Split brains that result in two simultaneous streams of consciousness cut off from each other are wrongly held to be grounds for doubting the existence of the divinely created soul. The mistake is based on two related errors: first, a failure to appreciate the soul's dependence upon neurological functioning; second, a fallacious belief that if the soul is simple, i.e. without parts, then there must be a unity to its thought, all of its thoughts being potentially accessible to (...) reflection or even unreflective causal interactions. But a soul theorist can allow neurological events to keep some conscious thoughts unavailable to others. (shrink)

Elizabeth Schechter explores the implications of the experience of people who have had the pathway between the two hemispheres of their brain severed, and argues that there are in fact two minds, subjects of experience, and intentional agents inside each split-brain human being: right and left. But each split-brain subject is still one of us.

Many have claimed that split-brain patients are actually two persons. I maintain that both the traditional separation argument and the more recent sophistication argument for the two persons interpretation are inadequate on conceptual grounds. An autonomy argument is inadequate on empirical grounds. Overall, theoretical and practical consequences weigh heavily in favor of adopting a one person interpretation.

After the first callosotomy surgeries were performed, the general consensus among the medical community was that severing the corpus callosum had relatively little, if any, effect on an individual's behavior. Nearly twenty years later, researchers discovered that, under experimental conditions, the two hemispheres could simultaneously maintain very different interpretations of the same stimulus. These findings immediately called into question the unity of subjective experience, a fundamental characteristic of human consciousness. How could the split‐brain patient not experience any disruption in (...) his experience as a unified self when the two hemispheres are physically and functionally disconnected? In this chapter, we review the research that has led to a conceptualization of the split brain as two minds within one body and the implications of this research for the scientific study of consciousness. We argue that consciousness is a neural function that emerges from the integration of information across available functional modules. (shrink)

In this paper I argue that split-brain syndrome is best understood within an extended mind framework and, therefore, that its very existence provides support for an externalist account of conscious perception. I begin by outlining the experimental aberration model of split-brain syndrome and explain both: why this model provides the best account of split-brain syndrome; and, why it is commonly rejected. Then, I summarise Susan Hurley’s argument that split-brain subjects could unify their conscious perceptual field by (...) using external factors to stand-in for the missing corpus callosum. I next provide an argument that split-brain subjects do unify their perceptual fields via external factors. Finally, I explain why my account provides one with an experimental aberration model which avoids the problems typically levelled at such views, and highlight some empirical predictions made by the account. The nature of split-brain syndrome has long been considered mysterious by proponents of internalist accounts of consciousness. However, in this paper I argue that externalist theories can provide a straightforward explanation of the condition. I therefore conclude that the ability of externalist accounts to explain split-brain syndrome gives us strong reason to prefer them over internalist rivals. (shrink)

To assess the ability of the disconnected cerebral hemispheres to recognize images of the self, a split-brain patient was tested using morphed self-face images presented to one visual hemifield at a time while making “self/other” judgments. The performance of the right and left hemispheres of this patient as assessed by a signal detection method was not significantly different, though a measure of bias did reveal hemispheric differences. The right and left hemispheres of this patient independently and equally possessed the (...) ability to self-recognize, but only the right hemisphere could successfully recognize familiar others. This supports a modular concept of self-recognition and other-recognition, separately present in each cerebral hemisphere. (shrink)

Some philosophers have argued that so long as two neural events, within a subject, are both of the same type and both carry the same content, then these events may jointly constitute a single mental token, regardless of the sort of causal relation to each other that they bear. These philosophers have used this claim—which I call the “singularity-through-redundancy” position—in order to argue that a split-brain subject normally has a single stream of consciousness, disjunctively realized across the two hemispheres. (...) This paper argues, against this position, that the kind of causal relations multiple neural events bear to each other constrains the mental tokens with which functionalists who are realists can identify them. (shrink)

This paper challenges the widely held theory that split-brain patients have ‘two-minds’ and can thus be described as being two distinct persons. A distinction is made between the singularity of mind and the coherence of mind. It is stressed that ‘a single mind’ is not something posited to explain coherence among mental contents, but is merely a mark that such coherence holds to a certain degree. However, there is no sharp dividing line regarding what counts as a single mind. (...) It is argued that mental coherence is always a matter of degree, and that our concept of a single mind can accomodate spit-brain phenomena. (shrink)

