Multiple auditory steady-state responses to AM and FM stimuli

Multiple auditory steady-state responses were recorded using tonal stimuli that were amplitude-modulated (AM), frequency-modulated (FM) or modulated simultaneously in both amplitude and frequency (mixed modulation or MM). When MM stimuli combined 100% AM and 25% FM (12.5% above and below the carrier frequency) and the maximum frequency occurred simultaneously with maximum amplitude, the MM response was one third larger than the simple AM response. This enhancement occurred at intensities between 50 and 30 dB SPL and at carrier frequencies between 500 and 4000 Hz. The AM and FM components of a MM stimulus generate independent responses that add together to give the MM response. Since AM responses generally occur with a slightly later phase delay than FM responses, the largest MM response is recorded when the maximum frequency of the MM stimulus occurs just after the maximum amplitude.