Effect of interstimulus interval on subjective categorical loudness judgements

The Independent Hearing Aid Fitting Forum uses the visual input/output locator algorithm and the Contour Test of Loudness Perception to achieve the goal of restoring normal loudness perception with amplification. This method presupposes that subjective categorical loudness judgments are valid and reliable when using the procedure outlined by the test developers. There is no indication in the instructions of the Contour Test of a specific duration for interstimulus interval (ISI). The effect of ISI on loudness perception is important to establish because of potential time-error effects. Time-error refers to the extent to which the judged magnitude of a second stimulus varies with the time interval by which it follows the first stimulus. Past research has indicated that judgments of a second stimulus are shifted in the direction of the intensity of the preceding stimulus and that this effect intensifies with shorter ISIs. The current experiment was designed to examine whether a change in ISI produced a change in the loudness rating of the subsequent stimulus for subjective categorical loudness judgment testing. A trend toward lower, median dB values for ratings 2 to 5 at 500 Hz in a group of subjects with normal hearing was noted when 1-sec intervals were used in comparison with longer ISIs. No trends for the effect of ISI were noted at 3000 Hz. The findings provide ISI recommendations for loudness judgment test administration.