Is P50 an epiphenomenon?

The mid-latency auditory evoked response (MLAER) consists of the P50, N100 and P200 components. P50 is widely used to examine sensory gating. There is growing evidence that phase-reorganization of the oscillatory components comprising EEG activity, and especially in the 4-8 Hz (theta) band, is responsible for MLAER generation. Consequently, all or part of the MLAER may be an epiphenomenon, indicating intervals of increased phase reorganization. In this study, we investigated the nature of P50. AEPs were obtained from 14 normal subjects using a double stimulus paradigm, and each artifact-free trial was decomposed into its constituent oscillatory components. Single trials were classified into 16 groups on the basis of the presence of N100/P200-like activity and the phase at stimulus onset in the 4-8 Hz band. It was found that P50 consists of low-frequency (theta) and high frequency (gamma) activity. The appearance of P50-related theta activity was closely related to the presence of N100. The P50-related theta activity appeared only when the theta activity goes through a positive zero crossing at the moment of stimulus presentation. Thus it appears that the theta component of the P50 should be considered as an epiphenomenon.