I.3: Plotting Channel Spectra and Maps
Plotting channel spectra and maps
To begin processing the data, we recommend first scrolling the data as shown before rejecting clearly 'bad' data stretches or data epochs, then studying their power spectra to be sure that the loaded data are suitable for further analysis. Note that the Matlab Signal Processing Toolbox needs to be in your Matlab path to use these functions.
Exploratory Step: Plot Channel Spectra and Maps.
To plot the channel spectra and associated topographical maps, select Plot > Channel spectra and maps. This will pop up the pop_spectopo.m window (below). Leave the default settings and press OK.
The function should return a spectopo.m plot (below). Since we only sampled 15% of the data (via the Percent data... edit box above), results should differ slightly on each call. (Naturally, this will not occur if you enter 100% in the edit box).
Each colored trace represents the spectrum of the activity of one data channel. The leftmost scalp map shows the scalp distribution of power at 6 Hz, which in these data is concentrated on the frontal midline. The other scalp maps indicate the distribution of power at 10 Hz and 22 Hz.
The pop_spectopo.m window menu (above) allows the user to compute and plot spectra in specific time windows in the data. The Percent data... value can be used to speed the computation (by entering a number close to 0) or to return more definitive measures (by entering a number closer to 100). Note that functions pop_spectopo.m and spectopo.m also work with epoched data. Another menu item, Plot > Channel properties, plots the scalp location of a selected channel, its activity spectrum, and an ERP-image plot of its activity in single-epochs.
The next section deals with some data preprocessing options available via the EEGLAB menu.