[Eeglablist] Critical pitfall of spectral power analysis?

[Daniele Marinazzo]

Indeed, thanks for elaborating.

Even if they were different mechanisms, relating to different aspects of
E/I balance, they would undoubtedly influence each other to an extent that
would make it difficult to separate them in the inference process following
a top-down analysis.

And then, extrapolating to limits, we have a continuum, at infinitely long
time scales everything is periodic, and at very low time scales, we have
spikes, which are instantaneous amplitude variations :)

On Tue, 2 Sept 2025 at 17:07, Gyurkovics, Mate <mategy at illinois.edu> wrote:

> Hi Daniele,
>
> some really interesting questions.
>
> If we say that 1/f activity has some neuronal origin, and if we connect it
> to E/I balance (the two things could even be collapsed at least if we
> consider scalp EEG, since E/I balance is basically how generically deep we
> can go with our inference from scalp recordings), then it's difficult to
> think that E/I balance is not involved in oscillatory activity as well.
>
> I actually agree with this point, and it's something I struggle with
> conceptually a bit, at least for scalp recordings. Oscillations are quite
> explicitly linked to E:I balance, at least in the sense that they are meant
> to emerge (to oversimplify this a bit) from the interplay of inhibitory and
> excitatory neurons, and represent excitation and inhibition reciprocally
> controlling the other. However, I think the key difference here is that
> this happens over time in a predictable fashion (inhibition goes up,
> excitation goes down, then excitation goes up, inhibition goes down, etc.).
> The E:I balance framework of broadband activity is more about the relative
> contribution of excitatory and inhibitory post-synaptic potentials to the
> signal we are measuring in a given moment (well, across a temporal
> integration window of course).
>
> This is all touched upon this paper by Voytek & Knight:
> https://urldefense.com/v3/__https://www.sciencedirect.com/science/article/pii/S0006322315003546__;!!Mih3wA!DFbmHddnjIjOc3ESdXFZAUfHFSDNjgj0zwXJNiaXZhmSRYR6Z8On4qoq9ncvdV7ewZPUGIbgRIkxgUgYKBxK2W95TAf6JmUT$  -
> however, I'm not sure how exactly their logic generalises to scalp
> recordings, plus it's quite complex and I'd hate to misrepresent it. But
> very briefly, if I remember the argument correctly, it links 1/f shape more
> to spiking properties (neuronal outputs), whereas oscillations to resultant
> changes in the electrical field (which may or may not oscillate at the same
> freq that neurons are spiking at). They do simply state: "This broadband 1/
> *f* is different from an oscillation (71
> <https://urldefense.com/v3/__https://www.sciencedirect.com/science/article/pii/S0006322315003546*bib71__;Iw!!Mih3wA!DFbmHddnjIjOc3ESdXFZAUfHFSDNjgj0zwXJNiaXZhmSRYR6Z8On4qoq9ncvdV7ewZPUGIbgRIkxgUgYKBxK2W95TB5q6SeH$ >),
> which are narrowband spectral processes within the PSD (70
> <https://urldefense.com/v3/__https://www.sciencedirect.com/science/article/pii/S0006322315003546*bib70__;Iw!!Mih3wA!DFbmHddnjIjOc3ESdXFZAUfHFSDNjgj0zwXJNiaXZhmSRYR6Z8On4qoq9ncvdV7ewZPUGIbgRIkxgUgYKBxK2W95TGj2bagk$ >)
> and which may have a separate neurophysiological mechanism from spiking
> activity."
>
> Top-down modelling approaches ([...]), usually start from the assumption
> that 1/f and broadband must be different things.
>
> You mean 1/f and narrowband, right? Yes, I have the same impression that
> that is the prevailing sentiment. I think this also brings us full circle -
> the question really is whether this is correct or not, and if yes, then
> division (which is inherent in dB conversion too) may not be an ideal
> solution for baselining.
>
> Thanks,
> Mate
>
>
> ------------------------------
> *Feladó:* Daniele Marinazzo <daniele.marinazzo at gmail.com>
> *Elküldve:* 2025. szeptember 2., kedd 12:16
> *Címzett:* Gyurkovics, Mate <mategy at illinois.edu>
> *Másolatot kap:* EEGLAB List <eeglablist at sccn.ucsd.edu>
> *Tárgy:* Re: [Eeglablist] Critical pitfall of spectral power analysis?
>
> Dear Mate, dear all,
> thanks a lot for continuing this interesting discussion.
> Regarding the independence of the mechanisms generating 1/f and
> oscillatory activity, what would these mechanisms be?
> If we say that 1/f activity has some neuronal origin, and if we connect it
> to E/I balance (the two things could even be collapsed at least if we
> consider scalp EEG, since E/I balance is basically how generically deep we
> can go with our inference from scalp recordings), then it's difficult to
> think that E/I balance is not involved in oscillatory activity as well.
> Top-down modelling approaches (not just Donoghue et al., but also early
> works such as this one from 1987
> https://urldefense.com/v3/__https://www.tandfonline.com/doi/epdf/10.3109/00207458808985730__;!!Mih3wA!DFbmHddnjIjOc3ESdXFZAUfHFSDNjgj0zwXJNiaXZhmSRYR6Z8On4qoq9ncvdV7ewZPUGIbgRIkxgUgYKBxK2W95TEJNmBTw$ 
