These simulations were produced using a model of peripheral processing, including cochlear filters with phase responses estimated based on psychophysical data in humans (Oxenham and Dau 2001b) and cochlear compression (for details, see Riecke et al. 2012). It can be seen that the M+ complex evokes a markedly “peakier” temporal response pattern than the M– complex—an effect due to the positive phase curvature of M+ compensating for the negative phase curvature of cochlear filters—whereas the negative phase curvature of M– adds to the negative phase curvature of the cochlear filters, resulting in an accentuated phase-dispersion effect (Recio and Rhode 2000; Summers et al. 2003). This effect has been used to explain why the masked threshold of a pure tone embedded in an M– complex is markedly higher than its masked threshold when embedded in an M+ complex having the same overall physical level (Kohlrausch and Sander 1995; Lentz and Leek 2001; Oxenham and Dau 2001a; Smith et al. 1986; see Fig. 53.1b, d).

Based on these simulation results, we predicted that if the continuity illusion depends on a fine-grained analysis of peripheral activity, then the level of the target below which the continuity illusion occurs (the “continuity-illusion threshold”) should be considerably lower for the M+ masker than for the M– masker, in line with the differences in simultaneous-masked threshold. Conversely, if the continuity illusion depends on a relatively coarse representation of the masker (e.g., a representation of its overall level or specific loudness), the illusion thresholds measured with the M+ and M– maskers should be reasonably similar, i.e., their difference should be considerably smaller than the differences in masked threshold.

This prediction was tested in three experiments. In the “continuity-illusion” experiment (C), a 1,500-Hz target tone (T) and a masker (M+ or M–), each lasting 200 ms, were played in an alternating sequence (TMTMTMT), and the listener’s task was to indicate whether the target was heard as continuing through the masker or not. The level of the target was varied adaptively to determine the continuity-illusion threshold (defined as the target level corresponding to the 50 % point on the psychometric function). The level of the masker was fixed at 50-dB SL per harmonic (relative to the listener’s hearing threshold at the frequency of the target).

In the “simultaneous-masking” experiment (S), the 200-ms target was presented simultaneously with, and temporally centered in, the 600-ms masker in one of two consecutive observation intervals (selected at random with equal probability), while the other observation interval contained only the masker. The task of the listener was to indicate which interval contained the target. The goal of this ­experiment was to provide a direct measure of the amount of simultaneous masking produced by the masker.

Lastly, the “forward-masking” experiment (F) was similar to experiment S, except that a 10-ms target was presented 5 ms after a 200-ms masker. The aim of this experiment was to provide an estimate of the strength of the long-term internal excitation produced by the masker (Carlyon and Datta 1997b). Thresholds in experiments S and F were measured with an adaptive two-alternative forced-choice procedure and a two-down one-up tracking rule estimating the 70.7 % correct point on the psychometric function (Levitt 1971). The level of the signal was varied, while the level of the masker was kept constant, as in experiment C.

In all adaptive-tracking procedures, the step size was initially set to 6 dB and reduced to 3 dB after the second reversal in the direction of change of the target level (from increasing to decreasing or vice versa) and to 0.5 dB after the fourth reversal. Each measurement began with the target level set sufficiently high (62-dB SL) so that the listener did not hear the target continue through the masker in experiment C and that the listener could easily detect the target in experiments S and F. The procedure was terminated after the tenth reversal. Threshold was computed as the arithmetic mean of the target levels at the last six reversals. For each listener and each masker condition, nine, six, and six thresholds were measured in experiments C, S, and F, respectively, in fully randomized order. Only in experiments S and F, visual feedback was provided to the listener after each trial.

Target and maskers were ramped on and off with 20-ms linear ramps, except for experiment F, in which the 10-ms target was ramped with 5-ms ramps (no steady state). In experiment C, the amplitude midpoints of the ramps of consecutive elements (T and M) were made to coincide so that the audibility of the gaps in the TMTMTMT sequence was reduced. The two observation intervals in experiments S and F were separated by a 500-ms gap.

Stimuli were generated digitally and presented monaurally via a soundcard and headphones (MDR-V900HD, Sony) in a sound-attenuating chamber. Stimulus presentation and response collection were controlled using the AFC software package (developed by Stephan Ewert at University of Oldenburg).