Biomedical subjects

K R Henry

Publications and source records attributed to K R Henry.

At least 19 recordsLinked to original sources

PubMed1992

One-tone suppression in the cochlear nerve of the gerbil.

One-tone rate suppression has been reported several times for auditory nerve fibers of mammalian and non-mammalian vertebrates. Because its properties are very similar to those of two-tone rate suppression, the possibility exists that one-tone rate suppression is the result of an interaction within the inner ear of the suppressing tonal stimulus and some ongoing extraneous acoustic stimulus. For this reason, reports of one-tone rate suppression often elicit suspicions that the investigators were not sufficiently careful in controlling leaks in their acoustic barriers or in the electrical pathways to their acoustic drivers. Recent reports of one-tone rate suppression in pigeon basilar-papillar fibers and goldfish saccular fibers were accompanied by descriptions of measures taken to avoid such leaks. In this paper, we describe one-tone rate suppression in a mammal, the Mongolian gerbil; and we demonstrate that the background spike activity being suppressed is not driven by either external sounds coming from outside the acoustic isolation test chamber or by non-stimulus electrical inputs to the acoustic driver. The suppressed background spike activity evidently arises from sources within the animal. These sources may be non-acoustic, associated with spontaneous pre- or post-synaptic ion-channel activity; or they may be acoustic sources--internal sound or vibration generators.

Acoustic Stimulation↗

PubMed1992

Modulation of cochlear nerve spike rate by cardiac activity in the gerbil.

Among primary auditory axons with characteristic frequencies (CFs) below 2500 Hz, a substantial subpopulation was found in which spike activity was driven by cardiac events. The presence of cardiac-driven activity was inferred from cycle histograms triggered on the peak of the electrocardiogram (ECG). This driven activity was either like a simple onset response (often followed by a reduction of spike activity to below background level), or as a longer lasting series of peaks and troughs. In two axons with high CFs (7 kHz and 12.5 kHz), cardiac-driven suppression was observed. Recordings made by a probe microphone revealed the presence of heart-related sound in the external ear canal. The onset of that sound coincided with the onset of cardiac-driven spike activity (and suppression).

Action Potentials↗

PubMed1992

Derived and enhanced compound action potentials at near-threshold levels: forward masking increases sensitivity of audiograms and tuning curves.

The amplitude of a cochlear nerve compound action potential (CAP) can be increased by forward maskers having levels close to the visual detection threshold of the CAP. This effect, termed enhancement, varies as a function of the frequency of the masker and probe stimulus, and is nonmonotonic with respect to the level of the masker. Other studies using the derived CAP have used a subtraction technique to evaluate the ability of simultaneous maskers having levels near the CAP visual detection threshold to influence the CAP produced by an above threshold tone. The present paper compares audiograms produced by the conventional nonmasked CAP visual detection threshold technique with audiograms produced by both forward masked derived CAPs and forward masked enhanced CAPs. In response to low and middle frequency stimuli, both masked CAP measures produce more sensitive audiograms than does the conventional nonmasking method. Forward masked amplitude tuning curves (TCs) were also produced, comparing the conventional 50% amplitude reduction and 20 microV amplitude reduction methods with TCs obtained with derived and enhanced CAPs. When the same criteria are used, both masked CAP measures result in sharply tuned amplitude TCs that are approximately 60 dB more sensitive than the conventional CAP technique. At near-threshold levels, the properties of forward masked enhanced and derived CAPs appear to be similar.

Acoustic Stimulation↗

PubMed1992

Suprathreshold comparisons of derived and enhanced compound action potentials.

The Derived cochlear nerve compound action potential (CAP) and the Enhanced CAP are both measures which demonstrate the ability of a forward masker to increase the amplitude of the CAP produced by a probe stimulus. Enhancement occurs whenever the amplitude of a masked CAP is larger than that of a nonmasked CAP, whereas the derived CAP is produced by the subtraction of the entire masked CAP waveform from that of the nonmasked waveform. Therefore, a derived CAP is created whenever the masker produces a difference of amplitude, latency, and/or waveform shape. The present experiments compare these two measures by observing the effects of 13 kHz maskers varying from levels of -10 to +70 db SPL on CAPs produced by 50 or 60 dB SPL, 13 kHz probe stimuli. Enhancement is characterized by a nonmonotonic increase of CAP amplitude (and sometimes a decrease of latency) as a function of increasing levels of the forward masker, whereas this pattern seldom occurs with the derived CAP. Enhancement is typically seen with forward making, but seldom seen with simultaneous masking, whereas the derived CAP is very similar under these two types of masking.