> <https://urldefense.com/v3/__https://www.tandfonline.com/doi/epdf/10.3109/00207458808985730__;!!DZ3fjg!4yzUWnNeatKBGE830z91fv-3XQOdXg9yENuPX6-EvSNasxuaO0hvTO49HTv0xWpVb-GWm5igQrszkn-a_lgDAgGznI7O$>),
> usually start from the assumption that 1/f and broadband must be different
> things. I see how this can be sort of epistemologically inevitable, even
> though there are instances in which a parsimonious model explaining both
> has been proposed (
> https://urldefense.com/v3/__https://journals.plos.org/ploscompbiol/article?id=10.1371*journal.pcbi.1010012__;Lw!!Mih3wA!DFbmHddnjIjOc3ESdXFZAUfHFSDNjgj0zwXJNiaXZhmSRYR6Z8On4qoq9ncvdV7ewZPUGIbgRIkxgUgYKBxK2W95TGmFxqtA$ 
> <https://urldefense.com/v3/__https://journals.plos.org/ploscompbiol/article?id=10.1371*journal.pcbi.1010012__;Lw!!DZ3fjg!4yzUWnNeatKBGE830z91fv-3XQOdXg9yENuPX6-EvSNasxuaO0hvTO49HTv0xWpVb-GWm5igQrszkn-a_lgDAm-R5AO4$>
> ).
>
> On Mon, 1 Sept 2025 at 23:18, Gyurkovics, Mate via eeglablist <
> eeglablist at sccn.ucsd.edu> wrote:
>
> Thanks again for all the interesting points. I'm certianly learning a lot
> on the physics side - and also about the PSD in different animals, I was
> genuinely unaware of all this, but sounds super interesting.
>
> I think the mechanism you are referring to, Makoto, is what's usually just
> described as "dendritic filtering", is it not? I'm aware of this process
> plus the low-pass filtering that happens as a consequence of spatial
> summation. These are the two main ones I was thinking of, so you are right,
> it was very imprecise on my part to talk about the low-pass filtering
> properties of the tissue.
>
> As demonstrated in a classical study by Lopes da Silva and van
> Leeuwen (1977), alpha oscillation is generated within the cortex which is
> only 4-5 mm thick.
>
> Just out of curiosity, what point was this sentence supporting?
>
> I'm very happy to learn the conceptual distinction between trivial and
> non-trivial contributions to the changes of 1/f power distribution. Thank
> you Mate. Your works are impressive.
>
> Very nice of you to say this, Makoto - I would, however, also like to
> stress that this particular distinction just reflects how I personally
> think about this problem (i.e., the contribution of the ERP to the
> spectrum), I'm sure reasonable people could disagree. (Although this is
> basically the logic we published in the Journal of Neuroscience paper
> linked above.)
>
> @Eugen - you raise many interesting points. (I certainly agree that in
> scalp recordings at leat, oscillatory activity is sparse, and most robustly
> occurs when the brain is not engaged with a task, in the form of alpha
> activity, but that is about as bold as I can be here.)
>
> is sinusoidal (with sharp peaks in the spectrum) activity and broadband
> (not necessarily 1/f) activity generated by different mechanisms?
>
> I think, to put it very simply, this is one of the fundamental questions
> underlying this discussion here. If they are separate AND fairly
> independent, that is when the conclusions of our paper hold. If they are
> separate but interact, our conclusions will sometimes hold, other times
> maybe not. If they reflect the same underlying mechanism, our conclusions
> would likely rarely be a concern. I personally think that they likely
> reflect mechanisms that are separate at least to some extent - plus
> 1/f-like broadban activity likely reflects several, not just one,
> generative mechanisms as highlighted by Makoto and others here too.
>
> Moreover, the function 1/f may simply be the result of the Fourier
> transform of a single excitatory or inhibitory postsynaptic potential.
>
> As far as I recall, this point is covered in Gao et al. (2017), the paper
> that first linked 1/f-like features to excitation/inhibition balance.
>
> Thanks,
> Mate
>
>
>
>
>
>
>
>
> ________________________________
> Feladó: Евгений Машеров <emasherov at yandex.ru>
> Elküldve: 2025. augusztus 30., szombat 9:07
> Címzett: Gyurkovics, Mate <mategy at illinois.edu>
> Másolatot kap: EEGLAB List <eeglablist at sccn.ucsd.edu>; Wirsing, Karlton <
> kwirsing at vt.edu>; Cedric Cannard <ccannard at protonmail.com>; 장진원 <
> jinwon06292 at gmail.com>; Makoto Miyakoshi <mmiyakoshi at ucsd.edu>
> Tárgy: Re: [Eeglablist] Critical pitfall of spectral power analysis?
>
> As a (naive and insufficiently substantiated hypothesis) — is sinusoidal
> (with sharp peaks in the spectrum) activity and broadband (not necessarily
> 1/f) activity generated by different mechanisms? Moreover, sinusoidal
> activity is a manifestation not of action, but of inaction of the brain.
> Physiological (alpha rhythm when closing the eyes, possibly also mu rhythm
> in the absence of proprioceptive signals, sleep spindles) or pathological
> (alpha coma and low-frequency sinusoids in the delta or theta ranges). It
> can be associated with the regulation of the level of constant potential
> and, in general, with metabolic processes carried out by the integral
