Two pairs of experiments studied the effects of attention and of unilateral neglect on auditory streaming. The first pair showed that the build up of auditory streaming in normal participants is gready reduced or absent when they attend to a competing task in the contralateral ear. It was concluded that the effective build up of streaming depends on attention. The second pair showed that patients with an attentional deficit toward the left side of space (unilateral neglect) show less stream segregation of tone sequences presented to their left than to their right ears. Streaming in their right ears was similar to that for stimuli presented to either ear of healthy and of brain-damaged controls, who showed no across-ear asymmetry. This result is consistent with an effect of attention on streaming, constrains the neural sites involved, and reveals a qualitative difference between the perception of left-and right-sided sounds by neglect patients. Auditory streaming is an example of the grouping or binding processes that have been extensively studied both in the auditory (e.g., Bregman, 1990; Darwin & Carlyon, 1995) and visual (e.g., Treisman & Gormican, 1988) domains. It is well-illustrated by the stimulus shown in Figure 1 (van Noorden, 1975), in which a pair of tones with frequencies A and B is presented in the sequence ABA-ABA-ABA. When the repetition rate of the sequence is slow, or when the frequencies A and B are close, listeners can hear a galloping rhythm corresponding to the repeating triplets (Figure 1, top panel). However, at faster rates and wider separations, the A and B tones split into two separate streams (Figure 1, bottom panel), and the galloping rhythm is lost (Anstis & Saida, 1985; van Noorden, 1975). Additionally, the tendency for this stream segregation to occur builds up over several seconds, so that listeners may hear the gallop at the beginning but not at the end of a long sequence. A question that has interested both auditory and visual researchers is the stage of perceptual processing at which grouping mechanisms occur. In particular, the extent to which group

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Four experiments measured sensitivity (d′) to differences in fundamental frequency (F0) between two simultaneously presented groups of frequency-modulated harmonics. Each group was passed through a bandpass filter in either a LOW (125–625 Hz), MID (1375–1875 Hz), or HIGH (3900–5400 Hz) frequency region. In the first two experiments, a dynamic F0 difference (ΔF0) was created by introducing a 180° disparity between the frequency modulations imposed on the two groups. Experiment 1 measured sensitivity to such ΔF0’s between a MID group with a baseline F0 of 125 Hz and all components summed in sine phase, and a HIGH group, in four conditions. When the baseline F0 of the HIGH group was also 125 Hz, performance was good when its components were summed in sine phase and bad when they were in alternating phase. Conversely, when the HIGH F0 was 62.5 Hz, performance was better for alternating phase than for sine phase, consistent with alternating phase doubling the internal representation of HIGH group’s F0. Similar results were obtained for a comparison between the LOW and MID groups. Experiment 2 measured sensitivity to ΔF0’s between either the LOW and MID groups or between the MID and HIGH groups, for baseline F0s of 88 and 250 Hz. Sensitivity was best when the combination of frequency region and F0 was such that both groups were resolved or both unresolved by the peripheral auditory system, and worst when the groups differed in ‘‘resolvability.’’ Experiment 3 replicated experiment 2 using a different paradigm, in which the two groups were always modulated coherently, and in which the ΔF0 was constant throughout the signal. Experiments 2 and 3 also measured sensitivity to differences in F0 between successively presented tokens of the same group. Experiment 4 showed that the high sensitivity to (simultaneous) ΔF0’s when both groups were unresolved could be attributed to listeners detecting the pitch pulse asynchronies that inevitably arise from ΔF0’s. Finally, a method for predicting sensitivity to simultaneous, across-frequency ΔF0’s from that to successive within-channel differences, on the basis of optimum combination of information, was applied to the results of experiment 3. The method succeeded in predicting sensitivity to ΔF0’s between two groups of resolved harmonics, but over-estimated performance when one group was resolved and the other unresolved. The results suggest that the F0’s of resolved and unresolved harmonics are processed by separate mechanisms, in contrast to the predictions of ‘‘autocorrelation’’ models of F0 encoding.

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Four adult users of the Mini System 22 cochlear implant participated in an experiment to investigate the perceptual independence of place-of-stimulation and temporal cues for pulsatile electrical stimulation. The motivation was the relatively poor rate discrimination ability of cochlear implantees compared to the higher accuracy of temporal coding revealed by electrophysiological measurements and the performance of normal hearing listeners. The hypothesis tested was that the central auditory system can combine consistent rate and place cues in a way that is more effective than using each cue independently. Difference limens for rate change, place change, and combined rate and place change (with consistent and inconsistent cues) were compared for stimulation at low and high rates. The results were compatible with place and rate cues being used independently in the combined rate-and place-change conditions, with no advantage found for the consistent-cue conditions.

The influence of hearing loss and aging on the perceptual organization of sound sequences was investigated by comparing the ability of young normal-hearing subjects and elderly subjects having either impaired or normal hearing for their age to form perceptual auditory streams from sequences of harmonic complex tones as a function of differences in fundamental frequency (F0). The sequences consisted of repeating triplets of harmonic complex tones separated by a silence (ABA-). In conditions in which the F0s of the A and B tone were so low that the harmonics could not be individually resolved by the peripheral auditory system even in the young normal-hearing subjects, those subjects showed similar stream segregation performance to the elderly hearing-impaired subjects. In contrast, when the F0s of the tones were high enough for the harmonics to be largely resolved at the auditory periphery in normal-hearing subjects, but presumably unresolved in the elderly subjects, the former showed significantly more stream segregation than the latter. These results, which cannot be consistently explained in terms of age differences, suggest that auditory stream segregation is adversely affected by reduced peripheral frequency selectivity of elderly individuals. This finding has implications for the understanding of the listening difficulties experienced by elderly individuals in cocktail-party situations.

A sound turned off for a short moment can be perceived as continuous if the silent gap is filled with noise. The neural mechanisms underlying this "continuity illusion" were investigated using the mismatch negativity (MMN), an event-related potential reflecting the perception of a sudden change in an otherwise regular stimulus sequence. The MMN was recorded in four conditions using an oddball paradigm. The standards consisted of 500-Hz, 120-msec tone pips that were either physically continuous (Condition 1) or were interrupted by a 40-msec silent gap (Condition 2). The deviants consisted of the interrupted tone, but with the silent gap filled by a burst of bandpass-filtered noise. The noise either occupied the same frequency region as the tone and elicited the continuity illusion (Conditions 1a and 2a), or occupied a remote frequency region and did not elicit the illusion (Conditions 1b and 2b). We predicted that, if the continuity illusion is determined before MMN generation, then, other things being equal, the MMN should be larger in conditions where the deviants are perceived as continuous and the standards as interrupted or vice versa, than when both were perceived as continuous or both interrupted. Consistent with this prediction, we observed an interaction between standard type and noise frequency region, with the MMN being larger in Condition 1a than in Condition 1b, but smaller in Condition 2a than in Condition 2b. Because the subjects were instructed to ignore the tones and watch a silent movie during the recordings, the results indicate that the continuity illusion can occur outside the focus of attention. Furthermore, the latency of the MMN (less than approximately 200 msec postdeviance onset) places an upper limit on the stage of neural processing responsible for the illusion.