Attentional modulation of electrophysiological activity in auditory cortex for unattended sounds within multistream auditory environments.

In three experiments, we addressed the issue of attention effects on unattended sound processing when one auditory stream is selected from three potential streams, creating a simple model of the cocktail party situation. We recorded event-related brain potentials (ERPs) to determine the way in which unattended, task-irrelevant sounds were stored in auditory memory (i.e., as one integrated stream or as two distinct streams). Subjects were instructed to ignore all the sounds and attend to a visual task or to selectively attend to a subset of the sounds and perform a task with the sounds (Experiments 1 and 2). A third (behavioral) experiment was conducted to test whether global pattern violations (used in Experiments 1 and 2) were perceptible when the sounds were segregated. We found that the mismatch negativity ERP component, an index of auditory change detection, was evoked by infrequent pattern violations occurring in the unattended sounds when all the sounds were ignored, but not when attention was focused on a subset of the sounds. The results demonstrate that multiple unattended sound streams can segregate by frequency range but that selectively attending to a subset of the sounds can modify the extent to which the unattended sounds are processed. These results are consistent with models in animal and human studies showing that attentional control can limit the processing of unattended input in favor of attended sensory inputs, thereby facilitating the ability to achieve behavioral goals.

Stream segregation of narrow-band noise bursts.

Normally hearing adults heard rapid alternations of a pair of band-limited noise bursts that had flat spectra (in terms of equal-loudness weighting of components) and sharp band edges. The bursts differed in center frequency (CF), but were matched on overall intensity, on bandwidth (BW) on a log frequency scale, and (roughly) on pitch strength. Listeners judged the ease with which the sequence could be held together perceptually in a single auditory stream (vs. forming separate high and low streams). Involuntary segregation was examined as a function of the following measures of frequency separation of the alternating noise bands: (1) the closest band edges, (2) the most remote band edges, (3) the CFs of the bands on a logarithmic scale, and (4) the BWs. Segregation was best predicted from the separation of the two CFs on a log-frequency scale (very strong effect). Increasing the BWs of the two alternating bursts (the same size, in log frequency, for both bands) also led to greater segregation (very weak effect).

Effects of time intervals and tone durations on auditory stream segregation.

Adult listeners rated the difficulty of hearing a single coherent stream in a sequence of high (H) and low (L) tones that alternated in a repetitive galloping pattern (HLH-HLH-HLH...). They could hear the gallop when the sequence was perceived as a single stream, but when it segregated into two substreams, they heard H-H- ... in one stream and L-L- ... in the other. The onset-to-onset time of the tones, their duration, the interstimulus interval (ISI) between tones of the same frequency, and the frequency separation between H and L tones were varied. Subjects' ratings on a 7-point scale showed that the well-known effect of speed's increasing stream segregation is primarily due to its effect on the ISI between tones in the same frequency region. This has implications for several theories of streaming.

Is a common grouping mechanism involved in the phenomena of illusory continuity and stream segregation?

Two auditory phenomena--stream segregation and illusory continuity through a wide-band noise interruption--were studied to determine whether the same principles of perceptual organization applied to both. A cycle was formed of a repeating alternation of two short bursts of narrow-band noise (NBN), one centered at a high frequency (H) and the other at a low frequency (L), with shorter bursts of wide-band noise (WBN) inserted between successive NBNs (H WBN L WBN H WBN...). In some conditions, listeners could hear a single NBN moving up and down behind the WBN bursts, although there was no NBN present with the WBN. Listeners rated the strength of this illusory continuity. Center frequency separation, rate of onsets, and bandwidth of the NBNs were varied. Increases in values of all three variables decreased illusory continuity. Other listeners rated the stream segregation of the H and L bands when successive NBNs were separated either by WBN bursts (as above) or by silences. The same three acoustic variables were manipulated. Increases in all three variables decreased the perception of a single stream. The similar disruptive effects on illusory continuity and on the one-stream percept in the stream segregation task support the idea that both phenomena depend on a common preliminary process of linking together the parts of a sequence that have similar frequencies.

Cumulation of the tendency to segregate auditory streams: resetting by changes in location and loudness.

