Self-consciousness and "Split" Brains: The Mind's I by Elizabeth Schechter (review)

Some philosophers offer theories of what consciousness is based on abstractions such as complexity or integration of information, or notions of accessibility. Others use thought experiments to argue for what consciousness could and could not be. Schechter's book is about how to understand a real phenomenon: the split-brain patient. Through clear and careful analysis of data and lucid argumentation centered around split brains, Schechter's book ends up being more than just a book about split-brains—it is an extended meditation on the nature of agency and the unity of consciousness.

The split-brain patient is a person who has undergone surgical resection of the corpus callosum, the main fiber tract that connects the left and right cerebral hemispheres, thus disconnecting the two halves of the cortex. Although split-brain patients may seem normal under most circumstances, careful experimental work has revealed that information that is presented solely to one hemisphere is generally inaccessible to the other hemisphere. This in itself is unsurprising, given what is known about brain anatomy. What is more remarkable is that each hemisphere seems to be aware of what is presented to it, but not to the other. This raises the question of what to say about the conscious and cognitive capacities of each hemisphere. Is the split-brain patient S one conscious subject, like you or I? Or is he two? Schechter introduces the fundamental puzzle raised by split brain patients: All three of the following intuitions seem to be true, but they are inconsistent:

1. 1. The duality intuition: A split-brain subject has two minds

2. 2. The unity intuition: A split-brain subject is one person

3. 3. The one-mind-per-person rule: Each person has exactly one mind Schechter ultimately supports 1 and 2 and rejects 3.

Schechter begins by arguing, first, that there are experiences associated with each hemisphere, and, second, that each hemisphere is associated with a distinct subject of experience. Some early neuroscientists working with split-brain patients argued that each hemisphere of the brain, designated LH and RH, housed its own consciousness. However, Schechter convincingly argues that the candidates for consciousness and agency are not brain-halves, but rather whole organisms minus the disconnected parts. So, she argues, there are dual consciousnesses in the split-brain patient: R, which consists of the whole person S minus the left hemisphere (S-LH), and L, the whole person minus the right hemisphere (S-RH). In so arguing, Schechter poses the interesting possibility (or actuality, if she is right) of distinct but overlapping conscious beings.

The consciousness claim is intimately tied to a claim about agency: She argues that split-brain patients house not only two subjects of experience but also two distinct intentional agents. She accommodates empirical results showing some commonalities in the mental states of these two agents by distinguishing between direct and indirect interactions between psychological entities, defending the claim that there are distinct psychological beings coembodied in one human animal. This is not a trivial claim, because despite the lack of cortical connections between the two hemispheres, both are richly connected through subcortical and rhombencephalic structures, as well as being integrated in a single body. Through a careful study of the scientific literature, Schechter shows why these connections do not undermine the duality intuition.

The unity intuition is more challenging to support. Schechter argues that R and L are both self-aware, implicitly, of one and the same body. In addition, she argues, both R and L explicitly think self-referential "I-thoughts," despite the fact that only L is a fluent language speaker (due to the left lateralization of language). And even though both are self-aware, it seems that neither R nor L is aware of the other cohabiting the same body. The crux of her argument is that each is therefore unable to distinguish itself from the other, while both identify the whole animal, S, as themselves. It is S that...