Two brains in one head?: The story of the split-brain phenomenon

[En español]

right left brain collage

Are you left-brained or right-brained?  Are you the hard-working, problem-solving, number-loving, office dweller?  Or the creative, passionate, playful, day-dreaming artist?  These images often made me rather livid as a science-inclined child because obviously I wanted to identify with the half of the brain spewing swirls of bright colors rather than the one depicting darkened cubicles or esoteric math equations.  The artists creating these polarizing images were just trying to boost their own egos and frame the rest of us as boring, right?  Did becoming a scientist really entail accepting that the core of my being was equation-ridden and tragically colorless?

Thankfully, no.  The idea that a person is right-brained or left-brained is a myth, perhaps only slightly less ridiculous than the widespread belief that humans only use 10% of their brains. (Sorry Morgan Freeman, but we use all 100%.)

However, the brain is indeed split into a right and left hemisphere, and the two are connected by a structure called the “corpus callosum”, a bundle of nerves through which information can be shared.  Imagine two incredibly powerful computers connected by a bundle of wires such that each computer has access to the data that the other one is processing.  How would you determine which computer was processing what?  You guessed it.  Cut the cords.

CorpusCallosum222Corpus Callosum Bulgular

In the 1960s, a surgeon named Joseph Bogen performed callosotomies (severance of the corpus callosum) on a group of epilepsy patients in the hopes of preventing their seizures from spreading from the region of origin to the rest of the brain. This group of patients—who, amazingly, exhibited no changes in temperament or intelligence—became a gold mine for researcher Roger Sperry and his student Michael Gazzaniga.  They designed a set of experiments to test the consequences of removing this connection between the left and right hemispheres, and Sperry ultimately won a Nobel Prize for his work.

These experiments takvisual pathways crosse advantage of the fact that visual information is segregated upon entering the human brain; information obtained by the right eye flows to the left hemisphere, while information entering the left eye streams straight to the right hemisphere.  Tracts carrying tactile information are similarly crossed; when you touch something with your right hand, the information goes to your left hemisphere first, and vice versa.  Of course, in those of us with an intact corpus callosum, all of this sensory information is ultimately shared so that the whole brain is in the know.  But this was not the case for those who had had their corpus callosum severed.  In this handful of patients, the right brain no longer knew what the left brain was doing.  This group of patients had—in many respects—two brains instead of one.

Are there differences in function between the left and right hemispheres? The answer is a resounding yes, and the surprising extent of this lateralization (when a process occurs more on one side than the other) can be demonstrated with a split brain patient in the simple experiment depicted below: if an image of a hammer is presented to the right hemisphere (i.e., it is shown to the left eye), and the patient is asked to verbally describe what he sees, he will say, “Nothing”.  However, if he is asked to use his left hand to pick up the object he sees, he will choose the hammer without hesitation.  What does this mean?  Clearly the right hemisphere (which controls the left hand), could identify the hammer.  Why did the subject say that he saw nothing?

This does not occur when an image is presented to the left hemisphere.  In this condition, when the subject is asked what he sees, his verbal response is prompt and accurate.  Therein lies the greatest difference between the two hemispheres: their ability for language. image018

In most of our brains, speech is generated almost exclusively from the left side.  Because the question “What do you see?” requires a verbal response, it is processed by the left side of the brain.  In the case of the apple, the left hemisphere also happens to be the one that received the image of the apple, and there is no problem.  However, in the case of the hammer, it is the non-verbal right hemisphere that holds the visual information. The language region in the left hemisphere is still fielding the question, but this time the left brain truthfully sees nothing.  It is almost as if the patient has become a pair of conjoined twins looking in different directions, only one of whom can speak. [Interestingly, one twin might believe in God while the other might be an atheist, but that’s another story for another day.]

Check out a real split-brain patient doing a variation of this task!

drawing 3d

While the left brain allows us to speak, the right brain takes the lead on processing spatial information.  These drawings from a split-brain patient illustrate this fact [1].   When a subject is presented with an image and then asked to copy it with his left hand, it is clear that he understands the 3D nature of the presented image (this particular subject is right handed, so the drawings are naturally a bit wobbly).  If, instead, the subject is asked to copy the image with his right hand, it is his spatially-inept left brain that is in control; he has no concept of three dimensions, rendering his drawings comically strange. Perhaps this is how the right hemisphere became associated with artsy, creative types.

The right hemisphere also has an edge when it comes to facial recognition.  Three split-brain patients were shown a set of 20 unfamiliar faces on either their left or right side.  After each face flashed on the screen, the subjects were instructed to choose the matching face from ten cards laid out on the table.  All three subjects were significantly faster and more accurate at this facial recognition task when the right side of the brain had access to the image (i.e., the screen showing the faces was to the subject’s left) [2].  Apparently the speaking conjoined twin has a bit of prosopagnosia going on.

Not all functions are lateralized, of course.  Despite the fact that ordinary touch information from each hand is only accessible to the opposite brain hemisphere, intense painful heat applied to one hand is accessible to both brain hemispheres [4].  Similarly, the presence of light on one side of the visual field is also detected by both sides of the brain [5]. And both sides are capable of a visceral reaction; a split-brain patient will slyly chuckle regardless of whether a nude picture is presented to the right or left side.  If it is the non-verbal right hemisphere that has access to the picture, the subject is unable to articulate the source of amusement, but that does not prevent the chuckle [1]. Both hemispheres are equally capable of get a kick out of a bit of nakedness.

While it’s fun to think about having two brains instead of one, why would the overwhelming majority of humanity with our corpus callosums intact care about how the right and left side of the brain divvy up functions?  In fact, lateralization of brain function might be part of what makes us human; the human brain is far more laterally specialized than that of any other animal.  Gazzaniga eloquently posits, “It may turn out that the oft-ignored corpus callosum, a fibre tract that is thought merely to exchange information between the two hemispheres, was the great enabler for establishing the human condition.” [5]

What does Gazzaniga mean by this?  He is saying that if an advantageous mutation occurred on one side of the brain, thereby allowing for the development of a new function, the complementary region on the other side could continue to perform the old function.  And because of the existence of the corpus callosum, information from both the old and new processes would be accessible to the whole brain–a mutation with no loss of function.  How does this connect with the birth of humanity?  Perhaps a series of mutations in the left hemispheres of our hominid ancestors enhanced capacity for speech, and it is language that endowed our species with the ability for complex social interaction, propelling human evolution forward.

And even though I now know that the existence of colorful right-brained people and calculating left-brained people is a total myth, the disappointed “left-brained” child in me takes sweet revenge in the fact that something as colossally important as language is associated with the left side.  Take that, side of frolicking and creativity and pretty, swirly colors.


  1. Gazzaniga, MS. The split brain in man.Scientific American (1967). at <>
  2. Gazzaniga, MS & Smylie, CS. Facial recognition and brain asymmetries: Clues to underlying mechanisms. Annals of Neurology (1983). at <>
  3. Benedetti, F, Poletti, S & Radaelli, D. Right hemisphere neural activations in the recall of waking fantasies and of dreams. Journal of sleep  … (2015). doi:10.1111/jsr.12299
  4. Stein BE, Price DD, Gazzaniga MS. Pain perception in a man with total corpus
    callosum transection. Pain. 1989 Jul;38(1):51-6. PubMed PMID: 2780063.
  5. Gazzaniga, MS. Cerebral specialization and interhemispheric communication.Brain (2000). at <>