In the previous section I outlined and defended Hurley’s claim that the passing of cross-cueing from the personal to the sub-personal level of explanation allows for the unification of split-brain consciousness. Although Hurley does offer these criteria, she does not explain how such a transition could occur. In this section I am going to suggest that it occurs via the learning of new perceptual sensorimotor skills. Enactive and ecological theories of conscious perception take an organism’s ability to perceive to be predicated on its possession and exercise of sensorimotor skills (Anderson 2014; Barrett 2011; Bruineberg and Rietveld 2014; Chemero 2016; O'Regan 2011; Noë 2004; Thompson 2007). They explain perception to be predicated on the possession and exercise of the knowledge-how required to become perceptually related to the environment. I want to suggest that we can explain how this perceptual know-how is acquired (in the case of split-brain syndrome) by applying the Dreyfus account of skill acquisition [Dreyfus 2002; Dreyfus and Dreyfus 1986/2014; cf. Dreyfus and Dreyfus 1980].Footnote 9 If we apply this account to enactive and ecological theories of conscious perception, then we will be able to provide an operationalisation of the passage of perceptual know-how from being applicable at the personal level of description to being applicable at the sub-personal level. Consequently, we could determine whether split-brain subjects do use perceptual sensorimotor skills which are partially constituted by external factors when they perceive.

Applying Dreyfus’ account of skill to split-brain syndrome

The Dreyfus account distinguishes between five stages of skill acquisition which are labelled as follows— “novice”, “advanced beginner”, “competence”, “proficiency”, and “expertise”. Dreyfus’ account explains skill learning as initially applicable at the personal level of explanation. When we begin to learn a new skill we engage in deliberate personal level activity. However, as the learner passes through the various stages of skill-learning, their possession and exercise of the skill comes to apply more and more at the sub-personal level of explanation. Once a skill has been mastered, the Dreyfus account takes it to be explained solely in terms of sub-personal mechanisms. Thus, stage one is thought to apply at the personal level of description whilst stage five is thought to apply wholly at the sub-personal level. This schema can therefore be used to provide a means through which Hurley’s criteria can be operationalised and tested. I am going to argue that if we apply this account of skill acquisition to split-brain syndrome, we will find that split-brain subjects exhibit evidence of learning a new sensorimotor skill— “constitutive-cross-cueing”. In exercising this skill the split-brain agent uses external factors to pass information across hemispheres, and so these external factors come to play the functional role once played by the corpus callosum. They can do so, I argue, by engaging in cross-cueing behaviours.

At the first stage of skill acquisition, which Dreyfus labels the “novice” stage, the learner is given specific information about scenarios out of context and instructed to follow specific rules in these specific scenarios. The novice chess player, for example, will be informed of the numerical value of each piece and instructed to never engage in a negative exchange. Novice learners will be unable to incorporate these instructions into the wider situational context of the activity in question, and so their knowledge is constituted by a series of autonomous, context-free, and rule-guided behaviours. For example, the novice chess player will be unable to recognise situations wherein a negative exchange would be appropriate. Rather, they will employ the rule ‘never engage in a negative exchange’ across the board when they play chess.

This stage would appear to best correspond to the state of the split-brain subject immediately post-surgery. If the subject does use external factors to pass information between hemispheres at this point in time, it would appear that the behaviour is engaged in deliberately. Although the subject is attempting to use external factors to replace the now-missing corpus callosum, their use of these factors would appear to be constituted by intentional and rule-guided behaviour. An example of cross-cueing which would appear to support this idea is an idiosyncratic behaviour carried out by subject N.G.:

[P]art way through test [N.G.] changed [her] manner of responding. Instead of pointing quickly with her hand, she would pause before her choices and move her hand only after a motion of her head which resulted in her pointing of her chin toward the choice subsequently pointed out by the hand. When this was recognised and she was asked not to move her head she resumed the undelayed pointing with her right hand. (Bogen 1990, p. 220)

N.G.’s behaviour appears to be a deliberate and personal level phenomenon because she would “pause before her choices”. Furthermore, when asked to cease her behaviour N.G. was able to do so immediately (presumably, if the behaviour were automatic and sub-personal N.G. would have more trouble ceasing it). N.G. appears to have explicitly decided upon the use of this strategy in the specific context of the experiment she is subject too. Therefore, N.G.’s exercise of the skill “constitutive-cross-cueing” is best categorised at the novice stage of skill development.

