I’ve written before about split brain patients, and what they mean for consciousness. Emily Esfahani Smith has a pretty good write up on the experiments and what they showed: How An Epilepsy Treatment Shaped Our Understanding of Consciousness – The Atlantic.
The patients were there because they all struggled with violent and uncontrollable seizures. The procedure they were about to have was untested on humans, but they were desperate—none of the standard drug therapies for seizures had worked.
Between February and May of 1939, their surgeon William Van Wagenen, Rochester’s chief of neurosurgery, opened up each patient’s skull and cut through the corpus callosum, the part of the brain that connects the left hemisphere to the right and is responsible for the transfer of information between them. It was a dramatic move: By slicing through the bundle of neurons connecting the two hemispheres, Van Wagenen was cutting the left half of the brain away from the right, halting all communication between the two.
One new piece of information I got from the article was how doctors originally came up with the idea of solving epileptic procedures by, essentially, cutting the brain in half.
He had developed the idea for the surgery after observing two epilepsy patients with brain tumors located in the corpus callosum. The patients had experienced frequent convulsive seizures in the early stages of their cancer, when the tumors were still relatively small masses in the brain—but as the tumors grew, they destroyed the corpus callosum, and the seizures eased up.
“In other words, as the corpus callosum was destroyed, generalized convulsive seizures became less frequent,” Van Wagenen wrote in the 1940 paper, noting that “as a rule, consciousness is not lost when the spread of the epileptic wave is not great or when it is limited to one cerebral cortex.
If you’ve ever read a write up on the split-brain patient experiments, then much of the rest of the article won’t be new info for you, but if you haven’t, I recommend reading it in full. The descriptions of the experiments themselves get a little awkward, with the left hemisphere of the brain controlling and receiving sensory input from the right side of the body and the right hemisphere controlling and getting input from the left side, but having attempted to describe these experiments myself, I can attest that it’s hard to avoid.
The main results of the experiments were to show that:
- The two hemispheres of the brain of a split brain patient could not communicate with each other, which seems to add additional evidence that the mind does not exist independent of the brain. These people appear to have two separate minds.
- Despite this inability to communicate, split brain patients were, more or less, fully functional, except, as in the experiments, where sensory inputs to their two hemispheres were isolated. This seemed to indicate that the two hemispheres could coordinate with each other by observing what the other side did through its side of bodily perceptions.
In other words, the mind is the brain. And there is no central indivisible command center in the brain; it is a decentralized information processing cluster. The brain itself is as centralized as things get, overall, for cognition.
The split brain patient experiments are pretty mind blowing. As the article discussed, the procedure is rarely performed anymore, so opportunities for new studies are going to be limited, at least in humans. But the opportunity to do those experiments, at least for a few decades, gave us insights into consciousness that every neuroscientist and philosopher of mind must now contend with.