[Eeglablist] Critical pitfall of spectral power analysis?

[Евгений Машеров]

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
>