Do We All Have Split Brains?

By Neuroskeptic | December 8, 2016 4:12 pm

When you’re doing two things at once – like listening to the radio while driving – your brain organizes itself into two, functionally independent networks, almost as if you temporarily have two brains. That’s according to a fascinating new study from University of Wisconsin-Madison neuroscientists Shuntaro Sasai and colleagues. It’s called Functional split brain in a driving/listening paradigm

In referring to ‘split brains’ in their title, Sasai et al. are linking their work to the literature on patients who have had a callosotomy, a radical brain operation that literally splits the brain in two by cutting the corpus callosum, the nerve tract that connects the left and right hemispheres of the cerebral cortex.

Only a small number of people with severe epilepsy have a callosotomy, but Sasai et al. argue that the healthy brain can ‘split’ itself when multitasking. Here’s how Sasai et al. illustrate the hypothesis of a ‘functional’ split between networks in the normal human brain, as opposed to a surgical left-right split:


Sasai et al. demonstrated this using fMRI. Thirteen participants who were scanned while performing a virtual driving task in which they had to navigate a road system. There was also a listening task: the audio stimuli were either a “GPS” voice that gave the participant instructions on where to drive to, or a “radio show” consisting of extracts from recent news articles, that were unrelated to driving.

The driving and listening tasks activated distinct regions of the brain, as shown here:

fmri_drivingThe key results came from participants who were simultaneously driving and listening to either the GPS or the radio show. Here’s what happened:

The integration between the two networks, assessed through a multivariate measure of functional connectivity based on coordinated shifts of multivoxel patterns across time, was higher in the integrated task compared with the split task. Furthermore, the integration of information between the two networks, assessed by the improvement in prediction accuracy of the joint dynamics of the two networks over their independent dynamics, was high in the integrated task and zero in the split task.

In other words, when the GPS voice was helping the participants to drive (“integrated task”), the brain ‘driving network’ and ‘listening network’ were acting in concert, with a high degree of functional connectivity. But when the drivers were listening to the radio show (“split task”), the two networks were largely independent – indeed, by one metric, which the authors call “integrated information“, they were completely seperate.

Sasai et al. go on to discuss the implications for our sense of self and consciousness

An intriguing question is what happens to consciousness when driving while listening in the split condition… does driving become unconscious, as on autopilot? Or, does a normally integrated conscious stream split into two separate conscious streams that coexist within the same brain, as indicated by studies of patients with an anatomically split brain? Integrated information is thought to be essential for consciousness (28), and the reduction of integrated information demonstrated here is at least compatible with a split in consciousness.

This is really interesting stuff. I’m not sure these results are directly relevant to consciousness, though. We don’t know for sure whether (say) the “driving network” is responsible for our conscious experiences related to driving. It’s possible that there is a “consciousness centre” elsewhere in the brain (the prefrontal cortex or the precuneus, perhaps), that integrates input from lower-level brain regions such as the “driving network”. If so, surely the disconnected nature of the lower-level networks would not necessarily preclude a unified consciousness.

ResearchBlogging.orgSasai, S., Boly, M., Mensen, A., & Tononi, G. (2016). Functional split brain in a driving/listening paradigm Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1613200113

CATEGORIZED UNDER: fMRI, papers, select, Top Posts
  • Maia

    “…the disconnected nature of the lower-level networks would not necessarily preclude a unified consciousness.” The other day I had a vividly clear experience that seemed to be an instance of this: I was reading a piece of music and playing my guitar while at the same time listening to a radio show. Usually, it seems like my brain switches back and forth rapidly between the two, but this time, for whatever reason, there seemed to be an “overall awareness” that popped up and observed the two streams of perception and understanding, which were simultaneous, but not mingling in any way and not switching, not interrupted, both fully functioning quite independently Just an anecdote, I know…but I happen to be trained in observing “inner experience” as it correlates or doesn’t, to “outer experience” . Maybe somewhat analogous to a phase shift into “superconductivity”, a body/mind flow state, and not an ordinary version of “multi-tasking”?

    • OWilson

      Could that explain the phenomena of driving to work, and not being aware of how you got there afterwards, or parking your car in seeming familiar surroundings and forgetting where you left it.

      And how about those kids that cannot complete a homework assignment without very loud music in their ears?

      Do we operate with dual core hardware, and can we aspire to quad core? :)

  • jay

    I know this comment isn’t immediately relevant to this post, but I couldn’t think of a better person to answer this question than you:

    This question is about the apparent activations and deactivations revealed by two contrasts in neuroimaging using cognitive subtraction. Suppose we are comparing activations in the two tasks: reading words – single letters. This reveals greater activation in brain areas A, B & C, and deactivations in D,E & F. We then compare reading single letters – reading words. This reveals greater activations and deactivations in different regions.

