Effects of noise bandwidth on the late-potential masking level difference

The masking level difference (MLD) is a psychoacoustic phenomenon derived from the subtraction of Sπ No thresholds (signals π radians out of phase and noise in phase at the two ears) from SoNo thresholds (signals and noise in phase at the two ears). The purpose of this study was to determine if the MLD derived from the late components (P1, N1, P2, N2) of the auditory evoked potentials was a physiological correlate of the behavioral MLD. Subjects were 15 young adults with normal hearing. Comparisons were made between behavioral and late potential thresholds to 500 Hz stimuli in So and Sπ conditions in quiet, and to 500 Hz stimuli in SoNo and Sπ No conditiones in narrow band (50 Hz) and wide band (600 Hz) noise. No significant differences were seen for behavioral or late-potential thresholds to So and Sπ conditions. The SπNo threshold was significantly lower than the SoNo threshold, yielding an MLD for both the behavioral and physiological responses. The magnitudes of both the behavioral and late-potential MLD were larger with the narrow band noise than with the wide band noise. Evidence, therefore, is provided that the late-potential MLD reflects similar processes as are responsible for the behavioral MLD.     

Response of frog midbrain neurons to tones amplitude-modulated by pseudorandom noise

Spike response in torus semicircularis units to the effects of uninterrupted characteristic frequency tones amplitude-modulated by pseudorandom noise were investigated during experiments on immobilizedRana ridibunda. Period histograms of modulating waveform of 512 msec duration (both modulating polarities) were produced for 32 units. Almost all neurons investigated responded exclusively to the positive half of the modulating signal. Difference histograms obtained by calculating period histograms for different polarities of the envelope faithfully reproduced the dynamics of signal amplitude in four units. The remainder responded only to envelope maxima, without reproducing amplitude dynamics among these; over half the units represented only some of the envelope maxima, moreover. Certain cells were found which retained their specific pattern of response to pseudorandom noise over a wide range of carrier intensities.N. N. Andreev Acoustical Institute, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 2, pp. 227–235, March–April, 1990.     

Efferent neurons of the auditory center in the midbrain of the frog Rana ridibunda

Individual cells which produce projections from the torus semicircularis in the frog have been visualized after injection of horseradish peroxidase (HRP) to various thalamic and isthmal areas. Labeled toral cells were observed if HRP had been injected to the posterodorsal areas of the thalamus or to the isthmal areas where lateral lemniscus fibers and cells of the premature lateral lemniscal nucleus are situated. Medium and large size cells in the rostrolateral torus semicircularis were mainly labeled. Thalamic injections of the HRP produced more labeled cells in the lateral part of the magnocellular nucleus, whereas isthmal injections produced labeled cells mainly in the lateral part of the laminar nucleus. A few HRP containing cells were observed in the principal nucleus of the torus. Specificity of the neuronal organisation of the auditory pathway in amphibians is discussed.     

Seasonal changes in frequency tuning and temporal processing in single neurons in the frog auditory midbrain

Frogs rely on acoustic signaling to detect, discriminate, and localize mates. In the temperate zone, reproduction occurs in the spring, when frogs emerge from hibernation and engage in acoustically guided behaviors. In response to the species mating call, males typically show evoked vocal responses or other territorial behaviors, and females show phonotactic responses. Because of their strong seasonal behavior, it is possible that the frog auditory system also displays seasonal variation, as evidenced in their vocal control system. This hypothesis was tested in male Northern leopard frogs by evaluating the response characteristics of single neurons in the torus semicircularis (TS; a homolog of the inferior colliculus) to a synthetic mating call at different times of the year. We found that TS neurons displayed a seasonal change in frequency tuning and temporal properties. Frequency tuning shifted from a predominance of TS units sensitive to intermediate frequencies (700-1200 Hz) in the winter, to low frequencies (100-600 Hz) in the summer. In winter and early spring, most TS neurons showed poor, or weak, time locking to the envelope of the amplitude-modulated synthetic call, whereas in late spring and early summer the majority of TS neurons showed robust time-locked responses. These seasonal differences indicate that neural coding by auditory midbrain neurons in the Northern leopard frog is subject to seasonal fluctuation.     

