The most commonly discussed indices of the OR (e.g., GSR, respiration, HR, CPV, PPV, and EEG) have been traditionally conceptualized as showing similar behavior in response to stimulus variation. Two of Sokolov’s hypothesized dimensions of stimulus variation affecting OR behavior are intensity and novelty, and these were focused upon in the present study. The six psychophysiological indices listed above were found to exhibit markedly different behaviors in their responses to manipulation of these two independent variables. These differences were so marked as to make the concept of a unitary OR untenable. © 1977, Psychonomic Society, Inc.. All rights reserved.