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PubMed2005

Neural population coding of sound level adapts to stimulus statistics.

Mammals can hear sounds extending over a vast range of sound levels with remarkable accuracy. How auditory neurons code sound level over such a range is unclear; firing rates of individual neurons increase with sound level over only a very limited portion of the full range of hearing. We show that neurons in the auditory midbrain of the guinea pig adjust their responses to the mean, variance and more complex statistics of sound level distributions. We demonstrate that these adjustments improve the accuracy of the neural population code close to the region of most commonly occurring sound levels. This extends the range of sound levels that can be accurately encoded, fine-tuning hearing to the local acoustic environment.

Acoustic Stimulation↗
PubMed2005

Pure tone audiograms and possible aminoglycoside-induced hearing loss in belugas (Delphinapterus leucas).

A behavioral response paradigm was used to measure pure-tone hearing sensitivities in two belugas (Delphinapterus leucas). Tests were conducted over a 20-month period at the Point Defiance Zoo and Aquarium, in Tacoma, WA. Subjects were two males, aged 8-10 and 9-11 during the course of the study. Subjects were born in an oceanarium and had been housed together for all of their lives. Hearing thresholds were measured using a modified up/down staircase procedure and acoustic response paradigm where subjects were trained to produce audible responses to test tones and to remain quiet otherwise. Test frequencies ranged from approximately 2 to 130 kHz. Best sensitivities ranged from approximately 40 to 50 dB re 1 microPa at 50-80 kHz and 30-35 kHz for the two subjects. Although both subjects possessed traditional "U-shaped" mammalian audiograms, one subject exhibited significant high-frequency hearing loss above 37 kHz compared to previously published data for belugas. Hearing loss in this subject was estimated to approach 90 dB for frequencies above 50 kHz. Similar ages, ancestry, and environmental conditions between subjects, but a history of ototoxic drug administration in only one subject, suggest that the observed hearing loss was a result of the aminoglycoside antibiotic amikacin.

Acoustic Stimulation↗
PubMed1990

Neural responses to auditory temporal patterns.

Sets of regularly repeating auditory stimuli elicit unique perceptions; listeners are able to identify specific temporal patterns. Some temporal patterns are unambiguous (only one pattern can be perceived), while others are ambiguous (numerous patterns can be detected). While the psychophysical properties of such percepts have been well studied, little is known about the underlying neurological bases of temporal pattern perception. In this experiment, the role of adaptation in temporal pattern perception is examined by studying neural responses in four cats to a temporal pattern that is perceptually unambiguous and one that is perceptually ambiguous. Measurements were made of the whole-nerve action potential, the auditory brainstem response, and potentials from the surface of the primary auditory cortex. The adaptation patterns corresponded with the perceptual organization of temporal patterns in humans at all levels of the nervous system studied.

Animals↗
PubMed1994

A perceptual study of source coding of Fourier phase and amplitude of the linear predictive coding residual of vowel sounds.

A practical question in a Fourier transform coding of speech signals is to what accuracy their amplitude and phase spectra have to be represented without perceptible distortions. In this paper a concern is with the audibility of quantization noise signals which are produced by quantizing the amplitude and phase spectra of vowel sounds. Experiments show that the detection of the noise targets with maskers of a low fundamental frequency is determined mainly by the sharpest temporal resolution of the auditory system in the high-frequency region. For maskers of a high fundamental frequency the detection is determined mainly by the sharpest spectral resolution in the low-frequency region. Noise targets with global random phase and amplitude are relatively more difficult to detect than those produced by a local randomization. Local random phase noise targets are generally more detectable than those produced by local amplitude randomization. The relative importance of phase and magnitude spectra in the Fourier transform coding is strongly dependent on the fundamental frequency of the vowel sounds and the window size used in the short-time Fourier analysis.

Auditory Perception↗
PubMed1986

[Experiences with the implantation of a multichannel electrode in the acoustic nerve].

