Certain non-isochronous sound trains are perceived as more isochronous when they start on beat.

Perceiving the duration of neighboring time intervals is vital for rhythm perception. We discovered a phenomenon in which the perceived equality/inequality of neighboring time intervals in a sound sequence is changed by its metrical interpretation. The target sound sequence consisted of eight short sound bursts marking seven neighboring time intervals, which were repetitions of two durations (T1 and T2) presented in alternation (T1-T2-T1-T2 …). There were three tempos, which corresponded to T1 + T2 being 210, 420, and 630 ms. The physical difference between T1 and T2 (T1 - T2) was varied systematically for each tempo in the ranges of -100 to 100 ms (when T1 + T2 was 210 or 420 ms) or -150 to 150 ms (when T1 + T2 was 630 ms). Participants reported the level of perceived equality/inequality of these neighboring time intervals. For each target sequence, four isochronous lower-pitched preceding sounds were added at different phases so that the beginning of either T1 (Beat-on-T1 condition) or T2 (Beat-on-T2 condition) coincided with the beat induced by these preceding sounds. When T2 was longer than T1 by up to 60 ms, the neighboring time intervals of the same target sequence were perceived as more "equal" in the Beat-on-T1 condition compared with the Beat-on-T2 condition. Such a difference in the perceived equality/inequality appeared significantly only at the intermediate tempo of T1 + T2 = 420 ms. The difference in equality/inequality perception at limited temporal conditions could be accounted for by the occurrence of an illusion in time perception called time-shrinking.

Overestimation of the second time interval replaces time-shrinking when the difference between two adjacent time intervals increases.

When the onsets of three successive sound bursts mark two adjacent time intervals, the second time interval can be underestimated when it is physically longer than the first time interval by up to 100 ms. This illusion, time-shrinking, is very stable when the first time interval is 200 ms or shorter (Nakajima et al., 2004, Perception, 33). Time-shrinking had been considered a kind of perceptual assimilation to make the first and the second time interval more similar to each other. Here we investigated whether the underestimation of the second time interval was replaced by an overestimation if the physical difference between the neighboring time intervals was too large for the assimilation to take place; this was a typical situation in which a perceptual contrast could be expected. Three experiments to measure the overestimation/underestimation of the second time interval by the method of adjustment were conducted. The first time interval was varied from 40 to 280 ms, and such overestimations indeed took place when the first time interval was 80-280 ms. The overestimations were robust when the second time interval was longer than the first time interval by 240 ms or more, and the magnitude of the overestimation was larger than 100 ms in some conditions. Thus, a perceptual contrast to replace time-shrinking was established. An additional experiment indicated that this contrast did not affect the perception of the first time interval substantially: The contrast in the present conditions seemed unilateral.

The occurrence of the filled duration illusion: a comparison of the method of adjustment with the method of magnitude estimation.

A time interval between the onset and the offset of a continuous sound (filled interval) is often perceived to be longer than a time interval between two successive brief sounds (empty interval) of the same physical duration. The present study examined whether and how this phenomenon, sometimes called the filled duration illusion (FDI), occurs for short time intervals (40-520 ms). The investigation was conducted with the method of adjustment (Experiment 1) and the method of magnitude estimation (Experiment 2). When the method of adjustment was used, the FDI did not appear for the majority of the participants, but it appeared clearly for some participants. In the latter case, the amount of the FDI increased as the interval duration lengthened. The FDI was more likely to occur with magnitude estimation than with the method of adjustment. The participants who showed clear FDI with one method did not necessarily show such clear FDI with the other method.

About the time-shrinking illusion in the tactile modality.

The aim of this study was to examine the occurrence of a so-called time-shrinking illusion in the tactile modality, while it had been tested so far mainly with auditory and visual stimuli. We examined whether the perception of an empty time interval marked by two brief tactile stimuli, S (240 ms), would be influenced by the presence of a preceding time interval, P (160, 240, or 320 ms). Results showed that S was underestimated when P was shorter than S. This underestimation appeared as a kind of perceptual assimilation between P and S, but S was not overestimated when P was longer. The underestimation was rather interpreted as a manifestation of the time-shrinking illusion.

Discrimination of two neighboring intra- and intermodal empty time intervals marked by three successive stimuli.

We investigated the discrimination of two neighboring intra- or inter-modal empty time intervals marked by three successive stimuli. Each of the three markers was a flash (visual-V) or a sound (auditory-A). The first and last markers were of the same modality, while the second one was either A or V, resulting in four conditions: VVV, VAV, AVA and AAA. Participants judged whether the second interval, whose duration was systematically varied, was shorter or longer than the 500-ms first interval. Compared with VVV and AAA, discrimination was impaired with VAV, but not so much with AVA (in Experiment 1). Whereas VAV and AVA consisted of the same set of single intermodal intervals (VA and AV), discrimination was impaired in the VAV compared to the AVA condition. This difference between VAV and AVA could not be attributed to the participants' strategy to perform the discrimination task, e.g., ignoring the standard interval or replacing the visual stimuli with sounds in their mind (in Experiment 2). These results are discussed in terms of sequential grouping according to sensory similarity.

