Central denervation hypersensitivity in the auditory system of the cat.

Data are reported for seven cats with a total of 29 electrodes permanently placed in or near the cochlear nucleus, the superior olivary complex, the nucleus of the inferior colliculus, and the medial geniculate body. Detection thresholds for pulsate electrical stimuli were measured using an operant behavioral procedure. Electrical stimulation thresholds were measured prior to and following bilateral destruction of the cochleas in all animals. In addition, four of the animals were tested using a site-of-stimulation discrimination prior to and following the cochlear lesion. Finally, hearing loss was evaluated in all cats after the completion of the experiments. Electrical stimulation thresholds showed a mean reduction of 7.9 dB throughout the brain stem auditory system fater cochlear destruction. The ability of the animals to perform the site-of-stimulation discrimination was not permanently impaired by the cochlear lesion. The data indicated the presence of increased sensitivity to electrical stimulation in most regions of the subcortical auditory system, although a lesser effect was found at the thalamic level. It was concluded that stimulation threshold provides an index relevant to the state of auditory neurons proximal to the electrode tip.     

Responses of neurons to click-pairs as simulated echoes: auditory nerve to auditory cortex

When two identical sounds are presented from different locations with a short interval between them, the perception is of a single sound source at the location of the leading sound. This "precedence effect" is an important behavioral phenomenon whose neural basis is being increasingly studied. For this report, neural responses were recorded to paired clicks with varying interstimulus intervals, from several structures of the ascending auditory system in unanesthetized animals. The structures tested were the auditory nerve, anteroventral cochlear nucleus, superior olivary complex, inferior colliculus, and primary auditory cortex. The main finding is a progressive increase in the duration of the suppressive effect of the leading sound (the conditioner) on the response to the lagging sound (the probe). The first major increase occurred between the lower brainstem and inferior colliculus, and the second between the inferior colliculus and auditory cortex. In neurons from the auditory nerve, cochlear nucleus, and superior olivary complex, 50% recovery of the response to the probe occurred, on average, for conditioner and probe intervals of approximately 2 ms. In the inferior colliculus, 50% recovery occurred at an average separation of approximately 7 ms, and in the auditory cortex at approximately 20 ms. Despite these increases in average recovery times, some neurons in every structure showed large responses to the probe within the time window for precedence (approximately 1-4 ms for clicks). This indicates that during the period of the precedence effect, some information about echoes is retained. At the other extreme, for some cortical neurons the conditioner suppressed the probe response for intervals of up to 300 ms. This is in accord with behavioral results that show dominance of the leading sound for an extended period beyond that of the precedence effect. Other transformations as information ascended included an increased variety in the shapes of the recovery functions in structures subsequent to the nerve, and neurons "tuned" to particular conditioner-probe intervals in the auditory cortex. These latter are reminiscent of neurons tuned to echo delay in bats, and may contribute to the perception of the size of the acoustic space.     

A reexamination of forward masking in the auditory nerve

Forward masking, as measured behaviorally, is defined as an increase in a signal's detection threshold resulting from a preceding masker. Previously, forward masking in the auditory nerve has been measured as a reduction in the neural response to a signal when preceded by a masker. However, detection threshold depends on both the magnitude of the response to the signal and the variance of the response. Thus changes in detectability cannot be inferred from response reduction alone. Relkin and Pelli (1987) have described a two-interval forced-choice procedure that may be used to measure the threshold for the detection of a probe signal in recordings of spike counts in single auditory neurons. These methods have been used to study the forward masking of characteristic frequency probe tones by characteristic frequency maskers as masker intensity was varied. Although the masker does reduce the detectability of the probe tone, it was found that the threshold shifts are much less than those observed behaviorally, particularly for intense maskers. In part, the small threshold shifts can be attributed to the reduction in response variance following the masker, which is the result of the adaptation of spontaneous activity. These results imply that behavioral forward masking must result from suboptimal processing of spike counts from auditory neurons at a location central to the auditory nerve.     

