Stimulus context affects auditory cortical responses to changes in interaural correlation

We use magnetoencephalography to study human auditory cortical processing of changes in interaural correlation (IAC). We studied transitions from correlated (identical signals at the 2 ears) to uncorrelated (different signals at the 2 ears) or vice versa for two types of wide-band noise stimuli: CHANGE signals contained a single IAC change (or none) and ALT signals alternated between correlated and uncorrelated at a constant rate. The relevant transitions, from correlated to uncorrelated or vice versa, are physically identical in both stimulus conditions, but auditory cortical response patterns differed substantially. CHANGE stimuli exhibited a response asymmetry in their temporal dynamics and magnetic field morphology according to the direction of change. Distinct field patterns indicate the involvement of separate neural substrates for processing, and distinct latencies are suggestive of different temporal integration windows. In contrast, the temporal dynamics of responses to change in the ALT stimuli did not differ substantially according to the direction of change. Notably, the uncorrelated-to-correlated transition in the ALT stimuli showed a first deflection similar to 90 ms earlier than for the same transition in the CHANGE stimuli and with an opposite magnetic field distribution. This finding suggests that as early as 50 ms after the onset of an IAC transition, a given physical change is processed differentially depending on stimulus context. Consequently, even early cortical activation cannot be interpreted independently of the specific long-term stimulus context used in the experiment.