Changes in the perceived duration of a narrowband sound induced by a preceding stimulus.

The authors show that a narrowband noise (NBN) is perceived as longer when presented immediately after a wideband noise (WBN), compared to when the WBN is absent. This effect depended on the WBN’s frequency spectrum overlapping that of the NBN, and it increased as the duration of the WBN increased up to 300 ms. It decreased when a silent gap was introduced between the WBN and NBN, but remained significant for an easily detectable gap of 40 ms. A correlate of the effect was observed in the mismatch negativity (MMN) to a deviant stimulus, consisting of a WBN + NBN, presented in a sequence of more common isolated WBNs. The MMN latency was longer for an on-frequency than for an off-frequency WBN; and, more importantly, the size of this difference correlated across participants with the difference in perceived duration. A rhythm-adjustment experiment showed that the presence of an on-frequency WBN immediately preceding a tone caused that tone to be heard as starting earlier than when the WBN was absent. The results are discussed in relation to the continuity illusion and models of duration encoding. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perceptual learning of noise vocoded words: Effects of feedback and lexicality.

Speech comprehension is resistant to acoustic distortion in the input, reflecting listeners' ability to adjust perceptual processes to match the speech input. This adjustment is reflected in improved comprehension of distorted speech with experience. For noise vocoding, a manipulation that removes spectral detail from speech, listeners' word report showed a significantly greater improvement over trials for listeners that heard clear speech presentations before rather than after hearing distorted speech (clear-then-distorted compared with distorted-then-clear feedback, in Experiment 1). This perceptual learning generalized to untrained words suggesting a sublexical locus for learning and was equivalent for word and nonword training stimuli (Experiment 2). These findings point to the crucial involvement of phonological short-term memory and top-down processes in the perceptual learning of noise-vocoded speech. Similar processes may facilitate comprehension of speech in an unfamiliar accent or following cochlear implantation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Genetic divergence and evolutionary instability in ospE-related members of the upstream homology box gene family in Borrelia burgdorferi sensu lato complex isolates

A series of related genes that are flanked at their 5' ends by a conserved upstream sequence element called the upstream homology box (UHB) have been identified in Borrelia burgdorferi. These genes have been referred to as the UHB or erp gene family. We previously demonstrated that among a limited number of B. burgdorferi isolates, the UHB gene family is variable in composition and organization. Prior to this report the UHB gene family in other species of the B. burgdorferi sensu lato complex had not been studied, and if this family is important in the pathogenesis or biology of the Lyme disease spirochetes, then a wide distribution among species and isolates of the B. burgdorferi sensu lato complex would be expected. To assess this, we screened for the UHB element by Southern hybridization and determined its restriction fragment length polymorphism (RFLP) patterns. The UHB element was found to be carried by all B. burgdorferi sensu lato complex species tested (B. burgdorferi, B. garinii, B. afzelii, B. japonica, B. valaisiana sp. nov., and B. andersonii), but the RFLP patterns varied widely at both the inter- and intraspecies levels. Variation in both the number and size of the hybridizing restriction fragments was evident. PCR analyses also revealed the presence of polymorphic, ospE-related alleles in many isolates. Sequence analyses identified the molecular basis of the polymorphisms as being primarily insertions and deletions. Sequence variation and the insertions and deletions were found to be clustered in two distinct domains (variable domains 1 and 2). In many isolates variable domain 1 is flanked by direct repeat elements, some as long as 38 bp. Computer analyses of the deduced amino acid sequences encoded within variable domain 1 predict them to be hydrophilic, surface exposed, and antigenic. The analyses conducted here suggest that the UHB gene family, as evidenced by the variable UHB RFLP patterns, is not evolutionarily stable and that the polymorphic ospE alleles are derived from a common ancestral gene which has been modified through mutation or recombination events. The characterization of ospE-related genes of the UHB gene family among B. burgdorferi sensu lato species will prove important in attempts to construct a model for UHB gene family organization and in deciphering the role of the UHB gene family in the biology and pathogenesis of the Lyme disease spirochetes.     