Human temporal auditory acuity as assessed by envelope following responses

Temporal auditory acuity, the ability to discriminate rapid changes in the envelope of a sound, is essential for speech comprehension. Human envelope following responses (EFRs) recorded from scalp electrodes were evaluated as an objective measurement of temporal processing in the auditory nervous system. The temporal auditory acuity of older and younger participants was measured behaviorally using both gap and modulation detection tasks. These findings were then related to EFRs evoked by white noise that was amplitude modulated (25% modulation depth) with a sweep of modulation frequencies from 20 to 600 Hz. The frequency at which the EFR was no longer detectable was significantly correlated with behavioral measurements of gap detection (r = -0.43), and with the maximum perceptible modulation frequency (r = 0.72). The EFR techniques investigated here might be developed into a clinically useful objective estimate of temporal auditory acuity for subjects who cannot provide reliable behavioral responses.     

Simultaneously measured behavioral and electrophysiological hearing thresholds in a bottlenose dolphin (Tursiops truncatus)

Dolphin auditory thresholds obtained via evoked potential audiometry may deviate from behavioral estimates by 20 dB or more. Differences in the sound source, stimulus presentation method, wave form, and duration may partially explain these discrepancies. To determine the agreement between behavioral and auditory evoked potential (AEP) threshold estimates when these parameters are held constant, behavioral and AEP hearing tests were simultaneously conducted in a bottlenose dolphin. Measurements were made in-air, using sinusoidal amplitude-modulated tones continuously projected via a transducer coupled to the pan region of the dolphin's lower jaw. Tone trials were presented using the method of constant stimuli. Behavioral thresholds were estimated using a 50% correct detection. AEP thresholds were based on the envelope following response and 50% correct detection. Differences between AEP and behavioral thresholds were within +/-5 dB, except at 10 kHz (12 dB), 20 kHz (8 dB), 30 kHz (7 dB), and 150 kHz (24 dB). In general, behavioral thresholds were slightly lower, though this trend was not significant. The results demonstrate that when the test environment, sound source, stimulus wave form, duration, presentation method, and analysis are consistent, the magnitude of the differences between AEP and behavioral thresholds is substantially reduced.     

Rate responses of auditory nerve fibers to tones in noise near masked threshold

The rate responses of auditory nerve fibers were measured for best frequency (BF) tone bursts in the presence of continuous background noise. Rate functions for BF tones were constructed over a 32-dB range of levels, centered on the behavioral masked thresholds of cats. The tone level at which noticeable rate changes are evoked by the tones corresponds closely to behavioral masked threshold at all noise levels used (-10- to 30-dB spectrum level). As the noise level increases, the response rate to the background noise approaches saturation, and the incremental rate response to tones decreases. At high noise levels, the rate responses to tones of low and medium spontaneous rate fibers are larger than those of high spontaneous rate fibers. Empirical statistics of auditory nerve fiber spike counts are reported; these differ from those expected of a Poisson process in that the variance is smaller than the mean. A new measure of discharge rate is described that allows rate changes to be expressed in units of a standard deviation. This measure allows tone-evoked responses to be interpreted in terms of their detectability in a signal detection task. Rate responses of low and medium spontaneous rate fibers are more detectable than those of high spontaneous rate fibers, especially at high noise levels. There appears to be sufficient information in the rate response of a small number of auditory nerve fibers to support behaviorally observed levels of detection performance.     

Comparison of electrophonic and auditory-nerve electroneural responses

Electrophonic and auditory-nerve electroneural responses were recorded from the inferior colliculus of the cat. The electrophonic response appeared at a latency 1.0-1.5 ms later than the electroneural response, due to the time requirements for cochlear transduction. The electrophonic response also demonstrated very slow growth of response amplitude with increasing stimulus current as compared to the electroneural response. Aminoglycoside perfusion of the cochlea eliminated the electrophonic component from the evoked response record and left the electroneural component relatively unchanged, indicating that the electrophonic is an acoustic stimulus that requires an intact auditory end organ for transduction.     