The second advanced beginner stage of skill development is reached when the learner can recognise contextually defined situations and apply maxims which are specific to that particular situation. For example, the advanced beginner will be capable of determining when an opponent’s King’s defence is weak and will be capable of applying the maxim ‘attack a weakly defended King’ in light of their determination. Whether or not a King’s defence is weak is something which can only be determined in the context of a specific game, and so the advanced beginner’s decision to attack the King is based on situation-specific knowledge. The advanced beginner has moved beyond the novice stage because they are now capable of recognising context-specific situations and responding to them in an appropriate manner.

Split-brain patient R.M. shows signs of being at the advanced beginner stage of learning the skill “constitutive-cross-cueing”. R.M. exhibited a number of (largely ineffective) cross-cueing behaviours when undergoing tests four months after surgeryFootnote 10:

When a pencil was placed in his left hand he held it appropriately but could not name it. It was then put into his right hand and he said, "a pencil." When a watch was put in his left hand, he said it was a "pencil" even when, with his left hand, he was holding the watch up to his left ear. A paper clip was put in his hand and he could not tell what it was; but when he put it in his right hand he immediately identified it. A pipe in the left hand was put into his mouth in an appropriate way; but it was called a "pencil" even after the bit was between his teeth. When an ashtray was put into his left hand he struck the table with it; it made a distinctive sound and he immediately told me what it was. When a pair of glasses was put in his hand, he could not name what he was holding until he tried to put them on. A handkerchief was put in his left hand; his left hand immediately put it into his left hip pocket…but he could not say what it was. (Bogen 1990, pp. 219-220).

R.M. is attempting to engage in cross-cueing behaviour and he does appear to recognise that, in the context of experimental situations, it is a good idea to exercise the skill “constitutive-cross-cueing”. However, R.M. is not very proficient in this skill and so his attempts to exercise it are largely unsuccessful.

At stage three of Dreyfus’ hierarchy the learner is labelled “competent”. Competent learners possess the ability to discern a number of task-relevant situations and they are capable of focusing on one task to the exclusion of the others. They will be able to act in accordance with the actions appropriate for that particular task whilst ignoring the actions which could be undertaken were they to be engaged in a different task. For example, the competent chess player will possess the ability to discern both that their opponent has a weak Kingside defence and that they themselves have a weak pawn structure. They will be capable of focusing and acting upon one of these situations to the exclusion of the other. If they decide to attack the opponent’s King and that attack further weakens their pawn structure, they will be able to ignore the guideline ‘rectify a weak pawn structure’ in order to succeed in the task of attacking the opponent’s King. Competent learners follow situation-specific maxims as opposed to situation-independent rules. Instead of prescribing specific actions for the completion of a given task, such situation-specific maxims instead prescribe general guidelines for context-dependent situations. The competent chess player’s attack on the opponent’s King will be guided by the ‘attacking the King’ maxim, and the moves they make will be determined by the context of the particular state of the game. Their moves will not be guided by rules which prescribe specific moves which should be made when attacking the King.

The competence stage of split-brain subjects learning the skill “constitutive-cross-cueing” seems to best describe a set of behaviours observed by Kingstone and Gazzaniga (1995). The word “o’clock” was flashed to the subject’s left hemisphere whilst the word “ten” was flashed to its right hemisphere. When the split-brain subject was asked to draw what they had seen, they (surprisingly) drew a picture of a clock which was showing the time was ten o’clock. Kingstone and Gazzaniga performed a series of further experiments on this subject. They concluded that the subject had been able to circumvent the experimental conditions by allowing each hemisphere to control the subject’s left hand. The left hemisphere was initially given control of the left hand and it drew a clock. The right hemisphere was then given control of the left hand, and it drew the hands of the clock to indicate ten o’clock. Interestingly, Kingstone and Gazzaniga commented that “the only integration to be found here occurred ‘on the sheet of paper in the drawing itself’” (Kingstone and Gazzaniga 1995, p. 324). If my thesis is correct, such external factors do indeed play a constitutive role in integrating the subject’s perceptual field.

