The present study addresses the separate activities of frontal and temporal MMN generators which might be differentially affected by a change in the probability of standard stimuli. As the probability of standard stimuli was increased, the frontal MMN component significantly increased in amplitude, while the temporal one was not affected. Correspondingly, the scalp current density (SCD) maps showed that the temporal MMN generator was activated even at low probability of standard stimuli, suggesting that even the weak memory trace could start the automatic mismatch process, whereas the frontal MMN generator was activated only with increased probabilities of standard stimuli, suggesting that the stronger the memory trace is, the easier it might trigger the involuntary switching of attention to stimulus change.

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Study Objectives: The mismatch negativity (MMN) component of the event-related potentials reflects the automatic detection of sound change. Only a few researchers have investigated the MMN elicitation during sleep in adult human and some of them reported that MMN amplitude was decreased in sleep compared to in waking. However, it is not clear that the decrease of MMN amplitude was due to increased drowsiness or long-term response decrement. Two experiments were conducted to clarify whether or not the MMN was elicited in each sleep stage. We presented auditory stimuli to subjects continuously from waking until sleep state (Experiment 1). Using the same experimental condition, we examined whether or not MMN amplitude was influenced by long-term stimulus presentation (80min.) and by vigilance level (Experiment 2). Conclusions:To obtain the reliable data, by using the automatic spectral analysis, we confirmed that MMN was elicited in REM sleep. MMN was not influenced by long-term stimulation. The result suggested that auditory stimuli could be processed in the preattentive sensory memory even in REM sleep.

To investigate a part of the structure of the memory trace, auditory event-related potentials (ERPs) were recorded from reading subjects while they were presented with two different stimulus-series simultaneously. A clear mismatch negativity (MMN) was obtained from each series, when the stimulus sequence consisted of a high-frequency series and a low-frequency series. Moreover, the MMN showed independent elicitation within each series. However, if the frequency range of one series overlapped with that of the other series, the amplitude of the MMN was prominently reduced, suggesting that the two processing functions indexed by MMN coexisted simultaneously in the preattentive acoustic system and were produced by the respective grouping of high-frequency tones and low-frequency tones.