A neural circuit transforming temporal periodicity information into a rate-based representation in the mammalian auditory system

Periodic amplitude modulations (AMs) of an acoustic stimulus are presumed to be encoded in temporal activity patterns of neurons in the cochlear nucleus. Physiological recordings indicate that this temporal AM code is transformed into a rate-based periodicity code along the ascending auditory pathway. The present study suggests a neural circuit for the transformation from the temporal to the rate-based code. Due to the neural connectivity of the circuit, bandpass shaped rate modulation transfer functions are obtained that correspond to recorded functions of inferior colliculus (IC) neurons. In contrast to previous modeling studies, the present circuit does not employ a continuously changing temporal parameter to obtain different best modulation frequencies (BMFs) of the IC bandpass units. Instead, different BMFs are yielded from varying the number of input units projecting onto different bandpass units. In order to investigate the compatibility of the neural circuit with a linear modulation filterbank analysis as proposed in psychophysical studies, complex stimuli such as tones modulated by the sum of two sinusoids, narrowband noise, and iterated rippled noise were processed by the model. The model accounts for the encoding of AM depth over a large dynamic range and for modulation frequency selective processing of complex sounds.     

Mapping of neurokinin-like immunoreactivity in the human brainstem

Background

Using an indirect immunoperoxidase technique, we have studied the distribution of immunoreactive fibers and cell bodies containing neurokinin in the adult human brainstem with no prior history of neurological or psychiatric disease.

Results

Clusters of immunoreactive cell bodies and high densities of neurokinin-immunoreactive fibers were located in the periaqueductal gray, the dorsal motor nucleus of the vagus and in the reticular formation of the medulla, pons and mesencephalon. Moreover, immunoreactive cell bodies were found in the inferior colliculus, the raphe obscurus, the nucleus prepositus hypoglossi, and in the midline of the anterior medulla oblongata. In general, immunoreactive fibers containing neurokinin were observed throughout the whole brainstem. In addition to the nuclei mentioned above, the highest densities of such immunoreactive fibers were located in the spinal trigeminal nucleus, the lateral reticular nucleus, the nucleus of the solitary tract, the superior colliculus, the substantia nigra, the nucleus ambiguus, the gracile nucleus, the cuneate nucleus, the motor hypoglossal nucleus, the medial and superior vestibular nuclei, the nucleus prepositus hypoglossi and the interpeduncular nucleus.

Conclusion

The widespread distribution of immunoreactive structures containing neurokinin in the human brainstem indicates that neurokinin might be involved in several physiological mechanisms, acting as a neurotransmitter and/or neuromodulator.     

Near-field responses from the round window, inferior colliculus, and auditory cortex of the unanesthetized chinchilla: manipulations of noiseburst level and rate.

Few studies have compared the response properties of near-field potentials from multiple levels of the auditory nervous system of unanesthetized animals. The purpose of this study was to investigate the effects of brief-duration noisebursts on neural responses recorded from electrodes chronically implanted at the round window, inferior colliculus and auditory cortex of chinchillas. Responses were obtained from seven unanesthetized chinchillas to a noiseburst-level and noiseburst-rate series. For the noiseburst-rate series, a 70 dB pSPL noiseburst was varied in rate from 10 to 100 Hz using conventional averaging procedures, and from 100 to 500 Hz using pseudorandom pulse trains called maximum length sequences (MLSs). Response thresholds were similar for the compound action potential (CAP), inferior colliculus potential (ICP) and auditory cortex potential (ACP). With decreasing noiseburst level, there were decreases in the amplitudes and increases in the latencies of the CAP, ICP and ACP. The shapes of the mean normalized amplitude input/output (I/O) functions were similar for the ICP and ACP, while the normalized I/O functions for the first positive peak (P1) and first negative peak (N1) of the CAP differed from each other and from the ICP and ACP. The slopes of the latency/intensity functions were shallowest for the CAP, intermediate for the ICP, and steepest for the ACP. With increasing rate, the latency shift was least for the CAP, intermediate for the ICP and greatest for the ACP. The amplitude of P1 of the CAP varied little with rate. All other potentials showed a pronounced decrease in amplitude at high stimulation rates. Excluding CAP P1, proportional amplitude decrease with rate was greatest for the ACP, intermediate for N1 of the CAP and least for the ICP. Responses were present in most animals at all recording sites, even for the highest rate (500 Hz) used in this study. For all potentials, the MLS procedure allowed the collection of a response at rates well above those where sequential responses would have overlapped using conventional averaging procedures.     