In four experiments, the accumulation, over time, of a tendency to hear separate high and low streams in a sequence of high (H) and low (L) tones, presented in a galloping rhythm (HLH-HLH-...), was studied. Each trial was composed of two parts, an induction sequence, then a test sequence, with no break between them. The test sequence was always heard at the far left. When the induction sequence and the test sequence were identical, the presence of the induction sequence increased the tendency for the test sequence to split into two streams. However, when the sequences differed in location (cued by differences in interaural timing or intensity over headphones and by loudspeaker placement in a free field) or when they differed in loudness, the accumulation of the segregative tendency was reset, and the test sequence sounded more integrated. When the induction sequence changed in location or loudness in gradual steps toward the value of the test sequence, resetting was much less. It appears that the accumulation of information about streams in different frequency regions is sensitive to sudden changes in parameters, even when they affect the frequency regions equally. This prevents the system from accumulating data across unrelated events.

Gallbladder diverticulum: a case report and review of the literature.

Gallbladder diverticulum is an unusual and uncommon disease process rarely discussed in the literature. This disorder may not be diagnosed until surgically resected. The presentation may entail nonspecific complaints over a prolonged period as highlighted in a case report of a 17-year-old woman with this disease. The incidence and pathophysiology of this disease process is correlated to the symptoms and signs of this patient. The surgical findings, procedure, and outcome are described.

The influence of different timbre attributes on the perceptual segregation of complex-tone sequences.

Spectral factors such as differences in harmonic content are powerful cues in the perceptual organization of tone sequences. Temporal features such as rise time, however, have been shown to be poor cues [W. M. Hartmann and D. Johnson, Mus. Perc. 9, 155-184 (1991)]. The relative influence of these timbral features on perceptual segregation was investigated. Complex tones were sequenced in a repeating ABA- "gallop" format, under four conditions in which tones A and B had the same or different timbres as defined by differences in numbers of harmonics and temporal-envelope features. A sequence started with A and B tones at the same F0. The F0 difference between A and B then increased over the course of a trial, until a listener terminated the trial indicating perceptual segregation into sub-sequences comprising A and B tones, respectively. The F0 difference required to reach this crossover point of segregation provided a measure of the efficacy of stimulus features of A and B as cues for perceptual organization. Sequences combining differences in harmonic structure and temporal envelope required the smallest F0 change for segregation. Sequences of tones with the same harmonic structure and temporal envelope required larger changes in F0, while the other conditions fell in the middle of this range. The F0-tracking method used in this study facilitates measurement of the relative contribution of different stimulus features to stream segregation. It also holds potential as a tool using the point of segregation as a measure of the magnitude of timbre differences brought about by different physical features of sounds.

Resetting the pitch-analysis system. 2. Role of sudden onsets and offsets in the perception of individual components in a cluster of overlapping tones.

Experiments on young adults studied the effects of suddenness of onset or offset on the discrimination of the order of pitches of individual tones in a 1-s, 4-tone cluster of overlapping pure tones. In experiment 1, the tones, all within a critical band, went on asynchronously. Each rose and decayed linearly in amplitude. Faster onsets, within the range 10 to 640 ms as measured on the first tone, increased the accuracy of the discrimination of the order of onsets, but 10-ms onsets were slightly worse than 40-ms onsets in early sessions. Experiment 2 found similar effects for the abruptness of offsets of tones in clusters whose components came on synchronously but went off asynchronously. Onset order was very much easier to detect than offset order. The auditory system may use neural onset and offset responses to reset itself and carry out new analyses at frequency-by-amplitude points of sudden amplitude change, thereby contributing to auditory scene analysis.

Allocating attention to frequency regions.

Three experiments were conducted to determine whether attention may be allocated to a specific frequency region. On each trial, a frequency cue was presented and was followed by a target tone. The cue indicated the most likely frequency of the forthcoming target about which the listeners were required to make a duration judgment. It was reasoned that if listeners are able to allocate attention to the cued frequency region, then judgments of any characteristic of a tone of the cued frequency should be facilitated relative to tones of different frequencies. Results indicated that duration judgements were made more quickly and accurately when the cue provided accurate frequency information than when it did not. In addition, performance generally declined as the frequency separation between cue and target increased. These effects are interpreted as an indication that listeners may use a frequency cue to allocate attention to a specific frequency region and that, under these conditions, the shape of the attentional focus conforms to a gradient. The possible similarities of covert orienting mechanisms in vision and audition are discussed.

Resetting the pitch-analysis system: 1. Effects of rise times of tones in noise backgrounds or of harmonics in a complex tone.

