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The received view within psychology and philosophy is that the split‐brain (commissurotomy) procedure leads to a breakdown in the unity of consciousness. Disunity models of the split‐brain can be divided into two classes: two‐streams models, according to which patients have two streams of consciousness, and partial unity models, according to which patients have a merely partially unified consciousness. Both models are motivated by the cognitive and behavioural disunities that patients exhibit in certain laboratory conditions, but they struggle to account for the cognitive and behavioural unity that patients demonstrate in everyday life. Preferable to disunity models is a full unity ‘switch’ model, according to which consciousness in the split‐brain rapidly switches between hemispheres. It is argued that only the switch model can account for both the behavioural disunities that split‐brain patients exhibit under experimental conditions and the behavioural unities that they exhibit outside of such contexts.

Michael S. Gazzaniga carried out original studies of human brain laterality and function in split-brain patients, work that has rich implications for consciousness, free will, and the self. He introduced the term cognitive neuroscience, helped develop the discipline, and founded the discipline’s flagship journal. More recently, he has effectively written several books for a broad audience about brain and mind, showing generations of readers the human face of science.

Roger W. Sperry earned a reputation for his ability to design critical experiments that demanded looking at the growing and functioning nervous system in new ways. Sperry's most important discoveries fall into two distinct domains. The first group began in the 1940s and concerned how axons grow to their proper places. Early in the 1950s, having established a name in neurobiology, Sperry became interested in the role of the corpus callosum, the massive band of axons connecting the right and left cerebral hemispheres. Other mid-20th-century researchers were soon able to prove that chemical growth and guidance factors exist. The individual most responsible for this ground-breaking work was Rita Levi-Montalcini, who discovered the nerve growth factor. Levi-Montalcini is important for neuroscience because she was able to demonstrate that Sperry was headed in the right direction when he brought up the possibility of chemical guidance in the 1940s. This chapter looks at the work of Sperry and Levi-Montalcini on neural growth, split brains, chemoaffinity, visual system, and consciousness.

The debate about the unity of consciousness in split-brain subjects has for the most part been pitched between two positions: that a split-brain subject has a single, unified stream of consciousness, or that she has two streams of consciousness, one associated with each hemisphere. A prima facie appealingly intermediate position, proposed most explicitly by Lockwood, is that a split-brain subject has a single but only partially unified consciousness. Philosophers have overwhelmingly rejected Lockwood’s proposal. In this chapter, Elizabeth Schechter issues a preliminary defense of the partial unity model (PUM) of split-brain consciousness. She argues that the major philosophical objections to that model apply no more to it than they do to the conscious disunity or dualitymodel. In particular, both models imply that a split-brain subject has two phenomenally conscious perspectives, and both raise questions about the relationship between having such a perspective and being a subject of experience.

If psychologists can, as they say, really identify perception (of an object) without awareness (of that object), they must have a reasonably clear operational grasp of what perception and awareness are. If so, philosophers can learn something from them. After settling on a test (at least a sufficient condition) for perception of x, the subjective test for awareness (you are not aware of x if you cannot detect it) is examined and found wanting by looking at such phenomena as split brains, change blindness, and unilateral neglect. An epistemic test is proposed in its place: S is consciously aware of x only if information about x is available to S as a reason for doing or believing something, as opposed to a reason why S does or believes something.

Robert Van Gulick explains how integration and unity play an important role in a number of current theories and models of consciousness. Normal consciousness is unified in a variety of ways but many disorders of disunity can also occur. What can we learn from them about consciousness and unity? What theories of consciousness might help us better understand the nature and basis of such disorders? Van Gulick first surveys the diverse types of conscious unity. He then briefly describes five theories of consciousness that involve integration, that is, Baars’s Global Workspace Theory (GWT), Tononi's Integrated Information Theory (IIT), Jesse Prinz's AIR theory (Attended Intermediate Representation), Tim Bayne's Phenomenal Unity thesis, and the combination of his own HOGS (Higher-order Global States) model with Virtual Self Realism (VSR). In that context, he discusses three specific pathologies of disunity -- neglect and extinction, dissociative identity disorder, and split-brains–and each is considered in relation to the integration based theories of consciousness.

Split-brain theorizing became the lingua franca of the 1970s and 1980s, with the left hemisphere considered the seat of rationality and language while the right hemisphere housed intuition and creativity. Expert and popular writing on cerebral asymmetry came to be directed to society’s privileged, who were encouraged to expand their right-brain potential with yoga, transcendental meditation, and biofeedback. At the same time, a substantial part of debates among neuropsychologists and related medical, social-scientific, and educational professionals revolved around the implications of such a revaluing of right-hemispheric skills specifically for African American, Latino, and Native American children. A remarkable array of experts began to affirm the existence of racial differences in intelligence while taking up a critique that “right-brained” (and often poor and minority) children were trapped in “left-brained” schools. Declaring IQ to be an inaccurate measure, psychologist Alan S. Kaufman in 1979 developed an influential alternative assessment scale specifically to expand what counted as intelligence and to include a range of creative, nonverbal, spatial, and emotional capacities—only to find that gaps in test scores between white and nonwhite children narrowed accordingly.

This chapter looks at developments in the split-brain studies during the past fifty years from a personal perspective. It describes experiments in split-brain research and the discovery of the effects of callosal disconnection. It highlights major findings in split-brain research that relate to the problem of consciousness and suggests that it is important to understand that the problem is constantly evolving and new dimensions are continually presenting themselves.

