Processing of abstract regularities in the human brain during dichotic listening.

The mismatch-negativity (MMN) component reflects automatic brain mechanisms encoding regularities in the auditory environment and detecting violations in them. Tone series, either descending or ascending in pitch, were used as stimuli. Occasional deviant stimuli (e.g., an ascending tone interspersed among descending tones) violated these regularities. The participants were presented either with a single descending binaural tone stream (identical input to both ears) or simultaneously with two different streams, an ascending stream to the left ear and a descending stream to the right ear. The effects of attention on the MMN were investigated by instructing the participants either to ignore the auditory stimuli or to detect the deviants in the designated ear. In the ignore conditions, the deviants in both the one-stream and two-stream conditions elicited the MMN. However, during the detection task, only the deviants presented in the attended ear elicited the MMN. The results demonstrate that the brain can encode opposite regularities from two simultaneously presented stimulus streams even when the participant’s attention is directed to another modality. However, the brain processes are not completely attention independent, as demonstrated by the lack of MMN to unattended-ear deviants when attention was firmly focused within the auditory modality to the other ear.