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PubMed1991

Auditory image perception and analysis: the basis for hearing.

The premise of this paper is that the auditory system's primary function is its ability to determine the sources of sound. Auditory image perception and analysis are defined as the basis for sound source determination. Few studies in the literature have focused on understanding these abilities and the paper argues that more attention should be paid to auditory image perception and analysis. Four questions are posed for understanding auditory image formation and seven physical variables are described which might be used for auditory image perception. The paper relates auditory image perception and analysis to a number of other topics in the hearing sciences in order to reinforce the argument that auditory image perception and analysis are the basis of hearing.

Acoustic Stimulation↗
PubMed1991

Aural intensity for a moving source.

Considerable, highly specific information is available to the auditory system concerning the trajectory of a moving sound source. This paper delineates the set of stimuli that motion-sensitive systems might use. General expressions for the sound intensity, the interaural intensity difference, and their first time derivatives, are derived for a source moving along an arbitrary trajectory. The general expressions are then made explicit for three special cases of motion of an omnidirectional constant level source: a source moving directly away from or toward the observer, a source moving around the observer's head, and a source moving in a straight line across the auditory field of the observer. The later special case combine characteristics of the two first ones. The functions are plotted and their characteristics compared. The combination of all four functions provides a unique signature for each source trajectory. The first time derivative of the monaural spectrum level function is found to be directly proportional to the velocity scaled by the distance of the source for omnidirectional sources of constant intensity. This makes the first time derivative of the spectrum level especially attractive as a component of a specialized source detection system in the brain.

Auditory Perception↗
PubMed1993

Human auditory cortical mechanisms of sound lateralization: I. Interaural time differences within sound.

Neuromagnetic responses to 600-ms binaural click trains, presented once every 1.1 s, were recorded with a 24-channel gradiometer from 6 healthy humans. During the first 300 ms, the left-ear stimulus led the right by 0.7 ms and the sound was lateralized to the left ear. At 300 ms, the interaural time difference (ITD) changed and the lateralization moved to one of 5 different locations between the ears. An N100m response peaked about 110 ms after the sound onset and an N130mc response (c to stress a response to the change) about 135 ms after the ITD change. The source locations of N100m and N130mc agreed with activity in the supratemporal auditory cortex; this was confirmed in one subject by superimposing MEG results on MR images. The sources of N100m and N130mc did not differ statistically significantly from each other, nor were there differences in N130mc sources to various lateralization changes. N130mc grew larger when the ITD change increased, in parallel with the increase in the change of the perceived location. We suggest that N130mc is analogous to N100m, but is delayed due to postmasking induced by the early part of the sound.

Acoustic Stimulation↗
PubMed1993

Human auditory cortical mechanisms of sound lateralization: II. Interaural time differences at sound onset.

Neuromagnetic responses were recorded over the right temporal cortex using a 24-channel gradiometer. Stimuli were binaural click trains, presented with six separate interaural time differences (ITDs). N100m to sound onset was larger and earlier for stimuli presented with left- than with right-leading ITDs. With stimulus lateralization taken into account, monaural and binaural stimuli evoked responses of roughly equal amplitude. In selective adaptation and oddball experiments, stimuli presented with different ITDs excited overlapping neuronal populations, but the amount of overlap decreased as the ITD between the stimuli increased. There were no systematic differences in the cortical source locations of the N100m as a function of ITD, however. Thus it appears that ITD-sensitive neurons in the human auditory cortex are not organized into a large-scale, orderly representation, which could be resolved by MEG.

Acoustic Stimulation↗
PubMed1995

Recovery of forward-masked responses in ventral cochlear nucleus neurons.

Single unit responses were obtained from 8 classes of cells in the ventral cochlear nucleus: Primarylike, Primarylike with characteristic frequencies below 1 kHz, Primarylike-Notch, Sustained Chopper, Transient-Chopper, Low-intensity Chopper; Onset with later activity and On-Chopper. Stimuli were paired tonebursts, a masker preceding a probe, separated by time delta t ms. The decrement in discharge rate to the probe was measured as a function of delta t and constituted the forward-masking recovery function. The recovery functions of primarylike units were similar to those reported for auditory nerve fibers, but recovered more slowly than all other classes of units in the ventral cochlear nucleus. Some units, such as onset units, were completely masked at short masker-probe intervals, while others, such as the low-intensity choppers, were less affected by the masker. More masking occurred in the first 2 ms of the response (onset rate) than in the overall response (average rate). Using shorter maskers and measuring the onset rate produced greater differences in masking functions between unit classes. Units with high spontaneous activity were more resistant to the effects of the masker than units with low and medium spontaneous activity. This was especially evident at high masker levels and short masker-probe intervals. Units other than primarylike often showed non-monotonic relationships between the firing rate evoked by the masker and the firing rate decrement in response to the probe, suggesting that both adaptation and inhibition are operating to produce the observed effects.