Free-field unmasking response characteristics of frog auditory nerve fibers: comparison with the responses of midbrain auditory neurons

Previous studies in the inferior colliculus have shown that spatial separation of signal and noise sources improves signal detection. In this study, we investigated the free-field unmasking response properties of single fibers in the auditory nerve--these were compared to those of inferior colliculus neurons under the same experimental conditions to test the hypothesis that central processing confers advantages for signal detection in the presence of spatially separated noise. For each neuron, we determined the detection threshold for a probe at the unit's best azimuth under three conditions: (1) by itself, (2) when a masker at a constant level was also presented at the unit's best azimuth, and (3) when the masker was positioned at different azimuths. We found that, on average, maskers presented at a unit's best azimuth elevated the probe detection threshold by 4.22 dB in the auditory nerve and 10.97 dB in the inferior colliculus. Angular separation of probe and masker sources systematically reduced the masking effect. The maximum masking release was on average 2.90 dB for auditory nerve fibers and 9.40 dB for inferior colliculus units. These results support the working hypothesis, suggesting that central processing contributes to the stronger free-field unmasking in the inferior colliculus.     

Coding of amplitude modulation in the auditory midbrain of the bullfrog (Rana catesbeiana) across metamorphosis

The functional development of the auditory system across metamorphosis was examined by recording neural activity from the torus semicircularis of larval and postmetamorphic bullfrog froglets in response to amplitude-modulated sound. Multiunit activity in the torus semicircularis during early larval stages showed significant phase-locking to the envelopes of amplitude-modulated noise bursts, up to modulation rates as high as 250 Hz. Beginning at metamorphic climax and continuing into the froglet period, phase locking was restricted to the more limited frequency range characteristic of adult frogs. The onset of operation of the tympanic pathway does not reinstate the highly synchronous neural activity characteristic of the operation of the fenestral pathway. Modulation transfer functions based on spike count did not show tuning for modulation rate in early stage tadpoles, but a greater variety of shapes of these functions emerged as development proceeded. Most of the different kinds of modulation transfer functions seen in adult frogs were also observed in froglets, but band-pass functions were not as sharply peaked. These data suggest that different neural codes for processing of the periodicity of complex signals operate in early stage tadpoles than in postmetamorphic froglets. Accepted: 7 October 1998     

Interval-integration underlies amplitude modulation band-suppression selectivity in the anuran midbrain

We examined the mechanisms that underlie band-suppression amplitude modulation selectivity in the auditory midbrain of anurans. Band-suppression neurons respond well to low (5–10 Hz) and high (>70 Hz) rates of sinusoidal amplitude modulation, but poorly, if at all, to intermediate rates. The effectiveness of slow rates of sinusoidal amplitude modulation is due to the long duration of individual pulses; short-duration pulses (<10 ms) failed to elicit spikes when presented at 5–10 pulses s^–1. Each unit responded only after a threshold number of pulses (median=3, range=2–5) were delivered at an optimal rate. The salient stimulus feature was the number of consecutive interpulse intervals that were within a cell-specific tolerance. This interval-integrating process could be reset by a single long interval, even if preceded by a suprathreshold number of intervals. These findings indicate that band-suppression units are a subset of interval-integrating neurons. Band-suppression neurons differed from band-pass interval-integrating cells in having lower interval-number thresholds and broader interval tolerance. We suggest that these properties increase the probability of a postsynaptic spike, given a particular temporal pattern of afferent action potentials in response to long-duration pulses, i.e., predispose them to respond to slow rates of amplitude modulation. Modeling evidence is provided that supports this conclusion.Abbreviations AM amplitude modulation - PRR pulse repetition rate - SAM sinusoidal amplitude modulation     

Process of adaptation in frog auditory system neurons

The responses of single neurones located in different parts of the auditory system of amphibians to tone signals of a small death of amplitude modulation were studied. It was shown that the firing rate generally diminished during both the first second of sounding (short-term adaptation) and subsequent several tens of seconds (long-term adaptation). In a considerable proportion of neurones, a sharp improving of the phase-locking of the response to modulation waveform was observed in parallel the drop in firing rate. These effects are expressed much more strongly in higher nucleus of the auditory system. A sharp accentuation of modulation waveform could be seen also in the completely adapted regime. In some cases, this effect was evident only after the addition of a random noise to the modulating function (stochastic resonance effect). These data were compared with physiological results obtained on mammals and with psychophysical observations.     

