David M. Harris

Action potential suppression, tuning curves and thresholds: Comparison with single fiber data

The collection of compound action potential (AP) threshold curves and their use to define the sensitivity of individual animals are described. Forward masking AP tuning curves (APTC) have also been collected in the chinchilla. Characteristics of APTCs are compared with single fiber frequency-threshold curves (FTC) in the same group of animals. The two sets of data are quite similar when the probe frequency used to collect the APTC is equated with a fibers characteristic frequency (CF). The major difference is that APTCs are usually broader than FTCs. A paradigm utilizing two maskers in a forward masking situation, developed to study psychophysical unmasking [19,37], has been modified for measuring AP suppression. AP suppression areas are described as similar to single fiber two-tone suppression areas when probe frequency and CF are above 3 kHz. Relationships among single fiber, AP and psychophysical thresholds, tuning curves and suppression areas are discussed.

Behavioral, compound action potential, and single unit thresholds: Relationship in normal and abnormal ears

Comparisons were made for two species (chinchilla and mongolian gerbil) among mean behavioral audiogram, mean just detectable action potential (AP) responses to tone bursts, and single-fiber response thresholds at the characteristic frequency, averaged in one-octave bands. In normal animals and in a group of Kayamycin-treated chinchillas, these mean measures appear to have a well-ordered relationship. Unit and AP thresholds are within 10 dB from one another throughout the frequency range. Behavioral thresholds are usually 15--20 dB more sensitive, but the three curves are roughly parallel except at the highest frequencies, where the behavioral threshold begins to increase approximately one-half octave above the physiological ones. Individual examples for four gerbils and four chinchillas having hair cell losses due to Kanamycin intoxication reinforce the notion based on mean data that in most cases AP thresholds can serve to predict the behavioral threshold configuration.

Developmental changes in frequency mapping of the gerbil cochlea: Comparison of two cochlear locations

The cochlear microphonic measured in scala tympani referenced to that at the round window is used to determine characteristic frequency (CF) at two locations in an age-graded series of Mongolian gerbils. No significant ontogenic shift in CF is seen at a second turn location, while an approximately 1.5 octave shift is seen in the mid-basal turn.

Frequency analysis by neuronal populations in the inferior colliculus of the rat

Response pattern changes of 52 single inferior colliculus (IC) neurons measured as a function of frequency and intensity changes of pure‐tone‐burst stimuli were analyzer in the form of peri‐stimulus‐time histograms (PSTH). Through pooling of single unit data and by direct measurement (multiunit PSTH), the typical response pattern of IC neuronal populations was described. This response was an initial damped oscillation (excitation‐inhibition) following stimulus onset which was followed by the establishment of a new baseline level of cellular activity that terminated at stimulus offset. A multiunit analysis of the tonotopic organization of rat IC revealed a pattern of activity within populations as predicted by Bekesy [Sensory Inhibition (Princeton U.P., Princeton, N.J., 1967)] and I.C. Whitfield [The Auditory Pathway (Camelot Press, London, 1967)]. The onset response was widespread, but sustained excitation was restricted to specific loci for specific tonal frequencies. These excitatory loci were bordered ...

Effects of aversive and rewarding electrical brain stimulation on auditory evoked responses in albino rat tectum

Abstract Auditory evoked responses in rat tectum yielded recovery functions to aversive electrical brain stimulation of either contralateral mesencephalic central gray or deep superior colliculus. Effects of stimulation on inferior colliculus auditory potentials are abolished by pentobartital and curare, implying tympanic muscle mediation due to reticular activation. Brain stimulation effects survive identical pharmacological manipulations when auditory responses are led from deep superior colliculus, showing independence from reticular processes or events occurring in inferior colliculus, one synapse previous. Rewarding brain stimulation also effects deep superior colliculus potentials, but interacts in an undetermined manner with nonspecific processes.