The question of whether sudden increases in the amplitude of pure-tone components would perceptually isolate them from a more complex spectrum was investigated in two experiments. In Experiment 1, a 3.5-sec noise was played as a masker. During the noise, two pure-tone components of different frequencies appeared in succession. Subjects were asked to judge whether the pitch sequence went up or down. The rise time of these components had only a small and inconsistent effect on discrimination. In Experiment 2, the 3.5-sec background signal was a complex tone. The amplitudes of two of its components were incremented in succession. Again, subjects judged whether the pitch pattern went up or down. This time there was a sizable, monotonic effect of the rise time of the increments, with more rapid increments leading to better discrimination. The difference between the two results is interpreted in terms of the auditory system's response to changing and unchanging signals and the role of its "sudden-change" responses in attracting perceptual processing to certain spectral regions.

Organization in audition by similarity in rate of change: evidence from tracking individual frequency glides in mixtures.

In audition, sound energy is assigned to separate auditory "streams" following principles of organization that closely parallel the visual gestalt principles that guide the perception of distinct forms or objects. Metzger (1934) provided evidence for organization in vision based on similarity in the velocity of moving forms. If two dots approach one another along one spatial dimension, they may appear to cross and continue beyond their meeting point if their velocities differ; otherwise, they usually appear to change direction abruptly and retrace their movements. If an analogous auditory principle exists, with rate of change in frequency substituted for velocity of movement, two frequency glides that sweep through the same frequency range in opposite directions should be able to perceptually cross if their rates of change differ; otherwise, they should usually appear to change direction and retrace the same frequency region. Four experiments provided data in support of this hypothesis, and the results were consistent across experiments with varying stimuli and methods of presentation. When properties of the stimuli favored organization according to a principle of frequency proximity, the effect of a principle of rate similarity was attenuated but still evident.

The perceptual segregation of simultaneous vowels with harmonic, shifted, or random components.

This experiment was an investigation of the ability of listeners to identify the constituents of double vowels (pairs of synthetic vowels, presented concurrently and binaurally). Three variables were manipulated: (1) the size of the difference in F0 between the constituents (0, 1/2, and 6 semitones); (2) the frequency relations among the sinusoids making up the constituents: harmonic, shifted (spaced equally in frequency but not integer multiples of the F0), and random; and (3) the relationship between the F0 contours imposed on the constituents: steady state, gliding in parallel, or gliding in opposite directions. It was assumed that, in the case of the gliding contours, the harmonics of each vowel would "trace out" their spectral envelope and potentially improve the definition of the formant locations. It was also assumed that the application of different F0 contours would introduce differences in the direction of harmonic movement (common fate), thus aiding the perceptual segregation of the two vowels. The major findings were the following: (1) For harmonic constituents, a difference in F0 leads to improved identification performance. Neither tracing nor common-fate differences add to the effect of pitch differences. (2) For shifted constituents, a difference between the spacing of the constituents also leads to improved performance. Formant tracing and common fate contribute some further improvement. (3) For random constituents, tracing does not contribute, but common fate does.

An experimental evaluation of three theories of auditory stream segregation.

Three theories of auditory stream segregation were evaluated. In two-part trials, subjects heard an induction sequence, whose effects upon an immediately subsequent test sequence were measured. The rhythm and total duration of Induction Sequence tones were varied in two experiments. The similarity between induction and test sequences aided segregation, but rhythmic predictability and longer tone durations did not. Frequency alternation during the induction sequence was not necessary to induce segregation in the test sequence. Furthermore, peripheral processes inadequately account for the segregation effects found. The data suggest that, once a distinct percept emerges from an auditory scene, properties derived from the percept (particularly changes) are fed back to control the ongoing analysis of that auditory scene. A neural adaptation to stimuli with constant properties may form part of this analysis.

The phonetic integration of speech and non-speech sounds: effects of perceived location.

