Hierarchical Neural Encoding of Temporal Regularity in the Human Auditory Cortex

Temporal regularity provides an important cue for the identification of natural sounds. Here, we measured auditory evoked cortical magnetic fields to investigate the neural processing of temporal regularity that cannot be tonotopically represented in the auditory periphery. Auditory steady state responses (ASSR) and sustained fields (SF) elicited by 40 Hz amplitude modulated periodic and non-periodic noises were analyzed. Periodic noises of 40-, 20-, and 5-Hz were prepared in the form of repeating frozen noises where the same noise segment appears at either each period (40 Hz), every second period (20 Hz), or every eighth period (5 Hz) of amplitude modulation. Compared to non-periodic white noises, periodic noises with repetition rates of 5-, 20-, and 40-Hz caused significantly increased SF amplitudes in both hemispheres. ASSR amplitudes were significantly enhanced for 20- and 40-Hz periodic noises in the right hemisphere while no enhancement was observed for periodic noises in the left hemisphere. The observed variation of the regularity effect between evoked response components and hemispheres may reflect the differences in the temporal integration window lengths adopted between ASSR and SF generators and also between the right and left auditory pathways.