Abstract
If we were to connect two individuals’ brains together, how would this affect the individuals’ conscious experiences? In particular, it is possible for two people to share any of their conscious experiences; to simultaneously enjoy some token experiences while remaining distinct subjects? The case of the Hogan twins—craniopagus conjoined twins whose brains are connected at the thalamus—seems to show that this can happen. I argue that while practical empirical methods cannot tell us directly whether or not the twins share conscious experiences, considerations about the locality of content processing in the brain entails that they most likely do so.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
Notes
A clip of the children reading in the Inseparable documentary (Pyke 2017: 31.00–32.00) suggests a roughly 1st grade level of reading at the age of 10.
It is not specified in the documentary whether these are sensory signals, or motor signals, or both. However, given the observational evidence, it seems to be at least motor signals.
This particular observation is relevant for theories of self-knowledge such as Carruthers (2009), since it indicates that inner speech is being transmitted through the sensory hub of the thalamus. Thus it could support the claim that we listen to ourselves producing inner speech.
This raises the fascinating question of whether the twins can access each other’s dreams.
Bayne distinguishes phenomenal unity from representational unity (where experiential aspects are bound under a common representational content, such as the redness and roundness of a ball) and subject unity (where experiences are subject-unified just in case they occur to the same person).
Montero (2017) appears to be defending a similar position on the twins’ case, although she does not analyse this in depth.
A common charge against the partial unity model is that it is inconceivable—we cannot imagine what it would be like to have a partially unified consciousness (e.g. Bayne 2010: Sect. 2.4). However, as Schechter (2014) points out, we similarly cannot imagine what it would be like to have two separate streams of consciousness. The whole point of both views is to give up the claim that there is a single subjective perspective. Another objection concerns indeterminacy: what could make it true that a subject had a partially unified consciousness as opposed to two separate streams. This is directly parallel to the problem of distinguishing sharing from divergence that I develop in Sect. 4.
Langland-Hassan (2015) also contrasts these possibilities when discussing the twins’ case.
Access consciousness is often linked with activity in the prefrontal cortex. See e.g. Lamme (2004), who argues for a neural distinction between access and phenomenal consciousness.
The same argument seems to refute the indeterminacy objection to the partial unity model of split brains. The possibility of differentially manipulating common experiences is a clear way to distinguish partial unity from two-stream views.
Though note that Dennett would deny even this spatially distributed model, since he believes that the consciousness of content has no ‘finish line’ in the brain, either or spatially or temporally.
See sources cited at the end of Sect. 2. There is also a case reported by Ploner et al. (1999) in which a patient, following damage to his somatosensory cortex was unable to locate the sensation of pain in his arm, while still able to report an unpleasant sensation ‘somewhere between his shoulder and hand’.
The pre-thalamic stages of Tatiana’s pain processing system may also be active but they need not be (this should be possible to confirm with current neural imaging).
There is another version of this objection according to which a content area accessed by both twins is somehow communicating with each twin in a distinct way simultaneously. For instance, Prinz appeals to synchronized rhythmic activity across neural populations. Perhaps one subset of the neural population is communicating with one twin, while another subset communicates with the other twin. We lack the fineness of resolution to tell that this is not occurring. Effectively, the objection here is that the twins are not really accessing the very same processing stages. I think all we can say in response is that we have no reason to believe it is even possible for a neural population to consciously process a single aspect of content while simultaneously adopting two distinct synchronisation patterns. The suggestion is merely an ad hoc way to defend divergence, in contrast to a simpler picture in which one pattern of neural activity in the content-processing site is able to contribute to both girls’ consciousnesses.
On the basis of the neural evidence, the same conclusion that content is not replicated in the frontal executive areas is drawn by Hirstein (2012, pp. 88–89; cf. 29; cf. Prinz 2012, pp. 101–102), though Hirstein believes that the frontal areas are the basis of the self which reflects on conscious content (e.g. 2012, pp. 22–24). Note that if the executive area with which content processing sites are supposed to interact is supposed to replicate that content, then this would simply be another version of a specific convergence zone, and is contradicted by the neural observations.