Acoustic Stimulation↗
PubMed1995

Brainstem lesions and click lateralization in patients with multiple sclerosis.

The ability to lateralize dichotic clicks with either interaural time delays (ITD) or interaural level differences (ILD) was tested in seven multiple sclerosis (MS) subjects who had normal audiograms. Along with the psychoacoustical tests, magnetic resonance images (MRI) of the subjects' brainstem were obtained. After matching each MRI section with the corresponding section of a computerized atlas of the brainstem, the parts of the auditory pathway affected by each MS lesion were determined. Of the seven subjects two performed normally with both types of interaural asymmetry and had no brainstem lesions involving the auditory pathway. Two subjects performed normally only with level differences, but perceived all the dichotic clicks with different ITDs in the center of the head; both had lesions involving the trapezoid body. Three subjects could not perform normally with either task, perceiving the clicks to the sides and never in the center for both ITDs and ILDs; all three had unilateral lesions of the lateral lemniscus. A multi-level decision making model is proposed to account for these results.

Audiometry↗
PubMed1995

Three-channel Lissajous' trajectory of the binaural interaction components of human auditory middle-latency evoked potentials.

Three-channel Lissajous' trajectories (3-CLTs) of the binaural interaction component (BI) of auditory middle latency evoked potentials (AMLEPs) were derived from 14 normally hearing adults by subtracting the response to binaural clicks from the algebraic sum of monaural responses. AMLEPs were recorded in response to 65 dB nHL, rarefaction clicks, presented at a rate of 3.3/s. A normative set of BI 3-CLT measures was calculated and compared with the corresponding measures of simultaneously recorded, single-channel vertex-left mastoid and vertex-neck derivations of BI and of AMLEP to binaural stimulation (B). 3-CLT measures included: apex latency, amplitude and orientation, as well as planar segment duration, orientation, size and shape. The results showed seven main apices and associated planar segments ('Be', 'Bf', 'Bg', 'Bh', 'Bi1', 'Bi2' and 'Bj') in the 3-CLT of BI. Apex latencies of the BI 3-CLT were comparable to peak latencies of the vertex-left mastoid and vertex-neck AMLEP and BI records, both in their absolute values and in intersubject variability. Durations of BI planar segments were approximately 5.0 ms. Apex amplitudes of BI 3-CLT were larger than the respective peak amplitudes of the vertex-mastoid and vertex-neck BI records, while their intersubject variabilities were comparable. The lateralization of BI components may indicate asymmetric processing of binaural auditory input, or may be connected with anatomical asymmetry such as skull thickness. Preliminary analyses did not reveal a clear correlation between the lateralization of the BI component 'Bi2' and the handedness of the subject. We suggest that BI components of AMLEP may be associated with the primary auditory cortex and subcortical ascending structures.

Acoustic Stimulation↗
PubMed1994

Responses of inferior colliculus neurons in C57BL/6J mice with and without sensorineural hearing loss: effects of changing the azimuthal location of a continuous noise masker on responses to contralateral tones.

Extracellular recordings were obtained from inferior colliculus neurons of young adult (2-month-old) C57 mice with normal hearing and middle-aged (6-month-old) C57 mice with sensorineural hearing loss as they responded to best frequency (BF) tones (signal) in the presence of a continuous background noise (masker). Rate/level functions were obtained for the signal alone, noise bursts alone, and the signal in continuous noise as a function of masker location. For both groups of mice, thresholds for BF tones were significantly elevated in the presence of noise at all three noise locations. Separating the signal and masker sources significantly improved masked tone thresholds of 2-month-old mice but not hearing-impaired mice. The decreased ability of middle-aged mice to benefit from separation of the signal and masker sources may reflect alterations in binaural processing as a result of sensorineural hearing loss.

