September 26, 2007; doi:10.1152/jn.00342.2007. Complex natural sounds (e.g., animal vocalizations or speech) can be characterized by specific spectrotemporal patterns the components of which change in both frequency (FM) and amplitude (AM). The neural coding of AM and FM has been widely studied in humans and animals but typically with either pure AM or pure FM stimuli. The neural mechanisms employed to perceptually unify AM and FM acoustic features remain unclear. Using stimuli with simultaneous sinusoidal AM (at rate f AM � 37 Hz) and FM (with varying rates ƒ FM), magnetoencephalography (MEG) is used to investigate the elicited auditory steady-state response (aSSR) at relevant frequencies (ƒ AM, ƒ FM, ƒ AM � f FM). Previous work demonstrated that for sounds with slower FM dynamics (f FM � 5 Hz), the phase of the aSSR at ƒ AM tracked the FM; in other words, AM and FM features were co-tracked and co-represented by “phase modulation ” encoding. This stud
