Responses of Auditory Nerve and Anteroventral Cochlear Nucleus Fibers to Broadband and Narrowband Noise: Implications for the Sensitivity to Interaural Delays

The quality of temporal coding of sound waveforms in the monaural afferents that converge on binaural neurons in the brainstem limits the sensitivity to temporal differences at the two ears. The anteroventral cochlear nucleus (AVCN) houses the cells that project to the binaural nuclei, which are known to have enhanced temporal coding of low-frequency sounds relative to auditory nerve (AN) fibers. We applied a coincidence analysis within the framework of detection theory to investigate the extent to which AVCN processing affects interaural time delay (ITD) sensitivity. Using monaural spike trains to a 1-s broadband or narrowband noise token, we emulated the binaural task of ITD discrimination and calculated just noticeable differences (jnds). The ITD jnds derived from AVCN neurons were lower than those derived from AN fibers, showing that the enhanced temporal coding in the AVCN improves binaural sensitivity to ITDs. AVCN processing also increased the dynamic range of ITD sensitivity and changed the shape of the frequency dependence of ITD sensitivity. Bandwidth dependence of ITD jnds from AN as well as AVCN fibers agreed with psychophysical data. These findings demonstrate that monaural preprocessing in the AVCN improves the temporal code in a way that is beneficial for binaural processing and may be crucial in achieving the exquisite sensitivity to ITDs observed in binaural pathways.     

Cervical Reflex Induced by Click Stimuli in Cats

We studied click-evoked potentials in the anterior horn of the spinal cord in 17 cats. A concentric needle electrode was inserted into the anterior horn of the spinal cord at levels C3-C6. Potentials evoked with 105 dB SPL clicks were recorded with a peak latency of 4.89-5.10 ms only at the C3 level. These responses were observed 45-60 dB SPL above the auditory brainstem response (ABR) threshold, and no potentials were evoked by stimulation of the contralateral ear. Average was performed 100 times with changes in stimulation frequency of 1-20 Hz. The amplitude of the potentials decreased with increasing stimulus frequency, but there were no changes in ABRs. The responses disappeared after destruction of the medial vestibulospinal tract at the obex level, but ABRs were still recorded. The spinal nucleus of the accessory nerves was located in the anterior horn of the spinal cord at levels C1-C6, and the sternocleidomastoid muscle motoneurons were found at levels C1-C3. The click-evoked potentials recorded in this study reflect responses of the spinal nucleus of accessory nerves through the vestibulospinal tract to click stimulation. The responses have the same characteristics as vestibular-evoked myogenic potentials that can be recorded using surface electrodes over the sternocleidomastoid muscles of humans.     

Effects of Signal Level and Background Noise on Spectral Representations in the Auditory Nerve of the Domestic Cat

Background noise poses a significant obstacle for auditory perception, especially among individuals with hearing loss. To better understand the physiological basis of this perceptual impediment, the present study evaluated the effects of background noise on the auditory nerve representation of head-related transfer functions (HRTFs). These complex spectral shapes describe the directional filtering effects of the head and torso. When a broadband sound passes through the outer ear en route to the tympanic membrane, the HRTF alters its spectrum in a manner that establishes the perceived location of the sound source. HRTF-shaped noise shares many of the acoustic features of human speech, while communicating biologically relevant localization cues that are generalized across mammalian species. Previous studies have used parametric manipulations of random spectral shapes to elucidate HRTF coding principles at various stages of the cat’s auditory system. This study extended that body of work by examining the effects of sound level and background noise on the quality of spectral coding in the auditory nerve. When fibers were classified by their spontaneous rates, the coding properties of the more numerous low-threshold, high-spontaneous rate fibers were found to degrade at high presentation levels and in low signal-to-noise ratios. Because cats are known to maintain accurate directional hearing under these challenging listening conditions, behavioral performance may be disproportionally based on the enhanced dynamic range of the less common high-threshold, low-spontaneous rate fibers.     

