The Aging Auditory System by Sandra Gordon-Salant Arthur N. Popper Robert D. Frisina & Richard R. Fay

Author:Sandra Gordon-Salant, Arthur N. Popper, Robert D. Frisina & Richard R. Fay
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

6.4.2 Precedence Effect

In a typical acoustic environment, the sound from a single sound source arrives at the ears of the listener as a combination of direct waves from the source combined with reflections from the surfaces in the room. In large rooms (e.g., auditoria), the reflections are heard as echoes, but in smaller rooms, the echoes are present but are not perceived. Suppression of the later arriving reflections is normal and is governed by the coding of interaural time and level cues by the central auditory system. In a common laboratory study of such binaural processing, sounds from two speakers are presented in an anechoic space. If the two sound sources are positioned in the horizontal plane and equidistant from the median plane, then the perceived location depends critically on the relative delay between the sounds from the two speakers. When the interspeaker delay is close to 0 ms, the percept is a single sound source at the midline. For brief clicks, as the delay is increased from 0 ms to ∼0.7 ms, the percept is a single sound source, the location of which is shifted toward the leading speaker. Blauert (1983) refers to this as “summing localization,” reflecting the fact that the perceived source position is a joint function of the position of the two speakers and the delay. For longer delays (e.g., 1 to 6 ms), the source position is perceived to correspond to the leading speaker. The latter has often been referred to as the “precedence effect” because the position of the first speaker takes precedence over the second (e.g., Wallach et al. 1949). This phenomenon is also referred to as the law of the first wave front and the Haas effect after the work by Haas (1951). For long interspeaker delays, the fusion between the two sounds begins to fail and the perception evolves into two distinct sound sources. Summing localization and the precedence effect are governed by factors such as stimulus type and timing and possibly by different neural processes (e.g., Blauert 1983). It has been suggested that summing localization under conditions of equal-level sounds from the two sources reflects interaural timing cues most likely coded at the level of the brainstem or midbrain. The precedence effect, on the other hand, is much more susceptible to stimulus manipulations as well as to practice effects and such “echo suppression” may reflect processing at the level of the auditory cortex.

Taking into consideration age-related declines in temporal processing (see Fitzgibbons and Gordon-Salant, Chapter 5), including the perception of interaural time cues (see Section 2.1), it is reasonable to expect some age-related differences in summing localization and/or the precedence effect. In perhaps the most comprehensive study of this effect, Cranford et al. (1993) studied apparent source location as a function of interspeaker delay for groups of younger and older listeners with clinically normal hearing or sensorineural hearing loss. Figure 6.5 summarizes the primary results for the four listener groups averaged across left- and right-leading conditions. Note that the younger

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