In a simple auditory reaction time experiment, evoked responses were averaged relative to auditory stimulus onset and to the right hand button press response. The response triggered average is displayed with all 128 signals of the geodesic sensor net superimposed (violet), the residual waveforms (red), and the first 5 PCA components (brown) in the data window on the left.
Six regional sources fully explain the data set. Their source waveforms (middle) show a clear separation despite the strong overlap and similarity of the waveforms at the scalp: Sources 1-2 model the evoked 3D current flow in the left (red) and right (blue) auditory cortex with the vertically oriented N100 (head scheme on the right) and the radially oriented N140 (second trace of each regional source). Activity in SMA (source 3, green) occurs around 60 ms, activity in the left motor cortex (light blue) around 40 ms prior to button press contact. Cerebellar activity (purple) peaks 12 ms before, visual cortical activity 60 ms after and cingulate gyrus activity 120 ms after the button press (second, vertical dipole of source 3, green).
The map shows the weak contribution of the SMA source 3 to the data at its peak 60 ms before button press. At this latency the scalp topography is already dominated by the strong N100 at the scalp and the SMA activity is not visible at the scalp without modeling.
Voltage or current source density maps can be displayed on the scalp; minimum norm maps on the standard cortical surface. For a given model, the model contributions of one or any combination of sources can be displayed as well as the unexplained residual map.
More details on the data processing, source analysis, and MRI/fMRI coregistration of this auditory reaction time experiment can be found in Tutorials / Hands On / Tutorial 3. Data are provided on our demonstration CD.