Recalibration of the auditory continuity illusion: sensory and decisional effects

Abstract

An interrupted sound can be perceived as continuous when noise masks the interruption, creating an illusion of continuity. Recent findings have shown that adaptor sounds preceding an ambiguous target sound can influence listeners' rating of target continuity. However, it remains unclear whether these aftereffects on perceived continuity influence sensory processes, decisional processes (i.e., criterion shifts), or both. The present study addressed this question. Results show that the target sound was more likely to be rated as 'continuous' when preceded by adaptors that were perceived as clearly discontinuous than when it was preceded by adaptors that were heard (illusorily or veridically) as continuous. Detection-theory analyses indicated that these contrastive aftereffects reflect a combination of sensory and decisional processes. The contrastive sensory aftereffect persisted even when adaptors and targets were presented to opposite ears, suggesting a neural origin in structures that receive binaural inputs. Finally, physically identical but perceptually ambiguous adaptors that were rated as 'continuous' induced more reports of target continuity than adaptors that were rated as 'discontinuous'. This assimilative aftereffect was purely decisional. These findings confirm that judgments of auditory continuity can be influenced by preceding events, and reveal that these aftereffects have both sensory and decisional components.

Figure 1. Stimuli and experimental design

A, Auditory stimuli consisted of an ascending tone in which portions were replaced by noise bursts, as illustrated by the sound spectrogram. B, Trials comprised an adaptor stimulus presented twice and a subsequent target stimulus. Listeners judged the continuity of the stimuli during visually cued response intervals. C, For the interrupted target stimuli, the relative amplitude of the tone and noise (SNR) was defined individually from thresholds for the continuity illusion of the interrupted tone (−6.8 dB on average). The uninterrupted target stimuli were physically identical to the interrupted target stimuli, except that the tone was physically uninterrupted. D, In experiment 1, the adaptors comprised relatively loud or soft noise, or no adaptors were presented. E, In experiment 2, the same stimuli as in experiment 1 were presented differently, i.e, the adaptors and target within each trial were presented either to the same ear or to opposite ears. F, For experiment 3, physically identical, perceptually ambiguous adaptor stimuli were used that were identical to the interrupted target stimuli. ‘Subjective’ adaptor conditions were created post hoc by sorting trials according to how listeners had judged the ambiguous adaptors (i.e., as either continuous [‘cont.’] or discontinuous [‘disc.’]).

Figure 2. Ratings of adaptor stimuli in experiments 1–3

The graphs show proportions of continuity reports of the adaptor stimuli in experiment 1–2 (A–B) and of the pre-adaptor stimuli in experiment 3 (C). The different stimuli are indicated by the upper schematic spectrograms. Listeners judged the loud adaptor (dark gray) mostly as ‘continuous’ (false alarm [FA]) and the soft adaptor (light gray) mostly as ‘discontinuous’. All graphs show means ± SE across listeners. *** P < 0.0005.

Figure 3. Ratings of target stimuli in experiment 1

A, The bar graph shows listeners’ sensitivity (as measured by d’) following the different adaptor stimuli or following silence (indicated by the upper stimulus spectrograms). Following the loud adaptors (dark gray) or silence (white), listeners could easily identify whether the target stimuli were truly interrupted or truly uninterrupted, as shown by d’-values far above zero in these conditions. Following soft adaptors (light gray), however, listeners perceived the different target stimuli as more similar, as reflected by significantly smaller d’-values in this condition. B, Significant aftereffects on listeners decision criterion (as measured by C) were observed, indicating that following soft adaptors, listeners used a more liberal decision criterion (i.e., they were more inclined to report the target as ‘continuous’). C, Following the soft adaptors, listeners rated the interrupted target mostly as ‘continuous’, and vice versa following the loud adaptors. Without prior adaptors, the interrupted target was ambiguous (i.e. FA rates were around the chance level of 50 %). D, Similar but smaller contrastive aftereffects were found for hit rates (i.e., continuity reports of the uninterrupted target). All graphs show means ± SE across listeners. *, **, ***: P < 0.05, 0.005, 0.0005; NS: not significant.

Figure 4. Ratings of target stimuli in experiment 2

Same layout as Figure 3, except that the no-adaptor condition is omitted, and both the loud adaptor condition and the soft adaptor condition are split according to their presentation mode. The latter term indicates whether the adaptors and the target within a trial were presented to the same ear or to opposite ears. The dichotic results from experiment 2 were overall similar to the diotic results from experiment 1 (Figure 3). The sensory aftereffect of the adaptors did not depend significantly on whether the adaptors and target were presented to the same ear or to opposite ears (A), suggesting that this aftereffect ‘transferred’ across the two ears. The presentation mode affected the decisional aftereffect of the adaptors (B), indicating a stronger impact on listeners’ decision criterion for adaptors that were presented to the opposite ear than the target.

Figure 5. Ratings of target stimuli in experiment 3

Same layout as Figure 3, except that the no-adaptor condition is omitted and the adaptor conditions were defined according to how listeners had judged the physically identical ambiguous adaptor stimuli (i.e., as either continuous [‘cont.’] or discontinuous [‘disc.’]). A, The ambiguous adaptors had no significant aftereffect on listeners’ sensitivity, indicating that they did not affect listeners’ perception of continuity. B, A significant aftereffect of the ambiguous adaptors on listeners’ decision criterion was observed, indicating that listeners were more inclined to judge the target as ‘continuous’ when they had judged the preceding adaptor as ‘continuous’ (dark gray) than when they had judged the same adaptor as ‘discontinuous’ (light gray). C, This assimilative decisional effect was also reflected in listeners’ FA rates: listeners rated the interrupted target as ‘continuous’ more often following reports of the ambiguous adaptors as ‘continuous’ than following reports of the same adaptors as ‘discontinuous’. D, For ratings of the uninterrupted target, a consistent aftereffect was observed. Overall these aftereffects of the physically identical adaptors contrasted with those of the physically different adaptors observed in experiments 1 and 2, indicating that the former were due to decisional factors rather than sensory factors (for details, see main text).