Although the MMN can be elicited in auditory modality, it remains unclear whether there is an analogous automatic deviant-related negativity elicited outside the auditory modality. The present study employed simultaneous audio-visual stimulus in the oddball paradigm to re-examine the effects of attention on audio, visual and audio-visual perception. This study was designed to investigate whether task-related processing of audio and visual features was independent or task-related processing in one modality might influence the processing of the other. Electroencephalographic (EEG) was recorded from 12 normal subjects. None of them had more than three years of formal musical training and none had any musical training within the past five years. Stimuli consisted of a set of four audio-visual stimuli that are distinguished by frequencies (Hz) for audio and features for visual appearing on the computer screen. ANOVA showed statistically significant of the interaction between electrode site and modality. The difference waves with 100-200 ms latency at the anterior sites were markedly different to the posterior sites. The emergence of posterior negativity in the audio-visual modality might not be attributed to visual discrimination process as it did not appear in the visual modality. The findings reveal the processing of a feature, hierarchically dependent on another feature in the audiovisual perception.

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