[Eeglablist] Advice on filtering strategy before ICA (EEGLAB pipeline; FAA, beta/alpha ratio/ PSD)

[Makoto Miyakoshi]

Hi Kathryn,

But my question about filtering is whether you recommend applying only a
high-pass
filter prior to ICA (as above), and then applying the full bandpass (e.g.,
including a low-pass) later, prior to PSD analysis?


Andreas Widmann answered that question once and for all in this 2015 paper.
10.1016/j.jneumeth.2014.08.002

The answer is, apply high-pass and low-pass separately, with an
(inevitably) short transition band width (TBW) to high-pass filter, and a
gentle TBW for low-pass filter.
For example, if you want to set a band-pass filter 0.5-55 Hz @-6dB, then

Highpass: cutoff 0.5 Hz (TBW 0.5 * 2 = 1.0)
Lowpass: cutoff 55 Hz (TBW 5, which makes pass-band edge at 55-5/2 = 52.5
Hz, and maximum suppression starts at 55+5/2 = 57.5 Hz)

For confirming definitions of terminologies, see this page. This page is a
summary of my questions and his answers.
https://urldefense.com/v3/__https://eeglab.org/others/Firfilt_FAQ.html__;!!Mih3wA!HPZkz51i3HC_zP7sK-ZAnbc3e7bwYY3dwxbgLZn8v25vAVwmBxhdk6YfF87fgdUWoKXpUNspBYhMnKmFvicAeN5kvEY$ 

The reason why you want to do this is that you do not want to use an
unnecessarily steep slope in the low-pass end, which makes the filter order
unnecessarily long.

Re-reference to linked mastoids prior to ICA (BioSemi recommendations)

My advice: do not use linked mastoids. Physically linked mastoids/earlobes
is out of the question, this is the worst because electrically your
subject's head is no longer shaped like a human head (heads are connected
bilaterally)!!! Digitally linked mastoids/earlobes is also problematic for
the following reason (from *Electric Fields of the Brain* p. 294, Nunez and
Srinivasan 2006).

The mathematical linked-ears or linked-mastoids reference may seem to
provide a "solution" to these problems associated with the physical linked
reference. However, does the original goal of using the average potential
of the two ears as the reference make sense? The motivation for this
procedure is not based on any physical properties of head volume
conduction. The measured potential difference between a pair of electrodes
depends on sources located near both electrode positions. With the
mathematical linked-ears reference, the recorded potential depends on
sources at three different locations. This approach may further complicate
the interpretation of scalp potentials and possible source locations rather
than simplifying it. One rationalization often argued to support the
linked-ears or linked-mastoids reference is its purported tendency to be a
"symmetric" reference with respect to both brain hemispheres, thereby
providing a tool for characterizing hemispheric asymmetries in EEG studies.
However, this is not generally true; the effect of the average mastoids
reference is artificially to correlate data from recording electrodes near
the two mastoids (Srinivasan et al. 1998), potentially reducing estimates
of dynamic hemispheric source asymmetries. We will consider several
simulations in the next section where we introduce the average reference
and compare it to the mathematical linked-ears reference.


Instead, use either average reference (if you have > 64 ch) or Dezhong
Yao's REST (if you use the low number of electrodes, you might want to use
this, but it is conceptually much more complicated than average reference).

Makoto

On Tue, Apr 28, 2026 at 11:35 PM Kathryn Bolton via eeglablist <
eeglablist at sccn.ucsd.edu> wrote:

> Hi EEGLAB mailing list,
>
> I hope you're having a good week so far! I’m a PhD student working on an
> EEG study examining affective responses to music in older adults.
> Participants listen to multiple 1-minute music samples, and I plan to
> compute frontal alpha asymmetry (FAA) and beta/alpha ratio (PSD-based
> measures) and relate these to valence/arousal ratings.
>
> I’ve structured my pipeline into three stages, partly to distribute work
> across research assistants:
>
>    - *Script 1:* preprocessing + ICA (run by RAs; outputs ICA-ready
>    datasets)
>    - *Script 2:* manual ICA inspection/cleaning (topography, spectrum,
>    timecourse), followed by channel interpolation
>    - *Script 3:* epoching, PSD computation, and extraction of FAA and
>    beta/alpha ratio
>
> In my original pipeline, I noticed that the component maps used for
> inspection in Script 2 appeared a bit off (e.g., nearly uniform colour
> across components). I’ve since revised Script 1 to the following workflow:
>
>    - Re-reference to linked mastoids prior to ICA (BioSemi recommendations)
>    - High-pass filter at 0.5 Hz only, removing a 50 Hz low-pass filter at
>    this stage
>    - Downsample to 512 Hz
>    - Automated detection and removal of bad channels (logged), prior to ICA
>    - Run ICA using rank-adjusted PCA
>
> This seems to have resolved the issue with the component maps. But my
> question about filtering is whether you recommend applying only a high-pass
> filter prior to ICA (as above), and then applying the full bandpass (e.g.,
> including a low-pass) later, prior to PSD analysis?
>
> I want to ensure that the filtering approach is appropriate both for ICA
> decomposition and for downstream frequency-based analyses (FAA and
> beta/alpha ratio).
>
> Thank you very much for your time and advice, I really appreciate it!
>
> Best regards,
> Kathryn
>
>
> *Kathryn Bolton, MA*
>
> Pronouns: she/her
>
> PhD Candidate | Clinical Psychology
>
> Department of Psychology | Cognitive Aging Lab (CAL)
>
> Toronto Metropolitan University
>
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