[FieldTrip] a few questions on separating fractal and oscillatory components in the data

Wed Feb 25 21:04:22 CET 2026

If anyone is still reading here, I would like to just add two
questions/observations on this topic

1. What does it say about the fit of the slope when the values of the
supposedly clean oscillatory part verge deep into negative values (up to
over minus 100, but similarly high in the other direction) and if this is a
trustable output how does one make it accessible to most statistical
analyses? E.g. what is a widely accepted procedure for transforming this to
all positive values? Personally, I am a bit skeptical that this is
compatible with good fit, but I am new to this, so my bad if I am wrong

2. I was particularly surprised that Gyurkovics found anything to correlate
negatively with the aperiodic component (exponent). In my data the
aperiodic signal "is king" and dictates the behavior of everything else, at
least as long as we consider absolute power. All frequencies below 30 Hz go
up and down at the same time and this coincides with the behavior of what
(for now in my case) amri_sig_fractal.m outputs as the aperiodic part.

Thanks everyone, hope this isn't a boring topic for this forum :)

Am Mi., 25. Feb. 2026 um 12:56 Uhr schrieb Ivaylo Iotchev <
ivaylo.iotchev at gmail.com>:

> Dear Diego / Dr. Lozano-Soldevilla
>
> thank you dearly! This is the kind of detail I was looking for :) I will
> definitely look closer into some of these papers. My first thought (but let
> see what will happen after reading) is to consolidate my decision to not
> detrend, at least as far as the spectral part of the hypotheses is
> concerned (the aperiodic signal is bigger and thus less distorted by poor
> fit, I assume). On the level of single trials which is currently the
> preferred scope, the deviations from the fit are likely catastrophic (as
> implicated by log-log)... if reviewers force me to separate the signal, I
> will see what is the best option for data pooled across trials, because
> with a trial by trial scope this is visibly not a good idea.
>
> Again thank you a million times for joining this discussion :)
>
> Best wishes, muchas Grazias!
>
> Ivo
>
> Am Mi., 25. Feb. 2026 um 12:44 Uhr schrieb Diego Lozano-Soldevilla via
> fieldtrip <fieldtrip at science.ru.nl>:
>
>> Hi Ivaylo,
>>
>> Following Jan-Mathijs’ advice, I’d like to add my 2 cents, as this is an
>> issue and I’ve encountered similar problems in the past.
>>
>> This is a hot-topic and you're not the only one having a hard time
>> modelling the 1/f under certain conditions. Since you know Gerster et al, I will
>> include here the rest of the relevant papers I came across during the
>> following interesting discussion led by the EEGlab community:
>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fsccn%2FOneOverF%2Fdiscussions&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467815779234%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=b%2Fg7ulV4HeSyznsJ7PNcSv3gkR9izaYx5AAWoGmiWaM%3D&reserved=0
>> <https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fsccn%2FOneOverF%2Fdiscussions&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467815800112%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=jTEfitrMfmuYHxVxm%2Fb4zP87R5vArEAfDPXRmmYVSBE%3D&reserved=0>
>>
>> Long story short, there's no easy way to model the 1/f and maybe, it is
>> better to avoid the modelling rather than to insist in order to avoid wrong
>> conclusions (see some papers below). Regarding your relative vs linear
>> subtraction, take a look at Máté Gyurkovics who faced this issue and
>> reported what happens with this additive vs multiplicative
>> (under)estimation. They found a weak negative rank-correlation
>> (within-subjects) between 1/f and narrow alpha power, in line with the
>> additive model. Here, dB conversion was an issue.
>>
>> On the other hand, Brake 2024 finds that:
>> "these two mechanisms thus require two distinct methods of detrending,
>> i.e., subtractive versus divisive detrending, respectively. Without prior
>> knowledge, it is unclear which method is required. Moreover, given the
>> amplitude of peaks above the background trend typical in EEG spectra, small
>> additive changes in the trend would alter the peak amplitudes minimally
>> compared to the errors introduced by incorrectly detrending (Fig. 5).
>> Overall, we conclude that spectral detrending should not be performed
>> unless there is clear physiological and biophysical justification and
>> validation." so maybe your data may be closer to the multiplicative
>> case.
>>
>> In my case, the spectra were somewhere in between, and I decided to drop