Developmental experience alters information coding in auditory midbrain and forebrain neurons

In songbirds, species identity and developmental experience shape vocal behavior and behavioral responses to vocalizations. The interaction of species identity and developmental experience may also shape the coding properties of sensory neurons. We tested whether responses of auditory midbrain and forebrain neurons to songs differed between species and between groups of conspecific birds with different developmental exposure to song. We also compared responses of individual neurons to conspecific and heterospecific songs. Zebra and Bengalese finches that were raised and tutored by conspecific birds, and zebra finches that were cross‐tutored by Bengalese finches were studied. Single‐unit responses to zebra and Bengalese finch songs were recorded and analyzed by calculating mutual information (MI), response reliability, mean spike rate, fluctuations in time‐varying spike rate, distributions of time‐varying spike rates, and neural discrimination of individual songs. MI quantifies a response's capacity to encode information about a stimulus. In midbrain and forebrain neurons, MI was significantly higher in normal zebra finch neurons than in Bengalese finch and cross‐tutored zebra finch neurons, but not between Bengalese finch and cross‐tutored zebra finch neurons. Information rate differences were largely due to spike rate differences. MI did not differ between responses to conspecific and heterospecific songs. Therefore, neurons from normal zebra finches encoded more information about songs than did neurons from other birds, but conspecific and heterospecific songs were encoded equally. Neural discrimination of songs and MI were highly correlated. Results demonstrate that developmental exposure to vocalizations shapes the information coding properties of songbird auditory neurons. © 2009 Wiley Periodicals, Inc. Develop Neurobiol 70: 235–252, 2010.     

Encoding of amplitude modulations by auditory neurons of the locust: influence of modulation frequency,rise time,and modulation depth

Using modulation transfer functions (MTF), we investigated how sound patterns are processed within the auditory pathway of grasshoppers. Spike rates of auditory receptors and primary-like local neurons did not depend on modulation frequencies while other local and ascending neurons had lowpass, bandpass or bandstop properties. Local neurons exhibited broader dynamic ranges of their rate MTF that extended to higher modulation frequencies than those of most ascending neurons. We found no indication that a filter bank for modulation frequencies may exist in grasshoppers as has been proposed for the auditory system of mammals. The filter properties of half of the neurons changed to an allpass type with a 50% reduction of modulation depths. Contrasting to reports for mammals, the sensitivity to small modulation depths was not enhanced at higher processing stages. In ascending neurons, a focus on the range of low modulation frequencies was visible in the temporal MTFs, which describe the temporal locking of spikes to the signal envelope. To investigate the influence of stimulus rise time, we used rectangularly modulated stimuli instead of sinusoidally modulated ones. Unexpectedly, steep stimulus onsets had only small influence on the shape of MTF curves of 70% of neurons in our sample.     

Distribution of auditory motion-direction sensitive neurons in the barn owl's midbrain

Barn owls have neurons sensitive to acoustic motion-direction in the midbrain. We report here that acoustic motion-direction sensitive neurons with receptive-field centres in frontal auditory space are not randomly distributed. In the inferior colliculus and optic tectum in the left (right) brain, the responses of about two-thirds of the motion-direction sensitive neurons were sensitive to clockwise (counter-clockwise) motion. The midbrain contains maps of auditory space that represent about 15 degrees of ipsilateral and all of contralateral space. Since a similar bias in motion-direction sensitivity was observed for neurons with receptive-field centres in ipsilateral as well as for neurons with receptive fields centres in contralateral auditory space, the brain side at which a motion-direction sensitive neuron was recorded was a more important predictor for the preferred direction of a cell than the spatial direction of the centre of the receptive field. Within one dorso-ventral electrode pass motion-direction sensitivity typically stayed constant suggesting a clustered or even a columnar-like organization. We hypothesize from these distributions that the right brain is important for orientating movements to the left hemisphere and vice versa.     