    My question is, why doesn’t the reverse contrast simply reveal the opposite of the first, that is, activations in D, E & F and deactivations in A, B & C?

    • Neuroskeptic

      That’s odd – if the two contrasts are truly the reverse of one another, they should give the same results but with the signs flipped!

      • jay

        That’s what I thought. How come papers will sometimes report the activation points for A-B but then, in some instances, report there were no activation differences for B-A? Is there a different sort of thresholding, or a mask used, for the second contrast compared to the first?

        • Neuroskeptic

          What they mean, in that case, is that there are no areas with positive activations for B > A.

          i.e. there are no areas where B is associated with higher signal than A.

          There may be areas where B – A is associated with deactivations (lower signal), but by convention these are often not explicitly mentioned, if they have already been reported as activations A – B.

          • jay

            Ah, I see. Thank you!
            But now your response has generated another question. Put most simply, would there ever be a case where B-A (second contrast) identified different activation points compared to the deactivations points revealed by A-B (first contrast)? If the two contrasts simply reveal the same regions, with their signs flipped, I don’t see how this would occur…?

          • Neuroskeptic

            Contrast B – A should always be the mirror image of contrast A – B, so the activations/deactivations would always be the same except ‘flipped’. If they were different I think it would be evidence that something had gone wrong or at least that some very unusual analysis approach was being used…

    • Neurosiscientist

      What software package are you using? The reverse contrast should produce the opposite images, as you expect.

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  • agnes debinski

    Now we can all see multi-tasking is not a matter of gender but it’s all about splitting your mind. When you listen to two sounds at the same time and you end up remembering both you are doing the same thing. Whenever two people are discussing two matters at the same time (so much fun!), you practice splitting your mind by doing that. I used to practice that with a male person and I dare say he was just as good at multi-tasking mind-wise as I was. Multitasking can be accomplished by everyone but a major prerequisite is that we are not exhausted – when you are tired, you even manage to mess up one task at a time, let alone more than one.

    • Maia

      Agreed. Although the multi-tasking attributed to women has usually been a version of doing several physical tasks at a time, along with one or two mental ones, not just two mental streams. I am not sure there is any credible evidence for this “gender theory” though, it seems more like glorified anecdote, and maybe even “excuse” as to why men aren’t as good at child-care (plus juggling several other things at the same time). Does “splitting” get better with practice? I would say yes, but we’ll have to do more reseach on this!

  • Alan Poirier

    Fascinating. I recall driving while engaged in a highly technical conversation on my hands-free mobile. The phone call lasted for about 30 minutes. When it was finished, I found myself in a completely different part of the city, some 20 miles from where I wanted to be. I had absolutely no recollection of how I had got there. Evidently, one side of my brain was doing all the driving, while I concentrated on the phone call. I was truly amazed.

    • Connie Porretta

      The exact reason why “hands-free” phones are no safer while driving. Distracted is distracted, period. Hang up everyone, and concentrate please.

      • Maia

        YOu can train attention so that Alan P.’s experience virtually never happens. But most people are not willing to put the time and energy into this training, so yes, hang up!

      • Alan Poirier

        Thanks Mom.

  • Justin Sattin

    I think that we do not have split brains, but rather have the capacity to engage in multiple activities simultaneously. Depending on what we’re doing, the underlying neural correlates may be more or less integrated. But there’s (arguably) a conceptual problem in stating that people have multiple conscious streams. I blogged about this here:

    Basically the same argument applies to to another recent post in this blog, about whether memories are really “stored” in synapses. Memory is the capacity to retain knowledge. This depends on the organism having certain intact neural structures such as (in humans at least) the hippocampus. There are likely microstructural prerequisites as well. But this doesn’t imply that memories are “stored” in synapses. They are not “stored” at all in the sense of storing information in a filing cabinet.

    • Neuroskeptic

      Thanks for the comment – interesting post!

  • makarov123

    I find the “consciousness” concept mentioned here interesting. Anecdotally, I have often noticed while driving and performing another cognitive task (thinking intensely about something, or listening to a radio show or book) that I cannot “remember” my drive.

    The experience is almost like “waking up” and realizing I have no memory of visual information from driving for the last, maybe 20-30 seconds. Obviously I was driving and staying on the road, I just don’t recall road features, whether I passed by pedestrians or cars on intersecting roadways, etc.

    This doesn’t happen very often to me, but the “waking up” feeling is quite jolting when it does happen.



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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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