Human auditory steady-state responses to tones independently modulated in both frequency and amplitude

Objective: Independent amplitude and frequency modulation (IAFM) of a carri er tone uses two different modulating frequencies, one for amplitude modula tion (AM) and one for frequency modulation (FM). This study measured the hu man steady-state responses to multiple IAFM tones. The first question was w hether the IAFM responses could be recorded without attenuation of the AM a nd FM components. The second question was whether IAFM stimuli would provid e a more effective demonstration of responses at intensities near threshold than the responses to AM tones. The third question was whether the respons es to multiple IAFM stimuli would relate to the discrimination of words at different intensities. Design: Multiple AM, FM, or IAFM stimuli were presented simultaneously. Res ponses were recorded between the vertex and the neck and analysed in the fr equency domain. The first experiment compared IAFM responses with AM and FM responses. The second experiment compared IAFM responses with AM responses between intensities 20 to 50 dB SPL. The third experiment related the IAFM responses to the discrimination of monosyllabic words at intensities betwe en 20 and 70 dB SPL. Results: Steady-state responses to the individual component of the IAFM sti muli were clearly recognizable although attenuated a little (14%) from the responses to AM or FM alone. Using IAFM stimuli was not different than simp ly using AM stimuli when trying to recognize responses at low intensities. The number of responses detected during multiple IAFM stimulation and the a mplitudes of these responses correlated significantly with word discriminat ion. Conclusions: IAFM of a carrier using two different modulating frequencies t one for AM and one for FM) elicits separate AM and FM responses that are re latively independent of each other. These separate responses can be used to detect whether a particular carrier has been processed in the cochlea, but they are not as effective as measuring responses to carriers that have bee n modulated in both amplitude and frequency at the same modulation frequenc y (mixed modulation). The detectability of eight different responses (four AM and four FM) to an IAFM stimuli relates well to the ability of subjects to discriminate words. IAFM stimuli therefore show promise as an objective test for assessing suprathreshold hearing.