This chapter deals with functional neurological models as the brain basis of consciousness. Here, the chapter asks whether there is anything about brain organization that tells about consciousness. The answer given is twofold, and comes from an examination of data from split-brain patients. First, this chapter proposes that those activities of the brain which underlie conscious experience are distributed over different functional modules. Second, it argues that data from patients with a sectioned corpus callosum indicate that a property of the left, but not right, cerebral hemisphere is an interpretation of the results of such modular activities. Split-brain phenomena such as those studied previously are intriguing and provocative (especially regarding the question of the unitariness of consciousness), although this interpretation is both complicated and contentious.

This chapter introduces the major philosophical debate about the split-brain phenomenon. Split-brain surgery severs the major white matter fiber tract connecting the two cerebral hemispheres. A number of individuals who underwent this surgery later agreed to act as participants in experiments designed to reveal its psychobehavioral consequences. The basic finding is that, after they are surgically divided in this way, the two hemispheres cannot interact in all the ways they once could: indeed, split-brain subjects sometimes give the impression of having two minds and spheres of consciousness, one associated with each hemisphere. A split-brain subject nonetheless seems to be one of us, at the end of the day. The aim of the book is to reconcile these apparently opposing intuitions by explaining how a split-brain person could have multiple minds.

This chapter critically surveys some of the leading philosophical and scientific theories of consciousness. The philosophical theories include dualism, representationalism, and higher-order accounts. Scientific theories include the global workspace, the left-brain interpreter, temporal binding, and accounts that link consciousness to the experience of a self. All these theories are found to be wanting, but they are used to develop a set of desiderata that an adequate theory of consciousness should meet. A theory should provide an account of subjective character, the fact that consciousness arises at the first-order, the fact that consciousness does not rely on central systems, the unity of consciousness, the possibility of selfless experience, the function of consciousness, the way different levels of explanation are integrated in consciousness, and puzzle-producing nature of phenomenal knowledge.

Humans have a specialized system that carries out interpretative synthesis, and it is located in the brain’s left hemisphere. The author calls this the ‘interpreter’: a system that seeks explanations for both internal and external events in order to produce appropriate behaviours in response. Before exploring this system, the author begins this chapter by providing an outline on how the human mind has been built through natural selection. The author asserts that if the view is correct, it can provide a better understanding of the nature of conscious experience. Early studies on split-brain patients are presented in this chapter as well.

The previous chapters argued that within a split-brain subject there are two subjects of conscious experience and intentional agents, R and L. This chapter explains who these two thinking beings are and how it is possible for two thinkers to be co-embodied. The basis of the 2-thinkers claim is, naturally, that R and L think, feel, decide, and so on, independently of each other. Of course, this does not mean that they do not causally interact; since they are co-embodied, they interact all the time. What split-brain experiments show, however, is that R’s mental activities interact with L’s largely only indirectly: one of them acts or reacts in some way, and the other senses or perceives this re/action. Mental activities are causal activities whose psychological kinds are defined by their powers to interact directly. Thus the thinking things in the split-brain case are R and L, and only derivatively S.

This chapter explains and discusses the assumption that minds are transparent to themselves (in a way that they aren't, of course, transparent to other people). Some such assumption has been perennially tempting across time and place, and continues to exert a powerful influence today, especially in philosophy. Indeed, an implicit transparency assumption might be an innately channeled property of the human mind. One goal of the chapter is to explain away our common-sense intuitions of mental transparency, thereby leveling the playing field between the interpretive sensory-access (ISA) theory and its competitors. Another goal is to show that the conflict between transparent-access accounts and the ISA theory cannot be avoided by consigning them to different explanatory levels or “spaces”.

This chapter concerns the relationship between the split-brain case and the non-split case. In the first half of the chapter, I consider arguments to the effect that if split-brain subjects have two minds apiece, then so do non-split subjects. Sometimes these arguments have taken the form of a reductio against the 2-thinkers claim for split-brain subjects. These arguments do not work: that a split-brain subject has two minds does not mean that I have two minds, although it does mean that I could. The second half of the chapter offers my own proposal for the respect in which R’s and L’s co-embodiment as one animal, S, makes a split-brain subject one of us: I argue that S must be the single object of both R’s and L’s implicit bodily self-awareness.

This chapter looks at four potential cases of mental combination, to examine what the theory sketched in the previous chapter might say about them. It starts with the “Nation-Brain” thought experiment, originally offered as a reductio ad absurdum of functionalism, where a few billion people agree to collectively simulate a single human mind. It then considers actual human social groups, the ways that they differ from this thought experiment, and the significance of these differences for questions of collective mentality. It next considers the split-brain phenomenon, where patients with a severed corpus callosum seem at times to exhibit two distinct consciousnesses in one head, and then finally comes back to the ordinary human brain, where two cerebral hemispheres, each capable of supporting consciousness without the other, are able to establish richly unified consciousness through their intact corpus callosum.

This chapter focuses on the attentional mechanism of the disconnected hemispheres. Jeffrey D. Holtzman, who conducted numerous attention studies on split-brain patients, found that a common attentional system was shared by two disconnected hemispheres. Sectioning the corpus callosum would hardly bring any change to the attentional capacity, but it is possible that each of the hemispheres may control its own attentional network if they are disconnected. A spatial cuing experiment revealed that the time to respond to a visual target was delayed when each of the hemispheres received two distinct attentional instructions compared with when both received the same cues. This experiment finally concluded that two disconnected hemispheres share a common attentional system.

This chapter briefly mentions how the book The Integrated Mind came to be published and explores further the regulation of emotional expressions in the brain. The author mentions that The Integrated Mind, which he coauthored with Michael Gazzaniga, basically deals with various studies of split-brain patients that help scientists to understand how the mind and brain normally work. He also discusses Leon Festinger’s theory of cognitive dissonance, and the cognitive theory of Stanley Schachter and Jerome Singer. According to Schachter and Singer, the cognitive interpretation of nonspecific emotional cues can be termed as emotion. They include in their theory that joy and fear had similar physiological states in the brain and body.