Russell R. Pfeiffer

Time‐Domain Measurements of Cochlear Nonlinearities Using Combination Click Stimuli

Spike discharge patterns of single cochlear nerve fibers in response to acoustic clicks imply that the cochlear neuron is excited by an oscillatory waveform—termed a Click Excitation Function (CEF)—presumably related to the impulse response of the basilar membrane. For a given fiber, the intensities of two clicks separated by k half‐periods of the fibers characteristic frequency can be adjusted so that a particular peak does not appear in the poststimulus time histograms to both polarities of the stimulus. If each click is assumed to produce identical, linearly superposing CEFs, then the relative amplitudes of adjacent peaks of the CEF can be determined by this nulling technique when k=1. To test this assumption, the relative amplitudes of peaks separated by k half‐periods can be obtained either indirectly from k=1 measurements or by direct measurement with click separations of k > 1 half‐periods. The results obtained by these two methods do not agree and, hence, are inconsistent with the linear superpo...

A Model for Two‐Tone Inhibition of Single Cochlear‐Nerve Fibers

An analog model by Engebretson and Eldredge accounts for nonlinear properties of the cochlear microphonic (CM) in response to single‐ and two‐tone stimulation. Recent evidence that shows strong similarities between CM and spike‐discharge patterns of single cochlear‐nerve fibers has prompted an association of two‐tone interaction of CM with two‐tone inhibition of nerve fibers, leading to a modification of the model for CM so as to make it applicable to cochlear‐nerve fibers. The result is a model that has clear two‐tone inhibition properties similar to those observed experimentally. Mathematical analysis applied to the model shows that the “inhibition” is easily explained by signal suppressions inherent to bandpass nonlinearities.

Cochlear nerve fiber responses: Distribution along the cochlear partition

Fourier analysis of discharge patterns in response to sinusoidal acoustic stimulation provides a consistent and repeatable measure of response phase and amplitude. The distribution of the fundamental components of response for large populations of fibers as a function of their characteristic frequency provides a link between the spatiotemporal characteristics of basilar membrane vibration and single fiber response.Subject Classification: 65.42, 65.40.

Response Patterns of Single Cochlear Nerve Fibers to Click Stimuli: Descriptions for Cat

Response patterns to click stimulation of 907 single cochlear nerve fibers, having characteristic frequencies below 2000 Hz, can be separated into two populations on the basis of salient features. Population I consists of approximately 93% of the fibers, and Population II consists of approximately 7% of the fibers. A statistical description of the correlation between properties of response patterns of Population I fibers and stimulus level and characteristic frequency is given. For the Population I fibers, with characteristic frequencies below 500 Hz, deviations from the precise interlacing of preferred times of spike discharges in response to rarefaction and condensation clicks, as well as some instances of biased response to condensation clicks are described. The features that set Population II fibers apart from those of Population I are given, and a correlation of these populations with anatomical details of innervation is suggested.

A system of nonlinear differential equations modeling basilar‐membrane motion

A phenomenological model for displacement of a point on the basilar membrane is developed by formulating a system of nonlinear differential equations: ẍi(t) + 2Di[1 + ηẋi2(t)]ẋi(t) + ω0i2xi(t) = Cxi−1(t), for i = 1, 2, …, 10, where x0(t) is the input (stapes displacement) and x10(t) is the output. This model, which behaves effectively linearly at low levels and nonlinearly at high levels, shows that a single nonlinear system is adequate to account for the following frequency‐dependent nonlinear phenomena of the peripheral auditory system: (1) limiting of the output level; (2) decrease of Q with increasing input level; (3) decrease of the most effective frequency with increasing input level; (4) changes in phase angle of the output with input level; (5) changes in shape of the click response waveform with input level; (6) two‐tone suppression with f1 = CF and f2 > CF; (7) generation of the combination tone 2f1 − f2 in response to two tones f1 < f2); (8) “amplitude” nonlinearity in response to click pairs; ...