Acoustic Stimulation↗
PubMed1994

Pure-tone masking profiles for human auditory brainstem and middle latency responses to 500-Hz tones.

A simultaneous masking paradigm was used to determine the frequency selectivity of human auditory brainstem (ABR) and middle latency (MLR) responses to 60 dB pe SPL 500-Hz probe tones in 12 normal adults. Masking profiles for simultaneous recordings of the ABR and MLR were obtained in the presence of pure-tone maskers presented at 60- and 70-dB SPL. Results show sharp amplitude profiles with maximum reduction in amplitude seen using the 500-Hz maskers. There were no significant differences in the masking profiles for the ABR and MLR waves to the 500-Hz probe tones. An additional measure of frequency selectivity, bandwidth at 50% reduction in amplitude (W50), also demonstrated no significant difference between the ABR and MLR waves. In summary, the results of this study and those of an earlier study (Mackersie et al., 1993) suggest no significant difference in the frequency selectivity of the ABR and MLR to low-intensity (60 dB pe SPL) 500- and 2000-Hz tones.

Acoustic Stimulation↗
PubMed1994

Effect of interaural time differences on middle-latency and late auditory evoked magnetic fields.

To determine if interaural time differences (ITDs) in binaural stimuli affect the middle-latency auditory evoked fields (AEFs) in the same manner as they affect the N100m deflection, neuromagnetic responses were recorded over the whole head using a 122-channel SQUID magnetometer. Binaural stimuli were lateralized to three positions, left, midline, and right, on the basis of ITDs. The N100m was significantly larger to stimuli with contralaterally-leading ITDs than to stimuli with no, or with ipsilaterally-leading ITDs. Neither the P30m nor the P50m deflections of the middle-latency response were significantly affected by ITD, although the P30m showed a tendency, similar to but smaller than that of N100m, to be larger to stimuli with contralaterally-leading ITDs. In some subjects, the source location of the P50m was anterior and inferior to the sources of the P30m and N100m, which are generated in the superior surface of the temporal lobe. Sound-related muscular artifacts were seen in the posterior recording channels of one subject, and the contribution of this activity to the signals over the temporal area was determined.

Acoustic Stimulation↗
PubMed1994

Sound calibration and distortion product otoacoustic emissions at high frequencies.

Distortion product otoacoustic emissions offer the potential for assessing inner ear function at high frequencies. However, commonly employed methods for calibrating the acoustic system used in these studies can lead to errors of +/- 20 dB or more in the estimate of eardrum sound pressure levels above 2-3 kHz [Siegel, J. Acoust. Soc. Am. 95, 2589-2597 (1994)]. We assessed the magnitude of these errors by measuring the distortion product emission 2f1-f2 (f1 < f2) in human subjects using either of two microphone locations to calibrate the stimulus. Either the emission probe microphone itself or a probe tube positioned near the eardrum were used in calibrations. The emissions collected with f1 in the vicinity of 5-7 kHz showed a pronounced peak in level relative to other stimulus frequencies when the emission probe was used for calibration. The peak at 5-7 kHz disappeared when the probe tube near the eardrum was used for calibration. The discrepancy in emission levels between the two calibration procedures was as large as 20 dB. The difference is attributed to the presence of standing waves in the ear canal. The systematic errors in estimating eardrum sound pressure level using the emission probe microphone undoubtedly contribute to the variability of emission measurements for high-frequency stimuli.

Acoustic Stimulation↗
PubMed1994

Modulation detection interference: across-frequency processing and auditory grouping.

Modulation Detection Interference (MDI) is the loss of sensitivity in processing amplitude modulation of a probe tone when a masker is similarly modulated. MDI was measured in four experiments to investigate two past claims concerning MDI: 1) That MDI represents across-spectral processing, and 2) that MDI is the consequence of the auditory system using common patterns of amplitude modulation to group spectral components into a single auditory source. Experiment I studied MDI when the envelope phase of the masker and probe modulators were different and was used to address the issue of the extent to which MDI is a consequence of spectral grouping based on common amplitude modulation. Measures of MDI for conditions in which the frequency separation between the probe and masker carriers was varied (Experiment II), estimates of modulation depth discrimination (Experiment III), and signal detection thresholds for brief sinusoidal signals masked by amplitude modulated tones (Experiment IV) were all used to address issues related to across-spectral processing of amplitude modulation. The conclusions of these studies is that MDI is largely an across-frequency phenomenon and that the role of auditory grouping based on a common pattern of modulation can not be ruled out as having a relationship to MDI.

