Investigation of the relationship among three common measures of precedence: Fusion, localization dominance, and discrimination suppression

Listeners have a remarkable ability to localize and identify sound sources in reverberant environments. The term "precedence effect" (PE; also known a s the "Haas effect," "law of the first wavefront," and ''echo suppression'' ) refers to a group of auditory phenomena that is thought to be related to this ability. Traditionally, three measures have been used to quantify the PE: (1) Fusion: at short delays (1-5 ms for clicks) the lead and lag percep tually fuse into one auditory event; (2) Localization dominance: the percei ved location of the leading source dominates that of the lagging source; an d (3) Discrimination suppression: at short delays, changes in the location or interaural parameters of the lag are difficult to discriminate compared with changes in characteristics of the lead. Little is known about the rela tion among these aspects of the PE, since they are rarely studied in the sa me listeners. In the present study, extensive measurements of these phenome na were made for six normal-hearing listeners using I-ms noise bursts. The results suggest that, for clicks, fusion lasts 1-5 ms; by 5 ms most listene rs hear two sounds on a majority of trials. However, localization dominance and discrimination suppression remain potent for delays of 10 ms or longer . Results are consistent with a simple model in which information from the lead and lag interacts perceptually and in which the strength of this inter action decreases with spatiotemporal separation of the lead and lag. At sho rt delays, lead and lag both contribute to spatial perception, but the lead dominates (to the extent that only one position is ever heard). At the lon gest delays tested, two distinct sounds are perceived las measured in a fus ion task), but they are not always heard at independent spatial locations l as measured in a localization dominance task). These results suggest that d irectional cues from the lag are not necessarily salient for all conditions in which the lag is subjectively heard as a separate event. (C) 2001 Acous tical Society of America. [DOI: 10.1121/1.1328792].