It may be the case that the twins share consciousness in this sense in addition to sharing conscious content though it would require a very careful articulation of just what the relevant function involves. For now I merely point out that the twins seem to collaboratively manage their shared conscious content, as when they use it to fluently move around, or jointly attend to their pain. Depending on how exactly the function is articulated, perhaps even listening to each other’s inner speech counts as shared integration of conscious content, or higher-order reference to it.
Documentary sources
Pyke, J. (2014). Twin life: Sharing mind and body. Twin Life Productions in association with Canadian Broadcasting Company.
Pyke, J. (2017). Inseparable: 10 years joined at the head. Curious Features in association with Canadian Broadcasting Company.
References
Ab Aziz, C. B., & Ahmad, A. H. (2006). The role of the thalamus in modulating pain. The Malaysian Journal of Medical Sciences, 13(2), 11–18.
Anderson, M. L. (2010). Neural reuse: A fundamental organizational principle of the brain. Behavioral and Brain Sciences, 33(4), 245–266.
Baars, B. J. (1988). A cognitive theory of consciousness. Cambridge, MA: Cambridge University Press.
Bayne, T. (2010). The unity of consciousness. Oxford: Oxford University Press.
Block, N. (1995). On a confusion about a function of consciousness. Behavioral and Brain Sciences, 18(2), 227–247.
Bramble, B. (2013). The distinctive feeling theory of pleasure. Philosophical Studies, 162(2), 201–217.
Carruthers, P. (2009). How we know our own minds: The relationship between mindreading and metacognition. Behavioral and Brain Sciences, 32(2), 121–138.
Carruthers, P. (2019). Human and animal minds: The consciousness questions laid to rest. Oxford: Oxford University Press.
Churchland, P. (1981). Eliminative materialism and the propositional attitudes. The Journal of Philosophy, 78(2), 67–90.
Courtiol, E. & Wilson, D. A. (2015). The olfactory thalamus: Unanswered questions about the role of the mediodorsal thalamic nucleus in olfaction. Frontiers in Neural Circuits, 9, article 49, 1–8.
Dainton, B. (2008). The phenomenal self. Oxford: Oxford University Press.
Damasio, A. R. (1992). The selfless consciousness. Behavioral and Brain Sciences, 15(2), 208–209.
Davis, L. (1997). Cerebral hemispheres. Philosophical Studies, 87(2), 207–222.
Dennett, D. (1991). Consciousness explained. New York: Little Brown & Co.
Dominus, S. (2011). Could conjoined twins share a mind? New York Times Magazine, 25th May 2011.
Gallagher, S. (2012). Multiple aspects in the sense of agency. New Ideas in Psychology, 30(1), 15–31.
Hardcastle, V. G. (2017). The binding problem. In W. Bechtel, G. Graham, & D. A. Balota (Eds.), A companion to cognitive science (pp. 553–565). Oxford: Blackwell.
Hershenov, D. B. (2013). Who doesn’t have a problem of too many thinkers? American Philosophical Quarterly, 50(2), 203–208.
Hirstein, W. (2012). Mindmelding: Consciousness, neuroscience, and the mind’s privacy. Oxford: Oxford University Press.
Ibañez, A., Gleichgerrcht, E., & Manes, F. (2010). Clinical effects of insular damage in humans. Brain Structure and Function, 214(5–6), 397–410.
Klein, C. (2015). What pain Asymbolia really shows. Mind, 124(494), 493–516.
Koch, C., Massimini, M., Boly, M., & Tononi, G. (2016). Neural correlates of consciousness: Progress and problems. Nature Reviews Neuroscience, 17(5), 307–321.
Koelsch, S., Jacobs, A. M., Menninghaus, W., Liebal, K., Klann-Delius, G., von Scheve, C., et al. (2015). The quartet theory of human emotions: An integrative and neurofunctional model. Physics of Life Reviews, 13, 1–27.
