The Silent Hemisphere: What Happens to a Severed Brain Half?
Remarkable cases exist where individuals undergo a hemispherotomy – a procedure involving the disconnection of one entire brain hemisphere – yet continue to live relatively normal lives. This is largely due to the brain’s remarkable plasticity, with the healthy hemisphere often compensating for lost functions. though, the fate of the disconnected hemisphere remains a profound mystery. Entirely isolated from sensory input and unable to communicate outwardly, the question arises: could this “silent” portion of the brain retain any form of consciousness?
Researchers at the University of Milan investigated this question by studying ten children who had undergone a hemispherotomy. Thay monitored brain activity using electrodes placed on the scalp, both before and after the surgery, while the children were awake. Comparing the activity patterns of the isolated and healthy hemispheres revealed a striking finding: the severed hemisphere began exhibiting brainwave patterns characteristic of deep sleep.
Specifically, the isolated brain tissue consistently produced slow waves, mirroring those seen during phases 2 and 3 of non-REM sleep – the deepest stages of sleep where dreaming is infrequent.This pattern persisted for months, even years, post-surgery. Simultaneously,the healthy hemisphere maintained a normal awake-state activity pattern,explaining the children’s continued consciousness and ability to interact with the world.
These findings suggest the isolated hemisphere likely experiences a significantly reduced, or even absent, level of consciousness.While brain activity alone cannot definitively determine consciousness, especially in a disconnected structure, the observed patterns strongly correlate with a state akin to deep, dreamless sleep.
The slow waves observed weren’t identical to those of natural sleep. They exhibited lower peaks and a slower rhythm, likely due to the absence of brain structures normally responsible for synchronizing and amplifying these waves – structures severed during the hemispherotomy.
Beyond the immediate implications for these patients, the study contributes to a broader understanding of the brain’s “default mode.” Researchers hypothesize that, in the absence of external stimuli, the brain naturally reverts to a resting state resembling deep sleep. This may also explain similar slow-wave activity observed in damaged brain areas following injuries like stroke, where portions of the brain become disconnected from their usual networks.
Future research aims to further investigate this phenomenon, with plans to utilize direct brain electrodes for more precise measurements of activity within the isolated hemisphere. This continued exploration promises to shed further light on the basic nature of consciousness and the brain’s inherent activity even in isolation.
