Split-Brain: Two Selves in One?

We tend to perceive ourselves as something unified and immutable—whether we call it our soul, essence, or self. Yet, whenever we try to explain it, the questions start. Is it just perception itself? Could it be composed of several elements or even just be an illusion? There has been little evidence to support any theory or question surrounding the self, which is hardly surprising since the debate has mainly revolved around our introspective intuition and thought experiments. One exception to this is the curious case of split-brain patients.

The so-called split-brain comes from subjects who have undergone a corpus callosotomy. During this surgical procedure, you sever the corpus callosum, a major connective tract between the two hemispheres. This was typically done to patients who suffered from frequent grand mal seizures to prevent the spread of the seizures across the hemispheres.

The corpus callosum is the largest white matter structure in the brain, yet there was little indication of it to the physicians and researchers. Patients were seen as surprisingly normal immediately after the procedure.

Testing the hemispheres separately could be done because each brain hemisphere primarily accounts for one-half of the body. For example, the left hemisphere (LH) accounts for the right half of the visual field and controls the right side of the body (brain science is never actually that clear-cut and straightforward in reality, but we will return to that).

Testing the Split-Brain

Through this targeted testing, they observed that the split-brain subjects’ perceptions and actions were more split than normal between hemispheres (one of the times an academic term is colloquial and sufficiently descriptive).

However, it is still clearest if illustrated with a prototypical example:

For most right-handed people, the left hemisphere (LH) is where most of the language areas are located. So, when split-brain subjects were presented with information only in their right visual field, they could report it verbally. So far, everything seems normal.

The oddities start when the information is only presented to the right hemisphere (RH). The subject would then claim to not see anything at all. When asked to use their left hand—which is primarily controlled by the RH—to scribble down what they saw, they did so, even while they verbally maintained that they didn’t know what was on that side or that there was nothing there (Sperry 1968).

It is as if there were two agents in one brain, one verbal and one non-verbal. The perceiver and actor we behaviorally interpret as a self appear to have been split into two. To add to the doubt this experiment puts on our sense of self, when asked why they (the RH) scribbled down what they did, they (the LH) would verbally post-rationalize. For example, as to why they scribbled a face, the subject would explain, “Oh, I saw one earlier” or “A smiling face is better than a frowning one” (Wolman, 2012).

So even when presented with this apparent split of their perception-action, they still try to maintain a unity of self through rationalization.

Early on, researchers were puzzled by how someone seemingly so normal could also act as if two. The notion of split-brain subjects as extraordinarily normal has carried on into contemporary textbooks.

However, while the physicians and the neuropsychologists were impressed by how “normal” the split-brain patients seemed in conversation and interaction (perhaps not that surprising considering it is most likely just an interaction with the language-capable hemisphere, LH), later accounts and a field study show a different story.

While talking about goals, beliefs, plans, etc., there may appear to be no problem, but acting them out is a different story. Split-brain subject’s daily lives appear to be wrought with indecisions and interruptions.

For some patients, simply dressing can consistently require 1-2 hours, grocery shopping or preparing a dish may take several hours and the actual completion of the intended task is not assured at all. Events like deciding to get eggs but ending up with milk or delivering a letter someplace on the way instead of the intended target. These are not one-off mistakes of an overworked mind; instead, they are constant (Ferguson, Rayport, and Corrie, 1985).

The Struggle for Unity

Based on these observational studies, this split has produced much competition and poor integration between the hemispheres, disrupting the continuity and unity of self.

This specific lack of integrative capacity between the two hemispheres can be further affirmed through experimental evidence as well.

When presented with the two parts of a compound word (two words that combine to make a different word, like sky and scraper = skyscraper), where each composite part is separated into each visual field, split-brain subjects have difficulties obtaining the compound word, but instead give the two words separately.

Even more difficult is comparing across the hemispheres, even just to see if the two targets are the same or different. This difficulty stands even for split-brain patients who are capable of answering verbally and with both hands for both visual fields (Pinto et al., 2017a). This indicates that the problem is specifically the cross-hemispheric integration of information.

Seemingly, the integration from the corpus callosum through inhibition and excitation helps shape the two hemispheres into unified action and belief (de Haan et al., 2020).

The theory that accounted for this and became prevalent in neurophilosophy was underlaid by the neuroscientific idea of hemispheric lateralization. The hemispheres are semi-independent, parallel, but specialized processors, and it has been shown that it is possible to operate well on a single hemisphere, even when the hemispherectomy is done in adult age (Liang et al., 2013). Evidence like this shaped a conceivable reality of dual consciousness.

However, being able to split consciousness by severing the corpus callosum may not sound like an appealing proposition. We could also take dual consciousness to be that we are always two due to the structure of the hemisphere, but that the corpus callosum makes it a unified, integrated mind.

