Single olivocochlear neurons in the guinea pig. I. Binaural facilitation of responses to high-level noise

Single medial olivocochlear (MOC) neurons were recorded from the cochlea of the anesthetized guinea pig. We used tones and noise presented monaurally and binaurally and measured responses for sounds up to 105 dB sound pressure level (SPL). For monaural sound, MOC neuron firing rates were usually higher for noise bursts than tone bursts, a situation not observed for afferent fibers of the auditory nerve that were sampled in the same preparations. MOC neurons also differed from afferent fibers in having less saturation of response. Some MOC neurons had responses that continued to increase even at high sound levels. Differences between MOC and afferent responses suggest that there is convergence in the pathway to olivocochlear neurons, possibly a combination of inputs that are at the characteristic frequency (CF) with others that are off the CF. Opposite-ear noise almost always facilitated the responses of MOC neurons to sounds in the main ear, the ear that best drives the unit. This binaural facilitation depends on several characteristics that pertain to the main ear: it is higher in neurons having a contralateral main ear (contra units), it is higher at main-ear sound levels that are moderate (approximately 65 dB SPL), and it is higher in neurons with low discharge rates to main-ear stimuli. Facilitation also depends on parameters of the opposite-ear sound: facilitation increases with noise level in the opposite ear until saturating, is greater for continuous noise than noise bursts, and is usually greater for noise than for tones. Using optimal opposite-ear facilitators and high-level stimuli, the firing rates of olivocochlear neurons range up to 140 spikes/s, whereas for moderate-level monaural stimuli the rates are <80 spikes/s. At high sound levels, firing rates of olivocochlear neurons increase with CF, an increase that may compensate for the known lower effectiveness of olivocochlear synapses on outer hair cells responding to high frequencies. Overall, our results demonstrate a high MOC response for binaural noise and suggest a prominent role for the MOC system in environments containing binaural noise of high level.     

Human salmonellosis in Singapore

1. Cats with one cochlea destroyed were trained to localize sound. After behavioral measures of the animal's accuracy of localization were made, cortical auditory areas were ablated unilaterally. 2. The results showed: a) like binaural localization, monaural localization of sound in space, as measured by the ability of an animal to move toward a sound source, depends on integrity of auditory cortex; b) it is only ablation of cortex contralateral to the functional ear that seriously affects localizing behavior; ablation of cortex ipsilateral to the intact cochlea has little or no effect on localizing behavior. 3. To explain the results, we suggest that auditory cortex is essential for an organized perception of space including the relation of the animal's position to other objects in space. We also suggest that auditory cortex contralateral to a given ear is necessary in order for the animal to recognize that a stimulus is presented to that ear of, when both ears are intact, to recognize that the stimulus to the given ear differs in some way (intensity, time of arrival, sequential arrangement of sounds) from the stimulus to the opposite ear.     

Modeling of percutaneous drug transport in vitro using skin-imitating Carbosil membrane

Extracellular recordings were made with microelectrodes from single neurons in the rat's dorsal nucleus of the lateral lemniscus (DNLL) and response characteristics were determined for monaural and binaural acoustic stimulation. The vast majority of DNLL neurons were narrowly tuned to sound frequency and their temporal responses to contralateral tone pulses fell into one of three broad categories: onset (57%), sustained (21%) or onset-pause-sustained (22%). Most DNLL neurons fired multiple action potentials to a single click delivered to the contralateral ear. The majority (77%) of DNLL neurons showed a monotonic increase in the number of spikes elicited by contralateral tone pulses of increasing sound pressure level; the remaining cells were weakly non-monotonic. No obvious tonotopic pattern was found in the distribution of characteristic frequency of neurons in DNLL. Most DNLL neurons exhibited either excitatory/inhibitory (74%) or excitatory/excitatory (9%) binaural response patterns. The remaining cells (17%) were monaural and driven exclusively by stimulation of the contralateral ear. The binaural neurons in DNLL were sensitive to both interaural intensity and interaural time differences as determined by presentation of dichotic tone bursts and clicks respectively. The responses of DNLL neurons could be distinguished on the basis of monaural and binaural response characteristics from those in surrounding areas including the sagulum, paralemniscal zone and the intermediate nucleus of the lateral lemniscus.     