Binaural detection with narrowband and wideband reproducible noise maskers. III. Monaural and diotic detection and model results

A single-interval, yes-no, tone-in-noise detection experiment was conducted to measure the proportion of "tone present" responses to each of 25 reproducible noise-alone and tone-plus-noise waveforms under narrowband (100 Hz), wideband (2900 Hz), monotic, and diotic stimulus conditions. Proportions of "tone present" responses (estimates of the probabilities of hits and false alarms) were correlated across masker bandwidths and across monotic and diotic conditions. Two categories of models were considered; one based on stimulus energy or neural counts, and another based on temporal structure of the stimulus envelope or neural patterns. Both categories gave significant correlation between decision variables and data. A model based on a weighted combination of energy in multiple critical bands performed best, predicting up to 90% of the variance in the reproducible-noise data. However, since energy-based models are unable to successfully explain detection under a roving-level paradigm without substantial modification, it is argued that other variations of detection models must be considered for future study. Temporal models are resistant to changes in threshold under roving-level conditions, but explained at most only 67% of the variance in the reproducible-noise data.     

Sound intensity processing by the goldfish

Capacities of the goldfish for intensity discrimination were studied using classical respiratory conditioning and a staircase psychophysical procedure. Physiological studies on single saccular (auditory) nerve fibers under similar stimulus conditions helped characterize the dimensions of neural activity used in intensity discrimination. Incremental intensity difference limens (IDLs in dB) for 160-ms increments in continuous noise, 500-ms noise bursts, and 500-ms, 800-Hz tone bursts are 2 to 3 dB, are independent of overall level, and vary with signal duration according to a power function with a slope averaging - 0.33. Noise decrements are relatively poorly detected and the silent gap detection threshold is about 35 ms. The IDLs for increments and decrements in an 800-Hz continuous tone are about 0.13 dB, are independent of duration, and are level dependent. Unlike mammalian auditory nerve fibers, some goldfish saccular fibers show variation in recovery time to tonal increments and decrements, and adaptation to a zero rate. Unit responses to tone increments and decrements show rate effects generally in accord with previous observations on intracellular epsp's in goldfish saccular fibers. Neurophysiological correlates of psychophysical intensity discrimination data suggest the following: (1) noise gap detection may be based on spike rate increments which follow gap offset; (2) detection of increments and decrements in continuous tones may be determined by steep low-pass filtering in peripheral neural channels which enhance the effects of spectral "splatter" toward the lower frequencies; (3) IDLs for pulsed signals of different duration can be predicted from the slopes of rate-intensity functions and spike rate variability in individual auditory nerve fibers; and (4) at different sound pressure levels, different populations of peripheral fibers provide the information used in intensity discrimination.     

Probe tone thresholds in the auditory nerve measured by two-interval forced-choice procedures

An important goal of auditory physiology is to relate the coding of signals in the auditory nerve to behavioral sensitivity. A useful step towards that goal is to measure physiological thresholds for the detection of tones in the neural spike train that are comparable to psychophysical thresholds. Detectability depends on the variability as well as the mean value of the response. A two-interval forced-choice task provides a criterion-free measure of detectability. On each trial of our experiments a probe tone was taken to be correctly detected if the number of spikes in response to the tone exceeded the number of spikes in an otherwise identical interval that did not contain the probe tone. (Analysis of the pulse-number distributions also allowed construction of ROC curves directly comparable to psychophysical ROC curves.) The proportion of trials that yielded correct detections was measured as a function of stimulus intensity to form a neurometric function, directly comparable to a psychophysical psychometric function. Threshold was defined as the intensity that produced a given proportion correct. The threshold intensity was also measured by an up-down procedure. Agreement between the two measures of threshold was excellent. Using the up-down procedure we could measure threshold in about 1 min, making it practical to measure the thresholds of a single neuron for many conditions. Comparisons of physiological and psychophysical ROC curves and neurometric and psychometric functions show systematic differences indicating that the animal makes its decisions inefficiently, perhaps by basing its decision on the maximum response among many neurons, rather than just the activity of the single most sensitive neuron.     

