consciousness in a way that goes beyond clinical diagnosis. Yet I think it has the potential to show us something important about the instantiation of consciousness in physical systems. For one thing, it may provide insights into which region or regions of the brain are independently conscious. For another, it may even undermine our idea that the conscious mind is a singular entity.

The intracarotid amobarbital procedure, or Wada test (named after the Japanese Canadian neurologist, Juhn Atsushi Wada, MD, who originated it in the late 1940s), is a preoperative diagnostic method for locating certain memory and language capacities in brain hemispheres by alternately anesthetizing each hemisphere to see what the complementary brain regions can do without it.

The diagnostic methodology is straightforward: if a patient exhibits impaired or absent speech capacities while only, say, the left hemisphere is anesthetized, but does not when only the right is anesthetized, doctors conclude that the patient’s left hemisphere is the language-dominant hemisphere. The test makes practical sense as a measure taken before ablative surgery because, while language production is typically a left-hemisphere function, this is not always the case, and in light of our interest in being able to speak, we’d like to know as best we can whether removing a portion of the brain will amount to eliminating our language capacities. (Over 90% of right-handed people, and about 70% of left-handed people, appear to use only their left hemisphere in language production. About 15% of left-handed people can speak no matter which hemisphere is impaired.) Memory is also tested in a similar way.

Doctors use this diagnostic information when deciding whether some (or even all) of a hemisphere should be surgically removed as a treatment (often last-ditch) for a number of debilitating ailments such as severe epilepsy or Rasmussen’s encephalitis.

The details of the test can vary from clinic to clinic, and it is limited by the very small risk which accompanies any invasive procedure (a catheter must be guided into one the two internal carotid arteries) and primarily because each hemisphere is only anesthetized for about 5-15 minutes. A typical test runs as follows.

Once the first hemisphere is anesthetized, a specialist shows the patient objects or pictures of objects and ask the patient to name and remember them. All along, the patient’s speech capabilities are also evaluated. Once the hemisphere “wakes up”, the patient is asked to recall these objects. Soon after, the other hemisphere is anesthetized (by retreating the catheter and guiding it into the other internal carotid artery). The same object-naming tests are run, this time with different objects. Again, the patient is asked to recall the objects once the hemisphere awakens.

The following sources provide more clinical details: Cleveland Clinic, Johns Hopkins Medicine, UW Medicine Regional Epilepsy Center, Yale School of Medicine Comprehensive Epilepsy Center. One gracious patient’s personal account is here (video).

Based on some conversations and correspondence (along with some prior beliefs about how people process such things in general), I suspect doctors and patients alike tend to see the Wada test as a situation in which the patient’s mind loses then regains certain cognitive capacities. If so, then this “mind” would exist as follows over time:

with both hemispheres, A and B, functioning before the test begins with only one hemisphere, A, functioning once the other, B, becomes anesthetized, with both A and B functioning again once B’s anesthesia wears off, with only B functioning once A becomes anesthetized, with both A and B functioning as A’s anesthesia wears off.

If the conscious mind persists through these stages, then we must ask where this professed mind is at each stage of the test. What—if anything—instantiates the conscious mind—if it makes sense to speak of such—at any stage? Here are the contending hypotheses as I understand them. (The list is not exhaustive of all possibilities, and one might find reason to mix elements of one or more with others.)

Instantiation by Unanesthetized Hemispheres: At any stage, the experiencing mind is instantiated by and only by whatever brain hemispheres are not anesthetized. That is, the composite of both hemispheres first instantiates the mind in stage 1, then one and only one hemisphere instantiates it in stage 2, then the composite of both again in 3, then the other hemisphere in 4, then finally the composite of both 5.

Instantiation by Unanesthetized Complements: At any stage, the experiencing mind is instantiated by and only by those portions of the entire brain which are not anesthetized. That is, the whole brain (subcortical regions included) first instantiates the mind in stage 1, then the whole brain minus one hemisphere instantiates it in 2, then the whole brain again in 3, then the other hemisphere in 4, then finally the whole brain again in 5.

Instantiation by Subcortical Region: At every stage, the experiencing “mind” (if it can be called such, itself lacking so many of the cognitive capacities the hemispheres enable) is instantiated by and only by some subcortical region of the brain (e.g. the thalamus or the limbic system) which is in functional interaction with the hemispheres and which becomes consciously aware of differences in experience as those hemispheres alternately shut down and start up again.