Auditory Processing of Spectral Cues for Sound Localization in the Inferior Colliculus

The head-related transfer function (HRTF) of the cat adds directionally dependent energy minima to the amplitude spectrum of complex sounds. These spectral notches are a principal cue for the localization of sound source elevation. Physiological evidence suggests that the dorsal cochlear nucleus (DCN) plays a critical role in the brainstem processing of this directional feature. Type O units in the central nucleus of the inferior colliculus (ICC) are a primary target of ascending DCN projections and, therefore, may represent midbrain specializations for the auditory processing of spectral cues for sound localization. Behavioral studies confirm a loss of sound orientation accuracy when DCN projections to the inferior colliculus are surgically lesioned. This study used simple analogs of HRTF notches to characterize single-unit response patterns in the ICC of decerebrate cats that may contribute to the directional sensitivity of the brain's spectral processing pathways. Manipulations of notch frequency and bandwidth demonstrated frequency-specific excitatory responses that have the capacity to encode HRTF-based cues for sound source location. These response patterns were limited to type O units in the ICC and have not been observed for the projection neurons of the DCN. The unique spectral integration properties of type O units suggest that DCN influences are transformed into a more selective representation of sound source location by a local convergence of wideband excitatory and frequency-tuned inhibitory inputs.     

Effects of the Ovarian and Contraceptive Cycles on Absolute Thresholds,Auditory Fatigue and Recovery from Temporary Threshold Shifts at 4 and 6 kHz

Absolute thresholds at 4 and 6 kHz were tested in three sessions before and after 20 min of exposure to 105 dB(A) pink noise in 12 young normal-cycling females, 11 young females on oral contraceptives and 8 young men. Women on contraceptives showed lower resting thresholds, larger temporary threshold shift (TTS) and higher recovery rates than normal-cycling females. The analysis of resting thresholds, auditory fatigue and recovery from auditory fatigue did not evidence any sex-linked difference. Significant differences linked to the phases of the menstrual cycle and of the contraceptive cycle were observed during recovery from auditory fatigue at 4 kHz and in resting thresholds at 6 kHz. Application of Kendall's coefficient of concordance confirmed these results. Absolute thresholds were highest at menstruation and lowest in the postovulatory phase in normal-cycling females and in women on oral contraceptives, during the days of pill ingestion. Oral contraception is probably a more important factor of change in hearing performance than the phases of the menstrual cycle.     

Relearning Auditory Spectral Cues for Locations Inside and Outside the Visual Field

Previous research has demonstrated that, over a period of weeks, the auditory system accommodates to changes in the monaural spectral cues for sound locations within the frontal region of space. We were interested to determine if similar accommodation could occur for locations in the posterior regions of space, i.e. in the absence of contemporaneous visual information that indicates any mismatch between the perceived and actual location of a sound source. To distort the normal spectral cues to sound location, eight listeners wore small moulds in each ear. HRTF recordings confirmed that while the moulds substantially altered the monaural spectral cues, sufficient residual cues were retained to provide a basis for relearning. Compared to control measures, sound localization performance initially decreased significantly, with a sevenfold increase in front–back confusions and elevation errors more than doubled. Subjects wore the moulds continuously for a period of up to 60 days (median 38 days), over which time performance improved but remained significantly poorer than control levels. Sound localization performance for frontal locations (audio-visual field) was compared with that for posterior space (audio-only field), and there was no significant difference between regions in either the extent or rate of accommodation. This suggests a common mechanism for both regions of space that does not rely on contemporaneous visual information as a teacher signal for recalibration of the auditory system to modified spectral cues.     

Auditory Nerve and Brain Stem Responses to Sound Stimuli at Various Frequencies

The responses of the auditory nerve and brain stem auditory nuclei have been recorded with ear lobe and scalp electrodes in response to unfiltered click stimuli which excite mainly high-frequency nerve fibers. In order to overcome this limitation, normal subjects and patients with hearing loss were stimulated with third-octave filtered clicks with center frequencies of 250, 500 and 1 000 Hz. Analysis of the responses suggests that these lower-frequency stimuli excite lower-frequency cochlear nerve fibers. It is concluded that low-frequency filtered clicks can be used to complement unfiltered clicks in order to obtain a more complete objective audiogram.     