Acoustic Stimulation↗

PubMed1992

Latency enhancement of the cochlear nerve compound action potential (CAP).

Forward maskers within two frequency-intensity domains are capable of decreasing (enhancing) CAP latency: one region flanks the low frequency tail, the other flanks the tip/high frequency slope regions of the latency tuning curve (TC). By contrast, amplitude enhancement typically does not flank the high frequency slope region.

Acoustic Stimulation↗

PubMed1992

Noise-induced auditory loss: influence of genotype, naloxone and methyl-prednisolone.

Inbred strains of mice have several advantages as models for human noise-induced hearing loss. However, the isogenic nature of inbred lines is very unlike the human condition, and may make this species less valuable as an auditory model. The present experiments start with two mouse genotypes having lifelong normal cochlear functions: The CBA/CaJ and the AUS/sJ inbred strains. These strains and their F1 hybrid offspring were examined for noise-induced elevation of the auditory brainstem response (ABR) threshold. The F1 line had an intermediate degree of loss and the most uniform high frequency cochlear loss. Methylprednisolone was found to protect the F1 from noise-induced losses, whereas naloxone did not.

Animals↗

PubMed1992

The mouse as a model for human audition. A review of the literature.

The mouse has several distinct advantages as an experimental model for human audition. Mice and humans express a similar presbyacusic and ototraumatic pattern. Several genetic mouse models also exist for conditions resembling human auditory disorders, such as otosclerosis. This paper reviews the strains of inbred mice (e.g. the CBA/J) which have recently been used as models for the normal human auditory system, describing their deficiencies. It is suggested that the F1 offspring of CBA/CaJ and AU/SsJ inbred mice would have advantages over existing models.

Acoustic Stimulation↗

PubMed1992

Auditory brainstem function of the F1 offspring of the cross of CBA/CaJ and AU/SsJ inbred mice.

Inbred strains of laboratory mice have several distinct advantages as models for examining conditions that influence the human auditory system, but the CBA/J mouse which has most often been used as a normal model has recently been found to have several disadvantages. This paper is the first report of the auditory brainstem responses (ABRs) of the F1 offspring of CBA/CaJ and AU/SsJ parents. At midlife, high-frequency ABR thresholds are lower in the F1 than in either parental genotype. Tuning curves obtained by forward masking of the ABR also display heterosis, i.e. they are narrower in the F1 than in either parental strain.

Animals↗

PubMed1991

Frequency-specific enhancement of the cochlear compound action potential: influence of the forward masker.

Under restricted frequency and intensity conditions, forward masking can result in the amplitude of the CAP being increased above its unmasked value (Henry, 1991). The present study provides a quantitative analysis of this enhancement effect. In response to forward maskers having the same frequency as the probe stimulus, central frequency (CF) enhancement varies as a function of the level of the forward masker: the lowest masker level at which it can reliably be detected is often well below the visual detection threshold of the CAP generated by the unmasked probe stimulus; the highest masker level at which it can reliably be detected corresponds to approximately 10 dB above the probe stimulus CAP threshold. A second low frequency (LF) enhancement region also exists, encompassing a narrow range of more intense maskers. CF enhancement can double the amplitude of the CAP, whereas LF enhancement is less pronounced. The magnitude of CF enhancement varies as a function of the duration of the forward masker, with longer durations generally increasing the magnitude of the effect. This duration effect, however, interacts with the level of the stimulus. Decreasing the interval between the end of the forward masker and the beginning of the probe increases the magnitude of CF enhancement.

Acoustic Stimulation↗

PubMed1991

Enhancement of the cochlear nerve compound action potential: sharply defined frequency-intensity domains bordering the tuning curve.