The third-formant (F3) transition of a three-formant /da/ or /ga/ syllable was extracted and replaced by sine-wave transitions that followed the F3 centre frequency. The syllable without the F3 transition (base) was always presented at the left ear, and a /da/ (falling) or /ga/ (rising) sine-wave transition could be presented at either the left, the right, or both ears. The listeners perceived the base as a syllable, and the sine-wave transition as a non-speech whistle, which was lateralized near the left ear, the right ear, or the middle of the head, respectively. In Experiment 1, the sine-wave transition strongly influenced the identity of the syllable only when it was lateralized at the same ear as the base (left ear). Phonetic integration between the base and the transitions became weak, but was not completely eliminated, when the latter was perceived near the middle of the head or at the opposite ear as the base (right ear). The second experiment replicated these findings by using duplex stimuli in which the level of the sine-wave transitions was such that the subjects could not reliably tell whether a /da/ or a /ga/ transition was present at the same ear as the base. This condition was introduced in order to control for the possibility that the subjects could have identified the syallables by associating a rising or falling transition presented at the left ear with a /da/ or /ga/ percept. Alternative suggestions about the relation between speech and non-speech perceptual processes are discussed on the basis of these results.

Auditory grouping based on fundamental frequency and formant peak frequency.

The perceptual grouping of a four-tone cycle was studied as a function of differences in fundamental frequencies and the frequencies of spectral peaks. Each tone had a single formant and at least 13 harmonics. In Experiment 1 the formant was created by filtering a flat spectrum and in Experiment 2 by adding harmonics. Fundamental frequency was found to be capable of controlling grouping even when the spectra spanned exactly the same frequency range. Formant peak separation became more effective as the sharpness (amplitude of the peak relative to a spectral pedestal) increased. The effect of each type of acoustic difference depended on the task. Listeners could group the tones by either sort of difference but were also capable of resisting the disruptive effect of the other one. This was taken as evidence for the presence of a schema-based process of perceptual grouping and the relative weakness of primitive segregation.

Auditory streaming and the continuity illusion.

In the present experiment, auditory stream organization was investigated in the presence of perceptually restored continuity. It was found that auditory streaming processes tend to yield the same perceptual organization independently of the presence or absence of perceptual restoration. Other observations include the dominance of frequency proximity over trajectory as a perceptual organization principle, and the effect of harmonic enrichment on perceptual grouping.

Fusion of auditory components: effects of the frequency of amplitude modulation.

The fusion of two amplitude-modulated (AM) tones presented simultaneously was studied. Subjects were presented with an AM tone (A) followed by a copy of itself (B) which was accompanied by another AM tone (C). In different experiments, the subjects were asked either to rate how clearly they could hear Tone B in the BC mixture or whether Tone B was present or not. The stronger the fusion of Tones B and C, the harder it was to "hear out" Tone B. It was found that fusion was strongest when Tones B and C were modulated at the same rate; segregation curves were obtained for the degree of mismatch of modulation frequency.

The perceptual segregation of simultaneous auditory signals: pulse train segregation and vowel segregation.

In the experiments reported here, we attempted to find out more about how the auditory system is able to separate two simultaneous harmonic sounds. Previous research (Halikia & Bregman, 1984a, 1984b; Scheffers, 1983a) had indicated that a difference in fundamental frequency (F0) between two simultaneous vowel sounds improves their separate identification. In the present experiments, we looked at the effect of F0s that changed as a function of time. In Experiment 1, pairs of unfiltered or filtered pulse trains were used. Some were steady-state, and others had gliding F0s; different F0 separations were also used. The subjects had to indicate whether they had heard one or two sounds. The results showed that increased F0 differences and gliding F0s facilitated the perceptual separation of simultaneous sounds. In Experiments 2 and 3, simultaneous synthesized vowels were used on frequency contours that were steady-state, gliding in parallel (parallel glides), or gliding in opposite directions (crossing glides). The results showed that crossing glides led to significantly better vowel identification than did steady-state F0s. Also, in certain cases, crossing glides were more effective than parallel glides. The superior effect of the crossing glides could be due to the common frequency modulation of the harmonics within each component of the vowel pair and the consequent decorrelation of the harmonics between the two simultaneous vowels.

Role of predictability of sequence in auditory stream segregation.

This study evaluates the role of item predictability in auditory temporal coherence. Thirteen normal-hearing subjects were required to hold together long tonal sequences as single strings of notes. Temporal and spectral predictability of successive notes in a sequence varied as a function of experimental condition. As the frequency separation of the notes in the sequence increased, the subjects found it more difficult to hold the sequence together as a single stream. There was no significant difference in subjects' abilities in performing this task as a function of experimental condition. That is, the predictability of successive notes appeared not to have a role in temporal coherence.