SEQUENTIAL STREAMING REDUCES THE EFFECT OF NON-SIMULTANEOUS MASKING ON AUDITORY INTENSITY RESOLUTION

The role of auditory object formation for intensity resolution was investigated by measuring intensity difference limens (DLs) for a pure-tone standard (1 kHz, 30 or 60 dB SPL) under four different masking conditions. All maskers were 935.4-Hz tones with a level of 85 dB SPL. In a two-interval intensity discrimination task, each interval contained a forward masker, a backward masker, or both. Finally, in the streaming condition, the two target tones (standard and standard-plus-increment) were embedded in an isochronous sequence of eight maskers. Maskers and targets in the streaming condition were expected to be perceptually organized as separate auditory streams, which should facilitate selective attention to the targets. In contrast, in the remaining conditions the maskers and the target presented in each interval were expected to be grouped together on the basis of temporal proximity. Compatible with this hypothesis, the masker-induced elevation in the intensity DLs was significantly smaller in the streaming condition than in the remaining conditions. This result supports the hypothesis that the effects of non-simultaneous maskers on intensity discrimination are reduced if the standard and the masker are perceived as separate auditory objects. Intensity resolution can be strongly affected by nonsimultaneous maskers. These maskers lead to phenomena like the midlevel hump in intensity discrimination: intensity difference limens (DLs) are strongly elevated for a midlevel standard compared to the same situation in quiet, but are hardly elevated for very low and high standard levels (e.g., Zeng, Turner, & Relkin, 1991). The first attempt to explain the midlevel hump was based on peripheral mechanisms located in the cochlea or in the auditory nerve. It failed on explaining the intensity-difference limens (DLs) elevations caused by contralaterally presented forward maskers, or backward masker