Sudden changes in spectrum of an echo cause a breakdown of the precedence effect

The effect of changing the frequency components of an echo relative to the sound source was examined in a two-choice discrimination task. Subjects sat in an anechoic chamber and discriminated the direction of the lag noise burst within a lead-lag pair presented over loudspeakers. The leading noise burst was broadband, and the lagging burst was either high- or low-pass filtered. On some conditions, this test burst pair was preceded by a conditioning train of burst pairs, which also had a broadband lead and either a high- or low-frequency lag. When the frequency content of the echo was held constant across the conditioning train and test burst pair, echo suppression that was built up during the repeating train was maintained for the test burst pair, shown by the subjects’ poor performance in detecting the location of the lagging burst. By comparison, subjects had little difficulty in localizing the lagging burst when the frequency content of the echo changed between the conditioning train and the test burst, indicating that any buildup of suppression during the train was broken when the lagging burst’s spectrum shifted. The data are consistent with an interpretation in which echo suppression is temporarily broken when listeners’ built-up expectations about room acoustics are violated.     

One sound or two? Object-related negativity indexes echo perception

The ability to isolate a single sound source among concurrent sources and reverberant energy is necessary for understanding the auditory world. The precedence effect describes a related experimental finding, that when presented with identical sounds from two locations with a short onset asynchrony (on the order of milliseconds), listeners report a single source with a location dominated by the lead sound. Single-cell recordings in multiple animal models have indicated that there are low-level mechanisms that may contribute to the precedence effect, yet psychophysical studies in humans have provided evidence that top-down cognitive processes have a great deal of influence on the perception of simulated echoes. In the present study, event-related potentials evoked by click pairs at and around listeners' echo thresholds indicate that perception of the lead and lag sound as individual sources elicits a negativity between 100 and 250 msec, previously termed the object-related negativity (ORN). Even for physically identical stimuli, the ORN is evident when listeners report hearing, as compared with not hearing, a second sound source. These results define a neural mechanism related to the conscious perception of multiple auditory objects.     

What the precedence effect tells us about room acoustics

Sound produced in a room is typically followed by numerous reflected sounds from nearby surfaces; yet we perceive a single sound source. This perceptual phenomenon, known as the precedence effect, has long been assumed to involve echo suppression, but the nature of this suppression remains unclear. In two experiments, we investigated whether information about the lagging sound's location was perceived. Our hypothesis was that such information is critical because reflected sound can function to inform the listener about objects and structures in the room. Listeners reported hearing more echoes under a stimulus situation that simulated sudden, unexpected changes in the location of the lagging sounds, as compared with stable stimulus conditions. Placement of lagging sounds' locations proved to be critical in that a sudden shift per se did not disrupt the fusion aspect of the precedence effect; the new location had to occupy a site that specified a new reflecting surface. Perception of echoes appears to be modulated by room acoustic information contained in reflected sound and listeners' expectations about this.     

Effect of click rate and delay on breakdown of the precedence effect

The precedence effect was tested as a function of echo-click delay and click rate after an abrupt switch in location between leading and lagging clicks. Click trains at three rates, 1/sec, 2/sec, and 4/sec, with delays ranging between 2 and 20 msec, were presented to subjects in an anechoic chamber. Duration of the click train after the switch in location was 12 sec, and echo click perceptibility was assessed throughout this period. The number of echo clicks heard was an increasing monotonic function of delay. The subjects reported a "fade-out" of echo clicks after a set number of clicks at each delay, regardless of rate. This result was interpreted as a buildup in inhibition of echoes produced by the ongoing click train. Suppression of echoes was stronger when the leading click originated from the right side than from the left side.     

Binocular single vision achieved by fusion and suppression

The occurrences of fusion and suppression were determined from stereograms which produced two retinal images located at equal distances but in opposite directions from the fovea. Subjects reported whether the dichoptic stimulus appeared single or not, and if single whether it appeared in the center of the visual field. The report of centrality is predicted by the fusion theory of single vision and that of noncentrality by the suppression theory. Experiment 1, with eight subjects, showed that for small disparities perceived singleness was the percept predicted by the fusion theory; for larger disparities, the percepts could sometimes be predicted by the fusion theory and other times by the suppression theory. Experiment 2, with 16 subjects, showed that with larger stimuli the percept predicted by the fusion theory is more likely to occur. Experiment 3, with four subjects, showed that the centrality was reported when the stimuli were presented for 100 msec. This result provided support for our interpretation that the centrality reports in Experiments 1 and 2 were not due to fixation error and suppression.     