Forward masking can either decrease or increase the response to a subsequent stimulus. In the gerbil, frequency-intensity domains of the maskers that decrease the amplitude of the compound action potential (CAP) can be plotted as the sharply defined CAP tuning curve (TC). Regions were also found over which masking increases (enhances) the amplitude of the CAP. Center-frequency (CF) enhancement domains were found in approximately 2/3 of the animals tested, in response to maskers having frequencies very near that of the probe stimulus, at levels ranging from below the CAP detection threshold to just below the tip threshold of the TC. Approximately 2/3 of the animals showing CF enhancement also displayed low-frequency (LF) enhancement, in response to a domain which borders the low-frequency tail of the TC.

Acoustic Stimulation↗

PubMed1991

Hypothermia differentially affects tuning curves generated by forward and by simultaneous masking.

In the gerbil maintained at euthermic (37.5 degrees C) conditions, forward masking produces a compound action potential tuning curve (CAP TC) which is less sensitive but more sharply tuned than that which is generated by simultaneous masking. These differences between forward- and simultaneously-masked CAP TCs are minimized at hypothermic (30 degrees C) conditions. The unmasking effect occurs at both temperatures, suggesting that hypothermia does not exert these changes by eliminating two-tone suppression.

Adaptation, Physiological↗

PubMed1991

Latency and amplitude compound action potential tuning curves for tonal stimuli with nontraditional envelopes.

To evaluate the influence of the acoustic context on the latency and amplitude tuning curves (TCs) of cochlear nerve compound action potentials (CAPs), tonal stimuli were generated with a variety of amplitude modulation envelopes. CAPs were produced by intensity increases (onsets) and decreases (offsets) from the low ambient sound level and by intensity changes from a preexisting tonal level. Onset CAPs from ambient levels generated V-shaped TCs. However, when simultaneous masking was used with onset CAPs which were produced by a 5- to 12-dB increase from preexisting levels of approximately 65 dB SPL, TCs were W-shaped and similar in appearance to those produced by simultaneous masking of offset CAPs. The forward masking of this same CAP resulted in a very sharp V-shaped TC. These data suggest that the preadaptation to 10 ms of a moderate level of a tonal stimulus can increase the tuning of ensembles of cochlear neurons to subsequent transient amplitude changes.

Acoustic Stimulation↗

PubMed1990

Latency and amplitude tuning curves of the N1 and N2 components of the cochlear nerve compound action potential.

Compound action potential tuning curves (CAP TCs) generated by masking the N1 component of the CAP provide a means of assessing the ability of the cochlea to selectively tune to certain stimuli. This paper examines some of the factors which can influence this TC when a moderately intense (i.e. 40-80 dB SPL) probe stimulus is used. At these levels, each of the four corners of the trapezoidal stimulus envelope is capable of generating a CAP. Also, short stimulus rise times can merge the CAPs produced by the first two corners, but this does not appear to have a major effect on the CAP TC. It was shown that the N2 component of the CAP for the first corner of the stimulus is equally capable of producing a well-tuned TC. Another study has shown that, in addition to amplitude decrements, one can use latency increases as a criterion for CAP TCs. We have demonstrated that latency TCs are more finely tuned than amplitude TCs at high levels, especially when the stimulus rise time is short.

Animals↗

PubMed1989

Transient responses to tone bursts.

Investigating theoretical conditions under which linearly-operating tuned structures produce click-like transient responses to onsets and offsets of trapezoidal tone bursts, we come to the following conclusions: (1) each of the four corners of the trapezoidal tone burst is capable of eliciting such a response; (2) the amplitude of the response and its dependence on the frequency of the modulated tone both depend on the phase of the modulated sinusoid at the time a corner occurs; (3) such responses will arise in structures having sufficiently steep band edges, provided that the frequency of the modulated tone is well outside the pass band of the structure--for a corner in cosine phase, the sustained slope of the low-frequency band edge must be greater than zero and that of the high-frequency band edge must be greater than 12 dB/Oct, for a corner in sine phase the sustained slope of the low-frequency band edge must be greater than 6 dB/Oct and that of the high-frequency band edge must be greater than 18 dB/Oct; (4) they will not arise in response to tone bursts whose frequencies fall within the pass band of the structure; (5) nor will they arise in response to a trapezoidal tone burst of any frequency applied to structures (such as simple microphones or drivers) following second-order dynamics and having both spectral zeros at infinity. We present theoretically derived relationships between the amplitude of transient responses and the tone-burst frequency, not only for the corners of trapezoidal tone bursts, but also for tone bursts of more general shapes. We conclude that, owing to its extraordinarily steep high-frequency rolloff, the filter associated with each cochlear axon is well suited to extracting temporal information from onset or offset singularities in modulated tones whose frequencies are above the characteristic frequency of the filter. Applying the theory to observed onset and offset responses to high-intensity tone bursts in auditory afferents of the Mongolian gerbil, we conclude that some of the responses we observed must have been sculpted in part by cochlear nonlinearities.

