Effect of click rate and delay on breakdown of the precedence effect

Abstract

The precedence effect was tested as a function of echo-click delay and click rate after an abrupt switch in location between leading and lagging clicks. Click trains at three rates, lIsec, 2/sec, and 4/sec, with delays ranging between 2 and 20 msec, were presented to subjects in an anechoic chamber. Duration of the click train after the switch in location was 12 sec, and echo click per­ ceptibility was assessed throughout this period. The number of echo clicks heard was an increas­ ing monotonic function of delay. The subjects reported a fade-out of echo clicks after a set num­ ber of clicks at each delay, regardless of rate. This result was interpreted as a buildup in inhibition of echoes produced by the ongoing click train. Suppression of echoes was stronger when the lead­ ing click originated from the right side than from the left side. Under normal room conditions, we localize a sound at its source, while failing to localize or even recognize the presence of numerous reflected sounds from surround­ ing surfaces. This perceptual phenomenon is known as the precedenceeffect or law ofthe first wavefront be­ cause we weight the original, or first-arriving, sound more heavily than any later-arriving sounds. In a classic study, Wallach, Newman, and Rosenzweig (1949) simulated nat­ ural room reverberations by playing the same sound through two loudspeakers, with one output leading the other by a few milliseconds. As the delay between the original sound and the lagging sound was increased, the listener eventually perceived the lagging sound at its lo­ cus. Blauert (1983, pp. 224-225) defined the echo threshold as the shortest delay at which a single auditory event breaks apart and the lagging sound is localized in a different direction from the original sound. Echo sup­ pression at short delays is produced by inhibitory processes, probably in the central auditory system (Zurek, 1987). Recently, Clifton (1987) reported that a breakdown of this echo suppression can be produced if the location of leading and lagging sounds is suddenly switched. The experimental situation was a train of clicks from two loudspeakers placed at ±90° off the listener's midline, with one output leading the other by about 5 msec (see Figure lA for diagram). The listener initially localized the clicks solely at the leading side. When the leading and lagging sides were switched within an interclick interval, the listener heard clicks from both loudspeakers for sev­ eral seconds before echo suppression was reestablished.