A "virtual electrode" mimics a real electrode inserted deep into neural tissue. The output measurement is a weighted sum of all the electrode's channels (spatial filter). This image shows the pattern of gain of a spatial filter listening to a target source (green "+" sign) while suppressing interfering signals (red "x" signs). The gain is zero (black) at the position of every interfering signal, enabling the virtual electrode to detect the target. However, the gain pattern depicted is much more complex than that of a real electrode, making it potentially sensitive to other distant sources. See the article by Alain de Cheveigné for more information on the unique properties of spatial filters, which he describes using a combination of theory and easy-to-visualize simulations. Cover image: Alain de Cheveigné.