> regulator generating oscillations (but we see oscillations directly only as
> an idle rhythm). Broadband activity seems to be directly associated with
> the functioning of individual neurons. Moreover, the function 1/f may
> simply be the result of the Fourier transform of a single excitatory or
> inhibitory postsynaptic potential. These mechanisms are interconnected, but
> different. Perhaps the mathematical apparatus for their study should also
> be different.
>
> Your truly
>
> Eugen Masherov,
> Burdenko Neurosurgery Institute
>
> > Thanks again everyone, for these very interesting points.
> >
> > Just to add to something that was said recently - yes, 1/f (or rather,
> 1/f^x) features are quite ubiquitous, I think practically any time series
> with some amount of autocorrelation will have a similar shape:
> https://urldefense.com/v3/__https://www.cell.com/trends/cognitive-sciences/fulltext/S1364-6613(14)00085-0__;!!Mih3wA!E0DB3LnNc6JHuXWc3N5M5iAUeusaX8LTN3dqbu9ZUeyzmDOwf3GVazliJU65YVxC6mC4VLMvHBOlkHKz4AeGoXXj$
> - its ubiquity is covered nicely in this lovely paper, as far as I remember.
> >
> > I also get most of Makoto's points about how just the location of the
> neuronal inputs, either in terms of proximity to the soma or in terms of
> cortical layers, will affect the strength of the low-pass filtering, and
> thus the shape of the 1/f scaling. This is super interesting, and this and
> dendritic filtering are certainly discussed in the literature to some
> extent. I am a bit more sceptical whether such subtle differences could
> contribute to 1/f changes in scalp recordings, but Makoto suggests they
> could and I trust his expertise.
> >
> > If you perform an ERP task, it would change 1/f power distribution, not
> surprisingly, because task-triggered cortico-cortical and thalamo-cortical
> inputs are recruited.
> >
> > This is a very interesting point. In our 2021 paper linked above, we
> also make the point that 1/f shape should change in an event-related
> design, but for a more trivial reason: ERPs are non-oscillatory (in the
> simple sense that they are transient bursts that do not repeat with a clear
> period), and will thus have a 1/f shape in the frequency domain (indeed,
> they do, there are some figures in the paper). Thus, 1/f scaling will
> change after an event trivially because there are well-known
> non-band-limited changes happening in the EEG (the ERPs). We tried to
> correct for the contribution of the ERPs and still found post-stimulus 1/f
> changes that we consider non-trivial (a steepening to be specific). These,
> then, could be explained by the mechanism that Makoto suggests (which we
> did not consider in the paper, as it seemed maybe a bit small-scale to
> explain scalp-derived effects) and/or by Gao et al.'s excitation-inhibition
> balance idea (this is the framework we used in the paper). It certainly
> cannot be explained by the general low-pass filtering properties of the
> tissue or similar more or less fixed variables, as those should not change
> so rapidly.
> >
> > I share much of your scepticism about oscillatory mechanisms (in scalp
> recordings), Makoto, but if we take the most typical generative mechanisms
> assigned to these phenomena (interplay of pyramidal cells and
> interneurons), they seem like they could potentially interact with these
> other mechanisms described above, or be fairly independent.
> >
> > So we've got this really complex picture, where there could be
> oscillations going on (maybe in alpha only), there could be (independent?)
> 1/f dynamics happening for multiple reasons, e.g., because of the location
> and/or the nature (E vs. I) of neuronal inputs changing, and there could be
> ERPs happening too, which might partly be phase-locked oscillations, and
> could also be related to where the neuronal inputs are located, so they
> "straddle" these different mechanisms quite a bit, probably. Not too sure
> about the ERPs to be honest.
> >
> > Two more minor points:
> >
> > I can't put up with the fuzziness of how the term 'oscillation' is used
> in the field now. Is a try-phasic burst, such as a classical event-related
> N1-P1 waveform, an oscillation?
> >
> > I agree completely that it is very unclear what constitutes an
> oscillation - basically, how many cycles are enough for something to be
> considered an oscillation, and how do we show that those cycles come from
> the same generative mechanism, and not just multiple successive events
> happening. This is less of a question for longer, more stable oscillations,
> e.g., alpha at rest.
> >
> > And as for Michael's question: my limited experience with this topic
> would certainly suggest that 1/f dynamics (for whatever reason) could
> change very rapidly, and often in a systematic fashion (e.g., predictably
> after a stimulus). They also do seem to change on much slower time scales
> as well, e.g., across the lifespan.
> >
> > Thanks,
> > Mate
> >
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