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At least 1,405 records · Page 78Linked to original sources
PubMed1982

Directionality of sound pressure transformation at the cat's pinna.

The directionality of the cat's pinna was studied by using the amplitude of the cochlear microphonic (CM) as a quantitative indicator of tympanic sound pressure level (SPL). It was found that tympanic SPL varied with the location of a free field stimulator in anechoic space. For high (tonal) frequencies, there was a circumscribed optimal area for tympanic SPL in the frontal ipsilateral sound field, in confirmation of previous findings with other techniques that the pinna has an acoustical axis. The directionality of the pinna, determined from the solid angle enclosed by the 5 dB isointensity-decrement line with respect to the optimal position, increased with frequency. For low tonal frequencies, no circumscribed optimal area in the frontal sound field could be distinguished, and tympanic SPL fell by only 10-12 dB for displacements of 90 degrees into the contralateral sound field. Excision of the pinna abolished the circumscribed optimal areas for tympanic SPL and revealed the pinna produces up to 28 dB amplification of acoustic signals delivered 'on-axis'.

Animals↗
PubMed1982

Quantitative analysis of intensity--rate and intensity--latency functions in peripheral auditory nerve fibers of northern leopard frogs (Rana p. pipiens).

The intensity--rate and intensity--latency functions were analyzed quantitatively from single auditory fibers in the eighth nerve of northern leopard frogs (Rana p. pipiens). There was a firing rate change of as high as 15 impulses/s and a latency shift of as much as 1.4 ms per dB change in sound intensity. These rates of change were different for units with the same best excitatory frequency and greater for each unit studied at lower intensities than at high intensities. The significance of the latency function on central binaural time processing was discussed.

Acoustic Stimulation↗
PubMed1983

The precedence effect.

When two similar transient sounds are presented binaurally in rapid succession, observers hear a single sound from a location which depends mainly on the properties of the first sound to reach the ears. This phenomenon, known as the precedence effect, was explored using stimuli consisting of 20 mus pulses presented using earphones; experiments were carried out on both the classical precedence effect (in which interaural delays provide the cues to lateralization) and on an amplitude-based precedence effect, where interaural amplitude differences provide the cues. Some experiments on the amplitude-based precedence effect led to unexpected but highly consistent anomalous results. The spectral characteristics of stimuli used in studies of both the classical and amplitude-based precedence effect were considered and, provided the delay between the two pairs of pulses used in the experiments is 600 mus or less, observers' behaviour is simply related to the amplitude and phase spectra of the stimuli.

Acoustic Stimulation↗
PubMed1983

Spatial receptive fields in the cat inferior colliculus.

Auditory spatial receptive fields of 122 single units in the inferior colliculus of 8 anesthetised cats were studied with free-field acoustic stimuli presented in the frontal hemisphere. The best frequency and best frequency threshold were determined for each unit with the speaker located in a position where the unit responded strongly. The intensity was then raised to 10 dB above threshold at the best frequency and the boundaries of the spatial receptive field were determined. For sounds of low intensity, receptive field size appeared to be a continuum with respect to best frequency. Units of high best frequency had small circumscribed fields located in the contralateral frontal hemifield. The boundaries of receptive fields for units of progressively lower best frequency expanded in all directions. Thus for intermediate frequencies, fields typically filled the contralateral hemifield whereas for low frequencies, units could be activated by stimulation from any position tested. At higher intensities, the boundaries of the receptive fields of units expanded. Circumscribed receptive field centres lay on a line corresponding to the acoustical axis of the contralateral pinna. For these units with small receptive fields, the free-field response to low intensity sounds appeared to be attributable more to the directional properties of the contralateral pinna than to significant binaural interaction.

Acoustic Stimulation↗
PubMed1984

Interaural intensity difference sensitivity based on facilitatory binaural interaction in cat superior colliculus.

Sensitivity to interaural intensity difference (IID) has generally been identified as a property of neurons exhibiting inhibitory binaural interaction, viz. contralateral excitatory and ipsilateral inhibitory input (EI cells). In the deep layers of the superior colliculus, however, almost 30% of IID-sensitive cells are characterised by facilitatory or mixed facilitatory/inhibitory interactions. Such cells typically have peaked IID sensitivity functions in contrast to the step functions characteristic of EI cells. There appears to be a continuum in IID sensitivity from pure step functions to sharply-peaked functions. The observation that a given form of IID sensitivity can be associated with patterns of binaural interaction other than that by which it is most commonly produced suggests that IID-sensitive neurons are better classified on the basis of the form of their IID sensitivity than their binaural input pattern. It seems probable that IID sensitivity based on facilitatory and mixed facilitatory/inhibitory binaural interactions is a general characteristic of the primary auditory pathway, although only fragmentary data are so far available.

