Medical What part of the brain is conscious?

[Q_Goest]

The 15 July 2005 issue of Science has an interesting article regarding something called anosognosia.

A strange and disturbing neurological condition, anosognosia, can cause obviously intelligent, awake and talking individuals to be unaware of paralysis on one side of their body. Berti et al. investigated patients with spatial neglect and found that about half of them were also anosognosic for their left hemiplegia because of lesions in the right brain hemisphere. Comparisons of the two groups with and without anosognosia revealed that damage to frontal areas (particularly brain areas 6 and 44, motor cortex BA4, and the somatosensory cortex) underpins the loss of awareness of motor impairment in these patients.

It seems very strange that one is unaware of their own paralysis while they are completely aware of the person they are being interviewed by asking them about it. The patient in this case would simply need to look down and realize that when consciously attempting to lift an arm, it would not move.

Similar problems might include the belief that limbs are still attached when in fact they've been amputated. That seems to be a similar phenomena wherein the brain is consciously aware of a limb that simply doesn't exist. Another problem I've seen is people with memory loss not realizing they even have a memory problem or constantly forgetting they have this problem.

If one were to model the brain as a mechanism of some sort with nerve inputs, it would seem that there is a borderline within the brain beyond which conscious awareness can not extend. In other words, if the input/output for the conscious portion of the brain simply goes silent and does not register anything, it would seem as if we can not be consciously aware of it or our consciousness is not impared by it except as that input/output may affect things such as memory.

We often view consciousness as being seated in the brain, but I wonder if there is any evidence which points to some limited location within the brain. The article in Science mentions specific locations, but unfortunately I'm not a neuroscientist and have no idea if this entire concept of there being a specific region of the brain responsible for consciousness has been examined. Any comments would be appreciated.

 

[hypnagogue]

The technical term for those parts of the brain whose activity is correlated with consciousness is "neural correlates of consciousness" (NCC for short). It is certainly the case that not all systems in the brain are NCCs, since the brain does a lot of information processing that can be shown to be unconsciousness, that is, not available to conscious awareness. Much more work needs to be done before the NCCs of the various features of consciousness can be exhausitively mapped out in detail, although the thalamocortical circuit seems to be a centrally important system as regards correlation with consciousness. (The thalamus is located roughly in the middle of the brain, and is often described as a 'relay station' to the cortex; all sensory input channels except smell feed into the thalamus. From the thalamus, there are dense neural connections to the cortex, where sensory information is processed further.)

Although it might seem intuitive and straightforward, the concept of what an NCC really is is somewhat subtle. A good discussion of what we should mean by an NCC can be found in David Chalmers' paper What is a Neural Correlate of Consciousness?. For some references on scientific research of NCCs, see http://consc.net/biblio/6.html#6.1a; for references on research of the NCCs of visual consciousness, see http://consc.net/biblio/6.html#6.1b.

 

[Q_Goest]

Thanks for the info, I'll check those out shortly.

 

[Antiphon]

Consciousness is so complex and so little understood that I believe it is
not productive to try to localize "it" in a specific part of the brain.

 

[El Hombre Invisible]

How do you suppose we attempt to make it more understood if we do not try to locate it? Even failure is informative.

 

[Q_Goest]

Consciousness is so complex and so little understood that I believe it is not productive to try to localize "it" in a specific part of the brain.

Yes, that's really my question, "is there a location, or more accurately, a given volume of the brain which is responsible for the phenomenon we call consciousness?"

It seems in the case of people with anosognosia, severed limbs and other maladies, there is a disconnect. Similarly, I suspect there are portions of the brain itself, such as those portions which might store memories, that don't contribute directly to the conscious experience.

I wonder what research has been done (especially on people that have undergone some brain damage) and what conclusions have been made. It seems the thalamus as mentioned by Hyp have been specifically mentioned in the literature.

I seem to remember a famous case of a man shot through the head with a steel spike who was working on a railroad over 100 years ago that survived and who's consciousness was relativly unaffected, but his personality was significantly affected. This is another case of consciousness not being affected, but only things that govern behaviour being altered.

So are there portions of the brain such as memory, or portions that govern behavior or other functions that DON'T contribute to the conscious experience?

