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Frequency modulation detection interference produced by asynchronous and nonsimultaneous interferers
Journal of the Acoustical Society of America, 108, 2329-36
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The effect of asynchronous and nonsimultaneous interferers on detection of sinusoidal frequency modulation (FM) was compared with the effect of a synchronous interferer. In a two-interval, two-alternative forced choice (2I-2AFC) adaptive procedure, listeners had to detect FM with a modulation frequency of 15 Hz, imposed on a 1-kHz sinusoidal carrier (the target). The 200-ms target was presented either alone (baseline condition), or with an interferer whose timing relative to the target was varied. The interferer was a 2.3-kHz sinusoidal carrier which was also frequency modulated at a rate of 15 Hz. Experiment one showed that thresholds for detection of FM increased significantly, both with a synchronous FM interferer ("classical FMDI"), and also with asynchronous interferers (starting 200 ms before and stopping 200 ms after the target). Moreover, "gapped" interferers that were turned off during presentation of the target (presented for 200 ms before and for 200 ms after the target but not simultaneously) produced the same significant increase in thresholds as an asynchronous interferer that was not interrupted. In contrast, thresholds were not affected by the presence of a gapped unmodulated sinusoidal interferer. Experiment two showed that increasing the duration of the silent gap (centered on presentation of the target) between FM interferers from 200 ms to 600 ms did not abolish the interference. Thus, nonsimultaneous FM interferers produced FMDI even when the silent gap between the interferers and target clearly led to the interferers and target being perceived as separate auditory objects. A possible explanation for the findings is the existence of an asymmetry in perception of steady and modulated sounds, as recently proposed by Cusack and Carlyon [Brit. J. Audiol., in press, (2000)]. Alternative explanations in terms of ringing in a hypothetical modulation filter bank and adaptation seem unlikely.