The authors developed a surgical approach to the acoustic nerve enabling the introduction of an electrode into the acoustic nerve. A multichannel electrode was implanted by this method in a deaf patient. The receiver casing for percutaneous transmission was fixed in the mastoid. Encouraging hearing results were obtained over a period of two months by electrical stimulation of the acoustic nerve.

Auditory Perception↗
PubMed1980

[Responses of cat caudate nucleus neurons to acoustic stimulation].

Reactions of 141 cat caudate nucleus neurons to auditory signals--tone (500 and 2000 Hz) and clicks with different frequency (0.2 and 0.8/s) and intensity (75, 80, 95 dB) were recorded extracellularly. Neuronal reactions were highly variable as to the character of responses (phasis, tonic), their structure (one or two phases of activation), latent periods (from 7.5 to 300 ms), discharge frequency in burst responses (from 90 to 800 imp/s). Analysis of the averaged poststimulus histograms and plots of the dynamic reaction modifications showed that in most units (74%) the responses became considerably better expressed in case of a decreased stimulus frequency: the response regularity and the number of spikes in each response rose. The neuronal reactions also increased and acquired more distinct time structure with an increase in the stimulus intensity, 17% of recorded units showed qualitative differences in the character of neuronal reactions to clicks and tones: phasic activation arosed to clicks and tonic change of pulse activity occurred to tones. Peculiarities of the caudate nucleus neuronal reactions to auditory stimuli with different parameters are discussed.

Animals↗
PubMed1997

Psychophysical studies with two binaural cochlear implant subjects.

Psychophysical studies have been completed with two binaural cochlear implant patients. In our earlier studies [van Hoesel et al., J. Acoust. Soc. Am. 94, 3187-3189 (1993); R. J. M. van Hoesel and G. M. Clark, Ann. Otol. Rhinol. Laryngol. Suppl. 106 104, 233-235 (1995)], lateralization experiments showed good sensitivity to interaural amplitudes but poor sensitivity to interaural time delays when compared with normal hearing subjects. In the studies presented here, both temporal and binaural intensity interactions were further explored. Interaural time delay (ITD) perception was investigated using direct measurement of the just-noticeable difference (jnd) in ITD. Both rate and place of stimulation were varied. Binaural rate discrimination was measured and compared with monaural rate perception. Binaural intensity interaction was explored for matched and unmatched place conditions by means of loudness summation and central masking studies. Results showed that ITDs for interaural time delays were large when compared to normal hearing, even when place of stimulation on each of the two sides was carefully matched. The jnds in ITD were similar for stimulation rates from 50 to 200 pps, and increased at 300 pps. Rate difference limens experiments showed similar results for diotic and monaural stimuli, but improved jnds for dichotic presentation at stimulation rates below 150-200 pps. Binaural intensity interactions showed loudness summation effects with both patients, for matched as well as unmatched place conditions. Central masking was also observed with both subjects, although it was not found to be place dependent.

Cochlear Implants↗
PubMed1984

Hearing loss in dogs after lesions of the brachium of the inferior colliculus and medial geniculate.

Seven dogs were tested for their sensitivity to pure tones following lesions of the brachium of the inferior colliculus and medial geniculate body. Bilateral section of the brachium of the inferior colliculus consistently resulted in an average hearing loss of as much as 37 dB in the midrange of the animals' audiograms. Lesions of the medial geniculate appear to produce a similar hearing loss if the ventral division of the medial geniculate is completely destroyed.

Animals↗
PubMed1980

Thalamocortical transformation of responses to complex auditory stimuli.