Animals↗
PubMed1984

Properties of spatial receptive fields in the central nucleus of the cat inferior colliculus. I. Responses to tones of low intensity.

Single neurones in the central nucleus of the inferior colliculus (ICC) of barbiturate-anesthetized cats were examined using free-field, pure-tone stimuli of low intensity at the neurones' best frequency. Receptive field size was inversely correlated with best frequency. Almost all neurones were maximally excited by stimulus positions in the hemifield contralateral to the recording electrode, irrespective of their best frequency. Simultaneous cochlear microphonic recording revealed that the neurones' best excitatory area was also the spatial region associated with maximum amplification by the contralateral outer ear. This amplification resulted in extremely low (less than -20 dB SPL in some neurones) best frequency thresholds. Response patterns were found not to vary markedly with speaker position. The results suggest that most ICC neurones are more sensitive to stimulation of the contralateral ear than to stimulation of the ipsilateral ear.

Animals↗
PubMed1984

Properties of spatial receptive fields in the central nucleus of the cat inferior colliculus. II. Stimulus intensity effects.

Single units in the central nucleus of the inferior colliculus (ICC) of barbiturate-anaesthetized cats were studied using pure-tone, best-frequency stimulation presented in the free field. At low stimulus intensities almost all neurones responded most strongly to stimuli positioned along the acoustical axis of the pinna contralateral to the recording electrode and there was little or no response to stimuli positioned in the ipsilateral hemifield. Four major classes of spatial response were distinguished when tones of moderate to high intensity were used. The simplest response (24% of the sample) to increasing intensity consisted of a monotonic increase in discharge level at all effective speaker positions and an expansion of the area of space from which a stimulus influenced the response (receptive field). A second class (21%) of units had a nonmonotonic increase in discharge level and an expanding receptive field with increasing intensity. Neither of these classes showed evidence of influence from the ipsilateral ear. The third class (26%) developed, at higher intensities, a second excitatory response region in the ipsilateral hemifield. The fourth class (20%) had receptive fields with fixed medial borders, irrespective of intensity. The third and fourth classes of units were thought to be binaurally influenced and to be sensitive to interaural phase and intensity differences, respectively.

Animals↗
PubMed1985

A functional organization of binaural responses in the inferior colliculus.

Binaural responses are segregated within an enlarged isofrequency region of the mustache bat's inferior colliculus. In the part of this region containing E-I responses, there is an orderly shift in the sensitivity of unit clusters to interaural intensity disparities, having implications for the representation of auditory space within isofrequency regions of the inferior colliculus.

Animals↗
PubMed1985

A monaural space map in the guinea-pig superior colliculus.

Under anechoic conditions, a horizontal array of loudspeakers was used to investigate the representation of auditory space in the guinea-pig superior colliculus. We have previously demonstrated that in animals with both ears intact, there is a topographical representation of the azimuthal dimension of auditory space in the deep layers of this nucleus. In the present study, we have investigated the contribution of monaural and binaural cues to the generation of the auditory space map. Occlusion of one ear or unilateral cochlear destruction resulted in omnidirectional responses in all cells to white-noise stimuli more than 20 dB suprathreshold. The sensitivity of cells to the location of sound at or near threshold was, however, unchanged and we demonstrate the presence of a threshold, monaural auditory space map. This monaural space map was destroyed by removal of the contralateral pinna and concha which resulted in all cells responding best, at threshold, to sounds opposite the external auditory meatus. Measurements of cochlear microphonic (CM) potentials, although variable, revealed that the pinna and concha may result in location-specific changes in the spectral pattern at the tympanic membrane.

Animals↗
PubMed1985

The four factors leading to binaural masking-level differences.

A simple extension of the Webster-Jeffress model is presented together with its predictions for the effects of various stimulus parameters on the size of binaural masking-level differences (BMLDs). The four factors leading to BMLDs (just-noticeable differences (JNDs), temporal effects in simultaneous masking, binaural interaction, and temporal effects in non-simultaneous masking) are described, new measurements of the effect of signal duration on the detectability of interaural delay are presented, and the high degree of correlation between observers' sensitivity to changes in level and their sensitivity to changes in interaural delay is demonstrated. A number of examples illustrating where knowledge of JNDs for level and interaural delay and their joint dependence on certain stimulus parameters are sufficient to predict BMLDs are discussed.

Auditory Perception↗
PubMed1986

Directional sensitivity of the auditory midbrain in the mustached bat to free-field tones.

