William P. Shofner

Peripheral basis of sound localization in anurans. Acoustic properties of the frog's ear.

Directional responses of single auditory fibers in the eighth nerve of northern leopard frogs (Rana pipiens) were studied in order to gain some insights into the acoustical properties of the frogs ear. In addition to the actual directional response of a fiber, a theoretical directional-response curve to the intensity-rate function of the unit. The difference in the two responses provided a measure of the directional characteristics of the frogs ear at the stimulating frequency which can be plotted in a polar diagram to show the directivity pattern of the frogs acoustic receiver. Directivity patterns were obtained from three groups of experimental animals under the following conditions: (I) mouth filled with moistened cotton; (II) contralateral ear coated with silicone rubber cement; (III) open mouth. Changes in the directivity patterns were observed with experimental manipulations and these were compared to those obtained from normal animals (Feng, A.S. (1980) J. Acoust. Soc. AM. 68, 1107-1114). The results suggest that the frogs ear behaves as a combination pressure-pressure gradient receiver.

Responses of ventral cochlear nucleus units in the chinchilla to amplitude modulation by low‐frequency, two‐tone complexes

For a tone that is amplitude modulated by two tones (fmod1 and fmod2), neither the stimulus waveform nor the half-wave rectified waveform has spectral energy at the envelope beat frequency (fmod2-fmod1). The response of ventral cochlear nucleus units in the chinchilla were recorded for best frequency tones that were amplitude modulated by low-frequency, two-tone complexes. Fourier analysis of poststimulus time histograms shows spectral peaks at fmod2-fmod1 in addition to the peaks at fmod1 and fmod2. The peaks in the neural spectra arise from compressive nonlinearities in the auditory system. The magnitudes of these spectral peaks are measures of synchrony at each frequency component. For all units, synchrony at fmod1 and fmod2 is greater than the synchrony at fmod2-fmod1. For a given unit, synchrony at fmod1 and fmod2 remains relatively constant as a function of overall level, whereas synchrony at fmod2-fmod1 decreases as the level increases. Synchrony was quantified in terms of the Rayleigh statistic (z), which is a measure of the statistical significance of the phase locking. In terms of z, phase locking at fmod1 and fmod2 is largest in chopper units, whereas onset-chopper units and primarylike units having sloping saturation in their rate-level functions show the smallest amount of phase locking. Phase locking at fmod2-fmod1 is also largest in chopper units, and smallest in onset-chopper units and primarylike units with sloping saturation.

Temporal representation of rippled noise in the anteroventral cochlear nucleus of the chinchilla.

This paper describes the temporal responses of anteroventral cochlear nucleus (AVCN) units in the chinchilla to rippled noises. Rippled noise is generated when a wideband noise is delayed and added (cos+ noise) or subtracted (cos- noise) to the undelayed noise. Renewal densities were constructed to evaluate synchronous discharges at the delay. In response to rippled noise, AVCN units which show phase locking to best frequency (BF) tones gave renewal densities having major peaks at the delay for cos+ noise, but nulls at the delay for cos- noise. Most AVCN units which did not show BF phase locking gave renewal densities that did not contain features related to the rippled noise delay; a few of these nonphase-locked units did show peaks in renewal densities for both cos+ and cos- noises. Synchrony at the rippled noise delay was also demonstrated with evoked potential recording. Autocorrelation functions of the neurophonic potential showed peaks at the rippled noise delay for both cos+ and cos- noises. In addition, peaks could be observed in the autocorrelation functions of neurophonic potentials for rippled noises with delays as short as 1 ms; peaks were never observed in renewal densities of single units for ripple delays as short as 1 ms. The results show that a temporal representation of rippled noise delay does exist in the AVCN and are consistent with current hypotheses regarding functions of AVCN subsystems. The temporal representation of the delay is a presumptive neural code for the pitches of rippled noises.

Pitch strength and Stevens’s power law

Recent studies have suggested that the saliency or the strength of pitch of complex sounds can be accounted for on the basis of the temporal properties in the stimulus waveform as measured by the height of the first peak in the waveform autocorrelation function. We used a scaling procedure to measure the pitch strength from 15 listeners for four different pitches of complex sounds in which the height of the first peak in the autocorrelation function systematically varied. Pitch strength judgments were evaluated in terms of a modification of Stevens’s power law in which temporal information was used from both the waveform fine structure and the envelope. Best fits of this modified power law to the judged pitch strengths indicate that the exponent in Stevens’s power law is greater than 1. The results suggest that pitch strength is primarily determined by the waveform fine structure, but the stimulus envelope can also contribute to the pitch strength.

