Spatial analysis of evoked potentials in man--a review

Steps in brain information processing are reflected on the scalp as changes of the electric potential which is evoked by the stimulus. However, for a given recording point on the scalp, there is no absolute amplitude or phase information of the electric brain potential. This means that the shape of an evoked potential waveform which is recorded from a given scalp location crucially depends on the location of the chosen reference. Only unbiased results of evoked potential data evaluation can be hoped to elucidate or map successfully into information processing models established by other methods, e.g. behavior measurements. Conventional recordings vs a common reference contain only one of many possible sets of waveshapes. In order to avoid ambiguities or bias of results, the entire evoked potential data set firstly must be analysed over space, and reference-independent parameters must be extracted. For each time point, the spatial distribution of the potentials is viewed as field map. The parameter extraction in a direct approach at each time point includes, e.g. locations of field peaks and troughs, voltage and gradient between them, and global electrical field power; or, parameters via the first or second spatial derivative of the electric field. In the second step, changes of these reference-independent field measurements are analysed over time. At component latency which is defined by maximal, global field power or by voltage range, mapped field distributions can be compared using maximal/minimal field value locations or complete maps. Significantly different field configurations establish the activity of non-identical neural generators. Classification of the field configurations (examination of orbits of field extrema over time) leads to the segmentation of series of field maps (multichannel EP data) into short epochs of stationary spatial configurations (i.e. spatially characterized components) with equal consideration of all recording points, and without the amplitude criterion. The application of these principles to the following problems is discussed: comparison of evoked potentials between different analysis times, in particular pre-stimulus and post-stimulus electric brain states; zero baseline for measurement; reference electrode; identification of evoked components in time and space. Illustrations of these problems include functional differences of input-analysing sub-systems, and the topography of cognition- and speech-related electric brain activity.