PIECES OF MIND: The Man with Two Brains

The corpus callosum acts as a bridge between the two hemispheres of your brain. What happens when the two halves no longer connect? For more than a decade, Dartmouth neuroscientist Michael Gazzaniga has followed the cognitive experiences of a man with severe epilepsy whose connection between his brain’s two hemispheres was severed to stop his seizures. Gazzaniga’s work with so-called “split-brain” patients has given him a unique perspective into how the brain perceives and communicates.


Inside your head, you possess three pounds of the most complex structure that we know of: your brain. The human brain has evolved over several million years to become the incredibly complex organ that it is in humans today. You might say the brain has come a long way from its hominid ancestors on the African savannas.

Still, we share many brain activities with our distant ancestors — dreams, memory, sleep and some form of language or communication have long characterized humans and made us individuals. The fight-or-flight response, for example, helped our ancestors defend and survive attacks by predators. Although we don’t have to fight predators today, the same response is activated every day in modern humans.

The brain of three million years ago found food, later learned to cook it, eventually learned to farm, make tools, build great cities — and invent the computer. Over the course of those three million years, both cranial capacity and brain size have expanded. The human brain has become increasingly complex, enabling us to enjoy a high level of problem-solving abilities.

“Pieces of Mind” takes you inside the human brain and shows you what fascinating questions scientists are exploring today about perception, memory, sleep, dreams and language.


The corpus callosum connects both hemispheres of the brain and enables the left and right sides of the brain to communicate. As you see in this episode of FRONTIERS, when this bridge is disconnected, the person functions quite well but experiences perceptual and cognitive dysfunction. If the corpus callosum is severed, the right hand truly does not know what the left hand is doing.

divided brain graphic

For the vast majority of us, the right hemisphere of the brain controls the left side of the body, and the left hemisphere controls the right side. What your left eye perceives registers in your right brain; what your right eye sees registers in your left brain. The right brain sends messages to the left, and vice versa, via the corpus callosum.

When the corpus callosum is severed, as it is in the patient’s brain as seen on FRONTIERS, there is perception but no messages travel back and forth between the brain’s two hemispheres. Thus, although both eyes “see” an image, each hand drawing the figure does only what its half of the brain perceives. The left hemisphere, which handles language in most people, does not receive or send messages about what the two halves see. The brain is, indeed, divided.

Signals from nerves on one side of the body arrive at the brain and cross over to the opposite side via the corpus callosum, which links the left and right hemispheres. The corpus callosum shown here is severed.


Using your biology text and any other resources, can you trace the pathway of a signal from a nerve to your brain? Some situations to map might be: what happens when you stub your toe, when you taste a piece of chocolate or when you worry about an exam.

Source: Public Broadcasting Service