In unanesthetized guinea pigs, thalamic (CGM), and cortical (auditory I) neurons were recorded simultaneously. Nine of 69 neuron pairs showed a positive cross-correlation of their spontaneous activities, with increased discharge probability of the cortical neuron beginning 2--5 ms after the discharge of the CGM-neuron. The individual neurons of such pairs had an identical CF and the same spectral responsiveness. The responses of cortical neurons to pure tones were much more phasic than those of the corresponding CGM-neurons. Thalamic neurons could be driven up to much higher AM- and FM-modulation frequencies (100 Hz) than cortical neurons, which usually ceased to follow AM-frequencies above 20 Hz. Stronger or weaker suppression of tonic response components in cortical and thalamic neurons and the lower AM-range of cortical neurons is related to stronger or weaker intracortical and intrathalamic inhibition respectively. Response characteristics to FM-stimuli are similar to those of AM-stimuli. All CGM and cortical neurons responded to a variety of natural calls of the same or of other species. Responses of CGM-cells represented more components of a call than cortical cells even if the two cells were synaptically connected. In cortical cells, repetitive elements of a call were not represented if the repetition rate was too high. High modulation frequencies within a call, such as those of the fundamental frequency, could still be separated in the response of some CGM-neurons, but never in those of cortical neurons. Both CGM and cortical cells responded essentially to transients (amplitude or frequency modulations) within a call, if spectral components of such elements were within the spectral sensitivity of the cell. Spectral components outside the spectral sensitivity range could result in suppression of spontaneous discharge rate. Responses of cortical and CGM-cells, and thus the representation of call elements by neuronal responses, varied with the intensity of a call. It is suggested that, at higher levels of the auditory system, essential information about the temporal features of complex sounds may be represented by neural responses to transients in various spectral regions.

Animals↗
PubMed1982

Cortical evoked potentials in response to brief modulation of signal amplitude. Experiments on auditory temporal resolution.

Human cortical evoked potentials were monitored with scalp electrodes as an indicator of the ability to resolve brief changes in an auditory signal. For a brief period in the middle of a noise pulse its intensity was increased or decreased. The magnitude and duration of this change was varied to establish (1) the threshold for the cortical evoked potential and (2) the effect on the evoked response (amplitude, latency) in the suprathreshold region. To evoke a stimulus-specific potential pattern, durations of about 16 ms were required for intensity changes of +3 dB. With an intensity step of +9 dB, the threshold duration was reduced to 4-6 ms. A brief increase in intensity was more associated with distinctly lower thresholds and larger response amplitudes than an equivalent reduction in intensity, duration being equal. These results confirm the critical durations found in psychoacoustic studies that offer valuable evidence as to the ability to resolve brief changes in an auditory signal.

Animals↗
PubMed1993

Extralemniscal co-activation is not indispensable for behavioral detection of auditory stimuli.

Thresholds for triggering summed auditory evoked responses (ERs) were measured in non-auditory (= extralemniscal--EL) nuclei receiving direct auditory projections from the lateral lemniscus. Primary EL ERs with onset latency of 3-6 ms reflecting activation of direct EL projections of lemniscal auditory nuclei were registered in caudal pontine reticular nucleus (CPRN), in deep layers of superior colliculus (SC) and in ventromedial hypothalamus (VMH). Secondary EL ERs (waves of EL ERs with onset latency above 10 ms) reflecting diffuse auditory EL co-activation of the brain, were registered besides the above mentioned nuclei also in the medial amygdala (MA). Threshold sound intensities for evoking primary EL ERs in CPRN, SC and VMH, for secondary EL ERs in all extralemniscal nuclei tested, and for conditioned avoidance behavior in a two-way shuttle box, were compared mutually. There were no significant mutual differences among thresholds for inducing secondary EL ERs in all EL nuclei tested. Thresholds for evoking secondary EL ERs were lower than those for evoking primary EL ERs in deep layers of the SC, equaled to thresholds for primary EL ERs in the VMH and were higher than thresholds for primary EL ERs in the CPRN. The results suggest that auditory EL projections into SC and/or VMH (but not into CPRN) might represent the primary triggering source for secondary EL ERs in various extralemniscal nuclei. Although conditioning lowered the threshold intensities for inducing secondary EL ERs, the threshold sound intensity for triggering conditioned behavior was lower than the threshold for secondary EL ERs.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Representation of acoustic events in the primary auditory cortex.