Acoustic Stimulation↗
PubMed1994

Responses of inferior collicular neurons of the FM bat, Eptesicus fuscus, to pulse trains with varied pulse amplitudes.

Under free field stimulation conditions, we studied the responses of inferior collicular neurons of the FM bat, Eptesicus fuscus, to pulse trains with varied pulse amplitudes. Each pulse train consisted of 7 pulses of 4 ms delivered at 24 ms interpulse-intervals (i.e. 42 pulses/s). For a control pulse train, all pulse amplitudes were equal to a neuron's best amplitude which, when delivered in single pulses, elicited maximal number of impulses from the neuron. The amplitudes of individual pulses of the remaining pulse trains were linearly increased or decreased at a slope of 0, 14, 28, 42, 56 and 69 dB/s. All 56 inferior collicular neurons discharged to pulse trains were of two main types. Type I (N43, 77%) neurons discharged to each pulse within a train while type II (N11, 20%) neurons discharged to the first pulse of a train stimulus only. Discharge patterns of the remaining (N2, 3%) neurons changed between type I and type II when stimulated with different pulse trains. The number of impulses discharged by a neuron varied with different pulse trains. In addition, the number of impulses discharged to each pulse by type I neurons also varied among individual pulses within the train. Only 14 neurons (25%) discharged maximally to the control pulse train. Responses of the remaining neurons to other pulse trains were either 30%-120% larger than (N17, 30%) or within 30% (N25, 45%) of the control pulse train response. Furthermore, half of 56 neurons selectively discharged to a most preferred pulse train with a response magnitude which was at least 50% larger than the response to the least preferred pulse train. Possible mechanisms underlying the different discharge patterns are discussed in terms of a neuron's recovery cycle, minimum threshold and inhibitory period relative to the temporal characteristics (pulse repetition rate and amplitude) of the pulse trains.

Acoustic Stimulation↗
PubMed1994

Human auditory cortical mechanisms of sound lateralisation: III. Monaural and binaural shift responses.

Neuromagnetic responses were recorded over the whole head with a 122-channel gradiometer. A pair of 150-ms 1-kHz tones separated by an interval of 150 ms was presented to one ear every 2 s. The other ear received either no input, an identical pair simultaneous to the first, an identical pair alternating with the first or a continuous 600-ms tone. The 'monaural shift' condition in which stimuli alternated between ears produced a clear perception of changing lateralisation, but the evoked response could be explained as merely the sum of simple monaural onset and offset responses; thus we found no evidence for a separate response to interaural intensity difference in this condition. The 'binaural shift' condition, in which intensity changed in one ear while the other received a continuous tone, evoked a transient response (N130m) at a latency of about 130 ms. N130m was larger over the hemisphere contralateral to the direction of shift, and larger than the corresponding monaural response, whether to an onset or an offset. We concluded that N130m also was not a separate directional response, but was analogous to a simple monaural response, the prolonged latency being due to masking and the enhanced amplitude to facilitation by the sustained response to the continuous tone.

Adult↗
PubMed1994

Representation of interaural level difference in the VLVp, the first site of binaural comparison in the barn owl's auditory system.

In the avian auditory system, the posterior division of the ventral nucleus of the lateral lemniscus (VLVp) is the first site where the levels of sound arriving at the two ears are compared. VLVp units are excited by sound at the contralateral ear and are inhibited by sound at the ipsilateral ear, and, as a result, are sensitive to interaural level differences (ILD). In this study, we investigate the functional properties of VLVp units and describe the topography of ILD sensitivity along the dorsoventral dimension of this nucleus. The responses of VLVp units were tested with monaural and binaural noise delivered through earphones. Excitatory and inhibitory responsiveness was quantified using several measures that assessed the effect of contra-ear stimulation and the effect of ipsi-ear stimulation on the contra-ear response. On the basis of these measures, we characterize the map of ILD sensitivity in the VLVp. The temporal pattern of unit responses were also analyzed. The discharges of VLVp units were regular and time-locked to the onset of a stimulus, a pattern of discharge reminiscent of the 'chopper pattern' observed in the lateral superior olive (LSO) of mammals. The temporal discharge patterns of a single VLVp neuron often distinguished between equivalent ILDs, resulting from different combinations of contra- and ipsi-ear levels, that were not distinguished by spike count alone. However, the temporal response pattern did not distinguish between all such combinations of contra- and ipsi-ear levels. The additional information was encoded by the pattern of activity across the entire population of VLVp neurons. This study describes similarities in the functional properties of VLVp and LSO units that suggest similar physiological mechanisms in avians and mammals for encoding similar acoustic information.