Unfortunately, Kingstone and Gazzaniga did not investigate the extent to which the subject’s co-ordination of hemispheric control of the hands was a deliberate and personal level decision or an automatic and sub-personal behaviour. So I can only provide a speculative explanation of the behaviour. That said, the behaviour is consistent with the subject having become used to applying the skill “constitutive-cross-cueing” even during experimental conditions. The subject’s behaviour was novel in the context of split-brain experiments, so there is reason to believe the subject is competent at the skill. My main reason for taking the subject to be only competent at the skill (as opposed to more advanced) is that the subject’s integrative behaviour was quite unimaginative. For example, when shown the words “hot” and “dog” the subject used the same technique (of switching hemispheric control of the hand) to draw a dog panting in heat. It would be more natural to interpret these words to refer to a piece of meat in a bun, and so a drawing of a sausage in a bun would be more appropriate. This indicates that the subject is not completely practised in applying the skill “constitutive-cross-cueing” during split-brain experiments. They are capable of applying the maxim ‘switch-hemispheric-control-of-hands’ during split-brain experiments. However, they are not particularly skilled at applying the maxim across different situations within the experiments.

Subjects who have reached the fourth stage of adaption are labelled as “proficient”. Proficient performers possess the ability to intuitively ‘see’ salient aspects of a given situation. However, they must still engage in explicit deliberation of maxims in order to determine the best course of action to undertake. A proficient chess player, for example, will be able to instinctively ‘see’ that the best course of action to take would be to mount an attack on the opponent’s King. They will still, however, need to plan this attack by considering the specific moves they should make. Split-brain subject L.B. appears to be at the proficient stage of learning the skill “constitutive-cross-cueing”. Consider the following behaviour, which he carried out during a split-brain experiment:

After a picture of a tree appeared in the left hemi-field, his hands formed a triangle, and he then said ‘teepee’. (Bogen 1990, p. 220)

L.B. appears to be well-practiced in engaging in split-brain experiments, to the point where he can ‘see’ from the context of the situation that cross-cueing would be appropriate. However, he is still using rather obvious bodily cues— he makes a rather noticeable triangular shape with his hands, as opposed to using the more subtle embodied cues and perceptual behavioursFootnote 11 which proliferate in extremely experienced split-brain subjects (Gazzaniga 2013)— and this makes it appear as if he must still explicitly deliberate on what action to perform. In other words, he can ‘see’ he is in a split-brain experiment but he must still make a personal level decision to engage in a specific cross-cueing behaviour. Having said this, such behaviour has only been observed in L.B. and so it seems he is at a particularly advanced stage of applying the skill ‘“constitutive-cross-cueing”. I therefore think the label proficient is most appropriate for L.B.

The final and most advanced stage of skill learning is labelled “expertise”. Experts simply instinctively ‘see’ the specifics of their situation, and they intuitively ‘know’ how best to respond to the situation in question. Experts exercise skills absorbedly— they perform automatically and lack any awareness of what they are doing. They do not need to think about what they are doing. Indeed, (according to Dreyfus) thinking will actually have a negative effect on the actor’s performance of the skill. Dreyfus argues that the best way to explain the behaviour of experts is to understand it to be wholly automatic, sub-personal, and to occur without thought. Rather than thinking about what they are doing, experts simply allow their bodies to perform the activity.

