Event-Related Potential Measures of a Violation of an Expected Increase and Decrease in Intensity

Unexpected physical increases in the intensity of a frequently occurring "standard" auditory stimulus are experienced as obtrusive. This could either be because of a physical change, the increase in intensity of the "deviant" stimulus, or a psychological change, the violation of the expectancy for the occurrence of the lower intensity standard stimulus. Two experiments were run in which event-related potentials (ERPs) were recorded to determine whether "psychological" increments (violation of an expectancy for a lower intensity) would be processed differently than psychological decrements (violation of an expectancy for a higher intensity). Event-related potentials (ERPs) were recorded while subjects were presented with auditory tones that alternated between low and high intensity. The subjects ignored the auditory stimuli while watching a video. Deviants were created by repeating the same stimulus. In the first experiment, pairs of stimuli alternating in intensity, were presented in separate increment (H-L...H-L...H-(H) under bar...H-L, in which H = 80 dB SPL and L = 60 dB SPL) and decrement conditions (L-H...L-H...L-(L) under bar...L-H, in which H = 90 dB SPL and L = 80 dB SPL). The paradigm employed in the second experiment consisted of an alternating intensity pattern (H-L-H-L-H-(H) under bar -H-L) or (H-L-H-L-(L) under bar -L-H-L). Importantly, the stimulus prior to the deviant (the standard) and the actual deviants in both increment and decrement conditions in both experiments were physically identical (80 dB SPL tones). The repetition of the lower intensity tone therefore acted as a psychological rather than a physical decrement (a higher intensity tone was expected) while the repetition of the higher intensity tone acted as a psychological increment (a lower intensity tone was expected). The psychological increments in both experiments elicited a larger amplitude mismatch negativity (MMN) than the decrements. Thus, regardless of whether an acoustic change signals a physical increase in intensity or violates an expected decrease in intensity, a large MMN will be elicited.