“One individual studied well, and thoughtfully, might enable you to draw conclusions that apply to the entire human species.”

-David Roberts, Professor of Surgery and Neurology at Dartmouth-Hitchcock Medical Center

The fascinating story of the split-brain patient dates back to the 1940’s. You might rightfully ask: “What is a split-brain patient?”


Split-brain patients are individuals who have been plagued by intractable epilepsy — so much so that they were willing to undergo split-brain surgery, which is essentially a procedure that severs the connections between the left and right hemispheres of the brain. This surgical procedure was meant to prevent the spread of seizure activity from its site of origin, thereby controlling the occurrence of debilitating epileptic seizures. The procedure is also known as a corpus callosotomy because the anatomical structure that connects the two hemispheres of our brains is called the corpus callosum, the so-called highway system of information transfer in the brain.

“It was a total shot in the dark.”

– Michael Gazzaniga

The first group that investigated these patients in the 1940’s claimed that there were no significant cognitive or behavioral impairments as a result of split-brain surgery. Fast forward to the 1960’s and along came Michael Gazzaniga, a driven young student at Dartmouth. During his junior year, a Scientific American article on how nerves grow piqued Gazzaniga’s interest, so he wrote a letter to the author, the one and only Roger Sperry, one of the biggest names in neurobiology. In his letter, Gazzaniga inquired about research opportunities — a move he now refers to as a “shot in the dark” — and landed an NSF summer fellowship at Caltech.


Gazzaniga as a student at Caltech in 1963

Sperry’s group at Caltech had been studying split-brain rats, cats, and monkeys for some time, and were observing dramatic effects on behavior, which raised a huge question mark in their minds about why earlier assessments of split-brain humans had not revealed significant post-surgical differences. They hypothesized that surgeries done in the 1940’s had not severed the corpus callosum and anterior commissure completely. Gazzaniga was thus tasked with coming up with novel and better ways of testing split-brain patients. So he did…

And his findings introduced the notion of functional lateralization to the field of neuroscience:gazz3

When split-brain patients were presented with visual information (such as an object or a word) in their right visual field, they were able to verbally identify the stimulus. Interestingly, if visual information was presented in their left visual field, patients were unable to do so — in fact they would typically say, “I don’t know.” To understand this phenomenon we must recall the following generalizations:

  1. Information in the right visual field is known to be processed by the left hemisphere, and information in the left visual field is known to be processed by the right hemisphere (see above figure).
  2. Certain aspects of language are known to predominantly reside in the left hemisphere of the brain.

From his observations, Gazzaniga came to the conclusion that split-brain patients were unable to verbally identify stimuli presented in their left visual field because, though the information would travel to the right hemisphere, it would not be transferred to the left hemisphere where ‘language resides’ due to the severed connection between the two hemispheres.

There is a twist however. Patients who stated that they “did not know” what the stimuli presented in their right visual field was were able to draw what they saw with their left hand. These observations along with many many follow-up studies testing for part-whole relations, apparent motion detection, mental rotation, mirror image discrimination, etc. led to the idea that there is perhaps a right hemisphere dominance for visuospatial processing. These ideas are not meant to be mutually exclusive for one hemisphere or the other. In fact, one patient clearly demonstrates that certain aspects of language, such as spelling, can also reside in the right hemisphere: P.S., a teenager split-brain patient, was asked “Who is your favorite girlfriend?” with the word ‘girlfriend’ flashing only in his left visual field. He was unable to answer the question verbally because the information remained in his right hemisphere; however his left hand (controlled by the right hemisphere) was able to select Scrabble letters and align them to spell’L-I-Z.’


Split-brain patients were the key to studying the functions of the two hemispheres independently, and Gazzaniga recognized the value in capitalizing on what this unique patient population had to offer to the advancement of neuroscience. Among his many accomplishments are serving on the President’s Council on Bioethics between 2001-09, basically founding the field of cognitive neuroscience with fellow psychologist/linguist George A. Miller, and being awarded the Guggenheim Fellowship for Natural Sciences. Come hear him talk on lessons learned from split-brain research this Tuesday, January 12 at 4 pm.

Ege A. Yalcinbas is a first-year student in the neurosciences graduate program currently rotating in Dr. Chalasani’s lab. Michael Gazzaniga was one of the first neuroscientists she read about in high school so she is excited to fangirl him at his talk on Tuesday.



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