Adaptation to auditory motion in the horizontal plane: effect of prior exposure to motion on motion detectability.

Thresholds for auditory motion detectability were measured in a darkened anechoic chamber while subjects were adapted to horizontally moving sound sources of various velocities. All stimuli were 500-Hz lowpass noises presented at a level of 55 dBA. The threshold measure employed was the minimum audible movement angle (MAMA)--that is, the minimum angle a horizontally moving sound must traverse to be just discriminable from a stationary sound. In an adaptive, two-interval forced-choice procedure, trials occurred every 2-5 sec (Experiment 1) or every 10-12 sec (Experiment 2). Intertrial time was "filled" with exposure to the adaptor--a stimulus that repeatedly traversed the subject's front hemifield at ear level (distance: 1.7 m) at a constant velocity (-150 degrees/sec to +150 degrees/sec) during a run. Average MAMAs in the control condition, in which the adaptor was stationary (0 degrees/sec,) were 2.4 degrees (Experiment 1) and 3.0 degrees (Experiment 2). Three out of 4 subjects in each experiment showed significantly elevated MAMAs (by up to 60%), with some adaptors relative to the control condition. However, there were large intersubject differences in the shape of the MAMA versus adaptor velocity functions. This loss of sensitivity to motion that most subjects show after exposure to moving signals is probably one component underlying the auditory motion aftereffect (Grantham, 1989), in which judgments of the direction of moving sounds are biased in the direction opposite to that of a previously presented adaptor.     

The precedence effect in three species of birds (Melopsittacus undulatus, Serinus canaria, and Taeniopygia guttata)

The perceived locations of paired auditory images, simulating direct sounds and their echoes, have been recently studied in budgerigars (Melopsittacus undulatus; M. L. Dent & R. J. Dooling, 2003a, 2003b). In this article, the authors extend those experiments to include measurements of the precedence effect using a discrimination paradigm in two additional bird species: canaries (Serinus canaria) and zebra finches (Taeniopygia guttata). Although time courses of summing localization, localization dominance, and echo thresholds were similar across all species, budgerigars had slightly higher overall levels of discrimination. The results from these experiments add further support that the precedence effect in birds is similar to that found in other animals and that the ability to suppress echoes that might degrade localization and auditory object perception may be a general property of the vertebrate auditory system.     

Adaptation to auditory motion in the horizontal plane: Effect of prior exposure to motion on motion detectability

Thresholds for auditory motion detectability were measured in a darkened anechoic chamber while subjects were adapted to horizontally moving sound saurces of various-velocities. All stimuli were 500-Hz lowpass noises presented at a level of 55 dBA. The threshold measure employed was the minimum audible movement angle(MAMA)—that is, the minimum angle a horizontally moving sound must traverse to be just discriminable from a stationary sound. In an adaptive, two-interval forced-choice procedure, trials occurred every 2-5 sec (Experiment 1) or every 10–12 sec (Experiment 2). Intertrial time was “filled” with exposure to the adaptor—a stimulus that repeatedly traversed the subject’s front hemifield at ear level (distance: 1.7 m) at a constant velocity (?150°/secto + 150°/sec)during a run. Average MAMAs in the control condition, in which the adaptor was stationary (0°/sec), were 2.4° (Experiment 1) and 3.0° (Experiment 2). Three out of 4 subjects in each experiment showed significantly elevated MAMAs (by up to 60%), with some adaptors relative to the control condition. However, there were large intersubject differences in the shape of the MAMA versus adaptor velocity functions. This loss of sensitivity to motion that most subjects show after exposure to moving signals is probably one component underlying the auditory motion aftereffect (Grantham, 1989), in which judgmentsof the direction-afmoving sounds are biased in the direction opposite to that of a previously presented adaptor.     

