Event-Related Potential Additivity as an Index of Overlap in Neurocognitive Resources.

Abstract

Scalp-recorded event-related potentials (ERPs) are poorly suited for source analysis. Consequently, it is difficult to assess the relative neural independence of two ERP waveforms using such methods. We report here an alternative method grounded in Helmholtz's Rule of Superposition, which stipulates that electrical fields summate where they intersect in time and space. Subjects read sentences that contained a word with one or two unexpected features. The similarity of the two features was systematically manipulated. We then compared ERPs elicited by the doubly anomalous words to composite waveforms formed by mathematically summing the ERP response to singly anomalous words. We quantified the independence of the neural response to the two stimulus features by applying a novel Additivity Index (ranging from -1 to 1). The computed Additivity Index value varied with presumed feature similarity, indicating that the index provides a valid and general method for quantifying the neural independence of scalp-recorded brain potentials.