[FieldTrip] Grand mean

[Schoffelen, J.M. (Jan Mathijs)]

Dear Sarah,

Thanks for clarifying your question. Now, could you provide the readership with some additional information about how you constructed these data structures, and how you tried to combine them, otherwise we (i.e. the people who’d want to think along with you to fix your problem) don’t know where to start.

Jan-Mathijs


On 14 May 2020, at 09:32, Sarah Kirchler <sarah.kirchler at gmail.com<mailto:sarah.kirchler at gmail.com>> wrote:

Dear Jan-Mathijs,

thank you for your answer. I see, i expressed myself not very clear.
I'd like to make the Grand mean for the difference and the sum between two specific frequencies I measured visually wih a SSVEP.
I'd like to do this because I'm interested in how the frequencies are processed in the brain regarding each other and a control measurement.
I'm not sure if I can make the difference already when I make the GM or later, when I plot the topography.
I tried to make this by building a datastructure that contains the GM of the one and the other frequency and then usw ft_math to compare them. But this doesn't work for me.
Maybe thats enough information for you to understand the problem better, I'm a beginner so I often explain my problems a little bit confusing, sorry for that.

Best,

Sarah

Am Mi., 13. Mai 2020 um 15:30 Uhr schrieb Schoffelen, J.M. (Jan Mathijs) <jan.schoffelen at donders.ru.nl<mailto:jan.schoffelen at donders.ru.nl>>:
Dear Sarah,

It is not clear what error message you get, nor why you think you should ft_timelockanalysis/ft_timelockgrandaverage/ft_topoplotER.

Some pointers:
-For a mathematical operation to be applied to numeric data that resides within a fieldtrip-style data structure you’d use ft_math.
-What you want to do (i.e. adding or subtracting estimates of frequency specific power estimated for different frequency bins) probably requires some manual intervention with the data, because FT will likely prevent you to do so, since it checks whether the frequency bins for the to-be-combined data structures match. I could type some recipe now, but since you are not too clear on the details so I’d rather not spend time on that now. Also, I’d like to give other people the opportunity to chime in here :).

Best wishes,
Jan-Mathijs

On 11 May 2020, at 16:43, sarah.kirchler at gmail.com<mailto:sarah.kirchler at gmail.com> wrote:

Dear Fieldtrip Users,

thank you for all the help you’ve given me already.
Now I am trying to make the sum and the difference of the Grand mean from/between two frequencies I measured with an EEG.
I read that it is possible to make a datastructure using {} within ft_timelockbaseline and then use the option cfg.operation(x1-x2 and so on) as a operation of function ft_topoplotER to make this sum/these differences.
Now my Problem is that I don’t need to make the baselinecorrection via ft_timelockbaseline because I don’t have the baseline in the datasegment I’m interested in. So I tried to build the datastructure for cfg.operation within ft_timelockanalysis but Fieldtrip doesn’t allow this and I get an error message.

My questions are now:
1. Can I make the difference also within the ft_timelockgrandavverage function or do I have to make the difference lawer in ft_topoplotER?
2. If so, how can I or form a datastructure I can use in cfg.operation or make the difference differently with some other function?

I hope you understand me,
to all you extraordinary brains: thank you in advance,

Sarah

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fieldtrip mailing list
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https://doi.org/10.1371/journal.pcbi.1002202
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