The impact of broadband noise on serial memory: changes in band-pass frequency increase disruption

Irrelevant sound consisting of bursts of broadband noise, in which centre frequency changes with each burst, markedly impaired short-term memory for order. In contrast, a sequence of irrelevant sound in which the same band-pass noise burst was repeated did not produce significant disruption. Serial recall for both visual-verbal (Experiment 1) and visual-spatial items (Experiment 2) was sensitive to the increased disruption produced by changing irrelevant noise. The results provide evidence that sounds that are largely aperiodic can produce marked disruption of serial recall in a similar manner to periodic sounds (e.g., speech, musical streams, and tones), and thus show a changing-state effect.     

Identification of temporal order within auditory sequences

Unpracticed Ss reported the order of sounds in sequences consisting of either three or four successive items repeated over and over without pause. With unrelated sounds each lasting 200 msec, correct reports of order were at chance level for oral responses and for card-ordering responses (each card bearing the name of one sound). The sequences with four unrelated items were studied in greater detail, and the threshold for identification was found to be 670 msec with oral responses and 300 msec with card-ordering responses. When two related sounds (tones) were used in four-item sequences, correct card-ordering was possible at 200 msec per item when the tones were temporally contiguous, but was not possible at this duration when the tones were separated by nonrelated sounds. Some special rules governing auditory sequence identification were suggested, and implications for theories of auditory perception discussed.     

A comparison of the effects of spatial separation on apparent motion in the auditory and visual modalities

In the present investigation, the effects of spatial separation on the interstimulus onset intervals (ISOIs) that produce auditory and visual apparent motion were compared. In Experiment 1, subjects were tested on auditory apparent motion. They listened to 50-msec broadband noise pulses that were presented through two speakers separated by one of six different values between 0 degrees and 160 degrees. On each trial, the sounds were temporally separated by 1 of 12 ISOIs from 0 to 500 msec. The subjects were instructed to categorize their perception of the sounds as "single," "simultaneous," "continuous motion," "broken motion," or "succession." They also indicated the proper temporal sequence of each sound pair. In Experiments 2 and 3, subjects were tested on visual apparent motion. Experiment 2 included a range of spatial separations from 6 degrees to 80 degrees; Experiment 3 included separations from .5 degrees to 10 degrees. The same ISOIs were used as in Experiment 1. When the separations were equal, the ISOIs at which auditory apparent motion was perceived were smaller than the values that produced the same experience in vision. Spatial separation affected only visual apparent motion. For separations less than 2 degrees, the ISOIs that produced visual continuous motion were nearly equal to those which produced auditory continuous motion. For larger separations, the ISOIs that produced visual apparent motion increased.     

Discrimination of time intervals presented in sequences: spatial effects with multiple auditory sources

This article discusses two experiments on the discrimination of time intervals presented in sequences marked by brief auditory signals. Participants had to indicate whether the last interval in a series of three intervals marked by four auditory signals was shorter or longer than the previous intervals. Three base durations were under investigation: 75, 150, and 225 ms. In Experiment 1, sounds were presented through headphones, from a single-speaker in front of the participants or by four equally spaced speakers. In all three presentation modes, the highest different threshold was obtained in the lower base duration condition (75 ms), thus indicating an impairment of temporal processing when sounds are presented too rapidly. The results also indicate the presence, in each presentation mode, of a 'time-shrinking effect' (i.e., with the last interval being perceived as briefer than the preceding ones) at 75 ms, but not at 225 ms. Lastly, using different sound sources to mark time did not significantly impair discrimination. In Experiment 2, three signals were presented from the same source, and the last signal was presented at one of two locations, either close or far. The perceived duration was not influenced by the location of the fourth signal when the participant knew before each trial where the sounds would be delivered. However, when the participant was uncertain as to its location, more space between markers resulted in longer perceived duration, a finding that applies only at 150 and 225 ms. Moreover, the perceived duration was affected by the direction of the sequences (left-right vs. right-left).     

