Repetition of complex frequency-modulated sweeps enhances neuromagnetic responses in the human auditory cortex

Frequency modulations (FM) play a decisive role in our everyday communication. To investigate the processing of FM direction we measured change-related auditory cortex responses with human magnetoencephalography. First, we tested for FM direction selectivity by presenting FM sweeps with the same FM directions in a repeated series (RS). These series were interrupted by a deviant with the opposite FM direction. Second, we tested for the representation of abstract rules and presented series of FM sweeps with alternating FM directions (AS). The AS series were interrupted by a deviant which was a repetition of the series' last FM sweep but broke the alternating pattern. For the RS, the deviant did not evoke significant change-related responses in the auditory cortex. However, for the first stimulus after the deviant, significantly stronger responses compared to standards were observed bilaterally in the auditory cortex at about 200 ms after stimulus onset. For the AS, we observed a similar bilateral change-related signal enhancement for a deviant FM sweep breaking the alternating series. Since this response enhancement occurred for both RS and AS even after a single FM sweep repetition, we conclude that these activities represent local signal enhancements rather than change-related responses due to abstract rule violation. In sum, our data indicate repetition enhancement due to spectro-temporal interactions between successive complex FM sweeps. These enhancement effects were observed for the first but not further repetitions suggesting a second-order repetition suppression of the initial repetition enhancement.