Neuronal responses to amplitude-modulated and pure-tone stimuli in the guinea pig inferior colliculus, and their modification by broadband noise

Neuronal responses were recorded to pure and to sinusoidally amplitude-modulated (AM) tones at the characteristic frequency (CF) in the central nucleus of the inferior colliculus of anesthetized guinea pigs. Temporal (synchronized) and mean-rate measures were derived from period histograms locked to the stimulus modulation waveform to characterize the modulation response. For stimuli presented in quiet, the modulation gain at low frequencies of modulation (approx less than 50 Hz) was inversely proportional to the neuron's mean firing rate in response to both the modulated stimulus and to a pure tone at an equivalent level. In 43% of units the mean discharge rates in response to the AM stimuli were greatest for those modulation frequencies that generated the largest temporal responses. These discharge-rate maxima occurred at signal intensities corresponding to the steeply sloping part of the neuron's pure-tone rate-intensity function (RIF). The change in mean-rate response to modulated stimuli, as a function of intensity, was qualitatively similar to the pure-tone RIF. Adding broadband noise to the modulated stimulus increased the neuron's temporal response to low modulation frequencies. This increase in modulation gain was correlated with mean firing rate in response to the modulation but did not bear a simple relationship to the noise-induced shift in the RIF measured for a pure tone.     

Hemodynamic responses in human multisensory and auditory association cortex to purely visual stimulation

Background  

Recent findings of a tight coupling between visual and auditory association cortices during multisensory perception in monkeys and humans raise the question whether consistent paired presentation of simple visual and auditory stimuli prompts conditioned responses in unimodal auditory regions or multimodal association cortex once visual stimuli are presented in isolation in a post-conditioning run. To address this issue fifteen healthy participants partook in a "silent" sparse temporal event-related fMRI study. In the first (visual control) habituation phase they were presented with briefly red flashing visual stimuli. In the second (auditory control) habituation phase they heard brief telephone ringing. In the third (conditioning) phase we coincidently presented the visual stimulus (CS) paired with the auditory stimulus (UCS). In the fourth phase participants either viewed flashes paired with the auditory stimulus (maintenance, CS-) or viewed the visual stimulus in isolation (extinction, CS+) according to a 5:10 partial reinforcement schedule. The participants had no other task than attending to the stimuli and indicating the end of each trial by pressing a button.     

Effects of stimulus duration on the response properties of Pacinian corpuscles: implications for the neural code.

Based on physiological and psychophysical data, it has been suggested that the neural code for threshold detection in the P channel, mediated by Pacinian corpuscles (PCs), may be 2-4 neural impulses/stimulus (Bolanowski et al., 1988). To further test the efficacy of this code, responses from PCs were measured at different vibratory burst durations. Since it is known that the P channel has the capacity to summate vibratory stimuli temporally within the central nervous system, the effect should also be present in the physiological results. Temporal summation predicts a decrease in threshold at a rate of -3 dB/doubling of stimulus duration. Responses from single PCs were integrated by counting neural spikes over the entire burst duration. Furthermore, real-time responses were integrated with a low-pass filter, more accurately modeling the central process. For the spike-counting scheme, a criterion of 4 impulses/stimulus showed a decrease in stimulus amplitude for increases in duration similar to that obtained psychophysically. The amplitude-duration function obtained with the low-pass filter, however, resulted in a function which did not follow that obtained psychophysically, regardless of the number of impulses/stimulus. Since the P channel is known to have a central integrator, it has been concluded that activity of a single PC afferent probably is not sufficient to signal threshold in the P channel.     

