Friday, March 21, 2014

The human animal and the cerebrum

Suppose your cerebrum was removed from your skull and placed in a vat in such a way that its neural functioning continued. So then where are you: Are you in the vat, or where the cerebrum-less body with heartbeat and breathing is?

Most people say you're in the vat. So persons go with their cerebra. But the animal, it seems, stays behind—the cerebrum-less body is the same animal as before. So, persons aren't animals, goes the argument.

I think the animal goes with the cerebrum. Here's a heuristic.

  • Typically, if an organism of kind K is divided into two parts A and B that retain much of their function, and the flourishing of an organism of kind K is to a significantly greater degree constituted by the functioning of A than that of B, then the organism survives as A rather than as B.
Uncontroversial case: If you divide me into a little toe and the rest of me, then since the little toe's contribution to my flourishing is quite insignificant compared to the rest, I survive as the rest. More controversially, the flourishing of the human animal is to a significantly greater degree constituted by the functioning of the cerebrum than of the cerebrum-less body, so we have reason to think the human animal goes with the cerebrum.

Another related heuristic:

  • Typically, if an organism of kind K is divided into two parts A and B that retain much of their function, and B's functioning is significantly more teleologically directed to the support of A than the other way around, then the organism survives as A rather than as B.

My heart exists largely for the sake of the rest of my body, while it is false to say that the rest of my body exists largely for the sake of my heart. So if I am divided into a heart and the rest of me, as long as the rest of me continues to function (say, due to a mechanical pump circulating blood), I go with the rest of me, not the heart. But while the cerebrum does work for the survival of the rest of my body, it is much more the case that the rest of the body works for the survival of the cerebrum.

There may also be a control heuristic, but I don't know how to formulate it.

10 comments:

Michael Rabenberg said...

"I think the animal goes with the cerebrum."

I agree, but there are at least two ways of interpreting this. One would be to say that (1) the animal becomes the cerebrum. Another would be to say that (2) the animal becomes that of which the cerebrum is a part--namely, the brain-vat composite. I incline to (2). I find it very hard to believe that I could be a cerebrum. I find it less hard to believe that I could be a brain-vat composite, for I tend to think of prosthetic limbs etc. as parts, not tools, of the organisms of which they are limbs.

Jeremy S. said...

If the brain stem is left intact and functioning in the cerebrumless body, has the cerebrum transplant actually given rise to a new animal (the one "left behind")? If so, is it a rational animal? Would this be an instance of asexual human reproduction (like monozygotic twinning might be)?

Alexander R Pruss said...

Jeremy:

I think it's not an animal. What teleology it had was to support the functioning of the cerebrum. Not just the functioning of *a* cerebrum, but of *this* cerebrum.

It's an interesting question whether it could get transplanted, though, and become the support for another cerebrum. (Unlike Michael, I don't think prosthetic limbs become part of the body.)

Jeremy S. said...

What about a case in which the cerebrum is destroyed while it is still inside the cranium of a human animal? Would destroying its cerebrum end the life of the animal, even if its functioning brain stem continued to coordinate metabolic activities? That seems implausible to me. Would you say, then, that in order to determine whether or not a cerebrum-less, apparently living, human body is an animal, it matters precisely HOW the original animal lost its cerebrum? Would this commit you to a sort of closest-continuer view?

Alexander R Pruss said...

Yeah, I suspect you can survive the destruction of your cerebrum.

So, yes, order matters. But that's not particularly surprising. You can survive having all your cells die and new ones grow back, but maybe not if it happens all at once.

I don't think being the closest continuer *constitutes* identity. But typically, your closest continuer, as long as it's close enough, will be you.

William said...

Biologically, the cerebrum cannot be conscious without the brainstem, so a separation of cerebrum (somehow kept with living cells--how?) and the continuation of the body with just a brainstem produces just one person, cut in two parts, both parts comatose and moribund.

A cerebrum (without brainstem) in a vat would be comatose unless some kind of prosthetic brainstem was added, and the result would be a machine hybrid which might be conscious, but I'm not sure.

If your evil neurosurgeon does this twice, in order to swap cerebrums on the body-brainstem parts somehow, I suspect that identity follows cerebrum as far as the answer to "who am I" would go.

Alexander R Pruss said...

I did not know we had strong evidence the stem is needed for consciousness. What is this evidence?

William said...

It's the "ascending arousal system" without which the rational animal cannot be awake. See Plum and Posner's textbook on stupor and coma, or for example here : http://www.ncbi.nlm.nih.gov/pubmed/23656993

Alexander R Pruss said...

Thanks, this is very interesting.

1. I have always wondered about claims about what is necessary for consciousness in light of the fact that I have a hard time imagining how one would experimentally differentiate between the hypotheses (a) x is unconscious and (b) x is conscious but is unable to overtly react to what she is conscious of and is not recording anything to memory.

Maybe there is a way. This question isn't something I pursued seriously.

2. We're conscious at least some of the time while asleep.

3. Consciousness is not required for a mental life, though I guess it is required for a flourishing human mental life.

4. But I think the really important question is how substantive the contribution of the brain stem is. Is it more like the oxygen that is contributed by the circulatory system, or is it more substantive?

William said...

The topic of partial consciousness in the vegetative state is mostly unkown, and there is controvwesy about it. I believe that the ascending activation system is required for dreaming, at least the type that occur in REM sleep. So the kind of consciousness that occurs when we dream probably could not occur in a cerebrum without a brainstem.

Parts of the upper brainstem (midbrain, thalamus) such as the 'tegmentum' are so cross-connected with the cerebrum it is unlikely if the two are really separable in cognitive or motor function.