An objective measurement of the build-up of auditory streaming and of its modulation by attention.

Three experiments studied auditory streaming using sequences of alternating “ABA” triplets, where “A” and “B” were 50-ms tones differing in frequency by Δf semitones and separated by 75-ms gaps. Experiment 1 showed that detection of a short increase in the gap between a B tone and the preceding A tone, imposed on one ABA triplet, was better when the delay occurred early versus late in the sequence, and for Δf = 4 vs. Δf = 8. The results of this experiment were consistent with those of a subjective streaming judgment task. Experiment 2 showed that the detection of a delay 12.5 s into a 13.5-s sequence could be improved by requiring participants to perform a task on competing stimuli presented to the other ear for the first 10 s of that sequence. Hence, adding an additional task demand could improve performance via its effect on the perceptual organization of a sound sequence. The results demonstrate that attention affects streaming in an objective task and that the effects of build-up are not completely under voluntary control. In particular, even though build-up can impair performance in an objective task, participants are unable to prevent this from happening. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Effects of temporal fringes on fundamental-frequency discrimination

It has recently been shown that the ability of listeners to encode the F0 of a "target" harmonic complex can be disrupted by another complex (the "fringe") presented immediately before and after it [R. P. Carlyon, J. Acoust. Soc. Am. 99, 525-533 (1996)]. This finding has been attributed to listeners overintegrating information about the fringe's F0 when estimating that of the target. Here difference limens for F0 (DLF0s) for a 100-ms harmonic complex target were measured using a two-interval two-alternative forced choice (2I-2AFC) adaptive procedure, in the presence and absence of 200-ms harmonic fringes. The target F0s in the signal and standard intervals were geometrically placed around a nominal F0 of 88 or 250 Hz; the fringe F0, constant throughout a block, was set at either of these two frequencies. The harmonics were bandpass filtered into one of three different frequency regions: low (125-625 Hz), mid (1375-1875 Hz), or high (3900-5400 Hz). In the low and mid regions, the fringes produced a large increase in DLF0s when they had a similar F0 to the target. This effect was absent or greatly reduced when the fringes and targets either had widely different F0s or occupied different frequency regions, and it was not reduced by providing additional spectral cues to the transition between fringes and targets. In the high region, the fringes produced large increases in DLF0s whether or not their F0 was similar to that of the targets. It is concluded that these results reflect a process of overintegration which, in the low and mid regions, is sensitive both to the F0 and to the spectral region of the stimuli. It is suggested that the different results in the high region may reflect the fact that, unlike in the low or mid regions, all the components of the targets and fringes were unresolved by the peripheral auditory system. Finally, the results of all the experiments are discussed in terms of auditory streaming.     

Dual temporal pitch percepts from acoustic and electric amplitude-modulated pulse trains

Two experiments examined the perception of unmodulated and amplitude-modulated pulse trains by normally hearing listeners and cochlear implantees. Four normally hearing subjects listened to acoustic pulse trains, which were band-pass filtered between 3.9 and 5.3 kHz. Four cochlear implantees, all postlinguistically deaf users of the Mini System 22 implant, listened to current pulse trains produced at a single electrode position. In the first experiment, a set of nine loudness-balanced unmodulated stimuli with rates between 60 and 300 Hz were presented in a multidimensional scaling task. The resultant stimulus spaces for both subject groups showed a single dimension associated with the rate of the stimuli. In the second experiment, a set of ten loudness-balanced modulated stimuli was constructed, with carrier rates between 140 and 300 Hz, and modulation rates between 60 and 150 Hz. The modulation rates were integer submultiples of the carrier rates, and each modulation period consisted of one higher-intensity pulse and one or more identical lower-intensity pulses. The modulation depth of each stimulus was adjusted so that its pitch was judged to be higher or lower 50% of the time than that of an unmodulated pulse train having a rate equal to the geometric mean of the carrier and modulation rates. A multidimensional scaling task with these ten stimuli resulted in two-dimensional stimulus spaces, with dimensions corresponding to carrier and modulation rates. A further investigation with one normally hearing subject showed that the perceptual weighting of the two dimensions varied systematically with modulation depth. It was concluded that, when filtered appropriately, acoustic pulse trains can be used to produce percepts in normal listeners that share common features with those experienced by subjects listening through one channel of a cochlear implant, and that the central auditory system can extract two temporal patterns arising from the same cochlear location.     

