AMPLITUDE-MODULATION FOLLOWING RESPONSE (AMFR) - EFFECTS OF MODULATION RATE, CARRIER FREQUENCY, AGE, AND STATE

Scalp responses to continuous amplitude-modulated (AM) tones were reco rded from adults and 1-month-old infants. The amplitude-modulation fol lowing (or envelope) response (AMFR) was quantified using magnitude-sq uared coherence. This measurement indicates the strength of the freque ncy-following response relative to background neural noise. The optima l modulation rate for generating the AMFR was determined by studying t he effects of stimulus modulation rate on the response. Stimulus AM ra te 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 i n both carrier frequency conditions. Infant AMFR coherence, in contras t, monotonically increased between 10 and 80 Hz (20-80 Hz for 2000 Hz) . Thus, within the frequency range examined, 40 Hz is optimal for gene rating the AMFR in adults, whereas 80 Hz is optimal in infants. Adults were tested while awake and infants were tested during periods of sle ep. Given the observed age difference in effective modulation rate, we examined modulation rate effects in a group of adults in both awake a nd sedated states. As in sleeping infants, 80 Hz was optimal for gener ating AMFRs in the sedated adults.