Could Travelling Waves Upset Cognitive Neuroscience?

By Neuroskeptic | July 9, 2015 8:53 am

A new paper published in Cognitive Processes argues that neuroscientists may need to look at brain activity from a new angle, in order to understand neural dynamics.

According to the authors, David Alexander et al. of Leuven in Belgium,

A ubiquitous methodological practice in cognitive neuroscience is to obtain measure of brain activity by analyzing the time course of activity alone, or the spatial topography of activity alone.

This usually results in throwing away most of the data as irrelevant: It is considered enough to analyze the time series at a site of interest, or to take spatial snapshots at some relevant times.

This practice boils down to treating brain data as if it were space–time separable.

If a signal is ‘space-time separable’, this means in effect that one can hold either space or time constant, and then measure the other.

For instance, in an EEG experiment, we typically consider the signal from one particular electrode (i.e. holding space constant) and plot a graph of how it varies over time. In a task-based fMRI experiment, we hold time constant and plot the spatial extent of activity at that time point.

By doing this, we are assuming that activity in the brain takes the form of standing waves.

However, Alexander et al. say that while we can treat brain activity in this way, we shouldn’t, because brain activity is dominated by travelling waves, activations or deactivations which move through the brain, and in which the temporal and spatial dimensions are therefore not distinct.

Recent research has suggested the importance of traveling waves of activation in the cortex… Critically, traveling waves are not space–time separable.

Traveling wave activity has been measured in the cortex at a number of scales, including columns, Brodmann areas, and whole cortex… Large-scale cortical waves have been shown to arise at a variety of frequencies, from the subdelta through to gamma bands.

Their prominence suggests that the correct frame of reference for analyzing cortical activity is the dynamical trajectory of the system, rather than the time and space coordinates of measurements.

Here’s my illustration of the problem, adapted from one of Alexander et al.’s figures:

brain_wavesOn the left we see a scenario in which a negative (blue) activation in one area is followed by a positive (red) activation in a nearby area. These ‘blobs’ are standing waves. But what if the positive and negative activations travel through the brain, as on the right?

Recognizing the importance of travelling waves means adopting a new approach to data analysis. So what do we need to do? Alexander et al. say that we need to find a coordinate transformation, that converts the space and time dimensions into a new coordinate space. They liken this to looking at the data from an angle:

brain_waves2Once we do this, we would finally be able to assume something like space-time separability. In particular, the coordinate transformation would allow us to assume that neural activity is additive, meaning that the subtraction method would be valid.

Currently, much of cognitive neuroscience is based on applying the subtraction method in space and in time. According to Alexander et al. this is probably a mistake, and means that we are ‘barking up the wrong tree’ in a general sense.

They conclude

We question the very notion that neurological entities are events occur at certain locations and times, rather than being comprised of trajectories that extend over locations and times…

If cortical activity is not space–time separable, then it seems likely that neither are perception or action. We take the view that measurements in neuroscience are not of events but of trajectories.

It’s fascinating stuff, although Alexander et al. don’t go into detail about how this works in practice. How are we to find the coordinate transform that allows us to rule these traveling waves, in any given case? And just what kind of additional understanding can we hope to find once we do so?

ResearchBlogging.orgAlexander DM, Trengove C, & van Leeuwen C (2015). Donders is dead: cortical traveling waves and the limits of mental chronometry in cognitive neuroscience. Cognitive Processing PMID: 26139038

CATEGORIZED UNDER: fMRI, methods, science, select, Top Posts
  • Nym w/o Qualities

    It continues to appear that neuroscience is in its alchemy phase, tinkering without really understanding. But you have to start somewhere.

    • Buddy199

      I’d say in 500 years we’ll be in the neuroscience alchemy phase, right now we’re still figuring out how to make a better Clovis point. An object that is unimaginably mysterious even to the unimaginably mysterious object itself.

      • Hominid

        You’re an optimist!

    • Hominid

      Neuroscience has become a meaningless term. It includes all manner of disparate sciences, pseudosciences, and technologies.

  • Uncle Al

    Social/soft sciences are funded as agendas, applied validations, re DSM 5 and Big Pharma. “This usually results in throwing away most of the data as irrelevant” This article illustrates how public exposure constrains inherent corruption: What does not support my imposition makes it stronger.

    Confounded variables were a major topic within Theory of Experimentation in the 1970s. It is good to see poobahs pontificating about people’s lives are only 40 years off track with methodologies and epistemologies of pseudopsychological data manipulation. Next stop, climatology and the Carbon Tax on Everything affording vast cash disappearances via Carbon Credit arbitrage.