>> the modelling because it caused more harm than good.
>>
>> If you aim to relate your results to the excitation-inhibition balance
>> thing, I would recommend reading Chakravarty et al 2026.
>>
>> Best,
>>
>> Diego
>>
>>
>>
>> Relevant papers here:
>>
>> Gerster, M., Waterstraat, G., Litvak, V. *et al.* Separating Neural
>> Oscillations from Aperiodic 1/f Activity: Challenges and Recommendations.
>> *Neuroinform* 20, 991–1012 (2022).
>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdoi.org%2F10.1007%2Fs12021-022-09581-8&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467815822811%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=282jGdMLSZc5kuSRf%2BzwqVtE%2BN17vjdvl2q53eCI28s%3D&reserved=0
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>>
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>>
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>>
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>>
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>>
>> Donoghue, T., Hammonds, R., Lybrand, E., Washcke, L., Gao, R., & Voytek,
>> B. (2024). Evaluating and Comparing Measures of Aperiodic Neural Activity.
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>>
>> Gyurkovics, M., Clements, G. M., Low, K. A., Fabiani, M., & Gratton, G.
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>>
>> da Silva Castanheira, J., Landry, M., & Fleming, S. M. (2025).
>> Quantifying Rhythmic and Arrhythmic Components of Brain Activity. openRxiv.
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>>
>> Brake N, Khadra A (2025) Contributions of action potentials to scalp EEG:
>> Theory and biophysical simulations. PLoS Comput Biol 21(2): e1012794.
>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdoi.org%2F10.1371%2Fjournal.pcbi.1012794&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467816194132%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=w8XcFzlmbmxCLc1IQH7dnrl186lQUYv1aCjXSZRI1zE%3D&reserved=0
>> <https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdoi.org%2F10.1371%2Fjournal.pcbi.1012794&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467816213374%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=T7bxIPpoyB%2Bj%2BaF%2FXSIYb%2FUjwobXXogocxzL1y8BGCY%3D&reserved=0>
>>
>>
>>
>> On Wed, 25 Feb 2026 at 11:56, Ivaylo Iotchev via fieldtrip <
>> fieldtrip at science.ru.nl> wrote:
>>
>>> Jan Mathijs,
>>>
>>> thank you dearly(!) for the fast response! Just to make my last thought
>>> clear: I expected oscillatory power and aperiodic power to not be strongly
>>> correlated with each other after separation, because I assume that
>>> correlation would mean one is still somewhat inside the other. For example,
>>> prior to separating them from each other, power in all bands likewise
>>> strongly correlated with each other, and in the same direction (theta
>>> with alpha, alpha with delta and so on...) which I understood as "the
>>> aperiodic part is the biggest part in my signal, so it makes the
>>> frequencies that float on top of it correlate with each other", does that
>>> make any sense to you, logically speaking?
>>>
>>> I would also value your opinion on *using division instead of
>>> subtraction* (i.e. raw signal./aperiodic signal, instead of raw signal
>>> - aperiodic signal). I think in my data, if I want to correct for aperiodic
>>> signals at all, it makes more sense, because my trials' spectra do not form
>>> clean straight lines in the log-log space as Gerster et al. recommend, so
>>> the tiniest errors in fitting the slope of the aperiodic signal could
>>> eliminate frequency information that is actually there (in my estimation
>>> the proportion of aperiodic to periodic power in my data is magnitudes
>>> bigger than already in humans, my animals have bigger muscles on the top of
>>> the scalp and the EEG is extracranial, which is why without using either
>>> relative power or aperiodically corrected power, frequency specific power
>>> is highly intercorrelated).
>>>
>>> If you have thoughts on any of these, let me know, otherwise thank you
>>> for the time you already put in this
>>>
>>> Bests,
>>>
>>> Ivo
>>>
>>> Am Mi., 25. Feb. 2026 um 10:55 Uhr schrieb Schoffelen, J.M. (Jan
>>> Mathijs) via fieldtrip <fieldtrip at science.ru.nl>:
>>>
>>>> Hi Ivaylo,
>>>>
>>>> It seems that you have certain expectations about correlations that are
>>>> not met by your data. I am not sure whether this warrants the conclusion
>>>> that ’the separation fails with the default approach’.
>>>>
>>>> It’s unclear what you correlate with each other, and why you expect
>>>> these measures to be (linearly?) correlated or not. All in all this makes