To ascertain the directional characteristics of the auditory system in the mustached bat, Pteronotus parnellii, we measured the summated neural response at the lateral lemniscus (N4) in response to pure tones at 30, 60 and 90 kHz, frequencies that are typical of the harmonics of this species' biosonar signal. Stimuli were presented at various vertical and horizontal locations in the contralateral hemifield. Intensity-response functions were measured at different horizontal locations for the second harmonic, and showed no variation in shape with variations in azimuth. There was little difference in directionality measured from either threshold or amplitude of N4 potentials. Our results show that areas of maximum sensitivity (best areas) were significantly different for each of the harmonics (P less than 0.05). The centers of the best areas were: first harmonic (30 kHz), 39 degrees azimuth and -19 degrees elevation; second harmonic, 20 degrees azimuth and 0 degrees elevation; and third harmonic, 12 degrees azimuth and -11 degrees elevation. Thus, with increasing frequency best areas shifted toward the vertical midline. Directionality to first harmonic stimuli was broader than to either of the two higher harmonics.

Acoustic Stimulation↗
PubMed1986

Directional hearing in the grassfrog (Rana temporaria L.). II. Acoustics and modelling of the auditory periphery.

In an earlier paper (Vlaming et al., 1984) we reported on optical measurements (laser-doppler interferometry) of the vibrations characteristics of the grassfrog's tympanic membrane. In the present paper these measurements were extended to include acoustic measurements concerning the functional role of the mouth cavity in frog hearing. Based on these measurements a model of the frog's acoustic periphery, consisting of three coupled linear oscillators with three entrance ports for sound, was developed and analyzed mathematically to give the various relevant transfer functions. The model is characterized by six parameters, all of which could be estimated from the available experimental data. For frequencies up to some 1500 Hz the model adequately describes the experimental data, both our own and earlier, seemingly conflicting data in the literature. For higher frequencies deviations occur, possibly due to nonuniform vibrations of the membranes. The model was used to evaluate the monaural directional sensitivity of the frog under free-field stimulation. Essentially it behaves as a combined pressure-gradient receiver, with highly frequency-dependent directional sensitivity. Directional sensitivity of the tympanic membrane could be modulated drastically by changing the resonance properties of the mouth cavity, without affecting the intrinsic membrane properties. This, theoretically, allows the frog to manipulate its direction sensitivity by actively tuning the volume of its mouth cavity. In order to account for discrepancies with known properties of low-frequency auditory nerve fibers an additional, extra-tympanic channel was included into the model. The extended model, the second-channel possibly involving the opercularis complex, provides a good quantitative fit to the available data on tympanic membrane movement as well as auditory nerve activity. Finally, the model enables to simulate a (moving) sound source in space, while stimulating the frog via closed couplers.

Acoustics↗
PubMed1988

Time is traded for intensity in the bat's auditory system.

Disparities in time and intensity are the two chief cues animals use for localizing a sound source in space. Echolocating bats belonging to the family Molossidae emit brief, ultrasonic signals for orientation that sweep downward about an octave over the duration of the pulse. Due to acoustic shadowing and the directional properties of the ears, pronounced interaural intensity disparities are created that vary as a function of azimuth. However, due to the small headwidth of these animals, azimuthal changes create small interaural time disparities that are at most 30 microseconds. The experiments in this report are concerned with the binaural processing of time and intensity disparities using brief FM signals that simulate the animal's natural echolocation calls. Binaural neurons receiving excitation from one ear and inhibition from the other (E-I neurons) were recorded from the inferior colliculus of Mexican free-tailed bats. The majority of units sampled were highly sensitive for temporal disparities of 100-300 microseconds, and a few had significant changes in discharge probability when interaural time was changed by 10-20 microseconds. However, all E-I neurons were also sensitive to intensity disparities. With only one exception, all E-I neurons traded time for intensity. On the average, each decibel difference in intensity could be compensated for by advancing or delaying the inhibitory sound by 47 microseconds. The main conclusion is that the auditory system processes interaural disparities by transforming level differences at the two ears into latency differences. Thus the discharge probability of each binaural neuron is determined largely by the arrival times of the discharges from the excitatory and inhibitory ears. In view of the substantial time-intensity trading ratios, the small interaural time disparities produced by azimuthal locations off the midline play no role in shaping the response properties of these neurons. Specific examples of how time-intensity trades can translate into a high spatial selectivity are presented.

Animals↗
PubMed1990

A combined sensitivity for frequency and interaural intensity difference in neurons in the auditory midbrain of the grassfrog.