 

[Q_Goest]

On a separate note, there must also be portions of the brain which perhaps perform specific tasks related to the conscious experience. I seem also to remember hearing about a woman who could not distinguish how fast oncoming traffic was traveling. She could see a vehicle, but for some reason couldn't determine how fast it was coming. Other people have problems not consciously seeing something, but are able to react despite this.

 

[Antiphon]

How do you suppose we attempt to make it more understood if we do not try to locate it? Even failure is informative.

We should attempt to understand it as something that emerges from the
complex interplay of many different parts of the organ, not as something
that is spatially confined to one or another parts of it. K?

 

[Q_Goest]

We should attempt to understand it as something that emerges from the complex interplay of many different parts of the organ, not as something
that is spatially confined to one or another parts of it.

When you say "parts of the organ" I presume you mean "brain" which has some boundry around it, as opposed to "parts of the organ" meaning body which includes the brain as a sub-part. Is this to presume that only neurons (or does this include glia?) are capable of consciousness then? Because if we rule out every other part of the body, including nerve cells such as the spinal chord, we are left with a group of neurons with an interesting interface. Somewhere it changes from neurons to not neurons. What is that interface, and where is it?

I tend to wonder though if this is a good assumption, that all the neurons in the brain are capable of consciousness, but no other cells are. Certainly not all the neurons in the brain are going to be used in the conscious experience at once, but many of them will. So there must be a switching on and off somehow. This switching on and off can also be affected by something as simple as xenon, used as an anesthetic for example. (Ref: Patent 5228434)

 

[Les Sleeth]

How do you suppose we attempt to make it more understood if we do not try to locate it? Even failure is informative.

Let's say you are trying to figure out what part of a radio produces language. You don't know where language comes from, but you have already assumed the radio itself generates language because that's the only place you hear it. The technology you have doesn't allow you to detect radio waves, so the idea that language might originate outside the radio, as a signal, seems non-parsimonious.

What you do, because you are committed a priori to a radio explanation of language, is try to show exactly where in the radio that language emerges so you can say "THERE," that's the seat of language. In this analogy, it is obvious that just because language emerges from the radio's speaker doesn't mean the speaker created the language.

Yet that is exactly what a great many thinkers assume about the brain even though we can't find any evidence that physicalness (i.e., the brain) is capable of generating consciousness. Affecting it, shaping it, organizing it, giving it a means for manifesting itself in this universe . . . yes. But creating consciousness . . . no.

 

[hypnagogue]

Les is correct to point out that observing a correlation between brain activity and conscious experience is not, in itself, sufficient grounds to deduce the nature of the causal relationship between the two. Of course, that latter topic is itself the subject of much discussion and debate in the sciences and philosophy.

However, I think we can and should proceed with this discussion without delving too much into the deeper metaphysical issues; those are further questions that are best left to other threads. Along these lines, saying that the brain is conscious, or that the brain produces or is causally responsible for conscious experience might seem relatively harmless, but in reality these phrasings are theoretically loaded. However, if we restrict our discussion to the neural correlates of consciousness, we can still have a fruitful discussion here that is faithful to the thrust of this thread's topic, while remaining neutral on the more complicated metaphysical issues.

 

[hypnagogue]

We should attempt to understand it as something that emerges from the
complex interplay of many different parts of the organ, not as something
that is spatially confined to one or another parts of it. K?

There is something to be said for this view. For instance, it is believed that various neural correlates of visual consciousness lie along the visual processing stream, beginning in area V1 of visual cortex in the occipital lobe and traversing the ventral processing stream to areas of the temporal lobe. Roughly stated, this means that certain activations of certain neural systems in these brain regions are correlated with certain features of experiential, visual consciousness. This might create the appearance of something of a modular view, on which the neural systems in the visual processing stream alone are enough to provide sufficient conditions for the existence of visual consciousness.

However, it is tacitly understood that for this correlation between neural systems in the visual processing stream and visual consciousness to hold, the relevant neural systems must be functioning in the wider context afforded by other neural systems. For instance, it is known that damage to the intralaminar nuclei in the thalamus leads to coma. If we assume for a moment that being in a coma implies the loss of unified, high level human consciousness, it follows that visual consciousness is lost as well. Thus, in order for neural systems in the visual processing stream to be correlated with visual consciousness, it seems that at least one background condition must be met: The intralaminar nuclei in the thalamus must be properly functioning. There are probably hosts of other such neural systems in the brain that must be functioning properly in order to provide the context in which certain neural systems are correlated with visual experience.