Features of ipsilaterally evoked inhibition in the dorsal nucleus of the lateral lemniscus

The dorsal nucleus of the lateral lemniscus (DNLL) is a binaural nucleus whose neurons are excited by stimulation of the contralateral ear and inhibited by stimulation of the ipsilateral ear. Here we report on several features of the ipsilaterally evoked inhibition in 95 DNLL neurons of the mustache bat. These features include its dependence on intensity, its tuning and the types of stimuli that are capable of evoking it. Inhibition was studied by evoking discharges with the iontophoretic application of glutamate, and then evaluating the strength and duration of the inhibition of the glutamate evoked background activity produced by stimulation of the ipsilateral ear. Excitatory responses were evoked by stimulation of the contralateral ear with best frequency (BF) tone bursts. Glutamate evoked discharges could be inhibited in all DNLL neurons and the inhibition often persisted for periods ranging from 10 to 50 ms beyond the duration of the tone burst that evoked it. The duration of the persistent inhibition increased with stimulus intensity. Stimulus duration had little influence on the duration of the persistent inhibition. Signals as short as 2 ms suppressed discharges for as long as 30 ms after the signal had ended. The frequency tuning of the total period of inhibition and the period of persistent inhibition were both closely matched to the tuning evoked by stimulation of the contralateral ear. Moreover, the effectiveness of complex signals for evoking persistent inhibition, such as brief FM sweeps and sinusoidally amplitude and frequency modulated signals, was comparable to that of tone bursts at the neuron's excitatory BF, so long as the complex signal contained frequencies at or around the neuron's excitatory BF. We also challenged DNLL cells with binaural paradigms. In one experiment, we presented a relatively long (40 ms) BF tone burst of fixed intensity to the contralateral ear, which evoked a sustained discharge, and a shorter, 10 ms signal of variable intensity to the ipsilateral ear. As the intensity of the 10 ms ipsilateral signal increased, it generated progressively longer periods of persistent inhibition and thus the discharges were suppressed for periods far longer than the 10 ms duration of the ipsilateral signal. With interaural time disparities, ipsilateral signals that led contralateral signals evoked a persistent inhibition that suppressed the responses to the trailing contralateral signals for periods of a least 15 ms. This suggests that an initial binaural sound that favors the ipsilateral ear should suppress the responses to trailing sounds that normally would be excitatory if they were presented alone. We hypothesize a circuit that generates the persistent inhibition and discuss how the results with binaural signals support that hypothesis.     

Development of the auditory orientation response in the albino rat (Rattus norvegicus).

The development of head orientation to auditory stimulation was examined in rat pups at Postnatal Days 8, 11, 14, 17, and 20. The animals were tested in a quiet environment with single bursts of 65 dB (SPL) broad-band noise. A reflexive head turn toward the sound was first seen on Postnatal Day 14 and subsequently on Days 17 and 20. This result demonstrates that the onset of directional auditory responses occurred between Day 11 and Day 14. The role of binaural cues in early sound orientation was examined in 17-day-old pups with monaural ligation of the external meatus. These animals were unable to localize a sound source and consistently turned toward the side of the unligated ear regardless of the position of the stimulus. Thus binaural cues were shown to be important for head orientation to sound in early development. In a separate study, head orientation to high and low frequency tone pips was examined. Directional responses were first seen on Day 12 for a 16-kHz tone and Day 14 for a 2-kHz tone. These results indicate an earlier onset for orientation to high frequency sounds in the rat. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Auditory evoked magnetic fields in cases with temporal lobe glioma