Interpretation of spontaneous spike discharge patterns of neurons in the cochlear nucleus

Experimental studies with microelectrodes have resulted in quantitative statistical descriptions of spike discharge patterns of both cochlear nucleus neurons and the cochlear nerve fibers that innervate them. A renewal stochastic process provides a useful descriptive model for the discharge patterns of cochlear nerve fibers in the absence of acoustic stimuli. If it is assumed that the total input to a neuron is a superposition of many of these renewal processes, it is possible to derive discharge patterns for the neuron, given the way the neuron responds to its inputs. For some neurons in the anteroventral cochlear nucleus (AVCN), a model which assumes that the neuron discharges each time an input discharge occurs appears to be adequate. Certain other neurons, found in the posteroventral cochlear nucleus (PVCN), have discharge patterns compatible with the assumption that each discharge requires the accumulation of a number of input discharges. These findings are consistent with the anatomy of neurons in these two regions. Some extensions and generalizations of the modeling techniques used are presented.

Distortion Compensating, Condenser‐Earphone Driver for Physiological Studies

A design for a condenser earphone driver is given. The driver predistorts electrical input signals to counteract the inherent square‐law operation of the condenser earphone. Thus, smaller‐diameter earphones, with their wider bandwidth characteristics, can be used at stimulus levels of interest without appreciable harmonic distortion.

On the Sound‐Pressure Transformation from Free Field to Eardrum of Chinchilla

The sound pressure at the eardrum of anesthetized chinchillas was measured with a probe‐tube microphone implanted into the external auditory meatus. The ratio of this pressure to the free‐field pressure measured at a point corresponding to the center of the animals head was determined as a function of frequency (sound pressure transformation function, SPTF). Two distinctly different types of SPTFs were found. They differed by as much as 15 dB in the frequency range 1–8 kHz. Several of the ears that exhibited one type of SPTF showed slight perforations in the tympanic membranes. It appeared that a static pressure difference across the eardrum was responsible for the other type of naturally occurring SPTFs. Both types of SPTFs could be artificially produced by either creating or by equalizing static middle‐ear pressure. [This work was carried out at the Massachusetts Institute of Technology and supported in part by the National Institute of Health (Grant), the Joint Services Electronics Program, and NAS...

Characteristics of the (f2 − f1) component in response patterns of single cochlear nerve fibers

At a fixed stimulus level, single cochlear nerve fibers are responsive to sinusoidal stimuli over a band of stimulus frequencies. Stimulation with the sum of two sinusoids, each within the response band, can cause (f2 − f1) and (f2 + f1) distortion products (as well as many others) to appear in period histograms with amplitudes and phase angles that are predictable from the phase versus frequency plot for single sinusoids by a memoryless polynomial nonlinearity model. On the other hand, stimulation with the sum of two sinusoids, each above the response band, can for some choices of f1 and f2 produce large (f2 − f1) distortion products in the period histogram without significant response at any other frequency including f1, f2, and (f2 +f1). In this case, phase‐versus‐frequency plots for the (f2 −f1) component are similar to those for single sinusoidal stimuli at a frequency of (f2 −f1). Also, the amplitude of the (f2 −f1) component exhibits tuning curves that lie within the tuning curve for single sinusoi...

Modulation Index as a Response Criterion for Discharge Activity

The fact that spike discharges of spontaneously active cochlear nerve fibers demonstrate phase locking in response to low‐frequency sinusoidal stimuli at signal levels substantially lower than those required to generate a change in spike discharge rate has led us to investigate the properties of the following response criterion. We evaluate the index of modulation of the spike discharge rate as a result of the sinusoidal stimulus. Period histograms are modelled as an inhomogeneous Poisson process. Using maximum likelihood techniques, we estimate the index of modulation and the phase difference between the stimulus and the response. We have acquired data similar to “tuning curves” showing as a function of frequency the stimulus level necessary to maintain an arbitrary fixed level of modulation along with the corresponding stimulus‐response phase relations for the complete response‐band of fibers with low CF ( 3 kHz). We compare these results to those obtained using other response criteria, and examine their relationship to cochlear mechanics and cochlear microphonics. [This investigation was supported by PHS research grants from the National Institutes of Health.]