Adaptation of differential sensitivity of auditory system neurons to amplitude modulation after abrupt change of signal level

Impulse activity of neurons of brainstem auditory nuclei (medulla dorsal nucleus and midbrain torus semicircularis) of the grass frog (Rana temporaria) was recorded under action of long amplitude-modulated tonal signals. After adaptation of neuronal response to acting stimulus (30–60 s after its onset), we performed a sharp change (by 20–40 dB) of the mean signal level with preservation of unchanged frequency and depth of modulation. We also recorded a change of density impulsation and of degree of its synchronization with the modulation period as well as the phase of maximum reaction at the modulation period and phase of the response every 2 or 4 s. In the adapted state, the sharp change of the mean level had been provided, while maintaining frequency and depth unchanged. During the adaptation to long signals with small modulation indexes the firing rate continuously decreased, but synchronization with envelope usually increased considerably. A sharp rise in the mean level resulted in an increase of firing rate, which could be accompanied either by a continuation of synchronization growth (the effect is more typical of the dorsal nucleus) or by a sharp fall in synchrony with its subsequent slow recovery (the effect is more typical of the torus semicircularis). Nature of the changes following the change of the intensity of the reaction could depend on the signal parameters (initial level, magnitude of the jump, frequency and depth of modulation). The connection between the observed physiological data and the psychophysics of differential intensity coding is discussed.     

Responses of midbrain auditory neurons in rats to FM and AM tones presented simultaneously

Speech and other communication signals contain components of frequency and amplitude modulations (FM, AM) that often occur together. Auditory midbrain (or inferior colliculus, IC) is an important center for coding time-varying features of sounds. It remains unclear how IC neurons respond when FM and AM stimuli are both presented. Here we studied IC neurons in the urethane-anesthetized rats when animals were simultaneously stimulated with FM and AM tones. Of 122 units that were sensitive to the dual stimuli, the responses could be grossly divided into two types: one that resembled the respective responses to FM or AM stimuli presented separately ("simple" sensitivity, 45% of units), and another that appeared markedly different from their respective responses to FM or AM tones ("complex" sensitivity, 55%). These types of combinational sensitivities were further correlated with individual cell's frequency tuning pattern (response area) and with their common response pattern to FM and AM sounds. Results suggested that such combinational sensitivity could reflect local synaptic interactions on IC neurons and that the neural mechanisms could underlie more developed sensitivities to acoustic combinations found at the auditory cortex.     

Bilateral collicular interaction: modulation of auditory signal processing in amplitude domain

In the ascending auditory pathway, the inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from many lower auditory nuclei, intrinsic projections within the IC, contralateral IC through the commissure of the IC and from the auditory cortex. All these connections make the IC a major center for subcortical temporal and spectral integration of auditory information. In this study, we examine bilateral collicular interaction in modulating amplitude-domain signal processing using electrophysiological recording, acoustic and focal electrical stimulation. Focal electrical stimulation of one (ipsilateral) IC produces widespread inhibition (61.6%) and focused facilitation (9.1%) of responses of neurons in the other (contralateral) IC, while 29.3% of the neurons were not affected. Bilateral collicular interaction produces a decrease in the response magnitude and an increase in the response latency of inhibited IC neurons but produces opposite effects on the response of facilitated IC neurons. These two groups of neurons are not separately located and are tonotopically organized within the IC. The modulation effect is most effective at low sound level and is dependent upon the interval between the acoustic and electric stimuli. The focal electrical stimulation of the ipsilateral IC compresses or expands the rate-level functions of contralateral IC neurons. The focal electrical stimulation also produces a shift in the minimum threshold and dynamic range of contralateral IC neurons for as long as 150 minutes. The degree of bilateral collicular interaction is dependent upon the difference in the best frequency between the electrically stimulated IC neurons and modulated IC neurons. These data suggest that bilateral collicular interaction mainly changes the ratio between excitation and inhibition during signal processing so as to sharpen the amplitude sensitivity of IC neurons. Bilateral interaction may be also involved in acoustic-experience-dependent plasticity in the IC. Three possible neural pathways underlying the bilateral collicular interaction are discussed.     

