The precedence effect

When two similar transient sounds are presented binaurally in rapid succession, observers hear a single sound from a location which depends mainly on the properties of the first sound to reach the ears. This phenomenon, known as the precedence effect, was explored using stimuli consisting of 20 μs pulses presented using earphones; experiments were carried out on both the classical precedence effect (in which interaural delays provide the cues to lateralization) and on an amplitude-based precedence effect, where interaural amplitude differences provide the cues. Some experiments on the amplitude-based precedence effect led to unexpected but highly consistent anomalous results. The spectral characteristics of stimuli used in studies of both the classical and amplitude-based precedence effect were considered and, provided the delay between the two pairs of pulses used in the experiments is 600 μs or less, observers' behaviour is simply related to the amplitude and phase spectra of the stimuli.     

A precedence effect in the perception of inter-aural cross correlation.

Does the precedence effect, well known in the field of sound localization or lateralization, also apply to other percepts based on binaural processing? We have compared, with one and the same experimental paradigm, a manifestation of the traditional precedence effect in lateralization with a possible similar effect in the perception of diffuseness or compactness of a sound image. With dichotic headphone stimulation, lateralization was controlled by the inter-aural time delay (IATD), and diffuseness/compactness by the inter-aural cross correlation (IACC). The experimental paradigm rests on the principle of estimating the over-all sensation of a 20-ms noise burst, which was subdivided in two parts, with the relevant dichotic information (IATD or IACC) in the leading part being opposite to that in the trailing part. When each part is 10 ms, it is found that the overall sensation is slightly dominated by the information in the leading part, both for lateralization and for compactness/diffuseness. This dominance of the leading part can be compensated by a certain decrease of its duration and/or amplitude relative to that of the trailing part. It is found that this quantitative measure for the 'strength' of the precedence effect for the present stimulus is essentially the same for IATD and IACC, suggesting that the precedence effect does not apply exclusively to sound localization or lateralization, but to at least one other percept based on binaural processing as well, namely the processing of inter-aural cross correlation.     

Neural correlates of the precedence effect in auditory evoked potentials

The precedence effect in subjective localization tasks reflects the dominance of directional information of a direct sound (lead) over the information provided by one or several reflections (lags) for short delays. By collecting data of both psychoacoustical measurements and auditory evoked potentials the current study aims at neurophysiological correlates for the precedence effect in humans. In order to investigate whether the stimulus features or the perception of the stimulus is reflected on the ascending stages of the human auditory pathway, auditory brainstem responses (ABRs) as well as cortical auditory evoked potentials (CAEPs) using double click-pairs were recorded. Potentials were related to the results of the psychoacoustical data. ABRs to double click-pairs with lead-lag delays from 0 to 20 ms and interaural time differences (ITDs) in the lag click of 0 and 300 micros show an emerging second wave V for lead-lag delays larger than 2 ms. The amplitudes of the first and second wave V are the same for a lead-lag delay of about 5 ms. For the lag-ITD stimuli the latency of the second wave V was prolonged by approximately ITD/2 compared to the stimuli without lag-ITD. As the amplitudes of the second wave V were not decreased for a lead-lag delay around 5 ms as could be expected from psychoacoustical measurements of the precedence effect, ABRs reflect stimulus features rather than the perceptive qualities of the stimulus. The mismatch negativity (MMN) component of the CAEP for double click-pairs was determined using a diotic standard and a deviant with an ITD of 800 micros in the lag click. The comparison between the MMN components and the psychoacoustical data shows that the MMN is related to the perception of the stimulus, i.e., to the precedence effect. Generally, the findings of the present study suggest that the precedence effect is not a result of a poor sensitivity of the peripheral bottom-up processing. Rather, the precedence effect seems to be reflected by the MMN, i.e., cognitive processes on higher stages of the auditory pathway.     

Effects of aging on the precedence effect in sound localization

The precedence effect in sound localization can be evoked by presenting identical sounds (e.g., clicks) from pairs of loudspeakers placed on opposite sides of a subject's head. With appropriate inter-loudspeaker delays, normal subjects perceive a fused image originating from the side of the leading loudspeaker. Separate tests at loudspeaker delays ranging from 0 to 8 ms were presented to groups of young and elderly subjects. At 0 ms delay, young subjects perceived the fused image to be located halfway between the loudspeakers; at progressively longer delays, the image was perceived closer to the leading loudspeaker. Significant numbers of elderly subjects exhibited discrimination difficulties with delays below 0.7 ms.     

