A unique recent case of conjoined twins having a neural bridge connecting their brains raises some philosophical questions concerning mind sharing and the mind-body problem. From the article by Dominus in NYT (she personally spent 5 days with the family):

"Their brain images reveal what looks like an attenuated line stretching between the two organs, a piece of anatomy their neurosurgeon, Douglas Cochrane of British Columbia Children’s Hospital, has called a thalamic bridge, because he believes it links the thalamus of one girl to the thalamus of her sister... When they were 2 years old, he performed a study in which Krista’s eyes were covered and electrodes were glued to her scalp. While a strobe light flashed in Tatiana’s eyes, Krista was emitting a strong electric response from the occipital lobe, which is where images are assembled. The test also worked when the girls switched roles... The results of the test did not surprise the family, who had long suspected that even when one girl’s vision was angled away from the television, she was laughing at the images flashing in front of her sister’s eyes.".

So there is good evidence that the brain bridge at least allows them to share "qualia". If a natural brain bridge can form it is likely that artificial ones can eventually be constructed also. If a researcher can observe external images of brain activity simultaneously with "qualia" in the mind wouldn't it make interaction between physical and mental subject to empirical study? Is it challenging to explain from dualistic mind-body perspective, does it strongly suggest a physicalist view?

The reporter herself is aware of philosophical issues with "sharing consciousness", she quotes Damasio's Self Comes to Mind:

The fact that no one sees the minds of others, conscious or not, is especially mysterious,” he writes. We may be capable of guessing what others think, “but we cannot observe their minds, and only we ourselves can observe ours, from the inside, and through a rather narrow window”.

But there is some evidence, admittedly tenuous, that this mental isolation may also be bridgable:

"Could the girls’ connection go beyond sensory impressions to higher thoughts, thoughts as simple as “I want water” or as complex as “I’m tired of ‘Good Night Moon’”? The family says that the girls often get up silently and suddenly and walk over to, say, a sippy cup, which Tatiana then immediately hands to Krista, who drinks from it... ‘I have two pieces of paper,” Krista announced. The girls sat at a small table in the living room, drawing, their faces, as always, angled away from each other. Each had one piece of paper. So I was surprised by Krista’s certainty: She had two pieces of paper?.. Was Krista using “I” to refer to both her and her sister?.. Do they think of themselves as one when they speak in unison, as they often do, if only in short phrases? When their voices joined together, I sometimes felt a shift — to me, they became one complicated being who happened to have two sets of vocal cords, no less plausible a concept than each of us having two eyes. Then, just as quickly, the girls’ distinct minds would make their respective presences felt..."

Let's say for the sake of the argument that brain bridge does allow mutual access to "higher thoughts" and "I". My impression of some philosophies (e.g. phenomenology, existentialism, Bergsonism) is that they would treat "consciousness sharing", especially via a material link, as contradiction in definition. Is there one, and can such finding be explained away as an illusion, or would it challenge these views?

  • "does it strongly suggest a physicalist view?" In order to disprove physicalism one would need to have different experiences when the brain structures responsible for experiences are identical. From the position of reductionism it does not make sense to talk about mind-body dualism on practice until this phenomenon is observed. – rus9384 Sep 23 '18 at 21:17

I think you can resolve most such potential contradictions by changing the wording around. In some systems, you may say it is "consciousness sharing." However, if that is a contradiction in that system, then it should be valid to think of there being only one "consciousness," which has a highly unique topology.

There could be contradictions if one starts from the axioms that "a human has consciousness" and "conjoined twins are two humans." If one starts from those assumptions, one is forced to call it "consciousness sharing," whether that term fits the model or not.

Most likely, what this suggests is that the most meaningful definition of "consciousness" may not be a binary yes/no feature, but something fluid. Personally I find that definition helpful, but that's probably a good topic for a different question.

