[FieldTrip] ITC and harmonic artefacts?

[Ivaylo Iotchev]

I am sorry Jan Mathijs, and thank you for bearing with me!

I mainly wanted to make sure that harmonics can meddle with ITC, but I also
found an answer to another sub-question which might be of interest to other
yet inexperienced with this type of analysis.

As you suggested, some more information first... we are looking at
responses to speech streams, wherein word boundaries occur at a rate of 1.3
Hz and syllables follow each other at a rate of 4 Hz... the reason a 1.3 Hz
response is not visible when trials are defined relative to syllable start
is that every word consists of 3 syllables, meaning that two out of three
trials cannot be expected to show frequency alignment with the recurrence
of word boundaries (since only every 3rd syllable marks the beginning of a
new word). Hence I discovered the maybe trivial to some users rule, that
you have to select your starting points relative to the question at hand,
i.e. when looking for word-level entrainment all trials should be defined
relative to the word-beginnings, otherwise there will be a lot of trials
that introduce noise.

I hope I make more sense now :) Thank you dearly!

Ivo

Am Mi., 10. Mai 2023 um 08:25 Uhr schrieb Schoffelen, J.M. (Jan Mathijs)
via fieldtrip <fieldtrip at science.ru.nl>:

> Hi Ivo,
>
> I understand less than half of what you are writing. If you don’t provide
> any additional details about what (and how) you are trying to achieve, the
> only thing we can do as readership of this list (about 1800 people), is to
> call in the cavalry.
>
> I asked my pal chatGPT, and this is what I got for an answer:
>
>
>
> "It sounds like you are observing an effect in your inter-trial coherence
> analysis that is influenced by the distance between the time-points
> defining the trial-start.
>
> One possibility is that you are observing harmonics of the frequencies of
> interest, as you suggested. Harmonics are multiples of a given frequency,
> and they can produce similar patterns of activity in the brain. Therefore,
> it's possible that your analysis is picking up on harmonics of the
> frequencies you're interested in.
>
> Another possibility is that there may be some kind of artifact in your
> data that is causing the effect. For example, if there is some kind of
> noise or artifact in the data that is strongest at particular time
> intervals, this could cause the coherence values to be higher or lower at
> those intervals.
>
> To help determine the cause of the effect you're observing, you could try
> a few things:
>
>    - Check your data for artifacts: Look at the raw data to see if there
>    are any obvious sources of noise or artifact that could be causing the
>    effect you're observing. You could also try preprocessing the data
>    differently (e.g., applying different filters, using different reference
>    electrodes) to see if this changes the results.
>    - Conduct a control analysis: Run a control analysis where you
>    randomly shuffle the trial-start times and then compute the coherence
>    values. This will help determine if the effect you're observing is due to
>    the specific trial-start times you've chosen or if it is a more general
>    effect.
>    - Consider other possible explanations: Think about other possible
>    explanations for the effect you're observing. For example, it could be
>    related to the way the brain is processing the stimuli you're presenting,
>    or it could be related to some other aspect of the experimental design.
>
> Overall, it's difficult to say exactly what is causing the effect without
> more information, but these are some things you could try to help narrow
> down the possibilities."
>
>
>
> Perhaps that will help to get you going.
>
> Good luck,
> Jan-Mathijs
>
>
>
> On 8 May 2023, at 13:04, Ivaylo Iotchev via fieldtrip <
> fieldtrip at science.ru.nl> wrote:
>
> Dear community,
>
> for some reason choosing the distancing between time-points defining
> trial-start for my inter-trial coherence analysis seems to affect the
> results... high coherence is observed for each frequency which is
> x/distance. What is funny is that trial duration and exact timing of
> pre-stimulus baseline and post-stimulus end are not influencing this
> picture, only the distance between the points that define trial start. For
> example if I space my trials starting points 750 milliseconds apart, I will
> get increases for 1.3 Hz, 2.6 Hz, 4 Hz... eventually 8 Hz... and so on...
>
> To some extent I wouldn't be surprised if these are effects related to
> harmonics and I frankly do not expect a zero value for frequencies that are
> harmonic to the frequencies of interest, but maybe I am running up an
> artefact??
>
> Bests,
>
> Ivo
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