Two-, three-, and four-interval forced-choice staircase procedures: estimator bias and efficiency

Threshold estimates for multiple-interval forced-choice staircase procedures were studied using computer simulations. A sigmoidal psychometric function shape governed the hypothetical subject's responses in the simulations. Parameters varied included the number of trials, the step size for stimulus level change, and decision rules that targeted 70.7% and 79.4% correct performance. Each threshold estimate was calculated by averaging the stimulus levels at which a reversal a stimulus level direction occurred. The results of the simulations suggest that, as the number of alternatives is increased from 2 to 4, the variability of repeated threshold estimates decreases or remains constant, and the accuracy of the estimator, in most cases, improves. A subset of the simulations was compared with data obtained in a detection-in-noise task. The behavioral data were consistent with the simulation results. Two major conclusions were reached. First, 3- and 4-interval forced-choice (IFC) procedures are more efficient than a 2IFC procedure with a decision rule that targets 70.7% correct performance even when the additional time required to complete 3- and 4IFC trials is considered. Second, the accuracy of 2IFC procedures can be improved by fitting the trial history of a staircase run using probit analysis.     

Auditory brainstem responses in the Eastern Screech Owl: an estimate of auditory thresholds

The auditory brainstem response (ABR), a measure of neural synchrony, was used to estimate auditory sensitivity in the eastern screech owl (Megascops asio). The typical screech owl ABR waveform showed two to three prominent peaks occurring within 5 ms of stimulus onset. As sound pressure levels increased, the ABR peak amplitude increased and latency decreased. With an increasing stimulus presentation rate, ABR peak amplitude decreased and latency increased. Generally, changes in the ABR waveform to stimulus intensity and repetition rate are consistent with the pattern found in several avian families. The ABR audiogram shows that screech owls hear best between 1.5 and 6.4 kHz with the most acute sensitivity between 4-5.7 kHz. The shape of the average screech owl ABR audiogram is similar to the shape of the behaviorally measured audiogram of the barn owl, except at the highest frequencies. Our data also show differences in overall auditory sensitivity between the color morphs of screech owls.     

Opioid modulation of GABA release in the rat inferior colliculus

Background  

The inferior colliculus, which receives almost all ascending and descending auditory signals, plays a crucial role in the processing of auditory information. While the majority of the recorded activities in the inferior colliculus are attributed to GABAergic and glutamatergic signalling, other neurotransmitter systems are expressed in this brain area including opiate peptides and their receptors which may play a modulatory role in neuronal communication.     

The representation of periodic sounds in simulated sustained chopper units of the ventral cochlear nucleus

The nature of the neural processing underlying the extraction of pitch information from harmonic complex sounds is still unclear. Electrophysiological studies in the auditory nerve and many psychophysical and modeling studies suggest that pitch might be extracted successfully by applying a mechanism like autocorrelation to the temporal discharge patterns of auditory-nerve fibers. The current modeling study investigates the possible role of populations of sustained chopper (Chop-S) units located in the mammalian ventral cochlear nucleus (VCN) in this process. First, it is shown that computer simulations can predict responses to periodic and quasiperiodic sounds of individual Chop-S units recorded in the guinea-pig VCN. Second, it is shown that the fundamental period of a periodic or quasiperiodic sound is represented in the first-order, interspike interval statistics of a population of simulated Chop-S units. This is true across a wide range of characteristic frequencies when the chopping rate is equal to the f0 of the sound. The model was able to simulate the results of psychophysical studies involving the pitch height and pitch strength of iterated ripple noise, the dominance region of pitch, the effect of phase on pitch height and pitch strength, pitch of inharmonic stimuli, and of sinusoidally amplitude modulated noise. Simulation results indicate that changes in the interspike interval statistics of populations of Chop-S units compare well with changes in the pitch perceived by humans. It is proposed that Chop-S units in the ventral cochlear nucleus may play an important role in pitch extraction: They can convert a purely temporal pitch code as observed in the auditory nerve into a temporal place code of pitch in populations of cochlear-nucleus, Chop-S with different characteristic frequencies, and chopping rates. Thus, populations of cochlear-nucleus Chop-S units, together with their target units presumably located in the inferior colliculus, may serve to establish a stable rate-place code of pitch at the level of the auditory cortex.