Jasper, started then working on a specific electrode positioning system to be used in all laboratories. A committee of International Federation of Societies for EEG and Clinical Neurophysiology (IFSECN), led by H. Observation of different regional cerebral rhythms encouraged the use of multiple electrodes and of more recording channels, but the standardization of the recording methods soon became necessary, so that the resulting data could be comparable with one another. Later on, other researchers highlighted how, in reality, EEG activity varied significantly, depending on the area of the scalp from which it was recorded. Berger kept using this method for many years, considering it an efficient system to measure the global cortical activity. When Hans Berger recorded the first human EEG, he only had two electrodes available, positioned in the anterior and posterior regions of the head. montage = banana_montage data_banana = ft_preprocessing ( cfg, data_orig ) reref = 'no' % use the cfg.montage option instead cfg. ![]() channel = 'all' % this is the default cfg. sourcemodel = sourcemodel leadfield = ft_prepare_leadfield ( cfg ) data_org = data_org. inwardshift = 20 % in mm, relative to the scalp surface which is at 85 mm radius sourcemodel = ft_prepare_sourcemodel ( cfg ) figure ft_plot_headmodel ( headmodel ) alpha 0.3 ft_plot_mesh ( sourcemodel ) ft_plot_sens ( elec, 'label', 'label', 'elecshape', 'disc' ) view () cfg = cfg. cond = % conductivities of each sphere headmodel. The sources consist of a layer of 642 evenly distributed dipoles that are shifted inward from the inner skull surface.Įlec = ft_read_sens ( 'template/electrode/easycap-M1.txt' ) headmodel = headmodel. The following example uses a spherical arrangement of the electrodes, in combination with a three layer sperical headmodel. This method requires a forward model for the sources that are assumned to have generated the EEG data, this can be computed using ft_prepare_leadfield. See A method to standardize a reference of scalp EEG recordings to a point at infinity by Dezhong Yao (2001) for details. The REST or Reference Electrode Standardization Technique for scalp EEG recordings approximates a reference at a point at infinity. However, for source reconstruction you would want to use clean data without bad channels, so the median reference is less applicable anyway. Consequently, the median reference in the forward model will never really fit that of the data. The median is not recommended when you plan to do dipole fitting or source reconstruction on the data, since the median of the data will be influenced by the noise, whereas the median of the forward computed leadfield will not be influenced by the noise. We recommend the median reference when you have a lot of channels and when it is hard to identify and disable the bad channels. refchannel = 'all' data_median = ft_preprocessing ( cfg, data_orig ) Īfter this preprocessing step, the median over all channels will be zero. You can use this to compute an average reference over all electrodes like this:Ĭfg = cfg. refchannel and subtracted from all channels that were selected in ft_preprocessing. refmethod = 'avg' the average is computed over the channels specified in cfg. The process and consequences of applying a certain re-referencing scheme which is also known as a “montage” to clinical 1020 EEG recordings is very nicely explained on the Learning EEG website. montage in combination with ft_prepare_montage. Alternatively, if you have a more complex referencing scheme or want more control over the re-referencing, you can specify cfg. ![]() reref = 'yes' and give the specific method as cfg. ![]() Hence, it is common to apply some re-referencing in the preprocessing of EEG and iEEG data.įieldTrip implements multiple methods for re-referencing in the ft_preprocessing function. Example eeg preprocessing laplace bipolar Re-reference EEG and iEEG dataĮEG and intracranial EEG (iEEG) data, which includes sEEG and ECoG, is often recorded relative to a reference electrode that is good for the signal quality and for noise suppression (e.g., with an electrode firmly attached on the mastoid behind the ear), but that is not neccessarily the most optimal for subsequent analysis or interpretation of the data.
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