Thought suppression in obsessive-compulsive disorder

Social cognition research has indicated that attempts to suppress thoughts can lead to a paradoxical increase in the frequency of that thought. This phenomenon has been a central component of cognitive-behavioural models of obsessive-compulsive disorder (OCD); however, research has yet to demonstrate deficient thought suppression ability in OCD patients. We examined whether individuals with OCD (OCs) exhibit a deficit in the ability to suppress thoughts. In Experiment 1, attempted thought suppression led to a paradoxical increase in self-reported thoughts for OCs, but not for nonanxious controls (NACs) or anxious controls (ACs). In order to rule out self-report biases, in Experiment 2 we utilized a lexical decision paradigm that measured priming strength of a target word under thought suppression conditions. Results paralleled those of Experiment 1: OCs showed decreased lexical decision latency of the 'suppressed' thought (thought to reflect either increased priming strength or disrupted processing of nonsuppressed thoughts), thus exhibiting a paradoxical effect of thought suppression. This effect was not seen in NACs or ACs. These findings suggest that deficits in cognitive inhibitory processes may underlie the intrusive, repetitive nature of clinical obsessions.     

Duration discrimination of empty and filled intervals marked by auditory and visual signals

Experiments 1 and 2 compared, with a single-stimulus procedure, the discrimination of filled and empty intervals in both auditory and visual modalities. In Experiment 1, in which intervals were about 250 msec, the discrimination was superior with empty intervals in both modalities. In Experiment 2, with intervals lasting about 50 msec, empty intervals showed superior performance with visual signals only. In Experiment 3, for the auditory modality at 250 msec, the discrimination was easier with empty intervals than with filled intervals with both the forced-choice (FC) and the single stimulus (SS) modes of presentation, and the discrimination was easier with the FC than with the SS method. Experiment 4, however, showed that at 50 and 250 msec, with a FC-adaptive procedure, there were no differences between filled and empty intervals in the auditory mode; the differences observed with the visual mode in Experiments 1 and 2 remained significant. Finally, Experiment 5 compared differential thresholds for four marker-type conditions, filled and empty intervals in the auditory and visual modes, for durations ranging from .125 to 4 sec. The results showed (1) that the differential threshold differences among marker types are important for short durations but decrease with longer durations, and (2) that a generalized Weber’s law generally holds for these conditions. The results as a whole are discussed in terms of timing mechanisms.     

Remembering environmental sounds: The role of verbalization at input

Two experiments investigated the role of verbalization in memory for environmental sounds. Experiment i extended earlier research (Bower & Holyoak, 1973) showing that sound recognition is highly dependent upon consistent verbal interpretation at input and test. While such a finding implies an important role for verbalization, Experiment 2 suggested that verbalization is not the only efficacious strategy for encoding environmental sounds. Recognition after presentation of sounds was shown to differ qualitatively from recognition after presentation of sounds accompanied with interpretative verbal labels and from recognition after presentation of verbal labels alone. The results also suggest that encoding physical information about sounds is of greater importance for sound recognition than for verbal free recall, and that verbalization is of greater importance for free recall than for recognition. Several alternative frameworks for the results are presented, and separate retrieval and discrimination processes in recognition are proposed.     

The effect of vibrato on the recognition of masked vowels

Five experiments on the identifiability of synthetic vowels masked by wideband sounds are reported. In each experiment, identification thresholds (signal/masker ratios, in decibels) were measured for two versions of four vowels: a vibrated version, in which FO varied sinusoidally around 100 Hz; and a steady version, in which F0 was fixed at 100 Hz. The first three experiments were performed on naive subjects. Experiment 1 showed that for maskers consisting of bursts of pink noise, vibrato had no effect on thresholds. In Experiment 2, where the maskers were periodic pulse trains with an F0 randomly varied between 120 and 140 Hz from trial to trial, vibrato slightly improved thresholds when the sound pressure level of the maskers was 40 dB, but had no effect for 65-dB maskers. In Experiment 3, vibrated rather than steady pulse trains were used as maskers; when these maskers were at 40 dB, the vibrated versions of the vowels were slightly less identifiable than their steady versions; but, as in Experiment 2, vibrato had no effect when the maskers were at 65 dB. Experiment 4 showed that the unmasking effect of vibrato found in Experiment 2 disappeared in subjects trained in the identification task. Finally, Experiment 5 indicated that in trained listeners, vibrato had no influence on identification performance even when the maskers and the vowels had synchronous onsets and offsets. We conclude that vibrating a vowel masked by a wideband sound can affect its identification threshold, but only for tonal maskers and in untrained listeners. This effect of vibrato should probably be considered as a Gestalt phenomenon originating from central auditory mechanisms.     

