[Eeglablist] Surface Laplacian (CSD) changes baseline period activity?

[Mate Gyurkovics]

Dear all,

does anyone have any input on this, maybe? Also, I was wondering if you'd
consider the microvolt / cm2 values on the figure believable? (I used the
laplacian_perrinX() function by Mike Cohen, with the default smoothing
parameter 1e-5.)

Thank you.

Best,
Mate

On Fri, 13 Sep 2019 at 17:30, Mate Gyurkovics <mgyurkovics1 at sheffield.ac.uk>
wrote:

> Dear all,
>
> I ran a reaction time task in two age groups while recording EEG from 64
> channels. I want to look at age related differences in the magnitude of the
> N2 component at electrode FCz. I epoched, baseline corrected, then averaged
> my data, and the ERPs looked fine - flat baseline periods, then a nice N2
> about 200-300 ms after target stimulus presentation. However, when I apply
> surface Laplace transformation (or current source density, CSD) to the
> data, the waveforms change a little bit. Nothing drastic, but in the
> younger age group the baseline period is no longer flat, it appears to be
> increasing slightly. Please see the following image:
> https://imgur.com/a/nN5s2bA, where the first dashed line indicates the
> presentation of the first stimulus on a trial, and time point 0 is the
> presentation of the target stimulus, in other words, activity between -200
> and 0 makes sense, the gentle upward slope before -200 does not, and it's
> not there a) in the untransformed data, b) in adults, c) in all electrodes,
> even in the transformed data.
>
> Intertrial intervals were jittered so participants could not predict when
> the next trial will come up. Each condition of interest contains over 100
> epochs, close to 140 actually.
>
> Does anyone know why this pattern might emerge? Is this cause for concern?
> Is it a side effect of the Laplace transform? I want to stress again that
> the non-transformed (average referenced) data looks perfectly normal.
>
> (Footnotes about the image: It is actually showing the grand average
> waveform across all participants and conditions, but the pattern is
> visible. It's a little more pronounced in the young group and not at all
> present in adults. Also, I know the units on the Y axis should be
> microvolts/cm2, I have accidentally forgotten to change the label.)
>
> Thank you for your help.
>
> Best,
> Mate
>