Attribute capture in the precedence effect for long-duration noise sounds

Listeners perceptually fuse the direct wave from a sound source with its reflections off nearby surfaces into a single sound image, located at or near the sound source (the precedence effect). This study investigated how a brief gap presented in the middle of either a direct wave or simulated reflection is incorporated into the fused image. For short (<9.5 ms) delays between the direct (leading) and reflected (lagging) waves, no sound was perceived from the direction of the lagging wave. For delays between 10 and 15 ms, both sounds were perceived, but the gap was heard only on the leading side. When the gap was only in the correlated lagging sound at short delays, it also was perceived as occurring on the leading side. Moreover, gap detection thresholds were the same for gaps in the leading and lagging sounds, suggesting that the perception of the gap was not suppressed, but rather incorporated into the leading sound. Finally, scalp event-related potentials were not associated with the precedence effect until the gap occurred. This suggests that cortical mechanisms are engaged to maintain fusion when attributes in direct or reflected waves change.     

The precedence effect for lateralization at low sensation levels

Using dichotic signals presented by headphone, stimulus onset dominance (the precedence effect) for lateralization at low sensation levels was investigated for five normal hearing subjects. Stimuli were based on 2400-Hz low pass filtered 5-ms noise bursts. We used the paradigm, as described by Aoki and Houtgast (Hear. Res., 59 (1992) 25-30) and Houtgast and Aoki (Hear. Res., 72 (1994) 29-36), in which the stimulus is divided into a leading and a lagging part with opposite lateralization cues (i.e. an interaural time delay of 0.2 ms). The occurrence of onset dominance was investigated by measuring lateral perception of the stimulus, with fixed equal duration of leading and lagging part, while decreasing absolute signal level or adding a filtered white noise with the signal level set at 65 dBA. The dominance of the leading part was quantified by measuring the perceived lateral position of the stimulus as a function of the relative duration of the leading (and thus the lagging) part. This was done at about 45 dB SL without masking noise and also at a signal-to-noise ratio resulting in a sensation level of 10 dB. The occurrence and strength of the precedence effect was found to depend on sensation level, which was decreased either by lowering the signal level or by adding noise. With the present paradigm, besides a decreased lateralization accuracy, a decrease in the precedence effect was found for sensation levels below about 30-40 dB. In daily-life conditions, with a sensation level in noise of typically 10 dB, the onset dominance was still manifest, albeit degraded to some extent.     

Conditions under which the Haas precedence effect may or may not occur

This investigation explored the stimulus conditions of the existence region in the free field of the Haas "precedence" effect. Experienced normal-hearing adults (N:5) listened to 50-msec bursts (0.2 msec rise-fall) of broadband noise from 2 loudspeakers at 1 m distance at ear level, at +/- 20 degrees re midline. Bursts from the loudspeakers were separated by interstimulus onset intervals (ISO-Is) between 0 and 50 msec. In one condition, uncorrelated noise was produced by separate generators; in a second condition (correlated noise), the signal from one generator was split and led to each speaker. Ss classified their experience of each presentation into one of five defined categories:: (1) single non-moving sound image; (2) 2 stationary but spatially distinct sound images; (3) single sound moving from lead to lag source; (4) as (3) but motion interrupted or broken; and (5) 2 successive sound images, with no apparent motion. In addition, Ss indicated direction (L-R; R-L) of any apparent motion. At 0 msec ISOI, directional judgments with either correlated or uncorrelated bursts were at or near chance level, as expected, and with correlated noises a single image was usually (71%) experienced, presumably at an apparent location at or near the midline. But with uncorrelated noises, 2 simultaneous but distinct sound images were usually (68%) perceived. At 2 msec ISOI, a weak precedence effect was exhibited for correlated noise, where the "single" response was often (55%) made; but even here the effect of the second burst was not "suppressed" entirely since direction judgments were 83% accurate. At 4 msec ISOI, "single" responses had declined to 36% for correlated and to 6% for uncorrelated noises, while by 8+ msec, "single" responses were dominated more or less completely by perceptions of dual sources. We conclude that the precedence effect is often confined to extremely brief ISOIs, and that the parameters of stimulus duration and complexity, of rise-fall times, absolute placement and angular separation of sound sources, subject differences, and a variety of types of perceptual experiences need to be addressed experimentally before the existence region of the precedence effect is well defined or uncritically accepted; and that it is too soon to attempt to posit neurophysiological mechanisms in explaining the phenomenon.     