Auditory evoked magnetic field (AEF) is known to be suitable to separate left and right hemispheric activities while auditory evoked potential is not. To evaluate cortical auditory function in ten patients with temporal lobe gliomas, we measured AEF for monaural tone stimuli using a helmet-shaped 66-channel MEG system. Latency of the N 100 m, the most prominent peak with a latency around 90 ms, was measured in the hemisphere contralateral to the stimulus onset. In five patients, the N 100 m latency was within our normal range (mean +/- 2 s.d.). In these five cases, tumor was located in the anterior or the inferior part of the temporal lobe. We observed significant delay of the N 100 m latency in four patients and disappearance of the N 100 m in another patient. In the later five patients, tumor extended to the superior and posterior part of the temporal lobe. AEF can be used to evaluate cortical auditory function noninvasively in cases with temporal lobe gliomas.     

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.     

Measurements of the temporal fMRI response of the human auditory cortex to trains of tones

Averaged single trials (AST) allowed the functional magnetic resonance imaging (fMRI) response to auditory stimuli to be measured at high temporal (1 s) and spatial (0.1 cm3) resolution. Using this paradigm we investigated the transient signal response to 100-ms tone bursts in trains of between 100 ms and 25.5 s in total duration. We have demonstrated that the fMRI response to such auditory stimuli is approximately linear for trains of 6 s and longer, but that shorter stimuli produce signals that are larger than might be expected from the response to the longer stimuli. This nonlinear behavior can be modeled if an adaptive response to each stimulus is assumed. A study using a novel paradigm was also performed in order to study the influence of scanner noise during fMRI experiments on the auditory system response to tones. This study demonstrated that the temporal response to 700-ms tone stimuli is modified when performed in the presence of scanner gradient noise, the modification being a small but significant increase (P < 0.05) in the magnitude of the response. Finally the ability to measure the onset of functional activation using the AST method was examined. It was found, with the aid of computer simulation that a sampling rate of one image per second is adequate to distinguish temporal responses. Using the data acquired in this study, onset times were calculated for the auditory cortex, and these results are consistent with current models of functional activation.     

Effects of ear plugging on single-unit azimuth sensitivity in cat primary auditory cortex. I. Evidence for monaural directional cues

1. Single-unit recordings were carried out in primary auditory cortex (AI) of barbiturate-anesthetized cats. Neurons, sensitive to sound direction in the horizontal plane (azimuth), were identified by their responses to noise bursts, presented in the free field, that varied in azimuth and sound pressure level (SPL). SPLs typically varied between 0 and 80 dB and were presented at each azimuth that was tested. Each azimuth-sensitive neuron responded well to some SPLs at certain azimuths and did not respond well to any SPL at other azimuths. This report describes AI neurons that were sensitive to the azimuth of monaurally presented noise bursts. 2. Unilateral ear plugging was used to test each azimuth-sensitive neuron's response to monaural stimulation. Ear plugs, produced by injecting a plastic ear mold compound into the concha and ear canal, attenuated sound reaching the tympanic membrane by 25-70 dB. Binaural interactions were inferred by comparing responses obtained under binaural (no plug) and monaural (ear plug) conditions. 3. Of the total sample of 131 azimuth-sensitive cells whose responses to ear plugging were studied, 27 were sensitive to the azimuth of monaurally presented noise bursts. We refer to these as monaural directional (MD) cells, and this report describes their properties. The remainder of the sample consisted of cells that either required binaural stimulation for azimuth sensitivity (63/131), because they were insensitive to azimuth under unilateral ear plug conditions or responded too unreliably to permit detailed conclusions regarding the effect of ear plugging (41/131). 4. Most (25/27) MD cells received either monaural input (MD-E0) or binaural excitatory/inhibitory input (MD-EI), as inferred from ear plugging. Two MD cells showed other characteristics. The contralateral ear was excitatory for 25/27 MD cells. 5. MD-E0 cells (22%, 6/27) were monaural. They were unaffected by unilateral ear plugging, showing that they received excitatory input from one ear, and that stimulation of the other ear was without apparent effect. On the other hand, some monaural cells in AI were insensitive to the azimuth of noise bursts, showing that sensitivity to monaural directional cues is not a property of all monaural cells in AI. 6. MD-EI cells (70%, 19/27) exhibited an increase in responsiveness on the side of the plugged ear, showing that they received excitatory drive from one ear and inhibitory drive from the other. MD-EI cells remained azimuth sensitive with the inhibitory ear plugged, showing that they were sensitive to monaural directional cues at the excitatory ear.(ABSTRACT TRUNCATED AT 400 WORDS)     

