Detection Thresholds for Amplitude Modulations of Tones in Budgerigar, Rabbit, and Human

Abstract

Envelope fluctuations of complex sounds carry information that is ­essential for many types of discrimination and for detection in noise. To study the neural representation of envelope information and mechanisms for processing of this temporal aspect of sounds, it is useful to identify an animal model that can ­sensitively detect amplitude modulations (AM). Low modulation frequencies, which dominate speech sounds, are of particular interest. Yet, most animal ­models studied previously are relatively insensitive to AM at low modulation ­frequencies. Rabbits have high thresholds for low-frequency modulations, ­especially for tone carriers. Rhesus macaques are less sensitive than humans to low-frequency ­modulations of wideband noise (O’Conner et al. Hear Res 277, 37–43, 2011). Rats and ­chinchilla also have higher thresholds than humans for amplitude ­modulations of noise (Kelly et al. J Comp Psychol 120, 98–105, 2006; Henderson et al. J Acoust Soc Am 75, ­1177–1183, 1984). In contrast, the budgerigar has thresholds for AM detection of wideband noise similar to those of human listeners at low ­modulation frequencies (Dooling and Searcy. Percept Psychophys 46, 65–71, 1981). A ­one-interval, two-alternative operant conditioning procedure was used to estimate AM ­detection thresholds for 4-kHz tone carriers at low modulation ­frequencies (4–256 Hz). Budgerigar thresholds are comparable to those of human subjects in a comparable task. Implications of these comparative results for temporal coding of complex sounds are discussed. Comparative results for masked AM detection are also presented.