听力正常成年人同时多频听觉稳态诱发反应研究

目的　了解正常听力成人同时多频听觉稳态诱发反应的特性。方法　 32名受试者 (6 4耳 ) ,年龄 2 2～ 32岁 ,平均 2 8.3岁 ,男 12名 ,女 2 0名 ,双耳纯音听阈测试各频率听阈在 2 0dB以内。以载波频率为 0 .5、1、2和 4kHz的纯音 ,调制频率分别为 77、85、93和 10 1Hz(左耳 )以及 79、87、95、10 3Hz(右耳 )的调幅声作为测试信号 ,双耳 8个频率同时给声刺激 ,同时自动判定并记录反应。各载波频率的阈值、反应幅度等数据采用SPSS统计软件分析。结果　①同时多频听觉稳态诱发反应阈高于纯音听阈 7～ 19dB。②反应阈经双因素方差分析 (侧别×频率 ) ,左右耳之间无显著性差异 (F =1.94 2 ;μ 1,179;P =0 .16 5 ) ,但各频率之间有显著性差异 (F =31.2 5 4 ;μ 3,179;P =0 .0 0 0 ) ,各频率反应阈均值之间进一步的两两比较显示 0 .5kHz反应阈与其它频率之间存在显著性差异。③以 6 0dBSPL作为分析强度 ,男女受试者在各个频率的反应幅度的差异经双因素方差分析 (性别×频率 )有显著性差异 (F =2 .94 8;μ 3,175 ;P =0 .0 34) ,0 .5kHz反应幅度最低 ,4kHz最高。性别之间有显著性差异 (F =16 .4 90 ;μ 1,175 ;P=0 .0 0 0 ) ,男性反应幅度高于女性。④各频率不同状态下背景噪声的差异经双因素方差分析 ,提示清醒状     

The Auditory Stimuli to Evoke a clear Average Response at Behavioral Threshold

One of the difficulties in evoked response audiometry is identification of the response at lower stimulus intensities. It was thought that frequency change could evoke the response somewhat independently of the intensity of the tone, and averaged responses evoked by frequency-modulated (FM) tones were studied in order to learn whether this stimulus was suitable as a stimulus for obtaining a clear response at threshold intensity

The typical response evoked by a FM tone, which was presented in a continuous pure tone of 1 000 Hz by changing its frequency at intervals, was characterized by there vertex positive and two negative waves, which were refered to as P₁, P₂, P₃, N₁, and N₂. The P₁, N₁, and P₂ peaks were the most prominent. The latency of the response decreased with both increase in frequency increment and increase in tone intensity. The P₁-N₁ and N₁-P₂ amplitudes increased with increase in frequency increment, and this increase was more conspicuous for tones of lower intensity. Amplitude increment with increasing tone intensity were also observed, but the increments were smallest for the condition of 200 Hz increment because of a large amplitude at O dB SL intensity

This study revealed that an FM tone of 20% increment could evoke a clear discernible response at threshold intensity in all subjects. Thus, the FM tone is very advantageous for evoked response audiometry, although the hearing sensitivity at a very exact frequency is not indicated     

Processing Temporal Modulations in Binaural and Monaural Auditory Stimuli by Neurons in the Inferior Colliculus and Auditory Cortex

Processing dynamic changes in the stimulus stream is a major task for sensory systems. In the auditory system, an increase in the temporal integration window between the inferior colliculus (IC) and auditory cortex is well known for monaural signals such as amplitude modulation, but a similar increase with binaural signals has not been demonstrated. To examine the limits of binaural temporal processing at these brain levels, we used the binaural beat stimulus, which causes a fluctuating interaural phase difference, while recording from neurons in the unanesthetized rabbit. We found that the cutoff frequency for neural synchronization to the binaural beat frequency (BBF) decreased between the IC and auditory cortex, and that this decrease was associated with an increase in the group delay. These features indicate that there is an increased temporal integration window in the cortex compared to the IC, complementing that seen with monaural signals. Comparable measurements of responses to amplitude modulation showed that the monaural and binaural temporal integration windows at the cortical level were quantitatively as well as qualitatively similar, suggesting that intrinsic membrane properties and afferent synapses to the cortical neurons govern the dynamic processing. The upper limits of synchronization to the BBF and the band-pass tuning characteristics of cortical neurons are a close match to human psychophysics.     

