Neuroscientists understand, at least in general, how the biological machinery of the brain can compute information. But how does a brain become aware of information? What is sentience itself? When a specific part of the brain is damaged, does the patient lose only a specific category of knowledge, such as vision or language, or can the patient ever lose some of the essence of awareness?
A clinical syndrome called hemispatial neglect may help to answer the question. It is one of the most fascinating, and horrible, syndromes in the medical literature. Neglect was first described early in the 20th century, and over the years much has been learned about it.
Imagine waking up in the hospital after a stroke to find that half your world is gone. The left side of space and everything in it has been erased from your consciousness. You can talk to the people who stand to the right side of your hospital bed, but when they walk to the left side they disappear from your mind. You dress the right side of your body but forget to dress the left. You think you've eaten everything on your plate, but have eaten only the food on the right side. You can't even conceive of a left side of the plate. When someone rotates the plate, food that you didn't acknowledge before suddenly appears. When you draw a clock, you crush all 12 numbers into the right side of the drawing and don't notice that anything is wrong. You have no insight into your own condition because, lacking any awareness of a left side of space, you can't realize what is missing.
This bizarre and crippling syndrome is not simple blindness. After all, blind people and sighted people who close their eyes know about the objects around them. Instead it is a mental blindness. It covers vision, touch, hearing, memory and concept.
Over the years, different varieties of neglect have been described and associated with damage to different brain regions. But the most dense, profound loss of awareness is associated with a region of the cerebral cortex roughly just above the ear on the right side of the brain. Much more rarely, neglect of the right side of space is caused by damage to the same general area on the left side of the brain.
Neglect is a peculiar syndrome. It suggests that awareness is not a unified item, but like many constructs of the brain it can be knocked apart into a right and a left half. It suggests that awareness is constructed at least partially by a specific region of the brain. It suggests a close relationship between awareness and attention.
The findings are controversial. That same general region of the brain has been found to play a role in social thinking -- in understanding the minds of other people. Why would a brain area involved in social intelligence also participate in one's own basic awareness? Which of the rival accounts is correct? I have argued in my scientific writing that the two functions are not rivals, and instead are closely related. Awareness, sentience itself, may be part of the toolkit we use to understand ourselves and each other. It may be a function of our social brain.
In my view, there really is such a thing as a spirit, a soul, but it is not as people have imagined it in the past. The soul is information of a special kind, wrapped up into a complex structure, instantiated in the circuitry of the brain. It is quirky and individual to each of us, and is precious because it is not eternal.