Split-brain subject C.C. appears to have achieved expertise in the skill “constitutive-cross-cueing”. C.C. was asked to perform the same tasks as subject R.M. (quoted in detail earlier). However, he was much more effective in his execution of the tasks. C.C. was often observed to engage in non-verbal cross-cueing behaviours “which sometimes led to the correct answer and sometimes to an answer which was related but not correct” (Bogen 1990, p. 219).Footnote 12 As Bogen notes, “it is quite instructive how an individual with a verbal IQ under 70 (65 at this time) can use minimal sensory cues to identify objects which are familiar to him.” [ibid, p. 219]. C.C. is not particularly well-equipped (cognitively) to actively decide to engage in cross-cueing. He is not cognitively well-equipped to successfully perform such behaviours. Finally, he does not appear to be deliberately engaged in their execution. Given these points, there is some reason to believe that C.C.’s cross-cueing behaviours are automatically deployed and so best described as applying to the sub-personal level of description. This is an attractive explanation of C.C.’s behaviour because C.C. was being tested eight years after surgery. We can therefore explain the difference between C.C.’s performance, and the performance of R.M. (who was tested only four months after surgery), by arguing that C.C. has achieved mastery of the sensorimotor skill “constitutive-cross-cueing”. C.C. excels in the skill “constitutive-cross-cueing” because he has practiced it for eight years. He can thereby be labelled an expert of this skill.

Indeed, one could go further here and argue that the general behaviour of split-brain subjects in everyday life is best described in terms of their possessing expertise in the skill “constitutive-cross-cueing”. Split-brain subjects are virtually indistinguishable from healthy controls in everyday life. Once they have adapted to the absence of the corpus callosum they primarily engage in aberrant behaviour only during experiments (however, even during experiments they still occasionally exercise the skill “constitutive-cross-cueing”, as we have seen). Split-brain subjects appear to deploy the skill “constitutive-cross-cueing” automatically, and the fact they are not aware that they do so indicates that this skill is being deployed absorbedly. Consequently, these subjects appear to display expertise in this skill. If subjects are experts at performing this skill, then it is best described as applying at the sub-personal level. The external factors used by the split-brain subject should therefore be considered to be constitutive of experience— they have replaced the corpus callosum and now play the functional role of integrating the anatomically split hemispheres and so enabling a singular centre of access consciousness.

Does recent empirical evidence require we revise the standard account of split-brain syndrome?

Pinto et al. have recently published a paper which prima-facie appears to conflict with the standard understanding of split-brain syndrome outlined in this paper, and the empirical work upon which this understanding has been based (Fig.1, Pinto et al. 2017). As previously explained, it has commonly been thought that if stimuli are localised to the left and right hemispheres of a split-brain subject, each hemisphere will engage in autonomously controlled and goal-directed cognitive behaviour in response to the particular stimulus that hemisphere has been exposed to. Furthermore, each hemisphere will do so whilst being completely unaware of the information ‘contained within’ and/or acted upon by the opposite hemisphere. However, when Pinto et al. subjected split-brain subjects to the same kinds of experimental tests outlined above, their results differed in extremely important ways from that of previous empirical work. Pinto et al. helpfully provide the following diagram which summarises their findings:

Fig. 1 The standard account of split-brain syndrome and Pinto et al.’s findings Full size image

In short, although Pinto et al. replicated the finding that split-brain subjects cannot compare stimuli which have been localised to different hemispheres (the first box), they found that subjects could correctly answer questions about a given stimuli regardless of which hemisphere was in control of answering the question and which hemisphere the stimulus in question was localised to (boxes two and three). According to Pinto et al. these experimental results show that whilst perceptual information is localised to a given hemisphere during the split-brain experiment, cognitive access is not localised to a given hemisphere. Thus, it is not true that each hemisphere is cognitively unaware of the stimulus presented to and/or acted upon by the opposite hemisphere.

If one adopts an internalist approach toward split-brain syndrome, these results are indeed peculiar. The brains of the subjects tested by Pinto et al. have previously been mapped and it has been confirmed that their hemispheres were fully split during surgery and that no new anatomical inter-hemispheric connections have subsequently developed in their brains. As such, if one adopts an internalist perspective, this empirical work appears to require one of two conclusions:

1. Most, if not all, of the previous experimental work on split-brain syndrome (which spans decades) was fundamentally misguided, misinterpreted, and/or carried out incorrectly. 2. Both the previous experimental work carried out on split-brain syndrome and the empirical work reported in the Pinto et al. paper is of good standing. As such, something extremely mysterious has occurred.

Of course, neither (1) nor (2) is appealing for the empirically minded philosopher or psychologist. However, if one adopts the externalist approach outlined in this paper, then a third option becomes available:

3. Both the previous experimental work carried out on split-brain syndrome and the empirical work reported in Pinto et al.’s paper is of good standing. The reason the more recent results diverge from earlier reported work is that the subjects in question have become more adept at exercising the perceptual skill ‘constitutive-cross-cueing’.

