Split Brains: Why disconnected hemispheres won't be sending independent emails


I recently became interested in Split Brain (severing the Corpus Callosum) in regard to consciousness. I was stunned to hear that a left hand might actually fight with a right hand indicating that the two hemispheres of the brain may have their own consciousness yet the right hemisphere lacking the ability to speak (since speech is in the left hemisphere). I then thought about how the right-brain might be able to communicate without speech and the most obvious thought was in typing. Since the right hemisphere can see the left field of vision, it should be able to type out its own thoughts and feelings even simultaneously with the left hemisphere typing out perhaps completely different thoughts and feelings with the right hand! A quick search yielded no experiments with split-brains and typing. Have any such studies been done? Can Split-Brains even type up an email with both hands? I’m extremely curious to know!
— Sean

In order for the left and right brain to each type out their own thoughts and feelings, each hemisphere must have the capacity for language. As you point out, the left brain usually has control over speech, while the right brain typically lacks this ability. Yet, in order for the “silent” right brain to reveal its thoughts and feelings through typing with the left hand, as you propose, the right brain must have capacity for language processing and expression even while it lacks motor control of speech.

There were some very cool experiments in the late 1970s with a teenage split-brain patient, “J.S.”, that first suggested that the right brain can have some access to written language even while it lacks control of speech (1, 3). In one experiment, scientists asked J.S. “who is your favorite girl?”, by verbally asking him “who is your favorite blank” and showing the written word “girl” only to the left visual field (and thus only to the right hemisphere of his brain). While the boy first shook his head and shrugged, as if he didn’t see any word, he was then able to take Scrabble tiles with his left hand and spell out the name “Liz”, with a slightly embarrassed giggle (3, 7). J.S. could also use his left hand to spell and write out the names of objects that were shown only to his right brain (1). Together, these experiments showed that J.S.’s right brain had access both to some language and to its own abstract thought.

After work with J.S., experiments demonstrated that in at least some split-brain patients, the left hand could use a pen to write out, or tiles to spell out, information from the right hemisphere, suggesting that these patients could also type out information from their right brains with their left hands (6). But, can the left hand write out a whole email complete with thoughts and feelings from the right brain, perhaps while the left brain writes out a separate set of views with the right hand? Probably not.

Though the right brains of several patients have the ability to name objects by spelling or writing with the left hand, the right brain generally isn’t that sophisticated in using written language. For example, when written words were shown to one hemisphere and patients were asked to pick a response card from a set of four that was related to the flashed word by a given rule (i.e. opposite: day – night), the right hemisphere of many patients had trouble getting the word right (6). From these experiments and others like it, scientists have concluded that even though the right brain can have some language capacity, this capacity isn’t that advanced. While the right brain can sometimes have its own “dictionary” and associate words with meanings, writing whole sentences is usually beyond its capabilities (2).

So, a split brain patient couldn’t type a separate email with each hemisphere. But could he or she even use both hands to type an email being viewed by both eyes on a standard screen? Again, probably not.

When we type with both hands, each hand contributes letters to a single word, requiring that there be coordination between the right and left sides of our brain and body. This is exactly the role of the corpus callosum. Severing the corpus callosum prevents the performance of sophisticated bimanual tasks, such as using keyboard to type a sentence, because information required to coordinate movement of the left and right hands can’t be transferred from one hemisphere of the brain to the other. We typically use both eyes to read the screen of a computer, and thus both the fluent left hemisphere and the inarticulate right hemisphere of a split-brain patient would have access to the screen. But, because of the split, no directions from the fluent left half of the brain can pass to the right. Overall, it would probably be a lot easier for a split-brain patient to type with just one hand, the right hand, or use voice or handwriting recognition devices, when he or she wanted to write an email.

A final note: Split-brain patients are a spectacular example of how the study of an extremely small cohort of rare individuals has yielded fascinating insight into the organization of the mind. The number of these patients, who have had the connection between the left and right hemispheres of their cerebral cortex completely severed as treatment for epilepsy, has never been greater than a dozen, and improvements in epilepsy treatment have reduced the frequency of this already rare procedure (7). Though this condition does occasionally occur with stroke or other localized injury to the brain, it is rare for such injury to be limited to just the connection between the two hemispheres. While studies involving patients with surgically split brains have yielded insights into how we learn from patterns and how we make moral judgments (4,5), as the few remaining surgery patients age, the window of time to learn from them is closing quickly.


1. Gazzaniga MS, LeDoux JE, Wilson DH. Language, praxis, and the right hemisphere: Clues to some mechanisms of consciousness. Neurology 27, 1144-1147 (1977).

2. Gazzaniga MS. Cerebral specialization and interhemispheric communication: Does the corpus callosum enable the human condition? Brain 123, 1293-1326 (2000).  

3. LeDoux JE, Wilson DH, Gazzaniga, MS. A divided mind: observations on the conscious properties of the separated hemispheres. Annals of Neurology 2, 417-421 (1977).

 4. Miller MB, Sinnott-Armstrong W, Young L, King D, Paggi A, Fabri M, Polonara G, Gazzaniga MS. Abnormal moral reasoning in complete and partial callosotomy patients. Neuropsychologia. 48, 2215-20 (2010).

5. Roser ME, Fiser J, Aslin RN, Gazzaniga MS. Right hemisphere dominance in visual statistical learning. J Cogn Neurosci. 23, 1088-99  (2011).

6. Sidtis, JJ, Volpe  BT, Wilson DH, Rayport M, Gazzaniga, MS. Variability in right hemisphere language function after callosal section: evidence for a continuum of generative capacity. Journal of Neuroscience 1, 323-331 (1981).

7. Wolman, D. A Tale of Two Halves. Nature 483, 260-263 (2012).