Evaluating the noise in electrically evoked compound action potential measurements in cochlear implants

Electrically evoked compound action potentials (ECAPs) are widely used to study the excitability of the auditory nerve and stimulation properties in cochlear implant (CI) users. However, ECAP detection can be difficult and very subjective at near-threshold stimulation levels or in spread of excitation measurements. In this study, we evaluated the statistical properties of the background noise (BN) and the postaverage residual noise (RN) in ECAP measurements in order to determine an objective detection criterion. For the estimation of the BN and the RN, a method currently used in auditory brainstem response measurements was applied. The potential benefit of using weighted (Bayesian) averages was also examined. All estimations were performed with a set of approximately 360 ECAP measurements recorded from five human CI users of the CII or HiRes90K device (advanced bionics). Results demonstrated that the BN was normally distributed and the RN decreased according to the square root of the number of averages. No additional benefit was observed by using weighted averaging. The noise was not significantly different either at different stimulation intensities or across recording electrodes along the cochlea. The analysis of the statistical properties of the noise indicated that a signal-to-noise ratio of 1.7 dB as a detection criterion corresponds to a false positive detection rate of 1% with the used measurement setup.     

Effects of Location, Frequency Region, and Time Course of Selective Attention on Auditory Scene Analysis.

Often, the sound arriving at the ears is a mixture from many different sources, but only 1 is of interest. To assist with selection, the auditory system structures the incoming input into streams, each of which ideally corresponds to a single source. Some authors have argued that this process of streaming is automatic and invariant, but recent evidence suggests it is affected by attention. In Experiments 1 and 2, it is shown that the effect of attention is not a general suppression of streaming on an unattended side of the ascending auditory pathway or in unattended frequency regions. Experiments 3 and 4 investigate the effect on streaming of physical gaps in the sequence and of brief switches in attention away from a sequence. The results demonstrate that after even short gaps or brief switches in attention, streaming is reset. The implications are discussed, and a hierarchical decomposition model is proposed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of attention and unilateral neglect on auditory stream segregation.

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 greatly 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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Generalization of perceptual learning of vocoded speech.

Recent work demonstrates that learning to understand noise-vocoded (NV) speech alters sublexical perceptual processes but is enhanced by the simultaneous provision of higher-level, phonological, but not lexical content (Hervais-Adelman, Davis, Johnsrude, & Carlyon, 2008), consistent with top-down learning (Davis, Johnsrude, Hervais-Adelman, Taylor, & McGettigan, 2005; Hervais-Adelman et al., 2008). Here, we investigate whether training listeners with specific types of NV speech improves intelligibility of vocoded speech with different acoustic characteristics. Transfer of perceptual learning would provide evidence for abstraction from variable properties of the speech input. In Experiment 1, we demonstrate that learning of NV speech in one frequency region generalizes to an untrained frequency region. In Experiment 2, we assessed generalization among three carrier signals used to create NV speech: noise bands, pulse trains, and sine waves. Stimuli created using these three carriers possess the same slow, time-varying amplitude information and are equated for na?ve intelligibility but differ in their temporal fine structure. Perceptual learning generalized partially, but not completely, among different carrier signals. These results delimit the functional and neural locus of perceptual learning of vocoded speech. Generalization across frequency regions suggests that learning occurs at a stage of processing at which some abstraction from the physical signal has occurred, while incomplete transfer across carriers indicates that learning occurs at a stage of processing that is sensitive to acoustic features critical for speech perception (e.g., noise, periodicity). (PsycINFO Database Record (c) 2010 APA, all rights reserved)