Nonetheless, the activity of the visual processing stream is related to visual experience in more direct and finer-grained ways than the activity of the intralaminar nuclei, so we would be missing something if we didn't distinguish between the two. Ideally what we want is to be able to draw as fine-grained a correlation between neural activity and subjective experience as possible, while still paying heed to the more global, background conditions that come into play. In the paper I referenced earlier, Chalmers succinctly incorporates these considerations (and others) into his general definition of a neural correlate of consciousness (NCC):

An NCC is a minimal neural system N such that there is a mapping from states of N to states of consciousness, where a given state of N is sufficient, under conditions C, for the corresponding state of consciousness.

For a good extended discussion on this topic (modularity of NCCs vs. global considerations), see the section of Chalmers' paper titled "Direct correlation."

 

[Antiphon]

The previous three posts are very good.

I seem to remember a famous case of a man shot through the head with a steel spike who was working on a railroad over 100 years ago that survived and who's consciousness was relativly unaffected, but his personality was significantly affected. This is another case of consciousness not being affected, but only things that govern behaviour being altered.

We need a more comprehensive working definition of consciousness.
Maybe this fellow's actual thought patterns changed. Or maybe they
didn't but his temper (an emotional-glandular interaction) was altered.

For humans anyway I would think that our emotional states should be
included in the description of consciousness.

And let's not forget that there is a subconscous layer that underpins this
which is only faintly understood if at all.

 

[neurocomp2003]

isn't hte entire brain consciousness? If i remember correctly, you can lesion off any part of the brain but youi will lose certain capabilities that that part of the brain processed. perhaps memory, perhaps learning, physical motion.

 

[LeonhardEuler]

isn't hte entire brain consciousness? If i remember correctly, you can lesion off any part of the brain but youi will lose certain capabilities that that part of the brain processed. perhaps memory, perhaps learning, physical motion.

The fact that you loose abilities when a certain part of the brain is lost does not imply that this part of the brain is concious. It may be that this part simply performs calculations and gives relevant results to the part of the brain that is concious.

 

[saretesotu]

the right part of the bran is concence that's what i know. see this girl got her left brain replaced cause of a desies and she still had a concence and went to rehab and the docs say so to

 

[neurocomp2003]

leonard: then what is your definition of consciousness? awareness like that of an animal?

 

[LeonhardEuler]

Being aware that you exist and having feelings.

 

[saretesotu]

exactly your sences come from the left side of you brian

 

[the_truth]

neo-cortex

 

[LeonhardEuler]

neo-cortex

How do you know that?

 

[neurocomp2003]

neocortex isn't as developed in animals is it?

 

[Zantra]

on a similar topic, in certain extreme cases neurosurgeons can perform a hemispherectomy- essentially removing half of the brain- it's usually done in children who are at an early developmental stage. If done early enough, the remaining half of he brain compensates for the missing half, and they can lead an almost "normal" childhool. That is to say they can lead a fairly normal life versus no life at all. It's truly amazing.

 

[Smurf]

on a similar topic, in certain extreme cases neurosurgeons can perform a hemispherectomy- essentially removing half of the brain- it's usually done in children who are at an early developmental stage. If done early enough, the remaining half of he brain compensates for the missing half, and they can lead an almost "normal" childhool. That is to say they can lead a fairly normal life versus no life at all. It's truly amazing.

How does that work exactly? How does that affect how the child thinks? like in regard to hand-eye coordination and such?

 

[Zantra]

How does that work exactly? How does that affect how the child thinks? like in regard to hand-eye coordination and such?

Well I don't know the details of exactly how it works, but essentailly the brain compensates by directing left and right coordination to a part of the brain that still functions- much the way a blind man's hearing can become more attuned. As far as thinking, I believe the one subject read about never really developed beyond the mentality of a child, and he also had to see a physical therapist to learn to compensate. There's obviously going to be some difficulty when half of your brain is removed.

 

[zoobyshoe]

The thalamo-cortical complex referred to by Hypnagogue is, in fact, somehow, the essential base of consciousness in the brain. As he mentioned, damage to the thalamus results in coma, which is not true of any other part of the brain, and merely physically disturbing the thalamus results in unconsciousness: neurosurgeons are careful to stay away from it when operating on epilepsy patients, a procedure done while they are awake so they can paticipate with localizing the seizure focus by giving verbal feedback to stimulation of different areas.