Tests of the precedence effect in sound localization reveal abnormalities in multiple sclerosis

The precedence effect in sound localization involves presenting identical sounds (e.g., clicks) from pairs of matched speakers situated on opposite sides of a subject's head, with the clicks from one speaker preceding those from the other by a short interval. With appropriate delays, normal subjects perceive a fused image which originates from the side of the leading speaker. This test was administered to 24 patients with multiple sclerosis (MS). Separate tests involving speaker delays ranging from 0 msec (simultaneous presentation) to 8 msec were presented. At 0 msec delay, normal subjects perceived the fused image to be located halfway between the two speakers; at progressively longer delays, the image was perceived closer to the leading speaker. In contrast to normal subjects, a large proportion of the MS subjects exhibited difficulties with the task. The discrimination deficit was limited to delays below 1 msec, suggesting a problem involving an increased threshold for lateralizing the fused image away from midline toward the side of the leading speaker. The neural instability produced by demyelination in MS patients might account for this pattern of results.     

The precedence effect for lateralization for the mild sensory neural hearing impaired.

Using dichotic signals presented by headphone, stimulus onset dominance (the precedence effect for lateralization) was investigated for six sensorineural hearing-impaired subjects. Stimuli were based on 2400-Hz lowpass-filtered 5-ms noise bursts. We used the paradigm, as described by Goverts et al. (2000, Hear. Res. 148, 88-94), in which a single noise burst is divided into leading and lagging parts, with opposite lateralization cues (viz. an interaural time delay of 0.2 ms). The occurrence of onset dominance was investigated by measuring the lateral perception of the stimulus ('left' or 'right') with fixed, equal durations of leading and lagging parts, while decreasing the absolute signal level or adding a filtered white noise. The dominance of the leading part was quantified by measuring the lateral perception of the stimulus as a function of the relative duration of the leading (and thus the lagging) part. This was done at about 40 dB sensation level (SL) in quiet and in filtered white noise, at a signal-to-noise ratio resulting in a SL of about 6 dB. Results are compared to normal hearing reference data at various SLs. Hearing-impaired data show a large variance and overall a decreased precedence effect in terms of both occurrence and quantification, which cannot be explained on basis of reduced audibility. Mean performance of the hearing-impaired subjects at 40 dB in quiet was similar to normal hearing performance in masking noise at a signal-to-noise ratio of 0 dB.     

Peripheral auditory processing,the precedence effect and responses of single units in the inferior colliculus

The purpose of this paper is to illustrate how interactions occurring within the auditory periphery are relevant to the interpretation of neurophysiological data obtained in recent studies seeking to find physiological correlates of 'the precedence effect'. Similar information was presented orally at the recent International Symposium on the Central Auditory System held in Salamanca, Spain. The physiological data of interest are responses from single neural units in the inferior colliculus recorded following stimulation by successive pairs of binaural clicks. We show how peripheral, monaural, within-filter interactions of such successive clicks can produce internal values of interaural temporal differences (ITDs) and interaural intensitive differences (IIDs) that can differ greatly from those present in the external stimulus. These interactions can produce unintended internal ITDs and IIDs that are well 'outside the tuning range' of the single unit being studied. When this occurs, the responses of the single units to the pairs of clicks would be expected to diminish. We also discuss how hair cell-related adaptation and compression can also lead to diminished responses. It is suggested that effects resulting from peripheral interactions should be taken into account or evaluated quantitatively before other factors, including central inhibition, are invoked.     