Acoustic Stimulation↗

PubMed1989

Cochlear nerve responses to waveform singularities and envelope corners.

One way that discrete acoustic events may be signaled to the central nervous system is through spike synchrony over a subpopulation of cochlear axons. Each of the four corners of a trapezoidally modulated tone burst is such an event. Ordinarily, each corner comprises both an abrupt change in envelope slope and a singularity in the modulated waveform. In this study, in addition to stimuli of this sort, we employed a stimulus waveform in which a corner occurred without a waveform singularity. We obtained masker tuning curves for the CAPs corresponding to both kinds of corners and single-unit responses to both kinds of corners. The results suggest that the subpopulation of cochlear axons excited by the singularity component of a corner is distinct from that excited by the abrupt change in envelope slope.

Acoustic Stimulation↗

PubMed1989

Detuning of cochlear action potential tuning curves at high sound pressure levels: influence of temporal, spectral and intensity variables.

Action potential (AP) tuning curves (TCs), generated by probe stimuli of 60-65 dB SPL with short rise and decay (r&d) times, are less sensitive (have elevated tip thresholds) and are detuned (the frequency is shifted away from that of the probe stimulus, towards a middle frequency of the audiogram). These effects are more pronounced with forward than with simultaneous masking. TCs generated by masking tonal and narrow band noise stimuli are nearly identical, even though the spectrum is much wider for the noise stimulus. Decreasing r&d time has the same effect on TCs generated from both noise and tonal stimuli, even when it only measurably increases the acoustic splatter of the latter. Detuning appears to be related to a temporal-intensity interaction.

Action Potentials↗

PubMed1988

Effects of acoustic and sensory variables on masking tuning curves of the offset auditory brain-stem response in the rodent.

Simultaneous sinusoidal masking of the auditory brain-stem response (ABR) which is generated by the offset of a tone produces a W-shaped masking tuning curve (TC) in the gerbil, rat, mouse, and guinea pig. The probe stimulus offset must be very rapid in most animals. Lesions of the contralateral ear, strychnine blockage of the olivocochlear bundle, or removal of the ipsilateral outer ear do not alter the basic properties of the offset masking TC. Increasing the simultaneous masker duration selectively increases the tuning of the offset masking TC peak. Masking only the latter portions of the probe stimulus does not alter the shape of the offset masking TC.

Animals↗

PubMed1988

Offset AP masker tuning curve and the FFT of the stimulus.

In previous experiments, it was noted that a cochlear compound action potential (CAP) can be produced by the offset of a tone, provided that the amplitude of the tone is modulated by a trapezoid with slopes that are typically much steeper than required to produce onset responses. Subsequently, such trapezoidal tone bursts with steep slopes were used as probe stimuli in simultaneous and forward masking experiments that were designed to evaluate the tuning characteristics of these offset CAPs. Masker tuning curves (MTCs) were generated by plotting the masker frequency necessary to reduce the amplitude of the offset CAP by 50%. Simultaneous masking of the offset CAP generated a W-shaped MTC, with two sharply tuned tips and one sharply tuned peak. Forward masking generated a sharply tuned V-shaped offset MTC. By contrast, for onset CAPs, both simultaneous and forward masking generated V-shaped MTCs. The very steep stimulus slopes required to produce an offset CAP are likely to generate much more acoustic splatter than the more gradual slopes required to produce an onset CAP, and this may be related to the different shapes of the onset and offset simultaneous MTCs. To explore this possibility, the relationship of the spectral characteristics (determined by fast Fourier transform, or FFT) to the shape of the MTC was studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