Split Brain, Undivided Consciousness?

By Neuroskeptic | January 31, 2017 6:12 am

A new paper challenges a decades-old theory in neuroscience: Split brain: divided perception but undivided consciousness

According to the famous work of Roger Sperry and Michael Gazzaniga, “split brain” patients seem to experience a split in consciousness: the left and the right side of their brain can independently become aware of, and respond, to stimuli. Split brain patients are those who underwent surgery to sever the corpus callosum, the nerve tract connecting the two hemispheres of the brain.

However, according to Dutch researchers Yair Pinto and colleagues in the new paper, the traditional view is mistaken: there is in fact no evidence of a split consciousness in these patients.

Pinto et al. carried out a series of studies on two split brain patients, “DDC” and “DDV”. Here’s how Pinto et al. summarize their results and how they differ from the conventional view:

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Traditionally, it’s said that a split brain patient can report seeing a stimulus shown in the left visual field, but only with their left hand, not the right. A stimulus in the right visual field can be reported with the right hand alone and also verbally; this is because language is located in the left hemisphere of the brain, and the left hemisphere receives visual input from the right visual field.

However, Pinto et al. found that their patients were able to report the presence of stimuli on either the left or right visual field using either hand, as well as verbally. In other words, the split brain patients performed perfectly normally on this task.

pinto_splitThe authors say that this “surprising finding” suggests that “severing the cortical connections between the two hemispheres does not seem to lead to two independent conscious agents within one brain”. Similarly, when asked about their subjective experiences (although perhaps these should be taken with a pinch of salt),

Both patients indicated that they saw their entire visual field (so not just the visual field to the left or right of fixation). Further, they indicated that they felt, and were in control of their entire body. Finally, they reported that their conscious unity was unchanged since the operation (i.e. no other conscious agent seemed to be present in their brain/body).

Yet the patients in this study did have some deficits. Patient DDC was unable to say whether two stimuli presented on opposite sides of the visual field were the same or different. This suggests that the transfer of information between his brain hemispheres was indeed impaired – raising the question of why this did not affect his awareness.

Pinto et al. conclude that

Even without massive communication between the cerebral hemispheres, and thus increased modularity, unity in consciousness and responding is largely preserved. This preserved unity of consciousness may be especially challenging for the two currently most dominant theories of consciousness, the Global Workspace theory and the Integration Information Theory.

Indeed, but I’d go further: these results seem tricky to reconcile with any obvious model, not just of consciousness, but of brain function in general. How can the left hemisphere of the brain respond to stimuli presented in the wrong (left) visual field? It doesn’t seem possible within the accepted model of the brain’s wiring, unless we assume that there are intact interhemispheric connections.

So are we forced to hypothesize some kind of spooky “non-local” consciousness field which can somehow bridge the gap between the left and right hemispheres…? Perhaps we don’t have to go that far. Pinto et al. note that both of their patients had the split brain surgery decades ago and that in theory it is possible that “patients somehow develop mechanisms or even structural connections to re-integrate information across the hemispheres” over time.

ResearchBlogging.orgPinto Y, Neville DA, Otten M, Corballis PM, Lamme VA, de Haan EH, Foschi N, & Fabri M (2017). Split brain: divided perception but undivided consciousness. Brain PMID: 28122878

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  • http://selfawarepatterns.com/ SelfAwarePatterns

    Aren’t there sub-cortical connections between the two sides of the brain? It seems like this might be compatible with the phenomenon of blindsight, where the sub-cortical regions process information that is partially available to the cerebral regions. It might be that, over the decades, those connections got recruited as a new, albeit limited, channel.

    • smut clyde

      The extreme case of neural flexibility finding a work-around is when the callosum was never present at all. From Whackyweedia:

      Those with callosal agenesis can still perform interhemispheric
      comparisons of visual and tactile information but with deficits in
      processing complex information when performing the respective tasks.

    • Jeremy Slater

      SelfAwarePatterns hits the nail on the head, I think. Patients demonstrating “blindsight” appear to take advantage of a secondary collicular visual system, as one example of multiple chances for cross-hemispheric integration of visual information (albeit limited in scope) prior to the generation of conscious visual perception. As noted by smut clyde, the existence of these subcortical channels is also supported by studies of patients with agenesis of the corpus callosum. These patients start out life with a “split brain” yet are rarely found to show signs of split consciousness, suggesting that early life neural plasticity takes advantage of subcortical pathways to maximize interhemispheric integration. We sometimes find patients with this condition in the epilepsy clinic – haven’t found one with evidence of dual consciousness yet 😉

  • SDJets

    I’ve thought it’s been long accepted that there is wide variation in the way that split-brain patients manifest their behavior. Part of this variation comes from the fact that different brains are organized differently pre-surgically (e.g., language is often though not always left-lateralized). Part of it is because the surgery itself varies from patient to patient. Does the original article address such variation?

