Dual Consciousness and the Split-brain

Adam Mehdi
12 min readSep 3, 2021
Many brains, many faces? [1 source]


Last post reasoned using the the almost-whimsical teletransportation thought experiment. Here, we will reify that rarefied philosophy with the real — yet oh so strange — corpus callosotomy surgery, and in doing so, plumb deeper into the philosophy of mind and metaphysics of identity.


Patient P suffers from epilepsy, intense seizures.[2] His doctor exercised every tool in her conventional armamentarium to no avail: the ketogenic diet didn’t stop the seizures; the nerve sedatives reduced the intensity but not enough; the vagus nerve stimulation worked at first, but its effects leveled off. But then, she cogitated, there is that highly nonstandard procedure so voluminously described in the neuropsych textbooks…

Corpus callosotomy. A surgery that partially or completely severs the corpus callosum, the only (known) neural bridge between the left and right brain hemispheres. Without it, the two hemispheres presumably cannot communicate; but with it, electrical discharge spans the entire brain, giving rise to seizures. The doctor resolved to administer the corpus callosotomy. And she did.

Patient P woke up from the surgery feeling normal, not like his brain has been split in two. His doctor performs some post-surgical examinations on him. (Note: All of these describe real experiments.)

  • She asks, “Can you stand up?” He does, flawlessly.
  • “Can you button your own shirt?” He buttons halfway up, but then, his left hand begins to unbutton the shirt from the bottom — alien hand syndrome.[3] He wages a War of the Warlocks with the sinister hand, and after a minute leftie calms down.

The doctor then brings him to a computer screen which displays a cute, smiling bear on the left side of the screen, and fixes his gaze so that the bear remains in his left visual field.[3] She asks:

  • “Can you name the emotion?” Patient P says that he doesn’t see anything.
  • “Can you mimic the emotion on your face?” The left side of his face curves up into a grin as the right side continues to express a marmoreal indifference.

Lastly, she replaces the bear with a moving dot on the left side of the screen.

  • She asked whether the object on the screen is fixed or moving. Patient P asserted it was moving.

The doctor examines the report. The hemispheres are definitely not connected at the corpus callosum, but they continue to exchange low-level information like basic movement as evidenced by the moving dot question… probably because of subcortical connections. Normal side-effects for a full corpus callosotomy, the doctor concludes.[4]

Though not used as widely nowadays due to its spooky side-effects, the operation really works. Patient P’s seizures alleviated fully. And as usual, alien hand syndrome went away after a short time.

During the next week’s checkup, P reported feeling completely normal, and his family described him as the usual, loving papa P, but without the seizures. Then he performs one final examination. The patient fixes his gaze on a computer screen. A grilled chicken breast appears on the right side, and driveway tiled over with snow is displayed on the left (use imagination!). The doctor asks the patient to turn around with eyes closed and then examine an array of objects arranged on a table: a chicken doll, power drill, dog bone and mini snow shovel. He is to pick the objects that correspond to the pictures. He picks up the chicken doll in his right hand and grabs the snow shovel with the left.[5]

The doctor asks why P made his selection. P responds that the chicken doll goes with the chicken breast, and the shovel is to clean up the chicken coup. The doctor pencils a note: “Textbook case of left-brain interpreter. Normal side-effects.”

What happened here? The left brain hemisphere controls language. So, anything we hear from P comes from his left hemisphere. And yet the right hemisphere can also understand instructions, and it controls the left hand, so it is the one who picked the snow shovel, based on the image of the snowy driveway on the left of the screen. The left hemisphere, blind to the left visual field, does not see the image, and yet it asserts a false explanation about the right hemisphere’s behavior. Does it not seem that there are two conscious entities at battle here, where the left brain hemisphere battles and conquers the right brain hemisphere with its control of language?

Or so the theory of dual consciousness goes. This is not settled ground — for instance, alien hand syndrome is seen in stroke patients, so split-brain behavior need not be explained with consciousness. However, I will henceforth use dual consciousness as a thought experiement, which do not presuppose truth but rather logical consistency, so I can tackle the paradoxes on which it casts light.

I will resolve two of these problems here: one concerning identity, one epistemological.

  • If there are really two consciousnesses in P’s head, which one is P?
  • Why does the left hemisphere give explanations to behaviors it does not understand?

Draft your explanation for each question before I dive into them!

Draft your explanations with any tools at your disposal — pencil,

notepad, mirror. [source 12]



The first question asks whether the original P survives as one of his newly-severed hemispheres. That depends on how we see the hemispheres. The left and right hemispheres are separated, so they are surely two different entities. But is each entity a genuine person, or a lower-level being?

