Amplitude-modulation following response (AMFR): effects of modulation rate, carrier frequency, age, and state.

Scalp responses to continuous amplitude-modulated (AM) tones were recorded from adults and 1-month-old infants. The amplitude-modulation following (or envelope) response (AMFR) was quantified using magnitude-squared coherence. This measurement indicates the strength of the frequency-following response relative to background neural noise. The optimal modulation rate for generating the AMFR was determined by studying the effects of stimulus modulation rate on the response. Stimulus AM rate was varied between 10 and 80 Hz for continuous tonal stimuli of 500 Hz, and between 20 and 80 Hz for continuous tonal stimuli of 2000 Hz. Optimal modulation rate was defined as the AM rate that provided the highest coherence estimate. Adult AMFR coherence increased between 10 and 40 Hz (20-40 Hz for 2000 Hz), and decreased between 40 and 80 Hz in both carrier frequency conditions. Infant AMFR coherence, in contrast, monotonically increased between 10 and 80 H (20-80 Hz for 2000 Hz). Thus, within the frequency range examined, 40 Hz is optimal for generating the AMFR in adults, whereas 80 Hz is optimal in infants. Adults were tested while awake and infants were tested during periods of sleep. Given the observed age difference in effective modulation rate, we examined modulation rate effects in a group of adults in both awake and sedated states. As in sleeping infants, 80 Hz was optimal for generating AMFRs in the sedated adults.