Abstract
Infrequent deviant sounds appearing in a repeated, ordered sequence of standard sounds elicit the Mismatch Negativity (MMN) event-related component - indexing regularity violation - if the Inter Stimulus Interval (ISI) is long enough to avoid perceptual grouping and subjects are unaware of the sequential pattern. When pattern structure is explicitly provided, MMN is greatly reduced in amplitude, suggesting that top-down grouping prevented regularity violation detection. However, at short ISIs stimulus-driven, rhythmical structure perception is thought to override the effects of top-down predictability cues. We tested this hypothesis using sequences of ordered five 50-ms sine tones, the first four being standard tones (440 Hz), and the last a frequency deviant tone (494 Hz). Two blocked ISI conditions were used: 700 ms and 200 ms. The EEG of 30 participants was recorded while they detected rare (5%) lower frequency target tones (349 Hz). Top-down pattern information was provided off-line by means of static visual pictures. Results for the 700-ms ISI blocks showed a significant reduction in the MMN amplitude in the informed condition. As for the 200-ms ISI blocks, while no differences in MMN amplitude were found comparing informed and non-informed conditions at frontal sites, a significant anticipatory effect on the latency of the MMN was found at temporo-parietal electrode-sites in the informed condition, showing that top-down predictability cues were not fully overridden by rhythmical pattern perception. Furthermore, the building-up of N1 deviant activity in the informed condition started at occipital sites and gradually moved to frontal sites, suggesting a cross-modal effect of static visual predictability cues held in working memory on auditory processing.