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    Sorry, but I do not understand. Let's say reporter's impression is correct and they can merge and unmerge their "I"s, say we even call it a single consciousness that can part and rejoin like a river. How does it make a difference? If it can be done and undone artificially at will it still removes the mental isolation on which ideas about qualia, irreducibility/inaccessibility of the mental, etc. are based. And if it is done via a material link mental would seem to derive from physical. Formal non-contradiction may not be enough, it has to be something that can be plausibly maintained. – Conifold Sep 4 '15 at 0:44
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    @Conifold I think it really depends on how you start thinking about the problem. If you start from the assumption that the mental is purely mental (not material), and that two people cannot share the same mind, then it is trivial to declare that conjoined twins cannot share the same mind. It was actually an axiomatic assumption from the start. However, when dealing with the extreme limits, it is not unreasonable to see things get complicated. Consider an individual with alien hand syndrom, such as is a common side effect of a corpus callosotomy (severing a link in the brain)... – Cort Ammon Sep 4 '15 at 2:21
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    in alien hand syndrome, it is not uncommon for an individual to declare that the hand is controlled by a "stranger," which I think is reasonable to map to "another mind." In fact, the philosophical term they use is "the loss of the sense of agency." However, consider if this was transient (I cannot tell from what I read whether it is ever truly transient, or if that would call for switching the corpus collosum back on, but consider it as a thought experiement). Would it be valid for an individual to consider themselves a "whole I" when in control of their hand, while still using "I"... – Cort Ammon Sep 4 '15 at 2:27
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    in situations where the hand is acting on its own accord? Now granted it is unclear whether the alien hand can qualify as conscious without speaking to it. However, given its ability to accomplish goal related tasks such as "stop smoking," one has to consider the possibility that it should be thought of as a separate mind, while technically sharing the same brain. – Cort Ammon Sep 4 '15 at 2:29
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    The only reason I can see not to treat it as such is if, in fact, one's definitions and beliefs directly prohibit such ideas. In such a case, it may be reasonable to question whether the issue is with the dual consciousness being studied, or with one's own assumptions. – Cort Ammon Sep 4 '15 at 2:30

Stephen Jay Gould has an interesting article on twins - Living With Connections - in his book The Flamingo's Smile: Reflections in Natural History. He concludes the article with the following observation:

With vastly greater rarity, the dividing cells of a fertilized egg begin to separate into two groups, but do not complete the process--and conjoined (or Siamese) twins develop. Conjoined twins span the entire conceivable range from a single individual bearing a few rudimentary parts of an imperfect twin, to superficially joined, complete individuals like Chang and Eng [twins referenced earlier]. Ritta and Christina [twins also referenced earlier] fall squarely in the middle of this continuum. With our modern knowledge of their biological formation, I fear we must reject Serre's [a 19th century biologist who studied twinning] solution, and admit instead that his dilemma cannot be answered.

We inhabit a complex world. Some boundaries are sharp and permit clean and definite distinctions. But nature also includes continua that cannot be neatly parceled into two piles of unambiguous yeses and noes. Biologists have rejected, as fatally flawed in principle, all attempts by anti-abortionists to define an unambiguous "beginning of life," because we know so well that the sequence from ovulation to spermatogenesis to birth is an unbreakable continuum--and surely no will define masturbation as murder. Our congressman may create a legal fiction for statutory effect but thay may not seek support from biology. Ritta and Christina [twins referenced earlier] lay in the middle of another unbreakable continuum. They are in part two and in part one. And this, I am sorry to say is the biological non-answer to the question of the centuries.

If this argument leaves you with an empty feeling after so much verbiage, I can only reply with the paradoxical phase that is, so often, the most liberating response to an old mystery: The question has no answer because you asked the wrong question. The old question of individuality in Siamese twinning rests upon the assumption that objects can be pigeonholed into discrete categories. If we recognize that our world is full of irreducible continua, we will not be troubled by the intermediate status of Ritta and Christina...

Let us value connections. As Dante analogized physical with ideological separation, perhaps we can learn from the indissoluble union of Ritta and Christina that our intellectual world revels in continuity as well.

From the perspective of what are considered normally formed individuals, we have two hemispheres in our brains that think differently. Our corpus callosum unites the divergent hemispheric processes in the two hemispheres into one consciousness. Each one of us has consciousness sharing already. Why should it not be that twins with neural connections have one consciousness?

  • Certainly, there is no problem with answering yes to the title question on some accounts, but how would it affect philosophies that rely on mental isolation for their arguments, such as dualism or transcendental idealism? – Conifold Sep 8 '15 at 0:27
  • @Conifold ...that is the elephant in the room that is not being spoken about...where are the demarcations of an individual consciousness or is there such a thing – Swami Vishwananda Sep 9 '15 at 4:54
  • I don't see a huge problem for dualism. OP quotes, "If... it is likely that artificial ones can... be constructed also." On dualism, even if two brains can be connected artificially, one would not assume that the two connected entities were reduced to a single entity. Why would one assume differently because such a connection happened naturally? Like two pianist sitting at two pianos; if one plays while the other presses the damper pedal, the second piano will produce sympathetic vibrations. This bridge may function similarly, allowing one person to "play both instruments." – Terry Lewis Oct 10 '16 at 20:23

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