Gazzaniga examines this question by reviewing the fascinating research that he and others have conducted on split-brain patients whose left and right brain hemispheres can no longer communicate with each other due to a separation (often surgical, employed to treat extreme epilepsy) of the corpus collosum, which is the body that transmits data from one side of the brain to the other.
(Technical sidebar: The Wikipedia article on split-brain provides a useful overview of how the right and left hemispheres of the brain work together, wherein the left hemisphere (typically considered analytic or logical) and the right hemisphere (typically considered holistic or intuitive) each controls and receives sensory inputs from the opposite side of the body. In split-brain patients, there's a sort of cognitive breakdown in the way that objects are perceived or understood by one side of the body when picked up or perceived by the opposite hemisphere of the brain; studying this breakdown has allowed Gazzaniga to develop insights into the way the two hemispheres interact.)
From Jeffrey Foss's review of the book comes these interesting insights:
Gazzaniga (with his teacher, Nobel laureate Roger Sperry) discovered the split in human consciousness that results from splitting the human brain into right and left hemispheres, a split that consciousness itself doesn't even notice. We have accepted our internal divisions long, long ago, and have, over the millennia, used them to explain our capacity for good and for evil. But whereas we can actually feel ourselves being influenced by Mars or Satan or our combative instinct, no amount of soul-searching can reveal to split-brain patients the resulting rent in their very selves.I'm deeply drawn towards scientific research which reveals what I find to be essential truths about the nature of consciousness and self: namely, that we possess no particular, identifiable self as such, and that the myriad thoughts and insights that we attribute to a seemingly separate entity called "our self" are simply a collection of evolution-serving, neurochemical, electrical and biological processes that are in place solely to continue the species, and not for any particularly meaningful purpose higher than that.
The explanation for this is quite simple. The left brain, where language processing occurs, is the mechanism of the soul searching itself, and cannot, in split brains, access or report the activity of the right brain and its input into the brain-as-a-whole.
The brain, split or unsplit, has no centre of control, no centre of consciousness, no centre period: no self. Gazzaniga marshals countless scientific studies of the brain that reveal it to be a rag-bag collection of specialized modules for everything from facial recognition and counting through to distinguishing self from other.
It's quite amazing how these modules make us identify the thoughts and actions of our brain as our own, even when the cause is known to be external control of our brain via transcranial magnetic stimulation. It's quite amazing, that is, to think that our sense of self is achieved by some dozens of such modules working in loose formation with one another -- in the absence of any real self at all.
So, as Gazzaniga and the many scientists of his sort see it, they, you and I are but the imaginary focuses created by our nervous systems in order to better serve the evolutionary demand of our trillions of component cells to survive and reproduce.
I find these insights to be enormously liberating. It gives me the license to stop worrying about what's happening; to loosen up my expectations over the way I think things should be; and to allow myself to just let go and let things unfold as they will, because "I" have no control to exert over the system. The universe is taking care of itself without any express input from "me," so why don't I just stop worrying about it?
Foss adds a sidebar to his review listing five essential books on the question of free will:
• Incognito: The Secret Lives of the Brain, by David Eagleman (2011)
• Freedom and Belief, by Galen Strawson (2010)
• Freedom Evolves, by Daniel Dennett (2003)
• How the Mind Works, by Steven Pinker (1997)
• The Origins of Virtue: Human Instincts and the Evolution of Cooperation, by Matt Ridley (1996)