In addition, the thalamo-cortical complex is what is disturbed in attacks of petite-mal epilepsy, called absense seizures.

A person with this kind of epilepsy will experience very brief, but total, failures of consciousness, usually several times a day, but possibly even more.

Nothing but consciousness is affected in these seizures: the rest of the person's brain functions normally; there are no convulsions or other symptoms beyond the brief loss of consciousness.

To an outsider such a person appears to simply be standing still, unresponsive, and staring intently at nothing in particular. A few seconds later they resume a normal demeanor, but may seem mildy disconcerted, or surprised.

To the sufferer, time seems to suddenly jump ahead, like a film which has had a few seconds cut out of it. Things may have changed their position suddenly in the person's visual field, or the conversation they are in may seem to have inexplicably switched topic or speakers.

It's important to note that this is a completely different experience than "spacing out" i.e. becoming so lost in a train of thought that you stop paying attention to our surroundings. In an absense seizure conciousness ceases to function at all: there are no thoughts to get lost in.

The seizure activity in absense seizures runs somewhere in the circuits between the thalamus and the cortex of the frontal lobes, and is usually no problem to pick up on an EEG, exhibiting a tell-tale 3.5 hz spike and wave pattern that is the hallmark EEG tracing of the absence seizure.

So, during the absense seizure, the whole brain seems to continue to function, except those parts responsible for something essential about consciousness, which I don't even know what to call: perhaps basic consciousness or baseline consciousness or foundation consciousness.

 

[zoobyshoe]

Similar problems might include the belief that limbs are still attached when in fact they've been amputated. That seems to be a similar phenomena wherein the brain is consciously aware of a limb that simply doesn't exist.

V.S. Ramachandran, a well known neurologist, has done a lot of study of amputees, (as well as the half paralyzed stroke patients) and believes he has unlocked the mystery of the "phantom limb" phenomenon. Although a person's limb may be missing, the part of the brain that processes sensory input from that limb is still there, as well as the part that processes proprioceptive imput from that limb.

Ramachandran believes that, with no imput from the missing limb, these areas become receptive to imput from the surrounding neural tissue and erroneously present that imput to consciousness as being from the missing limb.

Strangely enough, I saw a different neurologist on TV a few years ago arguing the same thing. (Does Ramachandran check the prior research?) He had a man with no arms there, and by touching the man's jawline he could produce a sensation in the man's "phantom thumb". This, the neurologist said, was due to the fact that the sense of touch or these two places are processed right next to each other on the sensory strip of the brain: stimulation to the sensory strip jaw area, via the jaw, could bleed right over into the deprived sensory strip area for the thumb.

This all makes a great deal of sense since phantom limb sensations aren't constant: they kick in and out, and are felt to different degrees at different times, and involve every sensation from touch to pain.

This, though, is really quite a different phenomenon than the half-paralzed stroke patients. The amputees are well aware a limb is missing, and the phantom senstions are appropriately inexplicable and incongruous to them. The stroke patients in question, on the other hand, have lost the capacity to apprehend they are half-paralyzed.

 

[zoobyshoe]

It seems very strange that one is unaware of their own paralysis while they are completely aware of the person they are being interviewed by asking them about it. The patient in this case would simply need to look down and realize that when consciously attempting to lift an arm, it would not move.

It's funny you should bring this up, because I recently linked to an article about the same kind of stroke patients in another forum to make a point about the limits of our ability to know if we are hallucinating or not.

From these patients along with several strange cases I've read about where something like the opposite occurs (that is: they "disown" a limb, and can't believe it is actually attached to their body, and is a part of them), as well as cases where people believe a limb that is neither missing nor paralyzed, is nevertheless in a position different from the one they can visually observe it to be in, and also from the common reports of the "out-of-body" experience, I was lead to some reading about the sense we all have, but are unaware of, called proprioception. This is a person's ability to sense what position they are in without having to look at themselves.

You may have supposed such a thing was the natural side-kick of the sense of touch, (and indeed, it is sometimes referred to as "internal touch") but is is actually a different dedicated system with it's own nerve receptors (proprioceptors) and it's own processing centers in the brain, (parietal lobes, somewhere, and angulate gyrus? - don't quote me, that's from memory).