One approach to the problem of specifying the contribution of the primary auditory cortex to auditory perception has been based on single-neuron recording techniques in animals. These experiments measure the response rates of individual neural elements to parametric variations in 1 or more stimulus dimensions. The patterns of response rates and response failures revealed by these manipulations are quantitative descriptions of the form and fidelity of the cortex's representation of those stimulus dimensions. This strategy has been used to advantage in studies of the cortical representation of the spectral content of auditory events, the spatial location of a sound, and the time structure of sounds. The data constitute new links between neural coding and behavioral performance in normal and impaired listeners.

Animals↗
PubMed1991

Temporal resolution of gaps in noise by the rat is lost with functional decortication.

In Experiment 1 (n = 8), the rat's ability to detect brief gaps in white noise was measured by gap-produced inhibition of an acoustic startle reflex, elicited 100 ms after the gap. After bilateral application of KCl to the cortex, gaps as long as 15 ms provided no reflex inhibition; in contrast, the inhibitory threshold was between 2 and 4 ms in the saline control condition. In Experiment 2 (n = 13), noise pulses of 40, 50, or 70 dB were presented 20-500 ms before the startle stimulus, and in Experiment 3 (n = 5) noise offsets occurred so that the startle stimulus was presented at the end of a 2-30-ms gap. Noise pulses and offsets both inhibited reflex expression equally in saline- and KCl-treated animals. Differences between the normal (saline) functions of noise offsets and gaps suggest additional sensory processing with the longer lead time. The loss of gap sensitivity after KCl application indicates that gap processing, unlike pulses and offsets, depends on cortical mechanisms.

Animals↗
PubMed1985

Central factors in the discrimination and identification of complex sounds.

The paper by Jesteadt and Norton [J. Acoust. Soc. Am. 78, 365-374 (1985)] described certain similarities between psychophysical and physiological measures of frequency selectivity. Although the hearing of naturally occurring sounds is dependent upon these peripherally based relationships, recent research has shown that other, more central, processes are also strongly involved in the perception of complex acoustic events. The present paper describes research on the discrimination of complex sounds other than those of speech or music. In contrast to the more peripherally determined limits on the listener's sensitivity for single tones and other simple stimuli, the processing of complex sounds requires the interaction of peripheral and central mechanisms. These issues are discussed in relation to recent studies of the responses of the cochlea to speech stimuli. It is suggested that the peripheral processor may be relatively transparent to the essential spectral-temporal properties of speech, whereas more central processing severely limits the rates and amount of information that can be extracted from complex sounds.

Attention↗
PubMed1986

Word deafness: one hundred years later.

Since its original description the diagnosis of word deafness has been greatly expanded. Confusion has arisen with regard to the usage of the related terms pure word deafness, auditory agnosia, and cortical deafness. Three new cases of word deafness are presented including one case with CT and necropsy correlation. These cases are compared with 34 previously reported cases of various cortical auditory disorders. Our review establishes that patients with word deafness who have had formal testing of linguistic and non-linguistic sound comprehension and musical abilities always demonstrated a more pervasive auditory agnosia. Despite the spectrum of auditory deficits and associated language abnormalities, patients with word deafness share common features including aetiology, pathology, clinical presentation and course. These common features justify inclusion of heterogeneous cortical auditory disorders under the rubric of word deafness. Despite some limitations the term "word deafness" should be retained for this syndrome, since inability to comprehend spoken words is the most distinctive clinical deficit. Word deafness is most frequently caused by cerebrovascular accidents of presumed cardiac embolisation, with bitemporal cortico-subcortical lesions. The sequence of cerebral injury is not predictive of resulting auditory deficits. Impairment of musical abilities parallels the severity of the auditory disorder.

Aged↗