  A natural sound can be described by dynamic changes in envelope (amplitude)
and carrier (frequency), corresponding to amplitude modulation (AM) and
frequency modulation (FM) respectively. Although the neural responses to both
AM and FM sounds are extensively studied in both animals and humans, it is
uncertain how they are co-represented when changed simultaneously but
independently, as is typical for ecologically natural signals. This study
elucidates the neural coding of such sounds in human auditory cortex using
magnetoencephalography (MEG). Using stimuli with both sinusoidal modulated
envelope (f_{AM}, 37 Hz) and carrier frequency (f_{FM}, 0.3 - 8 Hz), it is
demonstrated that AM and FM stimulus dynamics are co-represented in the neural
code of human auditory cortex. The stimulus AM dynamics are represented
neurally with AM encoding, by the auditory Steady State Response (aSSR) at
f_{AM}. For sounds with slowly changing carrier frequency ((f_{FM} < 5 Hz), it
is shown that the stimulus FM dynamics are tracked by the phase of the aSSR,
demonstrating neural phase modulation (PM) encoding of the stimulus carrier
frequency. For sounds with faster carrier frequency change ((f_{FM} >= 5 Hz),
it is shown that modulation encoding of stimulus FM dynamics persists, but the
neural encoding is no longer purely PM. This result is consistent with the
recruitment of additional neural AM encoding over and above the original neural
PM encoding, indicating that both the amplitude and phase of the aSSR at f_{AM}
track the stimulus FM dynamics. A neural model is suggested to account for
these observations.
