The largest fibre tract in the human brain connects the two cerebral hemispheres. A ‘split-brain’ surgery severs this structure, sometimes together with other white matter tracts connecting the right hemisphere and the left. Split-brain surgeries have long been performed on non-human animals for experimental purposes, but a number of these surgeries were also performed on adult human beings in the second half of the twentieth century, as a medical treatment for severe cases of epilepsy.
A number of these people afterwards agreed to participate in ongoing research into the psychobehavioural consequences of the procedure. These experiments have helped to show that the corpus callosum is a significant source of interhemispheric interaction and information exchange in the ‘neurotypical’ brain. After split-brain surgery, the two hemispheres operate unusually independently of each other in the realm of perception, cognition, and the control of action. For instance, each hemisphere receives visual information directly from the opposite (‘contralateral’) side of space, the right hemisphere from the left visual field and the left hemisphere from the right visual field. This is true of the normal (‘neurotypical’) brain too, but in the neurotypical case interhemispheric tracts allow either hemisphere to gain access to the information that the other has received. In a split-brain subject however the information more or less stays put in whatever hemisphere initially received it. And it isn’t just visual information that is confined to one hemisphere or the other after the surgery. Rather, after split-brain surgery, each hemisphere is the source of proprietary perceptual information of various kinds, and is also the source of proprietary memories, intentions, and aptitudes.
Various notions of psychological unity or integration have always been central to notions of mind, personhood, and the self. Although split-brain surgery does not prevent interhemispheric interaction or exchange, it naturally alters and impedes it. So does the split-brain subject as a whole nonetheless remain a unitary psychological being? Or could there now be two such psychological beings within one human animal – sharing one body, one face, one voice?
Prominent neuropsychologists working with the subjects have often appeared to argue or assume that a split-brain subject has a divided or disunified consciousness and even two minds. Although a number of philosophers agree, the majority seem to have resisted these conscious and mental ‘duality claims’, defending alternative interpretations of the split-brain experimental results. The sources of resistance are diverse, including everything from a commitment to the necessary unity of consciousness, to recognition of those psychological processes that remain interhemispherically integrated, to concerns about what the moral and legal consequences would be of recognizing multiple psychological beings in one body. On the other hand underlying most of these arguments against the various ‘duality’ claims is the simple fact that the split-brain subject does not appear to be two persons, but one – and there are powerful conceptual, social, and moral connections between being a unitary person on the one hand and having a unified consciousness and mind on the other.
The order of trait emergence in the evolution of cyanobacterial multicellularity