Lamme, V. A. (2004). Separate neural definitions of visual consciousness and visual attention; a case for phenomenal awareness. Neural Networks, 17(5–6), 861–872.
Langland-Hassan, P. (2015). Introspective misidentification. Philosophical Studies, 172(7), 1737–1758.
Lewis, D. (1976). Survival and identity. In A. Rorty (Ed.), The identities of persons (pp. 17–40). Berkeley: University of California Press.
Lockwood, M. (1989). Mind, brain and the quantum: The compound ‘I’. Oxford: Blackwell Publishers.
Lockwood, M. (1994). Issues of unity and objectivity. In C. Peacocke (Ed.), Objectivity, simulation, and the unity of consciousness (pp. 89–95). Oxford: Oxford University Press.
Montero, B. (2017). What combination problem? In G. Brüntrup & L. Jaskolla (Eds.), Panpsychism: Contemporary perspectives (pp. 215–228). New York: Oxord University Press.
O’Brien, G., & Opie, J. (1998). The disunity of consciousness. Australasian Journal of Philosophy, 76(3), 378–395.
O’Brien, G., & Opie, J. (2000). Disunity defended: A reply to Bayne. Australasian Journal of Philosophy, 78(2), 255–263.
Ohara, S., & Lenz, F. A. (2003). Medial lateral extent of thermal and pain sensations evoked by microstimulation in somatic sensory nuclei of human thalamus. Journal of Neurophysiology, 90(4), 2367–2377.
Parfit, D. (1971). Personal identity. Philosophical Review, 80(1), 3–27.
Ploner, M., Freund, H. J., & Schnitzler, A. (1999). Pain affect without pain sensation in a patient with a postcentral lesion. Pain, 81(1/2), 211–214.
Prinz, J. (2012). The conscious brain. Oxford: Oxford University Press.
Radden, J. (2011). Multiple selves. In S. Gallagher (Ed.), The Oxford handbook of the self (pp. 547–570). Oxford: Oxford University Press.
Roelofs, L. (2019). Combining minds: How to think about composite subjectivity. Oxford: Oxford University Press.
Ryan, D. (2014). Through her sister’s eyes: Conjoined twins Tatiana and Krista were extraordinary from the beginning. Vancouver Sun, 2nd January 2014.
Schechter, E. (2014). Partial unity of consciousness: A preliminary defense. In D. Bennett, D. J. Bennett, & C. Hill (Eds.), Sensory integration and the unity of consciousness (pp. 347–374). Cambridge, MA: MIT Press.
Schechter, E. (2018). Self-consciousness and ‘split’ brains: The minds’ I. Oxford: Oxford University Press.
Tye, M. (2003). Consciousness and persons. Cambridge, MA: MIT Press.
Unger, P. (1990). Identity, consciousness and value. Oxford: Oxford University Press.
Usrey, W. M., & Alitto, H. J. (2015). Visual functions of the thalamus. Annual Review of Vision Science, 1, 351–371.
Van Inwagen, P. (1995). Material beings. Ithaca: Cornell University Press.
Violentyev, A., Shimojo, S., & Shams, L. (2005). Touch-induced visual illusion. NeuroReport, 16(10), 1107–1110.
White, M. (1979). A new effect on perceived lightness. Perception, 8(4), 413–416.
Zeki, S. (2007). The disunity of consciousness. Progress in Brain Research, 168, 11–268.
Acknowledgements
Thanks to Philip Gerrans, Jon Opie and the two anonymous referees of this journal for their comments on earlier versions of this paper.
Author information
Authors and Affiliations
Corresponding author
Additional information
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
About this article
Cite this article
Cochrane, T. A case of shared consciousness. Synthese 199, 1019–1037 (2021). https://doi.org/10.1007/s11229-020-02753-6
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s11229-020-02753-6