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Almost all sounds in natural environments, including our voices, have an amplitude envelope that varies over time. This time-dependent variation [amplitude modulation (AM)] plays important roles in auditory perception, as is seen in speech perception (e.g., Shannon et al., 1995) and in perceptual grouping of frequency components (Darwin and Carlyon, 1995). To understand the processing of amplitude envelopes, several researchers have examined characteristics of perceptual adaptation produced by AM. For example, Kay and Matthews (1972) reported that the AM detection threshold increased following listeners' exposure to a long, modulated tone (i.e., adaptor). Wojtczak and Viemeister (2003) used a matching procedure to show that suprathreshold perception of modulated tones was also changed by adaptation. Moreover, to some extent, such aftereffects of AM exposure appear to be selectively linked to both the rate of modulation and the carrier frequency of the adaptor sound (Kashino, 1998; Richards et al., 1997; Wojtczak and Viemeister, 2003). To some, such findings suggest that aftereffects of AM adaptation arise from specific auditory processing of AM (e.g., Wojtczak and Viemeister, 2003).

An important problem related to adaptation by AM concerns the involvement of attention. Does auditory processing show constant adaptation irrespective of the direction of the listener's attention? In vision, several studies have reported that adaptation is indeed affected by attention (e.g., Yeh et al., 1996). For example, Chaudhuri (1990) examined the effect of attention on the motion aftereffect elicited by a moving array of random dots. The duration of the aftereffect was shorter when the observers shifted attention from the dots to alphanumeric symbols superimposed on the dots, compared to when they attended to the moving dots.

In the present report, the author examined possible effects of attention on adaptation by AM. Following research in visual perception, the author predicted that AM adaptation should be less pronounced when the listener's attention is directed to sounds other than the adaptor sounds during an adapting phase than when attention is focused directly on the adaptors.