One of the most distinctive physical features of the human brain is the fact that the cortex is divided into two hemispheres. The main connection between the two halves is a thick bundle of fibers called the corpus callosum. This is no quiet lane, it’s a major freeway constituting around 200 million neural tracts.

In an increasingly rare procedure, the callosum is sliced as a radical treatment for epilepsy. People who receive this treatment are referred to colloquially as split-brain patients and lab tests reveal profound effects on their mental functioning. In many ways, it’s as if the surgery leaves their mind divided in two.

A new paper reports on an elderly gentleman, referred to as H.W., who aged 88 presented at a clinic complaining of recent intermittent problems controlling his left hand and some mild memory difficulties. Preliminary tests found him to be high functioning. He scored 30 out of 30 on the “mini mental state examination”, which is used to pick up signs of dementia or confusion. But when the researchers - a team led by Natalie Brescian - scanned H.W.’s brain, they made a surprising discovery. He had no corpus callosum. The main channel between his two brain hemispheres was completely missing.

The medical name for H.W.’s rare condition is agenesis of the corpus callosum, meaning that he was born with this structure missing. Given the importance of the callosum for connecting the bicameral brain, you’d think this would have had profound neuropsychological consequences for H.W. In fact, a detailed clinical interview revealed that he’d led a normal, independent life - first in the military and later as a flower delivery man. Until recently, if H.W.'s testimony is to be believed, he appeared to have suffered no significant psychological or neurological effects of his unusual brain. The problems with his left hand, H.W. said, were new.

Brescian and her colleagues conducted comprehensive neuropsych tests on H.W. and on most he excelled or performed normally. This included IQ tests, abstract reasoning, naming tests, visual scanning, motor planning, visual attention and auditory perception. He did display memory problems and also some difficulties with fine motor control, especially when using both hands at once, and drawing. These issues, especially of motor control, are likely related to his congenital (from birth) condition, but they may also result from age-related neurological changes. The main message from the authors of this case study, though, is H.W.’s remarkable high-functioning, and his apparently unaffected life.

How can such a profound brain abnormality have so little functional consequence? The corpus callosum is not the only connection between the hemispheres, but it is by far the most important. “This case study underscores the plasticity of the developing brain,” the researchers said. Their theory is that the “congenital absence of the corpus callosum stimulates early cerebral organisation and the development of new or stronger inter hemispheric and perhaps intrahemispheric connections.” Their vagueness really betrays how little we still understand about brain development. For example, it’s not clear why H.W.’s brain adapted so well (as is sometimes the case in this condition) while other people with callosal agenesis display more serious and debilitating deficits.

H.W.’s story is consistent with the general rule that the brain adapts more readily to congenital or early acquired injuries or abnormalities, than to insults suffered later in life. For example, last year, researchers reported the remarkable case of a boy who recovered his language abilities to near-normal levels after losing almost his entire left hemisphere at age two and a half.

However, it’s also notable that the friends and relatives of many split-brain patients - whose callosum’s were cut in adulthood - often reported no obvious effects of the surgery. In such cases, the severance of the brain’s mighty communication cable did have profound consequences but these were only apparent in careful lab tests when researchers presented stimuli to just one hemisphere or the other. Somewhat strangely, Brescian and her team don't seem to have performed these kinds of specialized tests with H.W. Nonetheless, this remains a fascinating neurological case study that showcases the adaptability of the human brain.