EEG investigations of duration discrimination: the intermodal effect is induced by an attentional bias.

Previous studies indicated that empty time intervals are better discriminated in the auditory than in the visual modality, and when delimited by signals delivered from the same (intramodal intervals) rather than from different sensory modalities (intermodal intervals). The present electrophysiological study was conducted to determine the mechanisms which modulated the performances in inter- and intramodal conditions. Participants were asked to categorise as short or long empty intervals marked by auditory (A) and/or visual (V) signals (intramodal intervals: AA, VV; intermodal intervals: AV, VA). Behavioural data revealed that the performances were higher for the AA intervals than for the three other intervals and lower for inter- compared to intramodal intervals. Electrophysiological results indicated that the CNV amplitude recorded at fronto-central electrodes increased significantly until the end of the presentation of the long intervals in the AA conditions, while no significant change in the time course of this component was observed for the other three modalities of presentation. They also indicated that the N1 and P2 amplitudes recorded after the presentation of the signals which delimited the beginning of the intervals were higher for the inter- (AV/VA) compared to the intramodal intervals (AA/VV). The time course of the CNV revealed that the high performances observed with AA intervals would be related to the effectiveness of the neural mechanisms underlying the processing of the ongoing interval. The greater amplitude of the N1 and P2 components during the intermodal intervals suggests that the weak performances observed in these conditions would be caused by an attentional bias induced by the cognitive load and the necessity to switch between modalities.

Discrimination of brief gaps marked by two stimuli: effects of sound length, repetition, and rhythmic grouping.

We examined the effects of sound marker length, marker repetition, and rhythmic grouping on auditory gap discrimination. The discrimination ofthe duration of a gap between two markers was impaired by lengthening these markers (from 150 to 262.5 ms). Discrimination was impaired by lengthening the preceding marker relative to lengthening the following marker, while the impairment was not increased when both markers were lengthened compared with when only the preceding marker was lengthened. This indicates that the level of discrimination is not decided by a simple summation of the effects of the preceding and of the following marker's length. Moreover, discrimination of a gap between a short (S) and a long (L) marker and of a gap between a long and a short marker was improved by repeating the presentation of these gaps (ie by repeating the markers alternately as SLSLSL...): both types of discrimination led to near identical performance. Finally, under the repetition condition each type of discrimination was not related to the tendency for each individual to perceive the stimulus sequences as segmented into rhythmic chunks of a short tone followed by a long tone (as [SL][SL][SL]...), or those of a long tone followed by a short tone (as S][LS][LS][L...).

Perceptually plausible sounds facilitate visually induced self-motion perception (vection).

We examined whether and how sounds influence visually induced illusory self-motion (vection). Visual stimuli were presented for 40 s. They were made radially, expanding or contracting visual motion field and luminance-defined gratings drifting in a vertical or horizontal direction. Auditory stimuli were presented with the visual stimuli in most conditions; we employed sounds that increased or decreased in intensity, or ascended or descended in frequency. As a result, the sound which increased in intensity facilitated forward vection, and the sound which ascended/descended in frequency facilitated upward/downward vection. The perceptual plausibility of the sound for the corresponding self-motion seemed an important factor of enhancing vection.

Effects of temporal shapes of sound markers on the perception of interonset time intervals.

This study investigated how the temporal characteristics, particularly durations, of sounds affect the perceived duration of very short interonset time intervals (120-360 ms), which is important for rhythm perception in speech and music. In four experiments, the subjective duration of single time intervals marked by two sounds was measured utilizing the method of adjustment, while the markers' durations, amplitude difference (which accompanied the duration change), and sound energy distribution in time were varied. Lengthening the duration of the second marker in the range of 20-100 ms increased the subjective duration of the time interval in a stable manner. Lengthening the first marker tended to increase the subjective duration, but unstably; an opposite effect sometimes appeared for the shortest time interval of 120 ms. The effects of varying the amplitude and the sound energy distribution in time of either marker were very small in the present experimental conditions, thus proving the effects of marker durations per se.

Effects of sound-marker durations on rhythm perception.

Our aim in this study was to examine the influence of sound-marker durations on the perception of simple rhythm patterns. These comprised three successive sounds marking two neighbouring time intervals, T1 and T2, with their onsets. We varied the durations of each of the three sound markers to make them either 20 or 60 ms. Durations of T1 and T2 were also varied, but the total duration of T1 and T2 was fixed at either 240 or 480 ms. In experiment 1, participants compared the durations of T1 and T2. In experiment 2, the subjective duration of each interval was measured separately. We found that lengthening the duration of the sound marker which terminated an interval increased the subjective duration of that interval: lengthening the duration of the second marker increased the subjective duration of T1, and lengthening the duration of the third marker increased the subjective duration of T2. Lengthening the duration of the first marker increased the subjective duration of T1 when T1 + T2 = 240 ms, especially when T1 > T2. This effect of first-marker duration, which could not be observed with single intervals used in the control conditions, seemed to enhance the contrast between T1 and T2. The effects of marker durations are associated with previous time-perception studies, in which single time intervals were used. They are discussed in the context of rhythm-perception studies, in which more complex sound patterns have been used.