I laid out a criterion last article, the Layered Personhood, for exactly such questions.[6] According to it, each hemisphere must be conscious, creative and exhibit psychological continuity to be entities commensurate to people. It is a crude criterion, because it tries to cut through scientific problems we do not have good explanations of yet (e.g. How is consciousness created? What is the difference between creative and noncreative programs? Does a creative system presuppose consciousness?). However, we can test it by applying the conception to this problem, seeing if it makes sense. And if it does, we will have unfurled the paradox. Let’s apply it.

Earlier, we assumed the dual consciousness interpretation of the experiments, that each hemisphere is a momentary experiencer. Whether each hemisphere has the same consciousness as the original P is an open question, but in the least, we have assumed that each hemisphere are conscious entities as the premise of the thought experiment.

How about creativity and continuity? The left hemisphere quite clearly harbors these abilities. The left hemisphere creates new explanations in the chicken-shovel experiment; it interprets P’s selection of a shovel with the clearly creative, albeit false, idea that it is to “clean out the chicken coop.” And for continuity, P reported (so this is his left hemisphere talking) that he does not feel like his brain is severed in two. His family also did not perceive any substantial difference between pre- and post-callosotomy P. The plausible explanation is that P, or at least his left brain, retains psychological continuity through the surgery.[8]

Since his left brain is conscious, creative and continuous with him, the original P shares everything that matters for survival with his left brain. P continues in the left brain.[9] You might think they also have the same identity — that they are the same person. But a subtle, unintuitive point lurks in these waters: whether pre-surgery P shares the same identity as his left brain depends on whether the right brain is also creative and continuous with P.

If both the right and left brains are psychologically continuous with P, the problem is identical to that of the nonvaporizing teletransportation.[10] Identity requires uniqueness. To see why, suppose the right hemisphere is the same person as pre-surgery P, as well as the left hemisphere. Each presumably have the identity of P. But the left and right hemispheres are obviously not the same entities. So they can’t both have the same identity as P. This means there cannot be two entities of the same identity (the left and right hemispheres) coexisting, or else the identity breaks down.

But picture yourself in this situation. Imagine you are the left hemisphere of P, wanting to know your identity — whether you are the same P before the surgery or a different entity. How could you go about investigating? Determining the nature of the right brain, whether it is conscious, creative and continuous with P. If it is, the left and right hemispheres would have equal claim to P’s identity. Since we can’t privilege one over the other, and both cannot be P by definition, identity logically breaks down. You, as P’s left brain, do not have the same identity as P. In fact, P’s identity is not even inherited. As a matter of identity, P is not the same person coming out of surgery as he did coming in.

But all this is so only if the right hemisphere turns out to be creative and continuous with P. Let’s investigate whether that is plausible.

Here is where we are in our investigation.

The experiments thus far presented do not reveal enough about the right brain for a grounded explanation. Fortunately, another experiment does the job.

A doctor asks a split-brain patient what he wants to be in his life, and the left hemisphere asserts that he will be a “draftsman.” To access the right hemisphere, the patient was then asksed to spell out what he wants to be with his left hand. He penciled “automobile race.”

There can even be conflicting life goals between each hemisphere — draftsman versus racecar driver — because the right hemisphere is capable of its own interpretation of P’s memories. A new interpretation requires creativity. Also, the right hemisphere can only interpret P’s memories if it has access to those memories. From what we can see, the right hemisphere is both creative and psychologically continuous with P.

Being inherited by two coexisting entities, P’s identity breaks down. To restore it, each hemisphere is incented to destroy the other. Or, if they don’t take a fancy in destruction, each hemisphere should have preferred a different person on the other side of the P’s skull. Don’t you think this is absurd? I think this is absurd.

We have two options. We can either accept that absurdity or reach for the opposite horn of the dilemma: identity is just not what matters. Following Derek Parfit, it does not matter whether P before the surgery is the same person as some entity afterwards, only that his person is continued, i.e. an entity inherits his conscious, creative and psychological person. Whether that entity is him is irrelevant. The concept of numerical identity, as a basis for survival and personhood, is bankrupt.

Though if taken to be external, identity is not bankrupt per se. Remember that P’s family said P seemed the same person after the surgery? The same, loving papa P? That’s his social identity, solidified in the presence of his family. The identity of P, qua social construct, survives.[11] Note that this social identity is external, only born from being placed in social relations. As such, it should not be glommed or cathected to. Our social identity is not something we have, but rather something (necessarily) placed on us by others (though it is our default mode to identity with our social identity, which is in the form of a narrative).

The Layered Personhood conception threw light on the split-brain identity problem. Neither hemisphere is P. But it shouldn’t be said that P himself was destroyed, for his person continued through each hemisphere, both sharing P’s conscious, creative and psychological qualities. Yes, P’s numerical identity was destroyed since it was inherited to two entities, but that is nothing to worry over, and his social identity perpetuated.


