Volume 106, Issue 6, December 1999
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
106(1999); http://dx.doi.org/10.1121/1.428199View Description Hide Description
When two identical sounds are presented from different locations with a short interval between them, the perception is of a single sound source at the location of the leading sound. This “precedence effect” is an important behavioral phenomenon whose neural basis is being increasingly studied. For this report, neural responses were recorded to paired clicks with varying interstimulus intervals, from several structures of the ascending auditory system in unanesthetized animals. The structures tested were the auditory nerve, anteroventral cochlear nucleus, superior olivary complex, inferior colliculus, and primary auditory cortex. The main finding is a progressive increase in the duration of the suppressive effect of the leading sound (the conditioner) on the response to the lagging sound (the probe). The first major increase occurred between the lower brainstem and inferior colliculus, and the second between the inferior colliculus and auditory cortex. In neurons from the auditory nerve, cochlear nucleus, and superior olivary complex, 50% recovery of the response to the probe occurred, on average, for conditioner and probe intervals of ∼2 ms. In the inferior colliculus, 50% recovery occurred at an average separation of ∼7 ms, and in the auditory cortex at ∼20 ms. Despite these increases in average recovery times, some neurons in every structure showed large responses to the probe within the time window for precedence (∼1–4 ms for clicks). This indicates that during the period of the precedence effect, some information about echoes is retained. At the other extreme, for some cortical neurons the conditioner suppressed the probe response for intervals of up to 300 ms. This is in accord with behavioral results that show dominance of the leading sound for an extended period beyond that of the precedence effect. Other transformations as information ascended included an increased variety in the shapes of the recovery functions in structures subsequent to the nerve, and neurons “tuned” to particular conditioner–probe intervals in the auditory cortex. These latter are reminiscent of neurons tuned to echo delay in bats, and may contribute to the perception of the size of the acoustic space.
Evidence for the distortion product frequency place as a source of distortion product otoacoustic emission (DPOAE) fine structure in humans. I. Fine structure and higher-order DPOAE as a function of the frequency ratio106(1999); http://dx.doi.org/10.1121/1.428200View Description Hide Description
Critical experiments were performed in order to validate the two-source hypothesis of distortion product otoacoustic emissions (DPOAE) generation. Measurements of the spectral fine structure of DPOAE in response to stimulation with two sinusoids have been performed with normal-hearing subjects. The dependence of fine-structure patterns on the frequency ratio was investigated by changing or only (fixed or fixed paradigm, respectively), and by changing both primaries at a fixed ratio and looking at different order DPOAE. When is varied in the fixed ratio paradigm, the patterns of fine structure vary considerably more if plotted as a function of than as a function of Different order distortion products located at the same characteristic place on the basilar membrane (BM) show similar patterns for both, the fixed- and paradigms. Fluctuations in DPOAE level up to 20 dB can be observed. In contrast, the results from a fixed- paradigm do not show any fine structure but only an overall dependence of DP level on the frequency ratio, with a maximum for at close to 1.2. Similar stimulus configurations used in the experiments have also been used for computer simulations of DPOAE in a nonlinear and active model of the cochlea. Experimental results and model simulations give strong evidence for a two-source model of DPOAE generation: The first source is the initial nonlinear interaction of the primaries close to the place. The second source is caused by coherent reflection from a re-emission site at the characteristic place of the distortion product frequency. The spectral fine structure of DPOAE observed in the ear canal reflects the interaction of both these sources.
Evidence for the distortion product frequency place as a source of distortion product otoacoustic emission (DPOAE) fine structure in humans. II. Fine structure for different shapes of cochlear hearing loss106(1999); http://dx.doi.org/10.1121/1.428201View Description Hide Description
Distortion product otoacoustic emissions (DPOAE) were recorded from eight human subjects with mild to moderate cochlear hearing loss, using a frequency spacing of 48 primary pairs per octave and at a level and with a fixed ratio Subjects with different shapes of hearing thresholds were selected. They included subjects with near-normal hearing within only a limited frequency range, subjects with a notch in the audiogram, and subjects with a mild to moderate high-frequency loss. If the primaries were located in a region of normal or near-normal hearing, but DP frequencies were located in a region of raised thresholds, the distortion product was still observable, but the DP fine structure disappeared. If the DP frequencies fell into a region of normal thresholds, fine structure was preserved as long as DPOAE were generated, even in cases of mild hearing loss in the region of the primaries. These experimental results give further strong evidence that, in addition to the initial source in the primary region, there is a second source at the characteristic place of Simulations in a nonlinear and active computermodel for DPOAE generation indicate different generation mechanisms for the two components. The disappearance of DPOAE fine structure might serve as a more sensitive indicator of hearing impairment than the consideration of DP level alone.
106(1999); http://dx.doi.org/10.1121/1.428202View Description Hide Description
Auditory event-related potentials (ERPs) to speechsounds were recorded in a demanding selective attention task to measure how the mismatch negativity (MMN) was affected by attention, deviant feature, and task relevance, i.e., whether the feature was target or nontarget type. With vowel-consonant-vowel (VCV) disyllables randomly presented to the right and left ears, subjects attended to the VCVs in one ear. In different conditions, the subjects responded to either intensity or phoneme deviance in the consonant. The position of the deviance within the VCV also varied, being in the first (VC), second (CV), or both (VC and CV) formant-transition regions. The MMN amplitudes were larger for deviants in the attended ear. Task relevance affected the MMNs to intensity and phoneme deviants differently. Target-type intensity deviants yielded larger MMNs than nontarget types. For phoneme deviants there was no main effect of task relevance, but there was a critical interaction with deviance position. The both position gave the largest MMN amplitudes for target-type phoneme deviants, as it did for target- and nontarget-type intensity deviants. The MMN for nontarget-type phoneme deviants, however, showed an inverse pattern such that the MMN for the both position had the smallest amplitude despite its greater spectro-temporal deviance and its greater detectability when it was the target. These data indicate that the MMN reflects differences in phonetic structure as well as differences in acoustic spectral-energy structure of the deviant stimuli. Furthermore, the task relevance effects demonstrate that top-down controls not only affect the amplitude of the MMN, but can reverse the pattern of MMN amplitudes among different stimuli.