ESTIMATION OF INTERPOLATION ERRORS IN SCALP TOPOGRAPHIC MAPPING

Topographic maps are commonly constructed from electrical scalp record ings (such as EEGs and ERPs) using several different interpolation met hods. It is important to determine the accuracy of such maps. Previous assessments of interpolation methods have been based on global error measures and the visual appearance of the topographic maps. However, t he relationship of interpolation error to local contributing factors r equires a more detailed analysis. In this paper, we use simulations to explore and quantify the relationship of error to global and local fa ctors for different interpolation methods. We find that among the best interpolation methods, adequate electrode density is more important t han the method used. For shallow sources, we show that local interpola tion error is most correlated with potential gradient, and has a lesse r correlation with distance to nearest electrode. The greatest correla tion, however, is with the product of gradient and distance. Thus, int erpolation error can be controlled locally by making the interelectrod e distance inversely proportional to the expected potential gradient. With shallow sources, areas far from any electrode and having high app arent gradient are likely to have high interpolation error. Moreover, all areas far from any electrode may contain high interpolation errors , and should be interpreted with caution.