The Prisoner Inside Your Skull

The brain sitting inside your skull right now has two halves: the left hemisphere and the right hemisphere, as well as some stuff near the brain stem responsible for basic functions.

<sidenote> My rule about “what actions come from what brain areas”: if a zombie can do it, it’s probably controlled from the brain stem (hearts, lungs, motor control), if only you can do it (math, love, constantly thinking about that one embarrassing night), it’s probably the cerebrum - the “brainy” part. </sidenote>

Today I want to talk about the corpus callosum – the C-shaped little bridge of neurons that bridges the gap between the two hemispheres. It’s made up of a few hundred million neurons that are responsible for sending information from one hemisphere to another (oversimplified). Each hemisphere is made up of some 10-12 billion neurons in the cerebral cortex. This 60:1 ration between the 'thinking' neurons of each hemisphere and the 'messenger' neurons of the corpus callosum is about equivalent to the communication between Houston and Chicago being handled exclusively by the population of Bozeman, Montana .

A procedure to cut the corpus callosum down the meridian and thereby severing the communication between the brain's hemispheres is often employed as a form of palliative care for people suffering from severe seizures due to epilepsy. To help explain this form of patient care here’s a jargon-y sentence from Wikipedia (average word length is 7 characters):

“The role of the corpus callosum in epilepsy is the interhemispheric transmission of epileptiform discharges. These discharges are generally bilaterally synchronous in preoperative patients. In addition to disrupting this synchrony, corpus callosotomy decreases the frequency and amplitude of the epileptiform discharges, suggesting the transhemispheric facilitation of seizure mechanisms.”

Basically, during a seizure, an electrical firestorm is happening (epileptiform discharges), and not only does cutting this highway between the hemispheres localize this disruption, it actually reduces the severity and makes the seizures happen less often. This procedure is called a corpus callosotomy.

Not only does a callosotomy localize seizures, but it reduces severity and frequency

The neurological cost of this procedure was studied by a man called Roger Wolcott Sperry beginning in the 1950’s. At first, there were not many observable differences noted in so-called “split brain” patients who had undergone this callosotomy. Personality, emotionality, or intelligence didn’t significantly change noticeably from pre to post-operation. Upon looking closer, Sperry did begin noticing differences when presenting different information to different parts of the patient's visual field.

Before moving on, some quick brain anatomy: The visual cortex is located along the back of the brain, and spans across both hemispheres. Interestingly, the nerves from each eye cross over on the way to process information collected by your eyes. The information from the right part of your visual field gets processed in the back left, and the information from the left part of the visual field gets processed in the right hemisphere. Further, language processing mainly occurs on the left hemisphere of the brain. Highlighted in red in the lower animation is Broca's area, which processes language. So essentially, both hemispheres can see and think, but only the left hemisphere can talk.

Okay, back to Sperry's experiments. Sperry observed that in patients who have undergone a callosotomy, occasionally their hands would seem to disagree with each other – one hand would try to put a shirt on, while one would try to take it off. The right hand might be raised in violent intent, only to be restrained by the left hand. This points to something very deep, and very spooky– the right hemisphere has different ideas about whether one should put a shirt on and go to work, or whether one should use violence to get ones way. This ‘difference of opinion’ points to what can only defensibly be called a separate... being? conciseness? person? personality? - potentially with different values and opinions. Sperry himself put it this way:

“[The right hemisphere] is indeed a conscious system in its own right, perceiving, thinking, remembering, reasoning, willing, and emoting, all at a characteristically human level, and … both the left and the right hemisphere may be conscious simultaneously in different, even in mutually conflicting, mental experiences that run along in parallel.”

Sperry performed experiments in which separate hemispheres were presented with different visual information. One could sit a split brain patient in front of a pile of objects and show the right hemisphere text that reads “KEY,” and the left hemisphere text that reads “RING.” The results are processed differently in the different hemispheres:

Right hemisphere, observing the left visual field,

and controlling the left half of the body,

without access to speech processing:

The text reads “KEY.” I’ll use the left arm to pick up the key.

Left hemisphere, observing the right visual field,

controlling the right half of the body,

but with access to speech processing:

The text reads “RING.” I will say “Ring.”

When the patient is asked by the researcher to pass the object in their left hand to their right, and to tell the researcher why they picked it, the processing in the left hemisphere might look like this:

 Left hemisphere: I’ve been handed a key. The text reads “RING.” (Using speech center)

“Maybe you need a key to unlock the box the ring is in.”

Remember - without the ability to communicate through the corpus callosum, the left hemisphere has no idea what has been presented to the right hemisphere, and is totally unaware of the word KEY. The left hemisphere therefore confabulates an answer to the researchers question, using only the information it has.

Another variation of this experiment could be presenting a snowy scene to the right hemisphere, and a chicken coop to the left hemisphere. Among the pile of objects is a shovel. The right hemisphere would look at the snow and pick up a shovel, hand it to left hemisphere who might say “the shovel is for cleaning out the chicken poop.”

All this means that while you're reading this your left hemisphere might be having sort of a mind-blowing moment, while the mute right hemisphere might be rolling its eye and thinking “I can't believe that this is just dawning on them now.”

When I first read about the duality of our consciousness a few years ago, I thought something roughly akin to “that is an interesting little tidbit of information,” but upon reading further into it, and considering the implications, gravity, and spookiness of it started to dawn on me:

At all times, there is a mute consciousness inside your skull – listening, seeing, emoting, thinking, counting (up to about 20), all the while playing along and dutifully controlling the left part of your body. When you were young, before speech developed, the two “you’s” might have seemed pretty equal. Righty was better at spatial reasoning, and lefty was a bit better at counting. After a bit, lefty learned to talk, but righty couldn’t figure that one out, and remained mute as speech became the primary way to communicate with others. So what about cutting the corpus callosum allows the right hemisphere to start asserting itself and occasionally disagreeing with the actions taken by the left hemisphere? Why is this behavior never seen in preoperative patients? Does left brain assert control over right brain?

Also, this isn’t some theory, hypothesis, or speculation – it is considered well-evidenced by top physiologists and neurologists that you harbor two conscious entities inside your skull, one unaware that the house isn’t empty. Sleep well.

-          Scott

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