Multiple-band trigger features of midbrain auditory neurons revealed in composite spectro-temporal receptive fields

Receptive fields of single units in the auditory midbrain of anesthetized rats were studied using random FM-tone stimuli of narrow frequency-ranges. Peri-spike averaging of the modulating waveform first produced a spectro-temporal receptive field (STRF). Combining STRFs obtained from the same unit at different frequency regions generated a composite receptive field covering a wider frequency range of 2 to 3 octaves. About 20% of the composite STRFs (26/122) showed a pattern of multiple-bands which were not clear in the non-composite maps. Multiple-bands in a given composite map were often oriented in the same direction (representing upward or downward FM ramp) separated at rather regular frequency intervals. They reflect multiple FM trigger features in the stimulus rather than repetitive firing to a single trigger feature. Results showed that the subcortical auditory pathways are capable of detecting multiple FM features and such sensitivity could be useful in detecting multiple-harmonic FM bands present in the vocalization sounds.     

GABAergic inhibition shapes frequency tuning and modifies response properties in the auditory midbrain of the leopard frog

The functional role of GABAergic inhibition in shaping the frequency tuning of 96 neurons in the torus semicircularis of the leopard frog, Rana pipiens, was studied using microiontophoresis of the GABA_A receptor antagonist, bicuculline methiodide. Bicuculline application abolished, or reduced in size, the inhibitory tuning curves of 72 neurons. In each case, there was a concommitant broadening of the excitatory tuning curve such that frequency-intensity combinations that were inhibitory under control conditions, became excitatory during disinhibition with bicuculline methiodide. These effects were observed irrespective of the excitatory tuning curve configuration prior to bicuculline methiodide application. Results indicate an important role for GABA-mediated inhibition in shaping the frequency selectivity of neurons in the torus semicircularis of the leopard frog. Bicuculline application also affected several other response properties of neurons in the leopard frog torus. Disinhibition with bicuculline methiodide increased both the spontaneous firing rate (18 cells) and stimulus-evoked discharge rate (81 cells) of torus neurons, decreased the minimum excitatory threshold for 18 cells, and altered the temporal discharge pattern of 47 neurons. Additional roles for GABAergic inhibition in monaural signal analysis are discussed. Accepted: 25 August 1999     

Stimulus change detection in phasic auditory units in the frog midbrain: frequency and ear specific adaptation

Neural adaptation, a reduction in the response to a maintained stimulus, is an important mechanism for detecting stimulus change. Contributing to change detection is the fact that adaptation is often stimulus specific: adaptation to a particular stimulus reduces excitability to a specific subset of stimuli, while the ability to respond to other stimuli is unaffected. Phasic cells (e.g., cells responding to stimulus onset) are good candidates for detecting the most rapid changes in natural auditory scenes, as they exhibit fast and complete adaptation to an initial stimulus presentation. We made recordings of single phasic auditory units in the frog midbrain to determine if adaptation was specific to stimulus frequency and ear of input. In response to an instantaneous frequency step in a tone, 28 % of phasic cells exhibited frequency specific adaptation based on a relative frequency change (delta-f = ±16 %). Frequency specific adaptation was not limited to frequency steps, however, as adaptation was also overcome during continuous frequency modulated stimuli and in response to spectral transients interrupting tones. The results suggest that adaptation is separated for peripheral (e.g., frequency) channels. This was tested directly using dichotic stimuli. In 45 % of binaural phasic units, adaptation was ear specific: adaptation to stimulation of one ear did not affect responses to stimulation of the other ear. Thus, adaptation exhibited specificity for stimulus frequency and lateralization at the level of the midbrain. This mechanism could be employed to detect rapid stimulus change within and between sound sources in complex acoustic environments.