One of the main problems of the soul theory is how the soul, which has no material properties, interacts with body. Because it is difficult to understand how an immaterial being interacts with matter. In particular, as our scientific understanding of the way the brain works and how it affects our psychology expands, the question of whether a soul is needed for the mind manifests itself more strongly. In this context, current data on the close connection between the brain and (...) the mind is often used as an objection to the understanding of the soul. One of the recent data that has been claimed to have dealt a heavy blow against the understanding of the soul is the split-brain case. The split-brain case is the name given to the condition of patients whose brain hemispheres are surgically separated from each other. The difference of this case from other cases used against dualism is that it pointed out that the integrity of consciousness also depends on the brain. Because although there was a connection between many mental functions and the brain before, it was not between the unity of consciousness. For it was a new claim that the unity of consciousness also depends on the brain. On the contrary, arguments were used in favor of the existence of the soul, stating that the brain cannot provide the unity of consciousness due to its multi-part structure, and therefore an indivisible soul is necessary. Whereas, in the split-brain case, patients whose brain hemispheres were separated from each other could think and act like two different people. This is interpreted as contrary to the soul view, which argues that the individual and the soul are indivisible. The present study considers whether the split-brain case does indeed offer a strong argument against dualism. In this paper, first of all, more detailed information about the split-brain case will be given. Then, the arguments of those who use this case against dualism and why the issue seems problematic in terms of the soul view will be discussed. Afterwards, some solutions offered by dualists to this problem will be evaluated. There are three different strategies that dualists follow at this point. The first one rejects the idea that consciousness is divided as a result of the split-brain phenomenon, the second rejects the idea that the soul is indivisible, and the third way accepts both, but argues that they are not contradictory, because a single soul can have a divided consciousness. This paper argues that even if it is accepted that more than one stream of consciousness emerges as a result of the split-brain case, this cannot be seen as a finishing blow to dualism. Despite this, the case in question can be regarded as a strong argument against dualism. For a counter-argument to be strong, it does not need to conclusively demonstrate the falsity of what it opposes. It suffices to show the weakness of its probability of being true. In this respect, the split-brain case greatly weakens the probability of dualism being true. (shrink)

This paper challenges the widely held theory that split-brain patients have ‘two-minds’ and can thus be described as being two distinct persons. A distinction is made between the singularity of mind and the coherence of mind. It is stressed that ‘a single mind’ is not something posited to explain coherence among mental contents, but is merely a mark that such coherence holds to a certain degree. However, there is no sharp dividing line regarding what counts as a single mind. (...) It is argued that mental coherence is always a matter of degree, and that our concept of a single mind can accomodate spit-brain phenomena. (shrink)

Brain bisection raises the intriguing question about how many minds the split-brain patients have. Thomas Nagel and Derek Parfit, who have brought this question into consideration, come to similar conclusions in response to it. They both argue that the question has no answer, that there simply isn’t any countable number of minds that the split-brain patients have. In addition, Parfit argues that the split-brain cases can be adequately described only if we adopt a certain particular view about (...) the metaphysical nature of a person. The goal of this paper is to clarify both of those views and, in particular, to explain why Parfit’s preference for one model of personhood does not determine how many persons survive brain bisection. (shrink)

Many experimental findings with split-brain subjects intuitively suggest that each such subject has two minds. The conceptual and empirical basis of this duality intuition has never been fully articulated. This article fills that gap, by offering a reconstruction of early neuropsychological literature on the split-brain phenomenon. According to that work, the hemispheres operate independently of each other insofar as they interact via the mediation of effection and transduction—via behavior and sensation, essentially. This is how your mind and my (...) mind interact with each other, however, giving rise to the intuition that a split-brain subject has two minds. (shrink)

In this commentary I use recent empirical evidence and theoretical analyses concerning the importance of language and the meaning of self-recognition to reevaluate the claim that the right mute hemisphere in commissurotomized patients possesses a full consciousness. Preliminary data indicate that inner speech is deeply linked to self-awareness; also, four hypotheses concerning the crucial role inner speech plays in self-focus are presented. The legitimacy of self-recognition as a strong operationalization of self-awareness in the right hemisphere is also questioned on the (...) basis that it might rather tap a preexisting body awareness having little to do with an access to mental events. I conclude with the formulation of an alternative interpretation of commissurotomy according to self-awareness — a “complete” one in the left hemisphere and a “primitive” one in the right hemisphere. (shrink)

In this paper it is argued that the experimental data on commissurotomy patients provide no grounds for denying the singular personhood of commissurotomy patients. This is because, contrary to most philosophical accounts, there is no “unity of consciousness” discriminating condition for singular personhood that is violated in the case of commissurotomy patients, and because no contradictions arise when singular personhood is ascribed to commissurotomy patients.