Interactionism: At every stage, the experiencing mind is instantiated by and only by some non-brain entity, perhaps a “non-physical” entity.

No Such Mind: No experiencing mind persists through the Wada test.

Consider now reports such as this one about a patient’s first-person experience during a Wada test from Epilepsy.com (bracketed insert mine).

“When they did [anesthetized] my left side I could see all of the people and I think I knew who they were and I knew that they were doing the Wada test, I don’t remember feeling scared. I do remember my neurologist saying something like stay awake, try your best, etc. They lady with the box pulled things out, but I couldn’t talk. I don’t remember if it was like my post seizure experience when I knew the answer but couldn’t say it or if I just didn’t know the answer. I remember every one else having a look of suspense on their face, but I don’t remember having any emotional feelings. I don’t really remember the left side “coming back,” but obviously it did.”—Igroo 02

I will assume here that this report is sufficiently accurate. Granted, patients often confabulate their experiences and we should keep that in mind. However, we have two reasons to accept such recollections of the Wada test. First, many others who have undergone the Wada test report similar experiences. Second, confabulation could easily be detected by the doctors and other specialists who are present during the test, and who typically digitally record the sessions. If, for example, a patient describes in some detail how they could barely name a fork but would not even attempt to name a giraffe, it would be easy enough to check whether the patient was actually ever shown such items and whether they responded as described.

Here is another such report (same source).

“My neurologist showed me a bunch of objects and photos prior to the test, then during the test he would show me something, ask me to identify it and whether he had showed it to me before. During the procedure they had me hold my arm straight up. For the right side of the brain I didn’t notice anything different. For the left side – wow! When he showed me an object I looked at it and had that feeling you get when you can’t think of a word, like it’s on the tip of your tongue. Only that was true for all words – it was amazing! I had no words.”—jennagg

What I want to know is this: Who, or what, had no words?

More carefully, which portion of the brain—if any—instantiated the conscious experience of suddenly “having no words”, then minutes later, getting them back? (This, we should note, appears to be an entity accustomed to normally “having words”.) And which (seemingly other) portion of the brain—if any—is the portion which previously “didn’t notice anything different”? (Note: I know of no tests which attempt to locate cognitive capacities thought to be typically right hemisphere dominant, so it may be that person identified as jennagg did not have the opportunity to notice what was different.)

While other possibilities exist, one explanation of all we now know is that these patients are reporting on conscious experiences had by different portions of the brain at different times. This would hold if either of the first two hypotheses were true: Instantiation by Unanesthetized Hemispheres, or Instantiation by Unanesthetized Complements.

If the third hypothesis is true, Instantiation by Subcortical Region, then the true seat of the persisting consciousness mind lies there buried under the cortex. On this hypothesis, losing then regaining a hemisphere is something like closing then opening one eye.

While I do think there are decent or good reasons favoring each and every one of the five hypotheses listed above, the title of this post should make it clear which ones I currently find the most plausible, given what we now know about consciousness, the mind, and the neural material with which it is so intimately associated:

Multiple distinct, but possibly overlapping, regions of the brain can have their own independent conscious experiences of losing and regaining functional neural contact with other regions. And whole brains can carry the memories of such experiences and can thus later report on them.

If this is true, then we have access to an experimental method for exploring the sufficient conditions of consciousness for physical systems. Whenever first person reports about losing then regaining a brain region are generated, we can treat that as evidence (though admittedly not proof) of what that temporary loss is like for some complement of that region, and thus we can treat it as evidence that the complement of that region is itself independently conscious. (Not proof because the existence of what we regard as a report about a conscious experience does not necessitate the existence of a conscious experience; it is conceivable that the patient momentarily becomes a kind of philosophical zombie.)

We also have a reason to care about these distinct, possibly overlapping, proper parts of the brain which appear to be independently conscious; that is, to regard them as beings with intrinsic moral status. They are more like whole functioning brains with regard to this status than they are like spleens or fingertips. This is not to say that these evidently conscious proper parts of the brain have the same moral status as the whole brain, nor is it to say that they cannot differ in moral status among themselves. Nor is this to say that moral status requires consciousness. The main premise is simply that conscious things matter.