Neural Correlates of an Auditory Afterimage in Primary Auditory Cortex

The Zwicker tone (ZT) is defined as an auditory negative afterimage, perceived after the presentation of an appropriate inducer. Typically, a notched noise (NN) with a notch width of 1/2 octave induces a ZT with a pitch falling in the frequency range of the notch. The aim of the present study was to find potential neural correlates of the ZT in the primary auditory cortex of ketamine-anesthetized cats. Responses of multiunits were recorded simultaneously with two 8-electrode arrays during 1 s and over 2 s after the presentation of a white noise (WN) and three NNs differing by the width of the notch, namely, 1/3 octave (NN1), 1/2 octave (NN2), and 2/3 octave (NN3). Both firing rate (FR) and peak cross-correlation coefficient () were evaluated for time windows of 500 ms. The cortical units were grouped according to whether their characteristic frequency (CF) was inside (In neurons) or outside (Out neurons) a 1-octave-wide frequency band centered on the notch center frequency. The ratios between the FRs and the s for each NN and the WN condition and for each group of neurons were then statistically evaluated. The ratios of FRs were significantly increased during and after the presentation of the NN for the In neurons. In contrast, the changes for the Out neurons were small and most often insignificant. The ratios of the values differed significantly from 1 in the In–In and In–Out groups during stimulation as well as after it. We also found that the s of Out neurons were dependent on the type of NN. Potentially, a combination of increased and increased FR might be a neurophysiological correlate of the ZT.     

High Spectral and Temporal Acuity in Primary Auditory Cortex of Awake Cats

Journal of the Association for Research in Otolaryngology - Most accounts of single- and multi-unit responses in auditory cortex under anesthetized conditions have emphasized V-shaped frequency...     

单频和多频刺激骨导听性稳态反应的比较

目的测试听力正常青年人单频及多频刺激骨导听性稳态反应(BC-ASSR)阈值,比较两者是否有差异。方法选取听力正常青年人28例(56耳),进行骨导ASSR及纯音听阈测试,记录0.5、1、2、4kHz单频刺激及多频刺激的BC-ASSR反应阈及纯音听阈,比较两种刺激方式所得结果。结果在0.5、1、2、4kHz频率单频刺激骨导的ASSR反应阈分别为53、47、53、51dB SPL,多频刺激骨导ASSR反应阈分别为59、54、63、61dB SPL,两者间有高度显著性差异(P<0.01),且高频处的差异相对较大。结论BC-ASSR测试时,单频刺激和多频刺激方法所得听阈值存在一定的差异,且高频处的差异相对较大,因而临床应用测试时宜采用单频刺激方式。     

Auditory Nerve Fiber Responses to Combined Acoustic and Electric Stimulation

Persons with a prosthesis implanted in a cochlea with residual acoustic sensitivity can, in some cases, achieve better speech perception with “hybrid” stimulation than with either acoustic or electric stimulation presented alone. Such improvements may involve “across auditory-nerve fiber” processes within central nuclei of the auditory system and within-fiber interactions at the level of the auditory nerve. Our study explored acoustic–electric interactions within feline auditory nerve fibers (ANFs) so as to address two goals. First, we sought to better understand recent results that showed non-monotonic recovery of the electrically evoked compound action potential (ECAP) following acoustic masking (Nourski et al. 2007, Hear. Res. 232:87–103). We hypothesized that post-masking changes in ANF temporal properties and responsiveness (spike rate) accounted for the ECAP results. We also sought to describe, more broadly, the changes in ANF responses that result from prior acoustic stimulation. Five response properties—spike rate, latency, jitter, spike amplitude, and spontaneous activity—were examined. Post-masking reductions in spike rate, within-fiber jitter and across-fiber variance in latency were found, with the changes in temporal response properties limited to ANFs with high spontaneous rates. Thus, our results suggest how non-monotonic ECAP recovery occurs for ears with spontaneous activity, but cannot account for that pattern of recovery when there is no spontaneous activity, including the results from the presumably deafened ears used in the Nourski et al. (2007) study. Finally, during simultaneous (electric+acoustic) stimulation, the degree of electrically driven spike activity had a strong influence on spike rate, but did not affect spike jitter, which apparently was determined by the acoustic noise stimulus or spontaneous activity.     

