[Eeglablist] How many electrodes tolerated for interpolation?

[장진원]

Dear Prof. Richards,

I understand what you are pointing out. One of the main issues I am
encountering is bad channels resulting from transcranial direct current
stimulation (tDCS). In many cases, an anodal tDCS on the frontal cortex
results in some artifacts from tDCS ramp up and down; I am worried that
interpolation of such bad channels around the frontal area might
offset/substitute possible effects of tDCS on EEG signals.

By the way, I was a little bit worried about the reliability of
source-level analysis, as EEG functional or effective connectivity is, in
many cases, not "reliable" compared to microstates or spectral power. But I
found that many articles consistently discovered high test-retest
reliability and replicability of source-level analysis (Cannon et al.,
2012; Keyvan Mahjoory et al., 2017). I am not a fan of time-sensitive
analysis, such as functional connectivity, but source-level analysis is
essentially based on spatial configuration so it's much more reliable to
use.

Best Regards,
Jinwon Chang

2025년 11월 19일 (수) 오전 11:53, Richards, John <RICHARDS at mailbox.sc.edu>님이 작성:

> 1--source level EEG analysis is a transformation of the scalp potential to
> source locations. If the scalp potential has functionally significant
> relations with experimental variables or clinical conditions, so will the
> source locations.  Probably the same if the scalp potential is NS for
> experimental variables or clinical conditions, so will the source level be
> NS.  Reliability might be different, since each source location data is the
> sum of several electrodes.  Might be more reliable.
>
> 2--I'm not totally sure what you want to do with interpolation. There are
> MANY useful reasons for using it.  Typically EEG is spread across a number
> of electrodes, and not in just one location. If you have a bad channel or
> channels, you can safely estimate an amplitude in that channel by
> appropriate interpolation from groups of electrodes (e.g., spherical
> spline).  Since adjacent groups of electrodes usually have a similar
> signal, the interpolation of data for missing channels is appropriate.
>
> 3--Many researchers studying infant subjects or special populations
> (neurodevelopmental disorders) cannot get a large enough set of data for
> ERP, or other, and want to optimize their subjects' data. So they use some
> kind of rubric for estimating data from missing channels, e.g., no more
> than 10 channels of a 128 channel system with missing data.
>
> 4--The "lead-field" matrix in source analysis is computed from
> conductivities, electrodes, and head media (FEM, BEM).  Its sometimes
> onerous to compute.  E.g., all 128 electrodes are used to compute the LF
> and inverse spatial filter.  You cannot use the LF/inverse spatial filter
> with < the electrode positions used to compute it.  So using the same
> rubric (missing channels <= 10) you can use the data in the source
> computation when there are missing channels.
>
> 5--Nearly all people using a non-10-10 configuration (like EGI) wish to
> present data in the 10-10 nomenclature.  Typically one or more electrodes
> "near" the 10-10 electrode are used.  I have been using spherical spline
> interpolation from the 128 channels to the 81 10-10 channels, using the 128
> channel location and the 10-10 channel location to make the calculation.  I
> and colleagues have used this successfully in a number of papers.
>
> John
>
>
> ***********************************************
> John E. Richards
> Carolina Distinguished Professor
> Department of Psychology
> University of South Carolina
> Columbia, SC  29208
> Dept Phone: 803 777 2079
> Fax: 803 777 9558
> Email: richards-john at sc.edu
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>
> -----Original Message-----
> From: eeglablist <eeglablist-bounces at sccn.ucsd.edu> On Behalf Of 장진원 via
> eeglablist
> Sent: Tuesday, November 18, 2025 2:40 PM
> To: smakeig at gmail.com
> Cc: eeglablist at sccn.ucsd.edu; fernandez luis <isabelyluis2007 at me.com>
> Subject: Re: [Eeglablist] How many electrodes tolerated for interpolation?
>
> Dear Prof. Makeig,
>
> I appreciate your suggestion. Is source-level EEG analysis more reliable
> than scalp-level analysis? I wonder whether source-level EEG analysis (such
> as dipole fitting) achieves high test-retest reliability and replicability
> over different datasets, especially in clinical populations (depression vs
> healthy controls, as an example)? I am not sure whether source-level
> analysis is applicable in clinical research.
>
> Best Regards,
> Jinwon Chang
>
> 2025년 11월 18일 (화) 오후 1:19, Scott Makeig <smakeig at gmail.com>님이 작성:
>
> > I would question the value of channel interpolation - except for
> > making smooth pictures of particular scalp distributions. Individual
> > electrode-signal-difference scalp channels are inherently vague
> > measures that sum electrical activities generated in many unrelated
> parts of cortex.
> > Scalp channel signals are not more deserving of attention than
> > individual radio-frequency channels in an fMRI system ...
> >
> >  I do understand that (at least in the US) clinicians need to work in
> > terms that insurance companies will reimburse. But cortical
> > source-resolved EEG recording and analysis is technically quite
> > feasible, as our work at SCCN over the last 30 years has abundantly
> > demonstrated -- and allows more exact interpretation (and high
> > statistical certainty) than scalp-level data interpretation.
