Characteristics of amplitude modulation following responses in man

Amplitude modulation following responses (AMFRs) are a subset of envelope following responses (EFRs) and can be recorded from the human scalp. In AMFRs continuous amplitude-modulated (AM) stimuli with carrier frequency fc and modulation frequency fm are used. Responses arise at frequency fm and are largest for modulation frequencies in the range below 100 Hz depending on the subject’s age and state of arousal. Multi-channel recordings and brain electrical source analysis were carried out to get information about localization of AMFR generators and dependency on modulation frequency of the stimulus. Recording of AMFR seems to yield important information about frequency-specific hearing loss even in the low-frequency range. Responses are assumed to be evoked by neural excitation originating mainly from a basilar membrane region corresponding to the carrier frequency fc. However, in masking experiments, it could be shown that also regions of the basilar membrane related to frequencies up to several octaves above the carrier frequency contribute to the responses although stimulus bandwidth is very narrow. Models of cochlear mechanics were used to elucidate stimulus-response characteristics and effective stimulation bandwidth. Thresholds of AMFR in normal-hearing and hearing-impaired subjects are in close relation to psychoacoustically determined thresholds for the same stimuli.