Neural and Behavioral Sensitivity to Interaural Time Differences Using Amplitude Modulated Tones with Mismatched Carrier Frequencies

Bilateral cochlear implantation is intended to provide the advantages of binaural hearing, including sound localization and better speech recognition in noise. In most modern implants, temporal information is carried by the envelope of pulsatile stimulation, and thresholds to interaural time differences (ITDs) are generally high compared to those obtained in normal hearing observers. One factor thought to influence ITD sensitivity is the overlap of neural populations stimulated on each side. The present study investigated the effects of acoustically stimulating bilaterally mismatched neural populations in two related paradigms: rabbit neural recordings and human psychophysical testing. The neural coding of interaural envelope timing information was measured in recordings from neurons in the inferior colliculus of the unanesthetized rabbit. Binaural beat stimuli with a 1-Hz difference in modulation frequency were presented at the best modulation frequency and intensity as the carrier frequencies at each ear were varied. Some neurons encoded envelope ITDs with carrier frequency mismatches as great as several octaves. The synchronization strength was typically nonmonotonically related to intensity. Psychophysical data showed that human listeners could also make use of binaural envelope cues for carrier mismatches of up to 2–3 octaves. Thus, the physiological and psychophysical data were broadly consistent, and suggest that bilateral cochlear implants should provide information sufficient to detect envelope ITDs even in the face of bilateral mismatch in the neural populations responding to stimulation. However, the strongly nonmonotonic synchronization to envelope ITDs suggests that the limited dynamic range with electrical stimulation may be an important consideration for ITD encoding.     

单频和多频刺激的多频稳态反应比较

目的　了解单频与多频同时刺激方式的多频稳态反应阈值是否有差异。方法　分别对 30例听力正常的青年人进行纯音听阈测试 ,并采用单频和多频同时刺激 ,记录 0 .5、1、2、4kHz四个频率多频稳态反应的测试结果 ,并对结果进行统计学处理。结果　听力正常青年人单频与多频刺激的多频稳态反应阈值除 0 .5kHz处有显著差异 (P <0 .0 0 1)外 ,1、2、4kHz三个频率均无显著差异 (P >0 .0 5 ) ,两种刺激方式下得到的反应阈与纯音听阈的差距均在 15～ 2 5dB。结论　多频稳态反应有较好的频率特性 ,单频刺激与多频刺激两种测试方式所得到的反应阈相近 ,但临床应用时建议在 0 .5kHz处采用单频刺激记录     

Slow Cortical Evoked Potentials: Interactions of Auditory,Vibro-Tactile and Shock Stimuli

An interaction experiment, in which stimuli of one kind were interposed between stimuli of another variety, was performed on 6 subjects with 8 replications. Cortical vertex potentials evoked by shocks to the median nerve, by vibrotactile stimulation of a finger or by 2400 Hz filtered clicks were averaged by a computer. The inter-stimulus interval was 5 sec. Then stimuli of two different kinds were presented alternately at 2.5 sec intervals. The responses for each kind were averaged separately. The amplitudes, measured from N₁ to P₂, were compared for all combinations of the three modalities.

Cross-modality depression of response occurred (10% level of confidence) for tactile responses with interposed auditory stimuli and (1 % level of confidence) for shock responses with interposed tactile stimuli. A general trend toward depression was clear, with an overall average of 15%, although it was not consistent for all subjects or symmetrical among types of stimuli. This compares with 28% depression when the inter-stimulus interval for identical stimuli was reduced from 5 sec to 2.5 sec.