Acoustic Stimulation↗
PubMed1995

Acoustic features and acoustic changes are represented by different central pathways.

The central processing of acoustic stimulus changes can be observed neurophysiologically in the mismatch negativity auditory evoked potential (MMN). Stimuli differing in interaural phase were used to investigate the contributions of the primary and non-primary auditory pathways to the encoding of binaural stimuli and to investigate passively elicited measures of binaural processing in experimental animals. In guinea pigs, the MMN was obtained in response to 1000 Hz tones embedded in white noise (S:N = 2 dB). Using a modified oddball paradigm (that is, two stimuli presented in a series, each with a different probability of occurrence), stimuli were presented binaurally with both the tone and noise in-phase to the two ears (S0N0) as the standard stimulus ans the tone 180 degrees out-of-phase (S(PI)N0) as the deviant stimulus. The MMN, by definition, should occur only in response to a change, or 'mismatch,' between the standard and deviant stimuli. The response to the deviant stimulus in the oddball paradigm was compared to the response to the same stimulus when presented in a series alone. The responses to S0N0 and S(PI)N0 collected in a series alone, termed the intrinsic responses, were also compared. Responses were recorded from two surface epidural electrodes - one at the posterior midline and one over the left temporal lobe. AEPs from these locations have been shown to reflect the activity of primary and non-primary thalamo-cortical pathways respectively. A significant MMN was observed at the midline electrode, but no MMN was observed over the temporal lobe.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1995

Distortion-product otoacoustic emissions and their anaesthesia sensitivity in the European starling and the chicken.

The aim of the present experimental series was to provide further information on the distortion-product otoacoustic emissions (DP) of birds and contribute to a general understanding of DP generation. Basic characteristics of the DP 2f1-f2 and 2f2-f1 were measured in the ear canal of both awake and anaesthetized European Starlings and chickens. The effect of a third suppressive tone and the behaviour of the DP under anaesthesia were also studied. In general, the DP characteristics of both bird species resembled those of lizards and mammals, but first appeared at somewhat higher primary-tone levels. The best frequencies of third tones suppressing 2f1-f2 lay near the first primary tone (f1), but for 2f2-f1, the situation was more complex. Facilitation via a third tone was also seen for both DP, often at levels below those eliciting suppression. The DP 2f1-f2 disappeared completely at the onset of deep anaesthesia and recovered to its original magnitude when the anaesthesia was lightened, sometimes with a considerable delay. The compound action potential (CAP) was somewhat more sensitive to anaesthesia than the DP. Control experiments showed that the anaesthesia effect was not a result of hypoxia. Avian DP at low and intermediate sound levels are thus physiologically-sensitive manifestations of normal hair-cell function that are, in contrast to mammals, also anaesthesia-sensitive.

Acoustic Stimulation↗
PubMed1995

The effect of contralateral stimulation on cochlear resonance and damping in the mustached bat: the role of the medial efferent system.

In the unanesthetized mustached bat, stimulation of the ear with an acoustic transient produces damped oscillations which are evident in the cochlear microphonic potential. In this report we demonstrate how the decay time of these oscillations is affected by broadband noise presented to the contralateral ear (CLN). In the absence of CLN, the mean decay time was 1.94 +/- 0.23 ms, but during the presentation of CLN the decay time consistently decreased. The changes were finely graded, the higher the CLN, the greater the change. The effect could be maintained at a constant level for extended periods of time and this was evident when the CLN exceeded 40 dB SPL. The latency of the reflex for 64 dB noise was about 11 ms and near maximum changes occurred within 15 ms of CLN onset. Sectioning medial efferent nerve fibers in the floor of the fourth ventricle or the administration of a single dose of gentamicin eliminated changes produced by CLN. The prominence of CM responses to damped oscillations and the robust changes in response to CLN make the mustached bat an excellent model for studying the influence of the medial efferent system on cochlear mechanics.

Acoustic Stimulation↗