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Sir, More than half a century of scientific work with splitbrain patients has resulted in various profound insights on how the brain works. The generous dedication of these patients has truly transformed how we think about the brain and profoundly inspired the developing field of cognitive neuroscience. However, several questions raised by the work with split-brain patients still evoke controversy, one of the most prominent being whether separating the two cerebral hemispheres by cutting the corpus callosum may lead to two distinct conscious systems or not. In other words, is it possible to assume that a patient could be seen as two patients after the surgery? In their recent study, Pinto and colleagues argued that their empirical data represent evidence for the notion that while (visual) perception in split-brain patients is divided, the resulting pair of perceptual systems are integrated into one conscious agent (Pinto et al., 2017c). In a review that preceded their original manuscript, Pinto et al. (2017a) referred to this idea as the ‘conscious unity, split perception model’. In our recent review, we addressed the interpretation offered by Pinto and colleagues and offered an alternative explanation: the reported experimental observations may well be explained by cross-cueing between the hemispheres, without the need for a single integrated conscious control (Volz and Gazzaniga, 2017). Pinto and colleagues now argue against our perspective in a recent letter, stating that we did not present ‘substantive evidence’ for our interpretation (Pinto et al., 2017b). While we want to avoid a repetitive exchange on methodological aspects of the performed experiments, we do believe that something can be gained from the careful interpretation of split-brain findings, especially regarding their implications for our understanding of consciousness. Accordingly, we briefly address the points made by Pinto and colleagues in their response to the caveats we outlined regarding the interpretation of their work. One of the most essential points that Pinto et al. addressed is the lack of a formal definition of cross-cueing, rendering a meaningful discussion rather difficult. We agree with the authors, as the portrayal of cross-cueing in their manuscript does not overlap too much with ours: ‘. . . crosscueing (one hemisphere informing the other hemisphere with behavioural tricks, such as touching the left hand with the right hand) . . .’ (Pinto et al., 2017c). While such a simple description may potentially be accurate when testing a patient immediately after surgery, we have to emphasize the fact that the investigated patients underwent surgery many years prior to testing. Hence, the separated perceptual systems had ample time to learn how to compensate for the lack of commissural connections. For example, subtle cues may be given by minimal movements of the eyes or facial muscles, which might not even be visible to an external observer but are capable of encoding, for example, the location of a stimulus for the hemisphere that did not see it. Conversely, encoding the identity of a stimulus, i.e. what kinds of objects were presented to one visual field, seems far more complex. The resulting empirical prediction regarding Pinto et al.’s experiments would hence be that information on the location of a stimulus may be readily transferred between hemispheres via cross-cueing (enabling accurate stimulus localization in the ‘incongruent condition’), while the identity of a unihemispherically presented stimulus remains lateralized (not allowing stimulus identification in the ‘incongruent condition’). Indeed, this is exactly what Pinto and colleagues observed: the split-brain patient was able to report the location but not the identity of a stimulus in the incongruent condition, e.g. correctly locating a stimulus with the right hand or verbally (i.e. using the left hemisphere) even if it was exclusively doi:10.1093/brain/awx359 BRAIN 2018: 141; 1–3 | e15

Unifying control over the body: consciousness and cross-cueing in split-brain patients