The importance of using reference-independent descriptors of topographic P3 abnormalities in schizophrenia has been described by Strik et al.4,5 who also reported left-lateralized P3 reductions in patients using a silent count oddball paradigm with pure tones. We have proposed a two-step, generic analytic strategy to overcome these limitations.48,49 First, reference-dependent surface potentials are transformed into reference-free current source density (CSD; surface Laplacian) waveforms, which are an estimate of the local current flow normal to the scalp.39,40 While this transform not only eliminates redundant, volume-conducted contributions, yielding sharper topographies than ERPs,34,50 another core advantage is that, for a given EEG montage, any EEG reference will render the same, unique CSD waveforms, which completely removes the interpretational ambiguities stemming from the choice of recording reference. Second, to further disentangle temporally and spatially overlapping components, unique and orthogonal variance patterns are identified in these reference-free data by unrestricted Varimax-PCA using the covariance matrix,42,43,48,51 yielding neuronal generator patterns that may be directly related to known neuroanatomical circuits responsible for cognitive processing.52,53,55,56