Statistical and receiver operating characteristic analysis of empirical spike‐count distributions: Quantifying the ability of cochlear nucleus units to signal intensity changes

Analytical methods from signal detection theory were applied in an effort to quantify the ability of cochlear nucleus (CN) units to signal changes in intensity. Of particular interest was the relation between this ability and the different patterns of discharge that characterize auditory neurons. Single-unit responses to best-frequency (BF) tone bursts were recorded from neurons in the gerbil cochlear nucleus, and empirical spike-count distributions were generated. The mean-to-variance ratios for regular units were generally larger than those of irregular units. Receiver operating characteristic (ROC) curves were generated from empirical spike-count distributions. The area under the ROC curve [P(A)] was computed and used to define the performance of an observer detecting whether or not a change in firing rate has occurred, thus signaling a change in intensity. For a given change in mean spike count, units characterized by regular interspike-interval (ISI) histograms typically gave larger P(A) values than did units characterized by irregular ISI histograms. In addition, onset units gave larger values of P(A) than did irregular units for a given change in mean spike count. These results suggest that regular and onset units are better able to signal intensity changes than are irregular units.

Perception of the periodicity strength of complex sounds by the chinchilla

The perception of periodicity strength was studied in chinchillas using a stimulus generalization paradigm in an operant-conditioning, positive reinforcement behavioral task. Stimuli consisted of cosine-phase and random-phase harmonic complex tones, infinitely iterated rippled noises, and wideband noise. These stimuli vary in periodicity strength as measured by autocorrelation functions and are known to generate a continuum in the perception of pitch strength in human listeners. Chinchillas were trained to discriminate a cosine-phase harmonic tone complex from wideband noise and tested in the generalization paradigm using random-phase tone complexes and iterated rippled noises as probe stimuli. Chinchillas were tested in three different conditions in which the periods of the fundamental frequencies of the tone complexes were fixed at 2 ms, 4 ms, or 8 ms. Behavioral responses obtained from chinchillas were related to stimulus periodicity strength. For most animals, the behavioral responses to random-phase tone complexes were smaller than those to cosine-phase tone complexes. The behavioral responses were analyzed in terms of the Auditory Image Model of Patterson et al. [Patterson, R.D., Allerhand, M.H., Giguère, C., J. Acoust. Soc. Am. 98 (1995) 1890-1894], and the results suggest that the periodicity information in the stimulus envelope has a large influence in controlling the behavioral response of the chinchilla. Comparison of the generalization data obtained in the present study to magnitude estimation data obtained previously in human subjects suggests a greater influence of stimulus envelope for the perception of periodicity strength in chinchillas than for the perception of pitch strength in human listeners.

Critical bands and critical ratios in animal psychoacoustics: An example using chinchilla data

This paper suggests that critical ratios obtained in noise-masked tone studies are not good indicators of critical bandwidths obtained in both human and nonhuman animal subjects. A probe-tone detection study using chinchilla subjects suggests that they may be broadband processors in detection tasks as opposed to human subjects who use narrow-band, critical-band processing. If chinchilla and other nonhuman animal subjects are wideband processors, this can partially explain why their critical ratios are significantly greater than those measured in human subjects. Thus, large critical ratios obtained for nonhuman animals may indicate processing inefficiency rather than wide critical bands.

A quantitative light microscopic study of the bullfrog amphibian papilla tectorium: Correlation with the tonotopic organization

The height, width and cross-sectional area of the bullfrog amphibian papilla tectorial membrane were quantitatively analysed from frontal serial sections. The cross-sectional area (which is a measure of mass) of the tectorium does not appear to be linearly graded along the length of the papilla, but rather spatial gradations occur in more or less a step-wise manner. The spatially graded area (or mass) is well correlated with the width of the tectorium and their relationships with the tonotopic organization of the amphibian papilla are discussed.

Representation of a low-frequency tone in the discharge rate of populations of auditory nerve fibers

Discharge rates for populations of single auditory nerve fibers in response to 1.5 kHz tone bursts were measured in anesthetized cats. Separate plots of average rate vs. best frequency (rate-place profiles) were made for high, medium and low spontaneous rate (SR) auditory nerve fibers. At the lowest sound levels studied (34 dB SPL), all three SR groups show a peak in the rate-place profile centered around 1.5 kHz. At the highest sound levels studied (87 dB SPL), the average rates of the high and medium SR fibers are saturated across a wide range of best frequencies, but a peak around 1.5 kHz is maintained in the rate-place representation of the low SR fibers. These results show that in addition to the temporal information present in the discharge patterns of auditory nerve fibers, a rate-place representation of a single low-frequency tone exists in the auditory nerve over a wide range of sound levels.

Comparison of frequency discrimination thresholds for complex and single tones in chinchillas

Frequency discrimination thresholds were measured from five chinchillas for harmonic tone complexes having a fundamental frequency of 250 Hz. Stimuli consisted of the fundamental frequency and the second through 10th harmonics with individual components added in either cosine phase or random phase. In general, thresholds were independent of overall level for sound levels between 47 and 77 dB SPL, and there was no difference in thresholds observed between cosine-phase tone complexes and random-phase tone complexes. Discrimination thresholds were also obtained for a single 250-Hz tone for comparison with complex tone thresholds. Similar to data reported in human subjects, thresholds in chinchillas for tone complexes were lower than thresholds obtained using a single tone, although chinchillas required a larger frequency difference than human listeners. The results suggest that the mechanisms of frequency discrimination of complex tones are similar between chinchillas and human listeners.