Free-field binaural unmasking in ferrets.

Free-field detection by normal and monaural ferrets (N?=?4) of a 500-Hz tone presented over 1 laterally placed loudspeaker and partially masked by narrowband noise from 2 sources was studied at 2 angular separations of the noise sources (0° and 180°). Monaural listening was achieved either by plugging 1 ear canal or removing 1 cochlea. Normal ferrets showed an improvement in detectability of the tone when there was a 180° separation between the noise sources. This unmasking of the tone was abolished in both groups of monaural ferrets, suggesting that the unmasking was due to binaural processing. The development of an animal model demonstrating free-field binaural unmasking, in a species other than humans, will allow investigation into the functional consequences of experimental hearing loss. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Auditory spatial responses of young guinea pigs (Cavia porcellus) during and after ear blocking.

Tested 30 newborn guinea pigs to determine their ability to approach an auditory stimulus early in development. Observations of the behavior of 1–4 day old Ss in a circular 8-choice maze revealed a pronounced tendency to orient toward and approach a tape-recorded signal of guinea pig vocalizations. The occurrence of approach responses was reduced to chance in Ss tested with one ear occluded by wax ear plugs which attenuated but did not totally eliminate sound. The effect of monaural ear blocks was more severe than binaural blocks, which reflects the importance of binaural cues in the maintenance of approach responses to sound. In a 2nd study with 40 Ss the ability of older animals, 11–31 days of age, was examined. Directional approach responses to sound were also evident at this age, and ear plugs disrupted performance only under monaural conditions. Furthermore, in Ss raised from birth with monaural ear blocks but tested without ear plugs, there was a subsequent disruption of performance for at least 21 days. Results indicate the importance of binaural cues in the development of early auditory spatial reponses and suggest the need for appropriate binaural experience for subsequent localization of sounds. (52 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Tinnitus and event-related activity of the auditory cortex

A neuromagnetic study in tinnitus patients and normal-hearing controls was performed with a modified contingent negative variation (CNV) paradigm. While the warning stimulus S1 was a tone burst at an intensity well above threshold, the imperative stimulus S2 was presented at a near threshold intensity because, in the majority of cases, the perceived loudness of tinnitus is very close to the threshold for a pure tone of the same frequency. Subjects had to respond to S2 by pressing a button until its offset was detected. In this case, instead of the usual sudden cut-off of the CNV after the perception of S2, a slow negative deflection develops, the post-imperative negative variation (PINV). Its initial portion probably indicates the development of a second initial CNV because the subject had to attend also to the offset of S2. The neuromagnetic data were analysed both in the time domain and in the frequency domain (short-time spectral analysis of the classical EEG bands). The time domain waveform as well as the spectrotemporal patterns of the MEG bands exhibited deviations from the normal pattern in several tinnitus subgroups, depending on the characteristics of tinnitus (tonal vs. noisiform, monaural vs. binaural) and on the stimulation conditions (tinnitus side vs. non-tinnitus side).     