    • http://blogs.discovermagazine.com/neuroskeptic/ Neuroskeptic

      They do discuss the surgery, they say “In Patient DDC the complete corpus callosum and most of the anterior commissure was cut, and in Patient DDV the complete corpus callosum was removed. We selected Patient DDC for the extensive follow-up testing since his ‘split’ is the most severe… It is unlikely that our results can be explained by the anterior and posterior commissure still being (somewhat) intact, as this was also the case for many of the previously tested patients, and this did not seem to play an important role then.”

      As for pre-surgical differences, they didn’t discuss this.

      • Jonathan Doe

        It’s possible that redundant thalamo-cortical pathways can still allow this kind of information exchange between hemispheres even when the Corpus Callosum is entirely absent. Again, individual differences may play a big part here…

  • kieron George

    I thought the split consciousness was a temporary effect that only exists until the brain establishes alternative route to connect the hemispheres.

  • Stephen Dedalus

    In my opinion, most of the evidence for dual consciousness was bad anyway. Most of them originated from misinterpretations of Sperry-Gazzaniga work.
    It doesn’t help that people like VS Ramanchadran tell people bizarre anecdotes like that:https://www.youtube.com/watch?v=PFJPtVRlI64

  • smut clyde

    How can the left hemisphere of the brain respond to stimuli presented in the wrong (left) visual field?
    I remember Gazzaniga noting (somewhere in one of his later books) that a lot of unconscious cueing went on between the two hemispheres for some of the subjects, at a purely corporeal level, as the hemispheres kept their unitary consciousness synchronised. So if one hemisphere had access to visual information not available to the other, there might be a subtle physical twitch — felt by both sides — to signal it.

    • http://blogs.discovermagazine.com/neuroskeptic/ Neuroskeptic

      Thanks – the authors of this paper address that:

      “Another possible explanation to consider is that the current findings were caused by cross-cueing (one hemisphere informing the other hemisphere with behavioural tricks, such as touching the left hand with the right hand). We deem this explanation implausible for four reasons.

      First, cross-cueing is thought to only allow the transfer of one bit of information (Baynes et al., 1995). Yet, both patients could localize stimuli throughout the entire visual field irrespective of response mode (Experiments 1 and 5), and localizing a stimulus requires more than one bit of information.

      Second, visual capabilities differed per hemifield and comparison of stimuli over hemifields was not possible (Experiment 2). This suggests that transfer of visual information did not occur. Yet, in these same experiments response type did not affect performance, suggesting that unity in control was not driven by any form of transfer of visual information.

      Third, we explicitly set up the experiments to prevent cross-cueing (e.g. hands were not allowed to touch each other, or the other half of the body). Moreover, we did not observe any indications of cross-cueing occurring.

      Fourth, as cross-cueing is a slow process, ipsilateral responses driven by cross-cueing should be considerably slower than contralateral responses. Yet, in one experiment… average ipsilateral and contralateral responses were almost equally fast, and equally accurate (ipsilateral reaction times: 1229 ms, ipsilateral accuracy: 88.4%; contralateral reaction times: 1307 ms, contralateral accuracy: 97%; No significant difference between ipsilateral and contralateral reaction times: P = 0.13; or between ipsilateral and contralateral accuracy: P = 0.55)”

  • The Outcast

    With comments like ‘kind of spooky “non-local” consciousness field’ only goes to prove the limitations of the author of this article, indeed this site!

  • smut clyde

    Even without massive communication between the cerebral hemispheres, and thus increased modularity, unity in consciousness and responding is largely preserved.

    I can’t see why this is an issue. People have other disconnection syndromes and no-one wonders how their unity of consciousness persists.
    Me, I rate for Dennett’s “centre of narrative gravity” approach to Self. If “consciousness” is a kind of narrative construct, the outcome of trying to squeeze the distributed and multi-threaded nature of cortical activity into a single experiential stream, then the persistence of that construct despite obstacles to hemispheric synchronisation is not really a surprise.

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  • Olmy Olm

    As for non-local consciousness, several skeptics have conceded the evidence for it that a normal claim would require, with the caveat that this is an extraordinary claim that requires extraordinary evidence: http://www.skepticalaboutskeptics.org/examining-skeptics/editorial-skeptics-concede-evidence-for-psi/

    Psi is real. The only thing preventing people from accepting it is, as some of them honestly admit themselves, prejudice.

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Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.

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