> >
> > Scott Makeig
> >
> > On Tue, Nov 18, 2025 at 9:41 AM 장진원 via eeglablist <
> > eeglablist at sccn.ucsd.edu> wrote:
> >
> >> Thank for your kind reply. I understand it.
> >>
> >> Best Regards,
> >> Jinwon
> >>
> >> On Mon, Nov 17, 2025 at 7:23 PM fernandez luis via eeglablist <
> >> eeglablist at sccn.ucsd.edu> wrote:
> >>
> >> > > Hi Jinwon,
> >> > >
> >> > > Accurate EEG channel interpolation is methodologically feasible
> >> across a
> >> > broad spectrum of montage densities, including low-density
> >> configurations
> >> > such as 19- and 32-channel systems, intermediate-density arrays
> >> > such as
> >> 64
> >> > channels, and high-density systems such as 128 channels.
> >> > Importantly,
> >> the
> >> > methodological validity of interpolation does not primarily depend
> >> > on
> >> the
> >> > absolute number of electrodes, but on the availability of
> >> > non-artifacted neighboring electrodes with adequate spatial
> >> > distribution surrounding
> >> the
> >> > channel to be reconstructed.
> >> > >
> >> > >
> >> > > EEG interpolation is a spatial estimation procedure in which the
> >> signal
> >> > of an artifacted electrode is reconstructed using mathematically
> >> weighted
> >> > contributions from the nearest clean surrounding electrodes.
> >> Accordingly,
> >> > if the spatially adjacent electrodes are also artifacted,
> >> > interpolation becomes unreliable or methodologically inappropriate,
> >> > because the reconstruction process would be driven by distorted
> >> > input data—violating key assumptions underlying spherical spline
> >> > interpolation,
> >> inverse-distance
> >> > weighting, and other spatial estimation algorithms.
> >> > >
> >> > >
> >> > > In low-density montages (e.g., 19 or 32 channels), interpolation
> >> remains
> >> > technically feasible; however, the reduced spatial sampling
> >> > inherently limits the anatomical precision and spatial granularity
> >> > of the reconstructed signal. Nevertheless, interpolation in these
> >> > systems can yield clinically acceptable results as long as the
> >> > electrodes used as sources for reconstruction are clean, stable,
> >> > and sufficiently
> >> distributed
> >> > around the artifacted location.
> >> > >
> >> > >
> >> > > Intermediate-density systems such as 64-channel EEG offer
> >> > > improved
> >> > spatial resolution that allows more accurate reconstruction of
> >> > missing channels due to enhanced scalp coverage. High-density
> >> > montages, particularly 128-channel EEG systems, provide dense and
> >> > homogeneous
> >> spatial
> >> > sampling, minimizing interpolation error and generating
> >> > reconstructions that are more physiologically plausible and
> quantitatively reliable.
> >> This
> >> > level of spatial resolution is advantageous for applications
> >> > requiring high-fidelity scalp mapping, microstate analysis,
> >> > connectivity
> >> estimation,
> >> > and source localization.
> >> > >
> >> > >
> >> > > Despite differences in resolution across montage densities, a
> >> > fundamental methodological requirement remains invariant:
> >> > interpolation must be performed exclusively using clean,
> >> > non-artifacted surrounding electrodes. Reconstruction based on
> >> > artifacted neighbors compromises the physiological validity of the
> >> > estimated signal and undermines the mathematical assumptions
> intrinsic to spatial interpolation algorithms.
> >> > >
> >> > >
> >> > > Technical Comparison: Conventional vs. High-Density EEG
> >> > > Interpolation
> >> > >
> >> > >
> >> > > 1. Interpolation in conventional EEG (19–32 channels)
> >> > >
> >> > >
> >> > > Low-density EEG systems rely on sparse spatial sampling, which
> >> > > imposes
> >> > several methodological constraints:
> >> > >
> >> > >
> >> > > • Wide inter-electrode spacing
> >> > >
> >> > > • Higher vulnerability to local contamination
> >> > >
> >> > > • Reduced capacity to capture rapid spatial changes
> >> > >
> >> > > • Acceptable but limited reliability
> >> > >
> >> > >
> >> > > 2. Interpolation in high-density EEG (64–128 channels)
> >> > >
> >> > >
> >> > > High-density EEG (HD-EEG) significantly enhances the reliability
> >> > > of
> >> > interpolation due to:
> >> > >
> >> > >
> >> > > • Dense and homogeneous spatial sampling
> >> > >
> >> > > • Robustness to isolated corrupted channels
> >> > >
> >> > > • Improved modeling of spatial gradients
> >> > >
> >> > > • Near-physiological reconstruction in 128-channel systems
> >> > >
> >> > > Best
> >> > > Luis Fernandez, MSc
> >> > > Clinical Neuropsychologist
> >> > >
> >> >
> >> > > El 17 nov 2025, a las 20:41, 장진원 via eeglablist <
> >> > eeglablist at sccn.ucsd.edu> escribió:
> >> > >
> >> > > Hi all,
> >> > >
> >> > > I'm a clinical psychiatrist, so I am not really familiar with
> >> engineering
> >> > > concept of interpolation. I believe in high-density setting (128
> >> channel)
> >> > > interpolation of a few channels are acceptable, but what if more
> >> > > than
> >> 10
> >> > > bad channels in 64 channel setting? Is it tolerable or
> >> > > detrimental for maintenance of true signal?
> >> > >
> >> > > Best regards,
> >> > > Jinwon Chang
> >> > > _______________________________________________
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> >
> >
> > --
> > Scott Makeig, Research Scientist and Director, Swartz Center for
> > Computational Neuroscience, Institute for Neural Computation,
> > University of California San Diego, La Jolla CA 92093-0559,
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