[Eeglablist] time window

[Stephen Politzer-Ahles]

 I doubt it's possible to choose a time-locking point such that every
condition will show the MMN at the same time. There may be asymmetries such
that shorter-than-standard may have smaller/bigger/later/earlier MMNs than
longer-than-standard deviants (see
https://psycnet.apa.org/fulltext/2016-24934-001.html for a review of
factors that can cause MMN asymmetries). And in the case of
longer-than-standard deviants, the processes that generate the MMN might be
initiated when the person notices the deviant is longer *or* after the
person waits for the end of the deviant to see how much longer it is (I
suspect there are already studies that have been done which can tell you
which sort of process is more likely). So there's not a single point you
can lock to and know that the MMN is always going to be, e.g., 170ms after
that point.

What you do depends on what your research question is. If you are only
interested in comparing MMN amplitudes, you could always time-lock to the
same point (e.g., the sound onset). use a long epoch, and do peak-picking
(automatically detect the MMN peak for each deviant's MMN difference wave,
and measure the amplitude centered on that peak; note that a peak in the
wave may not necessarily correspond to the peak of the MMN component, per
Luck 2004). Or a better option would be to use a mass univariate test (see
Groppe, 2011) that doesn't require you to choose window for quantifying the
amplitude; you could use the cluster-based permutation test (Maris &
Oostenveld, 2007) if you are only interested in detecting whether there are
MMNs (and whether they are different), or the test with strong error
control (described in Groppe 2011) if you want to make claims about where
and when they happen in each condition.

A few other things to note: it is quite normal that the response changes
across channels, as one of the typical features of the MMN is that it is
larger at mid frontal electrodes than in other places. It is also pretty
normal that it differs across people (ERPs almost always do). Also, a
better MMN would be to subtract the ERP for one sound when it is not a
deviant from the ERP when it is a deviant, rather than subtracting the ERP
for a different sound (450ms) from the deviant sound (e.g. 270 ms); this
would entail, e.g., recording another block where 270, 350, 405, 450, 495,
540, and 630 occur in equal proportions so that there are no "standards"
and "deviants" (see Schroeger & Wolff 1996 in *NeuroReport*). If you have
already recorded your data, however, this is just something to consider for
future studies, rather than something practical to do with this dataset.

---
Stephen Politzer-Ahles
The Hong Kong Polytechnic University
Department of Chinese and Bilingual Studies
http://www.mypolyuweb.hk/~sjpolit/
<http://www.nyu.edu/projects/politzer-ahles/>


On Fri, Apr 19, 2019 at 9:23 PM Stephanie Alejandra Armstrong Gallegos <
sarmstrong2 at sheffield.ac.uk> wrote:

> Dear all
>
> I need to set a time window for the analysis of an MMN study. The study
> consisted of an oddball paradigm with auditory stimuli: the standard had a
> duration of 450 msec, and the deviants had 6 different durations, shorter
> than standard: 270, 350, 405 msec; and longer than standard: 495, 540, 630
> msec.
> The papers mention time windows around 100-250 msec. after stimulus onset,
> however in those cases the difference can be identify immediately from the
> start (e.g. the standard is a red dot, the odd is a blue dot); in my case,
> the difference in duration can be only noticed when the shorter stimulus
> has ended.
> So, I’m not sure how to set a time window that works for both the shorter
> and longer conditions. I plotted the data and the response seems to change
> across individuals and channels.
> Any ideas?
>
> thanks
> --
> Stephanie Armstrong
> PhD Researcher
> Psychology Department
> The University of Sheffield
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