Detection of TP53 mutation, loss of heterozygosity and DNA content in fine-needle aspirates of breast carcinoma

We recorded auditory-evoked magnetic responses with a whole-scalp 122-channel neuromagnetometer from seven adult patients with unilateral conductive hearing loss before and after middle ear surgery. The stimuli were 50-msec 1-kHz tone bursts, delivered to the healthy, nonoperated ear at interstimulus intervals of 1, 2, and 4 seconds. The mean preoperative pure-tone average in the affected ear was 57 dB hearing level; the mean postoperative pure-tone average was 17 dB. The 100-msec auditory-evoked response originating in the auditory cortex peaked, on average, 7 msecs earlier after than before surgery over the hemisphere contralateral to the stimulated ear and 2 msecs earlier over the ipsilateral hemisphere. The contralateral response strengths increased by 5% after surgery; ipsilateral strengths increased by 11%. The variation of the response latency and amplitude in the patients who underwent surgery was similar to that of seven control subjects. The postoperative source locations did not differ noticeably from preoperative ones. These findings suggest that temporary unilateral conductive hearing loss in adult patients modifies the function of the auditory neural pathway.     

Development of directional responses to sounds in the rat (Rattus norvegicus).

Exp I examined the tendency of 48 Sprague-Dawley rat pups to approach auditory stimulation. Tests were conducted in a circular maze with a central start area and 8 response areas at the periphery. Stimuli were played back over a loudspeaker behind 1 of 8 response areas. Signals included species calls at high and low intensities. Ss showed a tendency to approach the low-intensity playback of a species "social" vocalization but did not approach the other stimuli. Exp II investigated the importance of binaural cues for localization. 24 pups were tested with 1 or both ears blocked or neither ear blocked. Only the binaurally blocked and normal controls approached the sound source at above chance levels. The failure of Ss in the monaural group to approach the sound probably resulted from a disruption of binaural cues. The ability of Ss to localize sounds and process binaural cues is discussed in relation to the onset of hearing and in terms of physiological responses that are present early in ontogeny. (76 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perception of timing of kinesthetic stimuli

A psychophysical method was used to estimate the timing of perception of kinesthetic stimuli with different velocities in normal volunteers. A 1 ms auditory click occurred randomly before or after an imposed flexion movement at either 20, 40 or 60 deg/s of the metacarpophalangeal joint. Subjects reported whether the click was perceived before or after the movement onset (experiment 1) or perception of movement velocity (experiment 2). The time at which there was a 50% chance that subjects reported movement or velocity perception after the click was taken as an estimate of the time subjects perceived the stimuli. The difference in time of perceived movement velocity discrimination and movement onset was only significant when the velocity was 20 deg/s (52 ms). This suggests that movement onset and identification of the velocity of the faster movements are perceived nearly simultaneously.     

Chick (Gallus domesticus) approach preferences for natural and artificial sound stimuli.

Found that 2-day-old White Leghorn and New Hampshire chicks showed an unlearned preference for an ancestral maternal call over a brief, repetitive pure tone burst when choice preference tests were between stationary models emitting maternal call and tone burst sounds. However, other Ss of both breeds showed an unlearned preference for tone burst over maternal call when choice preference tests were between moving models emitting tone burst and call sounds. These same preferences were found in Ss that had been imprinted (exposed) to moving call and tone burst sounds on their 1st posthatch day. The tone bursts were briefer than the call note duration (25 vs 80 msec). Since very brief sound bursts are easier to localize, it is concluded that Ss preferred tone bursts over calls when sound sources were moving because of the greater ease of localizing tone bursts. Along with other recent data, the failure to find imprinting to a maternal call or to tone bursts (i.e., the call and tone burst preferences found were uninfluenced by a brief prior exposure to either sound) suggests the need to question whether or not auditory imprinting occurs in the domestic chick. (12 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Functional organization of auditory cortex in the mongolian gerbil (Meriones unguiculatus). II. Tonotopic 2-deoxyglucose