To me, one aspect of the experiments I recounted is most spooky: The ability of the left hemisphere to interpret the behavior of the right, and be so wrong at it. For example, the right hemisphere picks a snow shovel because it corresponded to a snowy driveway, but the left grafted its own explanation — the shovel is to clean the chicken coup. My second question asked why the left hemisphere acts like so. I won’t give a scientific theory but an epistemological account, an explanation for the behavior shown in the experiments, grounded in what we (critical rationalists) know about the process by which persons acquire knowledge.

That process comprises iterated guesswork and testing. P’s left brain observed a snow shovel in P’s left hand. Then, it guessed an explanation — many explanations — to try to explain the shovel. It tested each of those explanations against the information it has — that the snow shovel is there but no insight into how it came to be due to its lack of access to the right hemisphere’s information — and rejected those blatantly conflicting explanations. It continued the creative process: mashing, modifying and varying explanations and testing them against experience. Until, the left hemisphere reaches one without observable error. And truly, given the scant information accessible to the left hemisphere, its explanation is remarkably perspicacious, or at least obviously creative.

A sketch on the process of creativity: Varying explanations until reaching one without observable error.

The left brain’s explanation was that “the shovel is the clean out the chicken coop”.

This is a rough sketch of creativity, but contains broad brushstrokes in many areas. Creativity is still a mystery (if we understood it we would be able to program it); however, this is sufficient to render the puzzle less puzzling.

If P performed the examination before his callosotomy, he would have explained his behavior accurately: the chicken head goes with the chicken coup and the snow shovel is to clear out the snowy driveway. This is because the full-brain P would have had access to the right brain’s explanations and would have therefore rejected the left’s bad explanation. So, severing the corpus callosomy seems to have confined the environment in which subliminal theories are tested, and yet each unit (left and right brains) continue to generate explanations. What does this imply about the brain as a whole? The brain generates conjectures in many localized units in parallel and tests them against observation globally. From that process, creativity — the capacity to create new explanations — somehow emerges.


[1] An image created by a neural net optimized for trees. See Gabriel Goh et al. “Multimodal Neurons in Artificial Neural Networks.” (Distill Pub, 2021.)

[2] It seems to be a convention in philosophy to conjure up the most unimaginative, uninspired names possible. I’m following it. Hence “Patient P”.

[3] A left/right visual field is the left or right half of your range of vision. If your sight were a computer screen, the left visual field would be the left side of the screen with a boundary line drawn at the middle. The left brain hemisphere has access to only the right visual field, and the right has access to the left visual field. Also, the left brain hemisphere has access to the right side of the face and our language capacities. The right brain hemisphere controls the left side of the face and cannot speak, although it exhibits understanding. See Wikipedia contributors. “Dual consciousness.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 2021. (same article cited in [5]

[4] All of the examinations and responses were descriptions of real experiements, with a couple whimsical details grafted on (namely, a smiling bear). The report was also drawn from research from real experiments. My reference is the following survey on the split-brain literature:

Haan et al. (2020). Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness, Neuropsychology Review.

[5] This is the Gazzaniga experiment. See Wikipedia contributors. “Dual consciousness.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 2021.

[6] See the previous post for the full argument, but a compressed version of the Layered Personhood conception is as follows. It asserts that people are such in three layers — their conscious, creative and continuous qualities. We must consider people in the context of each layer, and two people are the same if they are sufficiently similar in each layer. And it presents a corrolarry: the three layers are the only aspects about a person that matters, and since identity is not one of them, identity is not what matters. Instead, it is external, a consequence of social relations that is important but not intrinsic to personhood.

[7] See “Dual Consciousness” (Wikipedia, https://en.wikipedia.org/wiki/Dual_consciousness).

[8] For the experiment’s paper, see LeDoux JE, Wilson DH, Gazzagniga MS. “A divided mind: observations on the conscious properties of the separated hemispheres.” Ann Neurol. 1977.

[11] This concept is the anthropological theory of identity. Some philosophers try to use the concept as a basis for a unique, numerical identity. However, I think it is clearly external to onelself and should be treated as such. Shoemaker, David, “Personal Identity and Ethics”, The Stanford Encyclopedia of Philosophy (Winter 2019 Edition).

[12] The image was generated by DALL-E, OpenAI’s text-to-image model of 12 billion parameters. OpenAI 2021 (“DALL·E: Creating Images from Text.”)



Adam Mehdi

Thinking about AI & epistemology. Researching CV & ML as published Assistant Researcher. Studying CS @ Columbia Engineering.