The attempt to model the structure of consciousness in split-brain subjects is ongoing. This paper concerns the recently proposed ?switch model? of split-brain consciousness, according to which a split-brain subject possesses only a single stream of consciousness, unified at and across time, that shifts from one hemisphere to the other from moment to moment. The paper argues that while the central explanatory element of the switch model may account for some aspects of split-brain consciousness, the best (...) general picture of split-brain consciousness is still offered by some version of the ?conscious duality model.? (shrink)

. Cognitive science challenges our understandings of self and freedom. In this article, adapted from a chapter in Minding God: Theology and the Cognitive Sciences , I review some of the scientific literature with regard to issues of self and freedom. I argue that our sense of self is a construct and heavily dependent on the kind of brain that we have. Furthermore, understanding the issue of freedom requires an understanding of the findings of cognitive science. Human beings are constrained (...) to be free; our biology in no small way determines the kinds of freedom that we are able to have. (shrink)

According to conventional wisdom, the split-brain syndrome puts paid to the thesis that consciousness is necessarily unified. The aim of this paper is to challenge that view. I argue both that disunity models of the split-brain are highly problematic, and that there is much to recommend a model of the split-brain—the switch model—according to which split-brain patients retain a fully unified consciousness at all times. Although the task of examining the unity of consciousness through the lens (...) of the split-brain syndrome is not a new one—such projects date back to Nagel’s seminal paper on the topic—the time is ripe for a re-evaluation of the issues. (shrink)

In this paper it is argued that the experimental data on commissurotomy patients provide no grounds for denying the singular personhood of commissurotomy patients. This is because, contrary to most philosophical accounts, there is no “unity of consciousness” discriminating condition for singular personhood that is violated in the case of commissurotomy patients, and because no contradictions arise when singular personhood is ascribed to commissurotomy patients.

In "Consciousness Explained," Dennett (1991) denies that split-brain humans have double consciousness: he describes the experiments as "anecdotal." In attempting to replace the Cartesian Theatre of the Mind" with his own "Multiple Drafts" view of consciousness, Dennett rejects the notion of the mind as a countable thing in favour of its being a mere "abstraction." His criticisms of the standard interpretation of the split-brain data are analyzed here and each is found to be open to objections. There exist (...) people who have survived left ["dominant"] cerebral hemispherectomy; by Dennett's criteria, they would not have minds. (shrink)

Contrary to received opinion among philosophers, psychologists, and neuroscientists, conscious duality as a principle of brain organization is neither incoherent nor demonstrably false. The present paper begins by reviewing the history of the theory and its anatomical basis and defending it against the claim that it rests upon an arbitrary decision as to what constitutes the biological substratum of mind or person.

We investigated binocular rivalry in the twocerebral hemispheres of callosotomized(split-brain) observers. We found that rivalryoccurs for complex stimuli in split-brainobservers, and that it is similar in the twohemispheres. This poses difficulties for twotheories of rivalry: (1) that rivalry occursbecause of switching of activity between thetwo hemispheres, and (2) that rivalry iscontrolled by a structure in the rightfrontoparietal cortex. Instead, similar rivalryfrom the two hemispheres is consistent with atheory that its mechanism is low in the visualsystem, at which each (...) hemisphere conducts asimilar analysis of its half of visual space. (shrink)

The neural substrates of context effects in word perception are still largely unclear. Interhemispheric priming phenomena in word recognition, typically observed in normal subjects, are absent in commissurotomized patients. This suggests that callosal fibers may provide contextual integration. In addition, certain characteristics of human frontal cortical fields subserving sensorimotor learning, as investigated by positron emission tomography, provide evidence for contextual integration not confined to the visual system. This supports the notion of common aspects of cortical computations in different cerebral areas.

The startling empirical data that concern us here are well known. Severing the corpus callosum produces a kind of mental bifurcation (Sperry 1968). In one experiment, a garlic smell is presented to a patient.

Consider the conscious states of a single subject at a time. Arguably, split-brain cases show that such states need not be jointly accessible. It is less clear that these cases also show that such states need not be jointly experienced. Michael Tye (2004) argues split-brain cases do have that implication, and Timothy Bayne and David Chalmers (2003) argue that they do not. I will develop two objections to Tye’s arguments. First, an analogy to blindsight on which he relies (...) is questionable. Second, even if his analogy succeeds, it shows only that a single person can simultaneously have two separate sets of phenomenally conscious mental states. That result fails to undermine the phenomenalunity thesis that Bayne and Chalmers defend, on which phenomenal consciousness is necessarily unified. (shrink)