The relation between spectral tuning and sensitivity for interaural intensity difference (IID) was studied for single units in the auditory midbrain of the grassfrog. The stimuli consisted of sequences of pure tones of different frequency and interaural intensity differences presented by means of a closed sound system. At best excitatory frequency, three types of binaural interaction were observed: E0 (one ear excitatory 23%), EE (both ears excitatory 9%) and EI (one ear excitatory, the other inhibitory 67%). For a considerable number of units different types of binaural interaction were observed for different stimulus frequencies. More than 30% of the binaural units had multiple excitatory and inhibitory regions in their spectrotemporal selectivity. E0 and EI units had uniformly distributed best frequencies, EE units generally had best frequencies near 1.0 kHz. The E0 and EE categories had response latencies less than about 70 ms whereas EI units could have longer latencies. Most EE and all EI category units had sigmoidally shaped IID-rate curves. About 40% of the units had a combined sensitivity for sound spectrum and IID which was invariant to overall stimulus intensity. For nearly all EI units the inhibitory influence of the ipsilateral ear was confined to frequencies in the 0.4-1.6 kHz range and was not correlated with a unit's best frequency. By means of a simple additive model we demonstrated that determination of sound source laterality can be achieved by ensemble coding in the auditory midbrain.

Action Potentials↗
PubMed1990

Neurons in the inferior colliculus of cats sensitive to sound-source elevation.

The sensitivity to variations in sound-source elevation was studied in 48 units, previously examined as to their azimuthal sensitivity, of the inferior colliculi of cats. Of these units, 36 were directionally-sensitive (firing rate varied by more than 50% across the range of positions studied) to both azimuthal and elevational changes. Elevation sensitivity was common to noise stimuli (19/25 units) and pure tones in excess of 6 kHz (17/17 units). Not one of the 8 azimuth-sensitive units with CFs below 6 kHz was directionally-sensitive to the elevation of CF stimuli. The 4 units omnidirectional to azimuthal variation were similarly insensitive to elevation. The shapes of functions relating sound-source elevation to spike count (elevation functions) varied across an apparent continuum, with some very sharply-peaked functions being observed. Peak spike counts almost invariably occurred at stimulus elevations above the horizontal plane. Comparisons of the widths of elevation and azimuth functions at the same sound pressure level were made for 36 units. The relative sharpness of elevation and azimuthal tuning varied across the population. The common association of sensitivity to both azimuth and elevation suggests that elevation sensitivity may be mediated partly by binaural comparisons.

Acoustic Stimulation↗
PubMed1991

Binaural masking level difference effects in single units of the guinea pig inferior colliculus.

We have studied the masking effects of a binaurally presented noise on the responses to binaural signals recorded from low-frequency cells in the inferior colliculus of the guinea pig. The spike rates to the masker and signal + masker were compared to quantify masking at different interaural time delays of the noise. The signal was a 50-ms tone burst at best frequency or a 50-ms segment of a synthetic vowel presented at the best interaural delay of the unit tested. At each noise masker delay, the noise level was adjusted to obtain a criterion spike difference. In most cases, the level required was lowest at the best delay for the noise. The mean difference between maximum and minimum masked thresholds across the cell population was very similar to the human psychophysical masking level difference under the same signal and masker conditions. In another series of tests, we measured the effect of the noise masker on the temporal pattern of the discharge to the signal. The signal used was a 500-ms segment of the synthetic vowel. In virtually all cases the addition of a continuous noise masker reduced the discharge rate synchronized to the fundamental frequency of the vowel. The degree of this reduction was dependent on the interaural time delay of the noise masker. For most units, maximum reduction was seen when the vowel and noise had the same interaural time delay. The similarity between the masking which we have shown physiologically and the reported in a variety of human psychophysical experiments suggests that the processing at levels up to and including the inferior colliculus contributes to the psychophysical BMLD.

Acoustic Stimulation↗
PubMed1991

Origin of the click-evoked binaural interaction potential, beta, of humans.

The human click-evoked binaural difference waveform has as its most prominent feature the peak, beta, which has been shown to be related to binaural perception. In normal human subjects, we investigated the effect upon beta of (1) delivering the clicks in the presence of high passed masking noise (4000 Hz cut-off) and (2) reversing click polarity. In the presence of the masker, little activity occurs at the time the click-evoked beta would be expected. No significant change in beta latency occurs when the click polarity is inverted. We conclude that beta is principally due to the high-frequency components of the broad band click, so that it is through the activity in high characteristic frequency auditory nerve fibers that click-evoked beta is generated. Because the medial superior olive is the major nucleus of the human superior olivary complex, our results suggest that beta is possibly generated by the high-frequency cells of the medial superior olive.

Acoustic Stimulation↗