Take the case of The Disembodied Lady reported in The Man Who Mistook His Wife For a Hat, by Oliver Sacks: a woman whose proprioceptive sense was destroyed by a freak illness now lives in a permanent "out-of-body" state. She no longer has any internal sense of where "she" is located. If she closes her eyes, she feels non-located. If she opens them, she can see from the perspective of her eyes, but doesn't "feel" located behind them. She can't move a limb without looking at it because she can't sense where it is to start with, where it is going, or aim it anywhere without visually tracking it the whole time. Sacks determined her parietal lobes were fine, and that it was the proprioceptors throughout her body that had been damaged: they could send her parietal lobes no information.

Somehow, simply having physical existence isn't enough to know we have a physical existence. To know we have physical existence, the brain must be informed by input from the sense of proprioception. The case reported by Sacks was, at that time, a freak: proprioception is usually so reliable that no such gross defects occur, and everyone remains blissfully unaware that we even need such a sense. (Since then, Sacks appends, many people have temporarily lost their sense of proprioception through a fad diet reccomending large amounts of vitamin B₆)

The stroke patients are a different kettle of fish than amputees with phantom limb sensations. The stroke patients don't seem to be able to apprehend that half their body isn't working, not even by looking at the unmoving arm or leg. This means they must be in the throes of a full-blown, elaborate visual hallucination in which the paralyzed half is seen by them to be doing everything they feel it is doing.

Ramachandran believes this is old fashioned, psychological denial. However, when we look at all these misplaced, missing, and disowned limb cases, a pattern emerges concerning proprioception.

In his next story, The Man Who Fell Out Of Bed Sacks tells of a patient who came in for weakness in the leg. He woke up next morning to find a "horrible" leg in bed with him. Thinking some drunken hospital employee must have put an amputated limb in bed with him as a joke, he threw it out of bed, but he himself went with it, and ended up on the floor.

He couldn't apprehend that it was his own leg. He couldn't feel his own leg, and didn't know what had happened to it beyond the fact it seemed to have disappeared, but this terrible limb wasn't it, and didn't belong to him.

Our sense of proprioception, the sense we don't even realize we have, usually works so perfectly that we never doubt it, or think of doubting it. It turns out to be higher on the hierarchy of what senses we believe than even our vision, such that a man will disown his own leg, and try to throw it out of bed, if his sense of proprioception ceases to extend into that leg.

I think that what's happening with the stroke patients is much more complicated than mere denial. I think that, despite muscular paralysis, their ability to process proprioception for the paralysed half is intact, but now independent of the limbs in question, and that all the feelings of motion and resultant change in location that are appropriate to bilateral functionality are being experienced by these patients: they feel like they are moving the paralyzed half.

Why do I think that? Because amputees report thay can move their phantom limbs, feel them in different positions, and go through the motions of interacting with objects (unsuccessfully, of course). Additionally, some seizure patients with simple-partial seizure activity in the parietal lobes, experience the sensation that limbs which are neither amputated nor paralyzed, are never-the-less in positions which are different from what visual inspection tells them. One man, in particular, couldn't shake the "feeling" that his arm was being held up in the air beside his head. He could easily see it was down at his side, but the feeling was so convincing he asked his wife to pull it back down for him.

The stroke patients seem to be so convinced by the feeling the paralyzed half is still functioning and moving, that they resolve the cognitive dissonance between that overwhelmingly convincing feeling and the sight of the paralyzed limbs, by unconsciously chosing to believe the feeling and to ignore the actual visual imput, replacing it with an hallucination appropriate to the sensory experience of being able to move. I would look for the reason they are susceptible to such hallucinations, while amputees are usually not, in the fact that the stroke patients have had "insults" to the brain.

( Not all stroke patents are like this, by the way. This subset is actually a minority.)

 

[hypnagogue]

The thalamo-cortical complex referred to by Hypnagogue is, in fact, somehow, the essential base of consciousness in the brain. As he mentioned, damage to the thalamus results in coma, which is not true of any other part of the brain, and merely physically disturbing the thalamus results in unconsciousness: neurosurgeons are careful to stay away from it when operating on epilepsy patients, a procedure done while they are awake so they can paticipate with localizing the seizure focus by giving verbal feedback to stimulation of different areas.

