Effects of a violation of an expected increase or decrease in intensity on detection of change within an auditory pattern

An infrequent physical increase in the intensity of an auditory stimulus relative to an already loud frequently occurring "standard" is processed differently than an equally perceptible physical decrease in intensity. This may be because a physical increment results in increased activation in two different systems, a transient and a change detector system (signalling detection of an increase in transient energy and a change from the past, respectively). By contrast, a decrease in intensity results in increased activation in only the change detector system. The major question asked by the present study was whether a psychological (rather than a physical) increment would continue to be processed differently than a psychological decrement when both stimuli activated only the change detector system. Participants were presented with a sequence of 1000 Hz tones that followed a standard rule-based alternating high-low intensity pattern (LHLHLH). They were asked to watch a silent video and thus ignore the auditory stimuli. A rare "deviant" was created by repeating one of the stimuli (e.g., LHLHLLLH. The repetition of the high intensity stimulus thus acted as a relative, psychological increment compared to what the rule would have predicted (the low intensity); the repetition of the low intensity stimulus acted as a relative, psychological decrement compared to what the rule would have predicted (the high intensity). In different conditions, the intensity difference between the low and high intensity tones was either 3, 9 or 27 dB. A large MMN was elicited only when the separation between the low and high intensities was 27dB. Importantly, this MMN peaked significantly earlier and its amplitude was significantly larger following presentation of the psychological increment. Thus, a deviant representing an increment in intensity relative to what would be predicted by the auditory past is processed differently than a deviant representing a decrement, even when activation of the transient detector system is controlled. The psychological increment did not however elicit a later positivity, the P3a, often thought to reflect the interruption of the central executive and a forced switching of attention.