Dissociable sensitivity of auditory evoked fields and 40-Hz auditory steady-state responses to spatial novelty

The 40-Hz auditory steady-state response (ASSR) reflects gamma-band oscillations and has been proposed as a candidate biomarker for psychiatric disorders. Previous work has shown that novelty detection related to temporal and physical stimulus features can modulate the magnitude of the 40-Hz ASSR. The present study examined whether spatial novelty induced by interaural time differences modulates the 40-Hz ASSR in parallel with transient auditory evoked responses. Magnetoencephalography recordings were obtained from 23 healthy participants while listening to binaural auditory stimuli presented at 40 Hz and lateralized to the left or right by interaural time differences in a pseudo-random sequence. Trials were classified according to whether the preceding stimulus was presented on the same or a different side, and further divided based on the number of preceding stimuli. Spatial novelty modulated the N100m, with larger amplitudes observed following stimuli on a different side and a graded increase with the number of preceding different stimuli. In contrast, neither the power nor phase synchronization of the 40-Hz ASSR was influenced by spatial novelty. These findings suggest that the sensitivity of the 40-Hz ASSR to novelty may depend on the dimension of stimulus change and may not directly mirror novelty-related activity reflected in the N100m.