Auditory object formation affects modulation perception

Most sounds in our environment, including speech, music, animal vocalizations and environmental noise, have fluctuations in intensity that are often highly correlated across different frequency regions. Because across-frequency modulation is so common, the ability to process such information is thought to be a powerful survival strategy in the natural world (Klump, 1996; Nelken et al., 1999).

Coherent modulations in one sound can aid in the detection of another sound (Hall et al., 1984; Durlach, 1963). On the other hand, modulation in one frequency region can also impede the detection or discrimination of modulation in other frequency regions (Yost et al., 1989). Although the neural substrates for across-frequency modulation processing remain unclear, recent studies have concentrated on brainstem structures (Pressnitzer et al., 2001).

In this study it is shown that sounds occurring after the target sound in time determine whether or not across-frequency modulation effects are observed. The results suggest that the binding of sound elements into coherent auditory objects precedes aspects of modulation analysis and imply a cortical locus involving integration times of several hundred milliseconds.

In other words, the modulation analysis necessary for signal detection is performed on objects, rather than frequency channels. The results place strong constraints on the search for neural correlates of this important aspect of auditory processing, and on future models of spectrotemporal processing.