By adopting an externalist account, one can provide a non-mysterious explanation as to why Pinto et al. derived the results that they did without calling into question the empirical credentials of virtually all the work previously carried out on split-brain syndrome. Indeed, the account sketched in this paper arguably predicts these recent empirical findings. The subjects reported on in Pinto et al. had each undergone split-brain surgery some time ago and each had plenty of experience participating in split-brain experiments. It is therefore quite likely that these results occurred because subjects have almost completely mastered the skill ‘constitutive-cross-cueing’, to the extent that it can be exhibited even during experimental conditions.

Pinto et al. do actually countenance the possibility that the results they report could have been due to cross-cueing. However, they reject this possibility for four reasons: (1) cross-cueing mechanisms are incapable of transmitting the amount of information possessed by subjects; (2) given that perceptual information about localised stimuli was not possessed by both hemispheres, it is unlikely that subjects possess knowledge of localised stimuli due to cross-cueing (if they did, then why was the perceptual information not transferred across hemispheres?); (3) the experiments were specifically designed so that cross-cueing could not occur and no cross-cueing was observed; and, (4) patient responses occurred too quickly for them to be explained by cross-cueing. (Pinto et al. 2017, p. 1236).

Pinto et al. defend point (1) with reference to a paper by Baynes et al. (1995). However, although this paper does appear to show that the ability to transfer information via cross-cueing mechanisms is limited, Baynes et al. specifically note that the subject under study in this particular paper is constantly improving his skill at using cross-cueing mechanisms within split-brain experiments, such that his performance in these experiments improves over time (Baynes et al. 1995, p. 1232, p. 1236, & p.1240). Thus, it is not only the case that Pinto et al. can’t rule out the possibility that cross-cueing strategies improve over time. In fact, the paper they reference to show that cross-cueing strategies can transfer only limited amounts of information actually argues for the claim that subjects exhibit improved performances on experimental tasks because cross-cueing mechanisms improve over time. Therefore, it specifically allows for the conclusion that the amount of information passed between hemispheres can increase as cross-cueing abilities improve. Although point two does show that the subjects have not been able to use cross-cueing to pass all information across hemispheres, it does not therefore show that no cross-cueing occurs. For example, consider that the results upon which this point is predicated do not differ substantially from those reported by Kingstone and Gazzaniga which were discussed earlier in this paper. That cross-cueing is not perfectly executed in a given situation does not thereby show that no cross-cueing whatsoever has occurred. Point three likewise does not show that cross-cueing does not occur, because there are plenty of examples wherein cross-cueing occurs in spite of experimental set-ups designed to remove its possibility, and it often takes further experimental work to find out the exact nature of a given cross-cueing strategy in a given experiment (cf. Gazzaniga 1969, 2013). Finally, it is not clear that the speed of patient response shows that cross-cueing could not have occurred. If subjects have learnt to use embodied or environmental mechanisms to play a constitutive role in unifying their conscious perceptual fields, then there is no principled reason to suspect that such external mechanisms would be any slower at passing information between hemispheres than the corpus callosum.Footnote 13

Interestingly, Pinto et al. do note that the time which had passed since the surgery has occurred could have played a role in their experimental findings. They suggest that perhaps “patients somehow develop mechanisms or even structural connections to re-integrate information across the hemispheres” (Pinto et al. 2017, p. 1236). This is precisely what I think has occurred, and the forgoing argument suggests a possible way in which it can be tested. It is unlikely that patient’s brains would have developed new connections— they are too old— but it is quite possible they could have developed a novel extended sub-personal system in response to having their corpus-callosum severed. Thus, rather than being problematic for the argument presented in this paper, these recent experimental results would appear to in fact support it.

‘Siamese twin’ objection

Another possible way to object to my argument would be to invoke the “Siamese Twin Objection”. According to this objection, if my argument is correct it may turn out to have too strong a conclusion— two human agents who engage in extremely closely integrated behaviour could come to possess a unified field of perceptual consciousness. Michael Lockwood raised this objection by using the example of Siamese twins (Lockwood 1994) and, interestingly, this counter-example appears to be somewhat of a reality: (Fig. 2).