Resume

Une expérience d'interactions, dans laquelle des stimuli d'un certain mode sensoriel étaient intercalés entre des stimuli d'un mode différent, a été effectuée sur six sujets avec huit répétitions. Un appareil à mémoires magnétiques a calculé les moyennes des potentiels corticaux du vertex, évoqués par des chocs appliqués sur le nerf médian, par une stimulation vibro-tactile d'un doigt ou par des clicks filtrés de 2 400 Hz. L'intervalle entre les stimuli successifs était de 5 secondes. Par la suite, les stimuli de deux modes différents alternaient en intervalles de 2,5 secondes. Les moyennes de réponses pour chacun de ces modes étaient séparemment calculées. Les amplitudes, mesurées de N₁ à P₂, étaient comparées dans toutes les combinaisons des trois modalités.

Une diminution transmodale de réponses se produisait pour les réponses tactiles avec des stimuli auditifs intercalés (significatif à un niveau de 10%) et pour des réponses aux chocs avec des stimuli tactiles intercalés (significatif à un niveau de 1%). Une tendance générate de diminution, ayant une grande moyenne de 15%, se manifestait clairement, bien qu'elle n'aie été soutenue par tous les sujets et ne s'avéraˇt pas symétrique parmi les types de stimuli. Par contre, en réduisant l'intervalle entre les stimuli successifs de 5 à 2,5 secondes, le taux de diminution était de 28%.     

Electrical Excitation of the Acoustically Sensitive Auditory Nerve: Single-Fiber Responses to Electric Pulse Trains

Nearly all studies on auditory-nerve responses to electric stimuli have been conducted using chemically deafened animals so as to more realistically model the implanted human ear that has typically been profoundly deaf. However, clinical criteria for implantation have recently been relaxed. Ears with “residual” acoustic sensitivity are now being implanted, calling for the systematic evaluation of auditory-nerve responses to electric stimuli as well as combined electric and acoustic stimuli in acoustically sensitive ears. This article presents a systematic investigation of single-fiber responses to electric stimuli in acoustically sensitive ears. Responses to 250 pulse/s electric pulse trains were collected from 18 cats. Properties such as threshold, dynamic range, and jitter were found to differ from those of deaf ears. Other types of fiber activity observed in acoustically sensitive ears (i.e., spontaneous activity and electrophonic responses) were found to alter the temporal coding of electric stimuli. The electrophonic response, which was shown to greatly change the information encoded by spike intervals, also exhibited fast adaptation relative to that observed in the “direct” response to electric stimuli. More complex responses, such as “buildup” (increased responsiveness to successive pulses) and “bursting” (alternating periods of responsiveness and unresponsiveness) were observed. Our findings suggest that bursting is a response unique to sustained electric stimulation in ears with functional hair cells.     

How Visual Cues for when to Listen Aid Selective Auditory Attention

Visual cues are known to aid auditory processing when they provide direct information about signal content, as in lip reading. However, some studies hint that visual cues also aid auditory perception by guiding attention to the target in a mixture of similar sounds. The current study directly tests this idea for complex, nonspeech auditory signals, using a visual cue providing only timing information about the target. Listeners were asked to identify a target zebra finch bird song played at a random time within a longer, competing masker. Two different maskers were used: noise and a chorus of competing bird songs. On half of all trials, a visual cue indicated the timing of the target within the masker. For the noise masker, the visual cue did not affect performance when target and masker were from the same location, but improved performance when target and masker were in different locations. In contrast, for the chorus masker, visual cues improved performance only when target and masker were perceived as coming from the same direction. These results suggest that simple visual cues for when to listen improve target identification by enhancing sounds near the threshold of audibility when the target is energetically masked and by enhancing segregation when it is difficult to direct selective attention to the target. Visual cues help little when target and masker already differ in attributes that enable listeners to engage selective auditory attention effectively, including differences in spectrotemporal structure and in perceived location.