It is important to note that even in this small group there are differences. First and foremost is the functional description, pioneered by Gazzaniga, Sperry and colleagues (Gazzaniga, Bogen, & Sperry, 1963 Gazzaniga, Bogen, & Sperry, 1962 Sperry, 1968), in which the intricacies, the exceptions, the effects of different testing conditions, and the experimental confounds have been delineated by decades of extensive research with a relatively small group of patients (Berlucchi, Aglioti, Marzi, & Tassinari, 1995 Corballis, 1994 Corballis et al., 2010 Corballis, 2003 Luck, Hillyard, Mangun, & Gazzaniga, 1989 Pinto, Lamme, & de Haan, 2017b Volz, Hillyard, Miller, & Gazzaniga, 2018). Since the earliest reports by van Wagenen and Herren ( 1940) and Akelaitis ( 1941, 1943) on the repercussions of a split-brain, two narratives have emerged. The term “split-brain” refers to patients in whom the corpus callosum has been cut for the alleviation of medically intractable epilepsy. In addition, it is suggested that the answers might not be a simple yes or no but that intermediate conceptualizations need to be considered. Does a split-brain harbor a split consciousness or is consciousness unified? The current consensus is that the body of evidence is insufficient to answer this question, and different suggestions are made with respect to how future studies might address this paucity. The main issue concerns the first-person perspective of a split-brain patient. Disagreement exists about the responsible mechanisms for this remaining unity. However, the breakdown is not absolute as several processes, such as action control, seem to remain unified. In short, callosotomy leads to a broad breakdown of functional integration ranging from perception to attention. This collective review paper aims to summarize the empirical common ground, to delineate the different interpretations, and to identify the remaining questions. Recently, the discussion regarding the consequences of cutting the corpus callosum (“split-brain”) has regained momentum (Corballis, Corballis, Berlucchi, & Marzi, Brain, 141(6), e46, 2018 Pinto et al., Brain, 140(5), 1231–1237, 2017a Pinto, Lamme, & de Haan, Brain, 140(11), e68, 2017 Volz & Gazzaniga, Brain, 140(7), 2051–2060, 2017 Volz, Hillyard, Miller, & Gazzaniga, Brain, 141(3), e15, 2018).

Corpus colossus