Actually, the intralaminar nuclei in the thalami are not the only portions of the brain whose proper functioning is necessary for consciousness:

"An astonishing amount of brain tissue can be lost from the great cerebral hemispheres without abolishing the state of consciousness, while tiny lesions in the slender axial core of the brain cause irreversible coma. We have known since the 1950s that a small area in the brain stem called the reticular formation is necessary for waking consciousness. When people with head injuries go into coma, it is often because tissue damage at the front of the head causes widespread swelling, choking off the blood supply even to the brain stem. When oxygen supply is lost to the reticular formation the result is coma; and because the nerve centers that control breathing and heart action are located very nearby, death often follows."

- from In the Theater of Consciousness: The Workspace of the Mind by Bernard J. Baars, p. 25-26

While it's important to the NCC search to discover brain areas that are necessary for consciousness, such necessary conditions can't give us the final story. Ideally, we want to find those brain regions whose proper functioning present minimally sufficient conditions for consciousness, given some background conditions.

To give an exaggerated illustration of why this is the case, we can say with confidence that a functional heart is a necessary condition for consciousness. Still, elevating the heart to the status of a primary physical correlate of consciousness seems to be miss the point-- it's not faithful to what we really mean by "physical (neural) correlate of consciousness." I have no doubt that the thalamocortical complex is more intimately involved with consciousness than is the heart, but if it is, it is primarily by virtue of presenting some set of sufficient conditions for consciousness, rather than merely necessary conditions.

 

[zoobyshoe]

Actually, the intralaminar nuclei in the thalami are not the only portions of the brain whose proper functioning is necessary for consciousness:

"An astonishing amount of brain tissue can be lost from the great cerebral hemispheres without abolishing the state of consciousness, while tiny lesions in the slender axial core of the brain cause irreversible coma. We have known since the 1950s that a small area in the brain stem called the reticular formation is necessary for waking consciousness. When people with head injuries go into coma, it is often because tissue damage at the front of the head causes widespread swelling, choking off the blood supply even to the brain stem. When oxygen supply is lost to the reticular formation the result is coma; and because the nerve centers that control breathing and heart action are located very nearby, death often follows."

- from In the Theater of Consciousness: The Workspace of the Mind by Bernard J. Baars, p. 25-26

Well, no. The reason the reticular formation is important to consciousness is because it moderates the functioning of the intralaminar nuclei in the thalamus:

The Reticular Formation
Address:http://www.geocities.com/medinotes/reticular_formation.htm

Scroll to bottom:

"Control of Consciousness

Ascending projections from the reticular formation terminate in the thalamus, subthalamus, hypothalamus, and basal ganglia.

The functions of most of these are poorly understood, but those to the thalamus seem to be particularly important.

They terminate in the intralaminar nuclei, which in turn project to widespread areas of the cortex.

Activity in this pathway is essential for the maintenance of a normal state of consciousness.
Bilateral damage to these fibres as they traverse or originate in the midbrain reticular formation results in prolonged coma."


In other words, the reticular formation, itself, is neither here nor there, except that it controls the level of activity of the thalamus (specifically, the intralaminar nuclei), which is vital to consciousness.

I'm going to guess that if you or I do more digging into this, we'll find the reason this, sort of. rheostat for the thalamus is located removed from that organ in the brainstem, it is because many other important functions having to do with sleep are also located in the brainstem.

(That whole page on the reticular formation is worth reading, if you can slog through the medical language: it does many, many things. I was particularly interested by the note that, in some cases, a single neuron in the formation may be doing all the work of a particular specific function it performs.)

While it's important to the NCC search to discover brain areas that are necessary for consciousness, such necessary conditions can't give us the final story.

I am sure no one has read to the end of the book yet, but I'm not sure I would characterize myself as looking for the neural correlates of consciousness. (That sounds like it may have subtle philosophical implications that I might not want to endorse if I knew what they were.)

My point was/is: the thalamo-cortical complex is vital to consciousness in a way nothing else in the brain is. I think the most vivid way to get that across is the descriptions of what happens when it is disturbed. Gross, probably clumsy analogy: it's as vital to consciousness as an engine is to a working car. No engine: you go nowhere.
------
The notion of "minimally sufficient conditions for consciousness" assumes a cut off point that I doubt exists. Each thing you subtract diminishes consciousness by removing a stimulus or function. I can't see any termination point (except obvious unconsciousness) as being anything but arbitrary. But maybe you have better insight into what they're looking for.