Pulsation threshold in the auditory nerve

Response patterns of single auditory‐nerve fibers are compared under two conditions. The first condition is similar to the stimulus condition customarily used to determine pulsation threshold psychophysically. A 100‐ms duration signal at a fibers characteristic frequency (CF) is presented at 10 dB above threshold at a rate of 5/s. A 100‐ms duration tonal masker is introduced during the interval between signal pulses, raised in intensity and PST response patterns collected at each masker level. The second condition is the same except that the signal is continuous rather than pulsed. In the pulsed‐signal condition when the masker‐evoked discharge rate equals the signal‐evoked discharge rate, the response pattern resembles the firing pattern to a continuous tone. When the masker‐evoked rate exceeds the signal‐evoked rate, the response pattern is indistinguishable from the pattern evoked by a continuous signal plus a pulsed masker. For different masker frequencies the results are essentially the same when ma...

Unmasking in the auditory nerve

Unmasking as a psychoacoustical procedure was developed by Houtgast [J. Acoust. Soc. Am. 51, 1885–1894 (1972)]. It is a two‐masker paradigm whereby a subjects masked threshold is determined with a probe tone (f1) preceded by two simultaneously occurring tones, one also at frequency f1, the other having variable frequency (f2). The f1 masker produces a forward masking effect on the probe and the degree of this masking is altered as the second masker influences the effectiveness of the first (f1) one. This paradigm has been applied to the study of single fiber responses and whole‐nerve action potentials (AP) from the auditory nerve of the chinchilla. For fibers certain combinations of the maskers (f1=CF, and f2) yield more masking than the f1‐masker alone. This occurs when the f2 masker is within the fibers response area. When f2 is within the fibers suppression area(s) the response to the probe tone is greater than with the f1 masker alone. That is, unmasking occurs under such conditions. The procedure ...

Comparison of current source density analysis with multi‐unit mapping in the inferior colliculus of the gerbil

In the central nucleus of the inferior colliculus single unit, 2‐deoxyglucose studies, and the anatomical structure, define a laminar organization related to frequency coding. The tonotopic axis appears to be along an axis of symmetry, frequency‐specific lamina are assumed to be activated synchronously, and the uniform distribution of neurons suggests homogeneous conductivity; conditions necessary for a one‐dimensional current source density (CSD) analysis of the distribution of current sources and sinks evoked along the tonotopic axis. We map tone‐burst‐evoked multi‐unit responses (multi‐unit PST histograms) at 100‐μ intervals as a microelectrode is lowered along the tonotopic axis. From evoked potentials, also collected at the recording sites, we compute the second derivative of the function of voltage with respect to space at a specific latency. This transformation represents the spatial profile of current sources and sinks. These data show that maximum multi‐unit evoked activity has the same latency a...

Comparison of auditory thresholds determined behaviorally and electrocochleographically in normal and kanamycin‐treated gerbils

The measurements of two electrical responses of the auditory periphery have been proposed for the objective assessment of the auditory sensitivity. The first is the classical method of measuring cochlear microphonic (CM) isopotential curves. The second is obtaining thresholds of whole‐nerve action potentials (AP) by using shaped tone bursts. To determine the validity and shortcomings of both methods, measurements have been conducted on normal and kanamycin‐treated gerbils and they are compared with behavioral audiometry. The study shows that between 500 and 24 000 Hz, AP threshold curves compare favorably with behavioral thresholds whereas CM isopotential curves generally do not. At very high frequencies, both responses fail to approximate the behavioral response pattern. Studies with kanamycin also support the idea that the whole‐nerve action potentials can be objectively used for the measurement of audiometric patterns. Such a proposal is also consistent with the observations that AP generated by low‐in...

A highly portable, lightweight windscreen for infrasound sensors

It is often desired to deploy infrasound sensors in remote locations for short periods of time. Because of noise created by wind flow past the sensor in the atmosphere at infrasound frequencies, it is also desirable to employ a mechanical windscreen with the goal of reducing the effect of the wind so as to enhance the acoustic signal-to-noise ratio. One of the challenges in wind screen design for temporary applications is to achieve reasonable portability in addition to adequate wind noise filtering. In this presentation, we report the performance a new windscreen design that is highly portable and that achieves performance on par with a commonly used dome-shaped windscreen. Performance comparisons of this newly designed windscreen, a typical porous hose windscreen, and a commonly used dome windscreen are investigated. Specifically, wind noise reduction of these three windscreens compared with a un-screened sensor are presented as a function of frequency and wavenumber using the measured wind speed as a p...