Discriminating time intervals presented in sequences marked by visual signals.

This article presents the results of three experiments on the discrimination of time intervals presented in sequences marked by brief visual signals. In Experiment 1A (continuous condition), the participants had to indicate whether, in a series of 2-4 intervals marked by 3-5 visual signals, the last interval was shorter or longer than the previous one(s). In Experiment 1B (discontinuous condition), the participants indicated whether, in a presentation of two series of 1-3 intervals, with each series being marked by 2-4 signals, the intervals of the second sequence were shorter or longer than those of the first. Whenever one, two, or three standard intervals were presented, the difference threshold was as high at 150 msec as it was at 300 msec with the continuous method but increased monotonically from 150 to 900 msec with the discontinuous method. With both methods, the increase was well described by Weber's law--the Weber fraction was roughly constant--between 600 and 900 msec (Experiment 2), whereas between 900 and 1,200 msec (Experiment 3), the Weber fraction increased.     

Discriminating time intervals presented in sequences marked by visual signals

This article presents the results of three experiments on the discrimination of time intervals presented in sequences marked by brief visual signals. In Experiment 1A (continuous condition), the participants had to indicate whether, in a series of 2–4 intervals marked by 3–5 visual signals, the last interval was shorter or longer than the previous one(s). In Experiment 1B (discontinuous condition), the participants indicated whether, in a presentation of two series of 1–3 intervals, with each series being marked by 2–4 signals, the intervals of the second sequence were shorter or longer than those of the first. Whenever one, two, or three standard intervals were presented, the difference threshold was as high at 150 msec as it was at 300 msec with the continuous method but increased monotonically from 150 to 900 msec with the discontinuous method. With both methods, the increase was well described by Weber’s law&#x2014the Weber fraction was roughly constant&#x2014between 600 and 900 msec (Experiment 2), whereas between 900 and 1,200 msec (Experiment 3), the Weber fraction increased.     

Auditory apparent motion under binaural and monaural listening conditions

This investigation examined the ability of listeners to perceive apparent motion under binaural and monaural listening conditions. Fifty-millisecond broadband noise sources were presented through two speakers separated in space by either 10 degrees, 40 degrees, or 160 degrees, centered about the subject's midline. On each trial, the sources were temporally separated by 1 of 12 interstimulus onset intervals (ISOIs). Six listeners were asked to place their experience of these sounds into one of five categories (single sound, simultaneous sounds, continuous motion, broken motion, or successive sounds), and to indicate either the proper temporal sequence of presentation or the direction of motion, depending on whether or not motion was perceived. Each listener was tested at all spatial separations under binaural and monaural listening conditions. Motion was perceived in the binaural listening condition at all spatial separations tested for ISOIs between 20 and 130 msec. In the monaural listening condition, motion was reliably heard by all subjects at 10 degrees and 40 degrees for the same range of ISOIs. At 160 degrees, only 3 of the 6 subjects consistently reported motion. However, when motion was perceived in the monaural condition, the direction of motion could not be determined.     

The detection of anisochrony in monaural and interaural sound sequences

Nakao and Axelrod (1976) and van Noorden (1975) showed that the threshold for discriminating an anisochronous duple rhythm (a series of clicks with a temporal offset on every other one) from an isochronous rhythm (no offset) is poorer when the clicks are presented alternately to the two ears than when they are presented to the same ears. Van Noorden reported that the difference between the thresholds in the alternating and nonalternating conditions varied with the tempo of the sequence. Nakao and Axelrod found invariance of this threshold difference with sequence speed. According to our quantification of temporal processing of interaural sequences, the latter result should be expected. We carried out five psychophysical experiments to establish interaural and monaural discrimination between isochronous and anisochronous rhythms. Across experiments, base time intervals of 60–720 msec were spanned. The main result was that we replicated the poorer discrimination for interaural sequences. This deterioration in discrimination was the same for all sequence speeds. It was also the case that the thresholds were almost constant up to a sound repetition rate of about 3 per second, but increased linearly with slower rates. This result supports evidence in the literature that temporal processing of sequences faster than about 3–4 sounds per second differs from temporal processing of slower sequences.