Tracking of a "moving" fused auditory image under conditions that elicit the precedence effect

Pursuit auditory tracking of a fused auditory image (FAI), based on stimulus conditions known to elicit the precedence effect phenomenon in sound localization, was investigated in 36 normal subjects and in a small group of subjects with known neuropathology. Movement of the FAI was simulated by incrementally varying the delay between two clicks presented, one each, from two loudspeakers placed on opposite sides of the listener. The group of normal listeners tracked the movement of the FAI without difficulty and with great accuracy; the perceived location of the FAI varied linearly with the interspeaker delay. The sensitivity of the task in detecting neural timing or integration deficits was investigated in 5 subjects with neuropathology, including subjects with unilateral temporal lobe lesions, multiple sclerosis, or dyslexia. These disorders, previously shown to disrupt neural timing, yielded characteristic patterns of tracking inaccuracy for this task. These subjects had no difficulty localizing either a moving unitary click source or sounds in daily life. These data support the suggestion that sound localization using stimulus conditions known to elicit the precedence effect places greater demands on neural timing and integration than conventional tests of localization, and may provide a more sensitive index of neural function.     

Effects of noise and reverberation on the precedence effect in listeners with normal hearing and impaired hearing

The purpose of this study was to determine the effects of reverberation and noise on the precedence effect in listeners with hearing loss. Lag burst thresholds (LBTs) for 4-ms noise bursts were obtained for 2 groups of participants: impaired hearing and normal hearing. Data were collected in reverberant and anechoic environments in quiet and noise, at sensation levels of 10, 20, 30, 40, and 50 dB. Results indicated a significant effect of reverberation on LBTs for both participant groups. LBTs increased with sensation level in the reverberant environment and decreased with increasing sensation level in the anechoic environment. There was no effect of hearing loss on LBTs. When the change in LBT due to noise was compared, the effect of noise depended on group and environment, with a greater effect of noise on the performance of listeners with impaired hearing. It is likely that the ability to fuse direct sounds and early reflections is degraded in listeners with impaired hearing and that this contributes to the difficulties experienced by these listeners in reverberation and noise.     

A study on the effect of infrasound

In order to examine the influence of infrasound that is becoming topical in society, human beings and guinea pigs were exposed to infrasound. After exposure, the hearing level, vestibular functions and autonomic nervous functions of human beings were examined, and endocochlear potential (EP) and cochlear microphonics (CM) of guinea pigs were examined. Next, after guinea pigs were exposed to intense audible low frequency sound that was born secondarily from infrasound and/or vibration of whole body concerning about air pressure change of infrasound, their EP and CM were examined. The results obtained were as follows: 1) By exposure of infrasound 10-15Ha 130-135dB LSPL for 30min. to human beings, their hearing level, vestibular functions and autonomic nervous functions were not changed. 2) After exposure of infrasound 15Hz 135-140dB LSPL for 24hrs.-72hrs. to guinea pigs, their EP and CM remained normal. 3) After each exposure of audible low frequency sound 90Hz 120dB SPL for 72hrs., 150Hz 110dB SPL for 72hrs. and 200Hz 100dB SPL for 72hrs. to guinea pigs, their EP became abnormal though their CM remained normal. 4) After exposure of 15Hz 500 mu +/- 30 mu vibration for 72hrs. to guinea pigs, both EP and CM remained normal. 5) After exposure of both audible low frequency sound 150Hz 100dB SPL and vibration 15Hz 500 mu +/- 30 mu for 72hrs. to guinea pigs, their EP became abnormal though their CM remained normal.     

Effects of age and hearing loss on gap detection and the precedence effect: narrow-band stimuli.

Deficits in temporal resolution and/or the precedence effect may underlie part of the speech understanding difficulties experienced by older listeners in degraded acoustic environments. In a previous investigation, R. Roberts and J. Lister (2004) identified a positive correlation between measures of temporal resolution and the precedence effect, specifically across-channel gap detection (as measured dichotically) and fusion. Across-channel gap detection may also be measured using frequency-disparate markers. Thus, the present investigation was designed to determine if the relation is specific to dichotic gap detection or may generalize to all types of across-channel gap detection. Gap-detection thresholds (GDTs) for fixed-frequency and frequency-disparate markers and lag-burst thresholds (LBTs) were measured for 3 groups of listeners: young with normal hearing sensitivity (YNH), older with normal hearing sensitivity (ONH), and older with sensorineural hearing loss (OIH). Also included were conditions of diotic and dichotic GDT. Largest GDTs were measured for the frequency-disparate markers, whereas largest LBTs were measured for the fixed-frequency markers. ONH and OIH listeners exhibited larger frequency-disparate and dichotic GDTs than YNH listeners. Listener age and hearing loss appeared to influence temporal resolution for frequency-disparate and dichotic stimuli, which is potentially important for the resolution of timing cues in speech. Age and hearing loss did not significantly influence fusion as measured by LBTs. Within each participant group, most GDTs and LBTs were positively, but not significantly, correlated. For all participants combined, across-channel GDTs and LBTs were positively and significantly correlated. This suggests that the 2 tasks may rely on a common across-channel temporal mechanism.     