This seeming science fiction idea impacted both neuroscience and neurophilosophy and two of the most popular theories of consciousness, integrated information theory and global workspace theory, have taken inspiration from it. With all that said, it is not like dual consciousness is not hotly disputed.

Especially so, as lateralization and neuroscientific evidence in all its splendor, what primarily convinced people of some sort of dual consciousness or self was the troves of anecdotal evidence of subjects acting as if with one mind but two consciousnesses/agents.

Stories of the Split: Anecdotal Insights

The anecdotes, transcripts, and videos of split-brain patients tell a more compelling story than any experimental evidence. There are even several accounts of researchers not reporting things because they sound too outlandish to be believable (Schecter, 2018).

One example is Mackay and Mackay (1982), where they essentially got a split-brain subject to play a game of guessing with himself. Where the RH was presented with a number, and the LH would guess, with RH indicating if the target was higher, lower, or correct. Seriously playing a game of hidden knowledge with yourself is frankly hard to conceive of outside of any multiple-agent claim.

These sorts of anecdotes can be found in most split-brain studies, especially the earlier ones, and most of them tend to indicate a duality of self. In fact, it is difficult for researchers who work with these subjects to even talk about them without employing a language implying two agents (Schechter, 2018).

While this is how it seems externally—internally, there is no evidence that the two hemispheres consider the other a separate self. They do not communicate explicitly or refer to the other as an agent. Supporting this, nothing in introspective reports by split-brain subjects indicate that they consciously feel different than “normal” and they certainly don’t feel like they are two.

Though they still, at times, act as if they recognize the other as at least an agent, interactions like slapping the left hand for trying to interfere while doing a task to stop it for a while (Bogen, 1987) make no sense unless one sees the hand as operated by something intelligent; it also wouldn’t make sense if one saw it as oneself.

Where have we ended up after more than 50 years of split-brain research? Since callosotomy is a very uncommon measure against epilepsy nowadays, we may be looking close to the end of this specific line of inquiry into the nature of self.

Notwithstanding, the debate was recently reignited by a new set of studies.

Pinto, de Haan, and Lamme (2017) showed that there is significant integration between the hemispheres, which allows them to judge if lines are perpendicular, find the largest circle, and track dot motion across the visual fields. The split-brain patients had largely no problems answering with either hand or verbally in both visual fields. Pinto and colleagues took this as evidence of split perception but under a unified consciousness.

The difficulty of a unified mind theory is explaining why it still can’t directly compare across the hemispheres or integrate higher-order stimuli rather than simply visuospatial ones.

Their study also showed the limitations of these case studies, as it made clear that the age of operation and the interval between it and the experiment may be a major factor in how disunified the mind is. Ostensibly, the brain could bridge the gaps through behavioral and environmental help improving over time (like cross cueing), new integration plasticity, or increased employment of subcortical integration (de Haan et al., 2020). To underline how much can change over time, some subjects even appeared to have developed rudimentary speech in the right hemisphere (Gazzaniga et al., 1996).

All these factors make it very difficult to get clean results and interpretations without controlling them and problematizing previous findings.

So, while it didn’t provide a slam dunk for unified consciousness, what it did do was further diminish the support by experimental evidence for dual consciousness. Even the prototypical example that introduced this article, which is a very specific version of an experiment, is merely anecdotal. It has never been done as an actual peer-reviewed experiment; it is more assumed to be true based on anatomy, animal studies, and anecdotal evidence supporting it. Pinto and colleagues’ results, at the very least, question whether it should be.

In the end, it essentially leaves us with three major theories. Two consist of a dual nature, either dual consciousness or dual agents (conscious and unconscious), and one is of a unified consciousness but split perception.

The problem with the dual consciousness claim is that it is difficult to test—or rather, it is incredibly hard to test any consciousness claim. But it is mainly supported by anecdotes. If one could tap into the anecdotal oddities on an experimental level, dual consciousness would have a much stronger argument.

Dual agent claims run into the problem that assuming RH is unconscious is difficult to reconcile with what it can do, seemingly stretching the perceived limits of unconscious capabilities. However, knowing exactly what consciousness is is necessary to define the unconscious.

Lastly, unified consciousness and split perception, what it struggles with is the observation and anecdotal evidence—subjects not being able to answer verbally what was on the left side when the right side was not presenting anything, yet could still answer with their right arm. Pinto et al., 2017, claimed that subjects initially claimed to not see anything at first—similar to previous findings—but by their methodology, they managed to change that.

This seemingly indicates something more akin to a switch consciousness, sort of tapping into the split perception sequentially at the correct prompting, like a perceptual illusion. Pinto and colleagues themselves explain it as more of a desynced consciousness, akin to watching a movie with an off-timed audio track, which makes the two impossible to integrate.

As Nagel, 1971, argued more than 50 years ago, when it comes to the split-brain—which appears to us as true now as it did then—whatever interpretation we make seems unacceptable.

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