>>>> it hard to reply in a targeted fashion.
>>>>
>>>> Regarding your statement of using medians for integration: this is what
>>>> the irasa folks came up with, so it’s standard practice in the irasa
>>>> universe.
>>>>
>>>> Best wishes,
>>>> Jan-Mathijs
>>>>
>>>>
>>>> > On 23 Feb 2026, at 17:11, Ivaylo Iotchev via fieldtrip <
>>>> fieldtrip at science.ru.nl> wrote:
>>>> >
>>>> > Dear community,
>>>> >
>>>> > this is a rather general question, because it is not limited to how
>>>> IRASA is used within Fieldtrip, but whoever feels like answering feel free
>>>> to tie your answers to Fieldtrip functions so that it is useful for
>>>> everyone here.
>>>> >
>>>> > I came across the slope phenomenon rather recently, and the first
>>>> general question is whether component separation always works and makes
>>>> sense for every type of data? I noticed that my frequency specific
>>>> components each remain positively correlated with the aperiodic part (and
>>>> each other), strongly so (r > .90) thus I suspect that the separation
>>>> failed with the default approach.
>>>> >
>>>> > For this reason I need to ask the legitimacy of a few alternative
>>>> approaches, that make sense to me, but might be entirely wrong and I just
>>>> don't see it
>>>> >
>>>> > 1. Divide the mixed spectrum by the aperiodic one, point by point.
>>>> For me this reduces the correlation between oscillatory and aperiodic
>>>> parts, but I do not see anyone else doing this
>>>> >
>>>> > 2. (in combination with the above) it seems to me that using medians
>>>> to integrate power across bins is a good idea, but again this does not seem
>>>> to be standard practice
>>>> >
>>>> > Just let me know what you think or know about this, and again feel
>>>> free to incorporate Fieldtrip usage in your answer to make this more
>>>> valuable for the community.
>>>> >
>>>> > Best wishes,
>>>> >
>>>> > --
>>>> > Dr. Ivaylo Iotchev. Researcher. Active in the fields of ethology,
>>>> psychology, and neuroscience
>>>> > Eötvös Loránd University, Budapest
>>>> > _______________________________________________
>>>> > fieldtrip mailing list
>>>> >
>>>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmailman.science.ru.nl%2Fmailman%2Flistinfo%2Ffieldtrip&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467816231534%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=%2FbtCFFmFN7IU2J6pWcRXRI3jD4V7cxrq75jGAasRQx4%3D&reserved=0
>>>> <https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmailman.science.ru.nl%2Fmailman%2Flistinfo%2Ffieldtrip&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467816249110%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=NsJPExzVj49EJau51Q2pgL%2FbTc%2F4iqqqwe4m6RN759g%3D&reserved=0>
>>>> >
>>>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdoi.org%2F10.1371%2Fjournal.pcbi.1002202&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467816266668%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=tbrZDFZq2oMposKPCH1bi168qGMR5jCBy6HAjnXmI7g%3D&reserved=0
>>>> <https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdoi.org%2F10.1371%2Fjournal.pcbi.1002202&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467816285924%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=05BEdMkc0w%2Bo2VDw8abLou9RlzY5e9%2By01eile00xCw%3D&reserved=0>
>>>>
>>>>
>>>> _______________________________________________
>>>> fieldtrip mailing list
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>>>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdoi.org%2F10.1371%2Fjournal.pcbi.1002202&data=05%7C02%7Cfieldtrip%40science.ru.nl%7Cdce9b0c3d1bc414311d308de74a929f0%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C639076467816352297%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C60000%7C%7C%7C&sdata=3HRJV%2F8gXOuJGoLBfGhvRygyb3ktjhRdAlB13STRoEA%3D&reserved=0
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>>>>
>>>
>>>
>>> --
>>> Dr. Ivaylo Iotchev. Researcher. Active in the fields of ethology,
>>> psychology, and neuroscience
>>> Eötvös Loránd University, Budapest
>>> _______________________________________________
>>> fieldtrip mailing list
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>>>
>> _______________________________________________
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>>
>
>
> --
> Dr. Ivaylo Iotchev. Researcher. Active in the fields of ethology,
> psychology, and neuroscience
> Eötvös Loránd University, Budapest
>

-- 
Dr. Ivaylo Iotchev. Researcher. Active in the fields of ethology,
psychology, and neuroscience
Eötvös Loránd University, Budapest
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