Changes in patterns of hemispheric electrophysiological activity as a function of instructional set

Investigations of differential hemispheric functioning which have used auditory event-related potentials (AERPs) typically employ paradigms in which two or more conditions are used. In these paradigms, conditions typically differ from each other with respect to both the physical aspects of the stimuli and the operations the subject must perform on these stimuli. Because both the subject's task and the physical properties of the stimuli change from one condition to another, it is unclear whether different patterns of AERP cerebral asymmetries obtained between conditions are primarily a function of differences in the stimuli (e.g., verbal vs. musical stimuli) or of endogenous factors such as the subject's cognitive approach to the processing of the stimuli. In the present study, two conditions with identical stimuli were employed but different instructions were given about how to process these stimuli. Music and paired tones were simultaneously presented in both conditions. In the Music Condition, subjects performed a task which involved attending to the music and ignoring the tones. In the Tones Sequence Condition, subjects counted specified sequences of tones and ignored the music. The results indicated that there were different patterns of left-right hemispheric AERP relationships between the two conditions. These findings support the hypothesis that endogeneous factors play a significant role in differential hemispheric functioning and that the AERP technique employed in this paradigm is a sensitive index of changes in electrophysiological activity associated with these endogeneous factors.