Enhancement of epileptiform activity in the EEG by 3-D adaptive spatial filtering: simulations and real data

An adaptive 'beamformer' spatial filter has been investigated for the enhancement of signals from deep sources in the brain suspected of being epileptiform discharges (EDs). A forward 3-D inhomogeneous spherical model that relates a dipolar source to recorded signals is used to determine the beamformer's spatial response constraints. The beamformer adapts, using the least mean squares algorithm, to reduce signal's from sources distant from the assumed source location and produces three outputs, the orthogonal components of the signal estimated to have arisen at or near the assumed location. Simulations were performed by using the same forward model to superimpose realistic EDs on normal EEG recordings. The simulations showed that the beamformer could enhance signals emanating from epileptogenic foci by a mean 120%. The beamformer was also shown to be relatively insensitive to depth and to inaccuracies in estimate of location (depth and angular location) of both radially and tangentially oriented dipoles. A preliminary clinical evaluation on real EEG records showed that the beamformer was able to enhance definite EDs by a mean 102%. Overall, these results demonstrate the value of the beamformer as a non-invasive means of enhancing activity from epileptogenic foci whose location is ill-defined.