[FieldTrip] Computing ERFs from HCP MEG Motor Task

Tue Jun 3 15:26:26 CEST 2025

Dear Fabio,

Make sure that the trigger info is correct and precise. Motor
responses usually are not phase locked to an external stimulus.
You could calculate induced frequency activity and see if you get a
contra-lateralized beta band desynchronization (that is not
phase locked to the onset of your stimulus).

best,

Stephan

El mar, 3 jun 2025 a las 13:56, Fabio Strappazzon via fieldtrip (<
fieldtrip at science.ru.nl>) escribió:

> Dear all,
>
> I'm trying to extract ERFs from HCP MEG minimally preprocessed data
> from the motor task (right hand trials) and before doing the trial
> averaging I'm computing very simple operations (trial selection,
> 0.5-60 Hz bandpass filtering and demeaning).
> I'm considering each run as independent, giving the fact that the
> sensor information is different across the two runs and this might
> affect the future source reconstruction.
> However, when I then compute the trial average, the evoked activity is
> not evident.
> Am I missing something?
>
> This is the script:
>
> clear all
> close all
> clc
>
> filesystem_dir % retrieves directories
> ft_init
> cd(scriptsdir)
> mySub = '177746';
> preprocdir = [datadir,mySub,'\MEG\Motort\tmegpreproc']; % preprocessed
> data dir
> subj_resdir = [resdir,mySub]; % results directory
> irun = 10;
>
> % loads minimally preprocessed data and trialinfo
> cd(preprocdir)
> load(sprintf([mySub,'_MEG_%d-Motort_tmegpreproc_TEMG.mat'],irun)) %
> run 1 (t0 is the EMG onset)
> load(sprintf([mySub,'_MEG_%d-Motort_tmegpreproc_trialinfo.mat'],irun))
> % WARNING: First columns are for EMG onsets, seconds for flash
> stimulus onset
>
> % selecting only right hand data
> cfg = [];
> cfg.channel = {'MEG'}; % read all MEG channels except EMG channels
> cfg.trials = find(data.trialinfo(:, 2) == 4); % select only right hand
> trials
> data_rh = ft_selectdata(cfg, data);
>
> % Wide-band BP filtering (prior to baseline correction):
> cfg = [];
> cfg.hpfilter = 'yes';
> cfg.hpfreq = .5;
> fdata_temp = ft_preprocessing(cfg, data_rh);
>
> cfg = [];
> cfg.lpfilter = 'yes';
> cfg.lpfreq = 60;
> fdata_rh = ft_preprocessing(cfg, fdata_temp);
>
> % Baseline correcting epochs;
> cfg = [];
> cfg.demean = 'yes';
> cfg.baselinewindow = [-0.3 0]; % -300 ms to 0 (chosen by HCP)
> fdata2avg = ft_preprocessing(cfg, fdata_rh);
>
> % ERF (avg across bsln corrected trials)
> cfg = [];
> avg_rh = ft_timelockanalysis(cfg, fdata2avg);
>
> % Plot
> cfg = [];
> cfg.showlabels = 'yes';
> cfg.fontsize = 6;
> cfg.layout = '4d248_helmet.mat';
> cfg.ylim = [-3e-13 3e-13]; % problem is present also when the scale is
> adjusted
> ft_multiplotER(cfg, avg_rh);
>
> Thanks in advance.
> Best regards,
>
> Fabio.
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-- 
Un cordial saludo,

Stephan Moratti, PhD
Profesor de Psicología Básica I
Universidad Complutense de Madrid
91 394 3141
smoratti at ucm.es

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