(My answer? Somewhere between 1 and 86,000,000,000…)

How many consciousnesses do I have? For most people, this is a no-brainer. The average person feels like they have a single, unified stream of consciousness. And for most people, this answer is sufficient. Most people behave like they have a single unified stream of consciousness: a single mind in a single brain in a single body. But one set of experiments suggests the “one-consciousness-per-brain” theory might be overly simplistic.

Patients suffering from split-brain syndrome have little or no direct connection between the right and left hemispheres of their brains. In day-to-day life, most split-brain patients function normally: they appear as if they have a single, unified consciousness. But in some experiments, each hemisphere appears to think independently, implying that there may be not one but two consciousnesses underneath.

I’ll give a brief example of how such an experiment could work (Parfit, “Divided minds”). Each hemisphere is connected to precisely one eye and one hand. Scientists take advantage of this by showing a blue color to the left eye and a red color to the right eye. When asked “how many colors did you see?” both hands write the same message, “only one.” But when asked “what color did you see?” one hand writes “blue” and the other “red.”

In these situations, split-brain patients seem to have two separate streams of consciousness. But in everyday life, patients show no signs of having more than a one. Philosopher Thomas Nagel argues that split brain patients don’t have one mind, but they also don’t have two. According to Nagel, the evidence from split-brain cases questions the idea that we have any specific number of minds at all.

I believe that the boundary of the mind is where information can no longer flow. Most of the time, no such information barriers exist, so people seem to have a single mind. If sections of the brain can no longer communicate, they cease to be part of the same mind. This fits with Nagel’s broad statement that people do not have a single, permanent, unified consciousness. Nagel also thinks that the split-brain experiments do not show evidence of one mind, or two minds, either. But during the split-brain experiments, information is not shared between the left and right hemispheres of the brain. So unlike Nagel, I think that the split-brain experiments do show proof of two minds.

Nagel makes two counterarguments to the idea that a split-patient can have one mind, then two minds during experimentation, and then one mind again. First, he says that there is no evidence for this explanation; it’s just convenient. We have no reason to suggest that a patient’s brain undergoes any anatomical change during experiments. According to Nagel, the idea that the brain has two minds during an experiment is an explanation that fits the experimental data well, but has no supporting evidence. I think Nagel goes wrong here when he asserts that some internal change must have taken place inside the patient to suddenly make a second mind appear. The change isn’t internal– it’s external.

When an image flashes in front of a split-brain patient’s left eye, signals propagate through their visual cortex to their brain’s right hemisphere. The patient’s right brain does some processing and instructs the left hand to do some writing. At no point does the signal traverse the corpus callosum (the band of connecting fibers) to the left brain (Linehard, “Split Brain Experiments”). The interviewer sends inputs through the left eye and receives outputs via the left hand. In this isolated environment, the interviewer converses directly with the right side of the patient’s brain.

While the right brain has its conversation, the left brain has a conversation of its own. By inputting signals through right eye and receiving them through the right hand, the interviewer can have a concurrent conversation with the left side of the brain. But the left shows no awareness of what the right side is doing, and vice versa. Information does not flow between the left and right brain. Thus, during split brain experiments, each hemisphere is an independent mind.

Nagel makes a second counterargument to the hypothesis that split-brain experiments are evidence of two minds. He says that if we could somehow show that the mind dissociates into two for experiments and recouples itself afterwards, this wouldn’t prove the hypothesis, since the patients display behavior of “two minds” and “one mind” at the same time. Throughout the experiment, a patient sits with a certain posture, follows and responds to instructions, and generally behaves exactly like a non-split-brain individual would in such a scenario. How could a mind be split into two and unified at same time?

It’s helpful to think of this as two independent, concurrent experiments. One is the split-brain experiment, where scientists isolate and communicate with each half of the brain individually. The other is the “experiment” of day-to-day life: the two halves of the patient’s brain work in unison to process inputs and produce outputs. Even though the two experiments are happening at the same time, they have different experimental designs. In the first experiment, each half of the brain receives a separate sensory input and each half generates its own independent output (through the hand it controls). In the second experiment, the patient receives input into both hemispheres and produces outputs with both hemispheres (through their total range of motor function). More specifically, throughout the first experiment, information does not pass between the left and right brain, but in the second experiment, it does. This is how split-brain patients can have two minds during the actual experiments but one mind throughout the duration of the study.

Then how many minds do we have? The split-brain case shows that we have two clumps of neurons in our heads, each of which exhibits behavior distinct and complex enough to be considered conscious. But why stop at two? The idea that we can have only up to two minds is a naturalistic convenience. Scientists experiment on patients with bisected cortices because, historically, patients with this condition have been available. How would our view on the topic change if our brains naturally divided into three sections instead of two?

Imagine a futuristic laboratory where scientists have the technology to communicate via electrical signals with individual neurons. Let’s say they also have a way to temporarily deactivate connections between neurons. With access to this futuristic machine, scientists could experiment on all sorts of split-brain architectures. What about trisecting the brain? Or dividing it into a few dozen golf ball-sized chunks? It seems likely that a brain split into three parts could still exhibit three separate consciousnesses. But it’s hard to say if the brain is complex and distributed enough to simultaneously demonstrate 100 separate consciousnesses.

Split-brain patients are proof that each of us has at least two minds. What’s the upper bound? We can be certain that an individual neuron is not complex enough to think. Therefore, the maximum number of possible consciousnesses within a single brain must be smaller than the number of neurons, some 86 billion. Without a rigorous definition of what constitutes a “mind,” we can conclude that the greatest number of minds that can be uncovered in an experimental setting is somewhere between 2 and 86 billion.

In everyday life, it seems to each of us that we have a single, continuous consciousness. But that consciousness is divisible. When isolated, individual sections of our brain can exhibit consciousness of their own. Patients with split-brain syndrome show evidence of two minds, each with its own perceptions, interests, and desires. It’s possible that future experiments will show evidence of three or more separate minds, which all work together in everyday scenarios. Perhaps each of us has thousands, or millions, or even billions of centers of consciousness, all within a single brain.

Sources

1. Nagel, Thomas. “Brain bisection and the unity of consciousness.” Synthese (1971): 396-413.
2. Parfit, Derek. “Divided minds and the nature of persons.” Science Fiction and Philosophy (2016): 91-98.
3. Linehard, Dina A. “The Embryo Project Encyclopedia.” Roger Sperry’s Split Brain Experiments (1959–1968) | The Embryo Project Encyclopedia, The Embryo Project Encyclopedia, 27 Dec. 2017, https://embryo.asu.edu/pages/roger-sperrys-split-brain-experiments-1959-1968.

This essay was written for “Minds, Machines, In Persons” class, taught by Prof. Zachary C. Irving, in Fall 2019.