A series of experiments will investigate adults' perception of an auditory phenomenon known as the precedence effect. The precedence effect occurs when two identical sounds are delivered from two spatial locations (e.g., two loudspeakers), with the onset of one leading the other by a few thousandths of a second. The listener localizes the sound solely at the leading location and does not hear the sound at the lagging location at all. When the time delay between onsets is increased, the echo threshold is reached and the lagging sound is heard as an echo. The precedence effect is due to the nervous system's active suppression of the echoes. These experiments will be directed toward establishing the conditions under which this suppression breaks down. Previous work has shown that a sudden switch in the location of leading and lagging sound will break down the suppression. These experiments will explore whether the presence of the echo before the switch is necessary and whether only the leading (or lagging) sound being changed in location is sufficient to break the suppression. Acoustic characteristics of the sound, such as intensity and frequency, will be varied to determine how they affect suppression. Inhibition of echoes is a critical feature of everyday listening in normal settings. Without echo suppression, it would be very difficult to detect the original source of a sound, leading to confusion of sound sources. Because the precedence effect is an inhibitory process of the central nervous system, brain damage produced by lesions or strokes could impair a person's ability to localize sound. Better understanding of the precedence effect and the stimulus conditions affecting it has relevance for theories of binaural hearing and has practical implications for patients with damage in the auditory areas of the brain.
