What determines the amplitude decrease of event-related potentials by repeated auditory stimulation?

Repetition of auditory stimuli results in a suppression of event-related potentials (ERPs) if it occurs within a time window of approximately 0.4s to 15s. Depending on the kind of the experiment and the view of the experimenter, this process might be termed sensory gating, habituation, adaptation or refractoriness. The different denomination already indicates that the functional (and anatomical) foundation of this response decrease is yet poorly understood. One paradigm for the investigation of these foundations is the application of stimuli trains, interspersed with longer intervals of silence. The event-related activity is then calculated for each stimulus position of the train separately. In intracranial recordings a decrease of the N100 amplitude was obtained only from the 1st to 2nd stimulus of a train. In that experiment stimulus material and interstimulus interval were comparable to sensory gating experiments. Similar as the N100, the P50 decrease was completed after the 2nd stimulus of the train. Both findings are not in line with the assumption of a habituation process and indicate that no additional information would be obtained in sensory gating experiments if trains instead of pairs of stimuli would be applied. An induced high frequency activity (80–160Hz) occurred in a latency range 80–350 ms. Analyses of single trials revealed that the activity consisted of 30–70 ms bursts of activity rather than an ongoing activation and the number of bursts as well as their amplitudes decreased by stimulus repetition. In line with the intracranial recordings, in a magnetoencephalography experiment a significant decrease of the auditory evoked field component N100m was observed only from 1st to the 2nd stimulus of a train. In the context of an ongoing and varying stimulation, stimulus repetition had no significant effect on the N100m, but elicited a mismatch negativity (MMN), which is usually evoked by discernible deviations from a repetitive auditory stimulation. Besides a short-term decrease (from one stimulus to another), cortical responses decrease continuously in course of an experiment, as shown for the N100m and, even more dramatically, for the neuromagnetic MMN. In conclusion, the investigation of various aspects of a response decrease by repeated stimulation elucidates very basic brain functions. The observation of alterations of these functions in neuropsychiatric disorders might provide useful information about pathological brain processes.