millions of neurons, reducing any potential spatial localization. This is exacerbated by the fact that the conductivities of the head tissues, varies across individuals and within the same individual due to variations in age, disease state, and environmental factors. The inference of the location of the current sources from electrode voltage measurements on the scalp is known as the EEG inverse problemand is comparable to reconstructing an object from its shadow: only generic features (the shape) are uniquely determined, others must be deduced on the ground of additional information. EEG is also very sensitive to subject movement and external noise. Electrodes used in EEG recording do not discriminate the electrical signals they receive. The recorded activity which is not of cerebral origin is termed artifact. Artifacts are noncerebral signals that often contaminate the recordings in both temporal and spectral domains within a wide
30 frequency band. The internal source of artifacts may be due to physiological activities of the subject (e.g., eyes movement, electrocardiographic activity, sweat or muscle artifacts) or its movement. External sources of artifacts are environmental interferences such as electrical noise from mains interference, bad contacts between electrode and skin, or interferences from recording equipment and cable movement.