Shivanath and Sharam can navigate their environment with relative ease and are capable of successfully co-ordinating their actions to complete everyday mundane behaviours (such as showering and dressing). Consequently, they could (presumably) co-ordinate their behaviour in such a way that their brains received sensory stimulation which accorded with Shivanath-and-Sharam-specific sensorimotor contingencies. If this is possible, then the argument proffered in this paper may appear to require that Shivanath and Sharam could come to possess a unified perceptual field in exactly the same manner as split-brain subjects. Furthermore, by a similar logic, my argument could also appear to license the existence of group consciousness— if groups of agents could co-ordinate their perceptual actions in a tightly integrated manner, then the individual members of the group could unify their conscious states such that there is ‘something it is like’ for the group (as an entity) to perceive.

In many ways, this objection is analogous to those which surfaced in the immediate aftermath of the original publication of the extended mind argument (Clark and Chalmers 1998). It was commonly objected that the extended mind had unpalatable consequences, and as such should be rejected. For example, the argument appeared to entail that the entire contents of the internet could count as part of an individual agent’s memory, or that groups of agents (such as institutions) could themselves count as being minded entities. There are roughly three stances which could have been taken in response to such objections: deny the more extreme consequences by tightening up one’s definition of when minds can reasonably be said to extend; accept these extreme conclusions as a surprising but sound consequence of a valid argument; or, leave the possibility of such conclusions on the table as a theoretical possibility, but as one which requires further argumentative work to arrive at. I think that a similar set of stances is available when defending my argument from the Siamese-twin objection.

One could outright reject the possibility of group consciousness by providing some further conditions as to when consciousness can occur, and explaining why these conditions are met in split-brain syndrome but not in Siamese twins or groups of agents. Andy Clark, for example, has argued against the possibility of consciousness extending beyond the brain by claiming that only brains can support the extremely precise, time-sensitive information-processing required for consciousness [2009]. One could provide a similar argument in the context of my account of split-brain subjects (of course, doing so requires diverging from Clark’s position because he does not think consciousness can extend beyond the brainFootnote 14). Michael Anderson has recently provided an empirically detailed account of how brains co-ordinate their own activities with bodily and environmental factors in order to efficiently spread cognitive load and successfully engage in goal-directed activities (Anderson 2014), and this account has been applied to and used to explain consciousness (Downey 2016). Anderson’s account may require that parts of the brain need to physically be in close proximity to successfully co-ordinate activity, such that even the relatively short distance between the brains of Siamese-twins rules out the possibility of their forming a single extended sub-personal system.Footnote 15 Of course, the brains of individuals within a group are physically removed to an even greater extent, so such an account of consciousness would likewise rule out the possibility of the group unifying their states of consciousness through the formation of one extended sub-personal perceptual system.

Alternatively, one could simply accept the possibility that Siamese-twins and groups of agents could unify their consciousness in exactly the same manner that I have argued split-brain subjects do. Whilst this would be a surprising and counter-intuitive conclusion, it is not clear that it should be rejected solely on that basis. Just as the extended mind is (arguably) logically entailed if one accepts a functionalist account of mind (Wheeler 2010), it could turn out that the existence of group consciousness is logically entailed if one accepts a functionalist account of consciousness (Schwitzgebel 2015). Thus, the existence of group consciousness may simply be a consequence of a sound and valid argument.

At present, I think it is best to conclude that the argument developed here appears to allow for the possibility of group consciousness, but that much more work needs to be done before this question can be decided either positively or negatively. After all, providing an argument for or against group consciousness (in the present context) requires that one provide specific details about specific examples and explain why they are relevantly like the split-brain case. For example, one would have to show that Shivanath and Sharam exhibit the same kinds of cross-cueing behaviours and relevantly similar group perceptual behaviour as split-brain subjects before concluding that they possess a singular group perceptual consciousness. Consequently, resolving this possibility in any direction requires much further work, and so I suggest that the Siamese-twin counter-example be left open as an avenue for future research.