    • Buddy199

      If I had a dollar for every time I’ve said that.

      • Uncle Al

        I gave a seminar entitled “Ontological Recapitulation of Bredt’s Rule.” Really. Imagine a derived Master of Liberal Arts degree, “Bridgehead Olefin Small Ring Social Engagement.” Diversity in everything!

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          • Uncle Al

            In process.

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          On #bitcoin-assets we regularly discuss your work, including the essays, the parity violation experiment, and the future fate of our common enemies… Come and pay us a visit, Uncle Al. And dust off your PGP key.

          • Uncle Al

            “8^>) I’ll stop by as soon as I push back some alligators.

            The vacuum chiral anisotropy experiment is one day in a Raman scattering rig looking for racemate non-degenerate rotational spectra in a 1 kelvin vacuum supersonic expanded molecular beam. Required racemate must be otherwise spectroscopically quiet (rigid polycyclic with rotational symmetries) and insanely chiral. Off the shelf!

            Org. Chem. 62(12), 4162 (1997), DOI: 10.1021/jo962267f
            Starting material
            Eur. J. Org. Chem. 11, 2590 (2006), DOI: 10.1002/ejoc.200600019
            Product. I’ve talked with de Meijere
            Phys. Chem. Chem. Phys. 16(4), 1415 (2014). DOI: 10.1039/C3CP54175D
            Molec. Phys. 111(14-15), 2154 (2013) DOI: 10.1080/00268976.2013.793888
            Somebody with the equipment. I must talk with Boudon.

          • HenryC

            I actually could read that, and it does have meaning.

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      If you are in need for extra profit from $50-$300 on daily basis for freelancing from your house for few hours each day then this may interest you…

  • fahd09

    Aren’t these effects already detectable with multivariate methods like PCA or ICA (which give spatiotemporal maps of brain activity) ?

  • Mihai Voicu

    It makes sense that the way neuron signals are interracting is more important than how the signal is processed by certain neurons:

    • Hominid

      Or both are important.

  • HenryC

    Even with my limited knowledge of neuroscience I know that thoughts, memories and thinking is a matter of activating a network of neurons and the different paths and thoughts are an active network and the transitions of states matter. I never did understand why people thought static pictures meant anything significant.

  • Xiangbin Teng

    fMRI has poor temporal resolution so you can analyze it in a traveling ware like way but you won’t see anything meaningful. MEG with source reconstruction would include spatio-temporal information and people do explain data in a temporal and spatial sense. But, in stead of making up a term like traveling wave, could we just say ‘ prefrontal area lights up first, and 100 ms later visual cortes lights up, and then motor arear’…

  • s k

    Its interesting to see another view…static and dynamic observations are similar to photography and video…both are accurate views with their own constraints. Perhaps it would be helpful to conjoin McgilChrist’s hemispheric views to travelling waves…and watch the adventure.

  • Neuroscientist

    Idea is fundamentally flawed, bc it assumes that brain is a homogenous medium. Instead, highly targeted long distance projections constrain how signals propagate. Only during sw sleep does traveling wave analysis apply.

    • Neuroskeptic

      Interesting point. Do you have any references for that?

      • Neuroscientist

        Traveling waves in sws: Murphy … & Tononi from a few years ago (PNAS?)

        • Neuroskeptic

          Ah, got it: Source modeling sleep slow waves.

          But is there evidence that traveling waves are only found in SWS?

          • Neuroscientist

            That paper describes the physiological state, unique to sws, that affords the propagation of traveling waves — a ‘breakdown of effective connectivity’. That traveling waves wouldn’t be the norm in the awake brain is

          • Neuroscientist

            … analogous to how one can know that tws don’t propagate from break pedal to break pads in wheels of your car

          • simonsterdotcom

            There is evidence to the contrary: Although in that case it’s only hippocampus, not synchrony among areas

          • JR KING

            I think there’s actually evidence of travelling waves in awake conditions too. e.g.

            Plus (warning: #selfadvertisement), temporal generalization very often demonstrate sequential responses across brain regions, which, I think can be considered as a traveling waves (at least under certain conditions): here’s an open-access example for anyone to try:

            That being said, and while their methodological point is interesting to highlight, it is quite far fetched from their title and conclusions…

    • anna.dahl2
  • KA

    These comments are hilarious

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About Neuroskeptic

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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