THE EFFECTS OF MAGNITUDE AND DIRECTION OF STIMULUS CHANGE ON AUDITORYEVENT-RELATED POTENTIALS ELICITED BY DEVIANT SIGNAL STIMULI

The present study sought to identify components of the auditory event- related potential (ERP) elicited by stimuli that serve as signals for overt discriminative responses. Sokolov's view of a selective neural f ilter for deviant stimuli predicts that responses to deviant signal st imuli will be graded in proportion to the amount of change from the st andard and independent of the direction of that change. The demonstrat ion of a bi-directional and graded ERP response requires at least two levels of stimulus change in each direction. The present study incorpo rated two deviants that were lower in pitch (lowest, low) and two that were higher in pitch (high, highest) in order to evaluate the degree (linear, quadratic, etc.) of the function relating ERP response to ton al deviance. Stimulus changes on both the direction and magnitude dime nsions were also varied on a trial-by-trial, rather than on a block-by -block, basis which eliminated potential confounds with block or sessi on differences, and discriminative responses were required to both sta ndard and deviant tones, thereby investing both categories of stimuli with signal value. The amplitude of the P3 component associated with d eviant stimuli showed close correspondence to the (quadratic) function predicted from the selective filter model. A late negative slow wave (NSW) at Fz and a positive slow wave (PSW) at Pz differentiated devian t tones from the standard but did not distinguish between the deviants themselves. A fronto-central NSW observed at Fz and Cz for initial st andard tones was greater than the predominantly frontal NSW elicited b y the deviant tones. The topographical differences in NSW elicited by the initial standard tone and by all deviant tones suggest that differ ent processes are reflected in the NSW response to these Stimuli.