The tonotopic organization of the auditory cortex in the Mongolian gerbil was mapped with 2-deoxyfluoro-D-glucose (2DG) using narrow-band frequency-modulated tones of different centre frequency (FM tones) and tones periodically alternating between two different frequencies (alternating tones) as stimuli. Continuous tone bursts of a constant frequency and repetition rate were used in initial experiments. Continuous tones produced 2DG patterns similar to those observed in animals that were not specifically stimulated. With tone bursts of constant frequency and repetition rate variable patterns were observed, some of which could be interpreted only in retrospect in the light of results obtained with FM tones and alternating tones. These stimuli, in contrast, produced differential metabolic responses which in conjunction with 2DG data from monaural animals and electrophysiological data made it possible to distinguish a primary auditory field AI with its dorsal region Ald, an anterior auditory field AAF, a ventral field V, a dorsoposterior field DP and a ventroposterior field VP, a dorsal field D, and in addition an anteroventral field AV. In the largest field (AI) and the smaller rostrally adjacent field AAF, frequency-specific dorsoventral bands of labelling (isofrequency contours) were mapped quantitatively. Bands shifted as a function of frequency relative to each other and to an independent spatial reference line in the lateral hippocampus. Spatial analysis of the single bands obtained with FM tones, and of the double bands obtained with alternating tones in both fields, revealed roughly mirror-imaged tonotopic maps of AI and AAF. In AI the progression from low to high frequencies was from caudal to rostral and in AAF the gradient was reversed, leading to a common high-frequency border of the two fields. In AI, the spatial resolution for frequencies below 16 kHz was in similar intervals per octave and higher for frequencies below 1 kHz. AI showed a somewhat higher spatial resolution for frequencies (at least below 1 kHz) as well as longer isofrequency contours than AAF. The 2-deoxyglucose patterns provided average tonotopic maps and topological data on various fields, as well as reliable landmarks in the gerbil's auditory cortex.     

Psychophysical and speech perception studies: a case report on a binaural cochlear implant subject

Further improvements in speech perception for cochlear implant patients in quiet and in noise should be possible with speech processing strategies using binaural implants. For this reason, presented here is a series of initial psychophysical and speech perception studies on the authors' first binaural cochlear implant patient. For an approximate matching of the places of stimulation on the two sides, the patient usually reported a single percept when the two sides were simultaneously stimulated. Lateralization was strongly influenced by amplitude differences between the electrical stimuli on the two sides, but only weakly by interaural time delays. Speech testing, comparing monaural with binaural electrical stimulation, showed a binaural advantage particularly in noise.     

Effects of attention and unilateral neglect on auditory stream segregation.

Two pairs of experiments studied the effects of attention and of unilateral neglect on auditory streaming. The first pair showed that the build up of auditory streaming in normal participants is greatly reduced or absent when they attend to a competing task in the contralateral ear. It was concluded that the effective build up of streaming depends on attention. The second pair showed that patients with an attentional deficit toward the left side of space (unilateral neglect) show less stream segregation of tone sequences presented to their left than to their right ears. Streaming in their right ears was similar to that for stimuli presented to either ear of healthy and of brain-damaged controls, who showed no across-ear asymmetry. This result is consistent with an effect of attention on streaming, constrains the neural sites involved, and reveals a qualitative difference between the perception of left- and right-sided sounds by neglect patients. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hemispheric asymmetries in the perception of rapid (timbral) and slow (nontimbral) amplitude fluctuations of complex tones.

Hemispheric asymmetries for processing rapid (timbral) and slow (nontimbral) amplitude fluctuations of complex tones were investigated in 32 right-handed nonmusicians. Two monaural matching-to-sample tests with contralateral white noise and attention directed to 1 ear were used, 1 with tones presenting slow fluctuations of amplitude and 1 with tones presenting rapid fluctuations of amplitude perceived as different timbres. Stimuli were generated by altering the amplitude envelope of a steady state complex tone. Dependent variables were reaction time and accuracy. The results suggest an important role for the right hemisphere in the perception of temporal variations of intensity of sounds both when these variations are rapid and perceived as timbral qualities and when they are slow and perceived as changes of loudness. (PsycINFO Database Record (c) 2010 APA, all rights reserved)