Effects of age and hearing loss on gap detection and the precedence effect: broadband stimuli.

Older listeners with normal-hearing sensitivity and impaired-hearing sensitivity often demonstrate poorer-than-normal performance on tasks of speech understanding in noise and reverberation. Deficits in temporal resolution and in the precedence effect may underlie this difficulty. Temporal resolution is often studied by means of a gap-detection paradigm. This task is similar to binaural fusion paradigms used to measure the precedence effect. The purpose of this investigation was to determine if within-channel (measured with monotic and diotic gap detection) or across-channel (measured with dichotic gap detection) temporal resolution is related to fusion (measured with lag-burst thresholds; LBTs) under dichotic, anechoic, and reverberant conditions. Gap-detection thresholds (GDTs) and LBTs were measured by means of noise-burst stimuli for 3 groups of listeners: young adults with normal-hearing sensitivity (YNH), older adults with normal-hearing sensitivity (ONH), and older adults with impaired-hearing sensitivity (OIH). The GDTs indicated that across-channel temporal resolution is poorer than within-channel temporal resolution and that the effects of age and hearing loss are dependent on condition. Results for the fusion task indicated higher LBTs in reverberation than for the dichotic and anechoic conditions, regardless of group, and no effect of age or hearing loss for the nonreverberant conditions. However, higher LBTs were observed in the reverberant condition for the ONH listeners. Further, there was a correlation between across-channel temporal resolution and fusion in reverberation. Gap detection and fusion may not necessarily reflect the same underlying processes; however, across-channel gap detection may influence fusion under certain conditions (i.e., in reverberation).     

CT-Cephalometry—A study in Indian population

Determination of obstructive site in obstructive sleep apnoea (OSA) is of paramount importance is planning the management. Cephalometric evaluation of lateral X-rays when combined with clinical assessment and fibreoptic examination of the airway helps in locating the site of obstruction. The usual technique of cephalometry has been modified so as to give a better delineation of the soft tissues. Holding a 2mm card board in the mouth and using barium paste helped in more accurate calculations. Using our technique, various parameters have been quantified and a number of controls were studied and normal range derived. Further improvement in cephalometry has been done by using C.T. cephlometry topogram technique. A topogram is a scan done on a running table top cranio-caudally. Using the topogram technique 38 OSA patients were evaluated for all the parameters. The technique, its advantages over traditional cephalometry and the values obtained in the study are discussed in this paper.     

Vestibular decruitment. A study of the phenomenon in a normal population.

Vestibular decruitment is a phenomenon characterized by the presence of less intense responses following stronger vestibular stimulation, and its presence has been considered as indicative of central vestibular pathology. In this study conducted in 24 normal individuals, with normal caloric responses, decruitment was observed in 58.3% of the cases. These decruitments were of types I and II as defined by Torok. Type III decruitment was not seen in these patients. In view of the present findings the actual meaning of the phenomenon must be investigated in larger numbers of patients with peripheral or central vestibular disorders.     

全国老年听力残疾人群现状调查研究

目的掌握我国老年听力残疾人群的现状及流行病学特征，提出适合我国国情的听力残疾防治对策。方法对第二次全国残疾人抽样调查数据进行流行病学分析，描述人群的分布特征。结果老年听力残疾现残率为11．04％，残疾等级以三、四级为主，占79．13％；致残原因中，老年性耳聋占66．87％，原因不明性耳聋占8．91％，中耳炎占8．62％。结论加强听力相关疾病的预防和治疗，降低听力残疾发生风险，对提高老年人生活质量有重要意义。