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Biomedical subjects

B M Johnstone

Publications and source records attributed to B M Johnstone.

At least 73 records · Page 4Linked to original sources
PubMed1982

Temporary threshold shift modified by binaural acoustic stimulation.

Monaural losses in hearing sensitivity induced by an intense pure tone could be reduced if an acoustic stimulus of the same frequency was simultaneously delivered to the other ear. The reduction was eliminated when the contralateral stimulus was set at a frequency other than the ipsilateral trauma frequency and also after the administration of strychnine, a known blocker of auditory efferent activity. This suggests that acoustic activation of auditory efferents is responsible for the reduced ipsilateral sensitivity loss.

Acoustic Stimulation↗
PubMed1982

Reduced temporary and permanent hearing losses with multiple tone exposures.

Acoustic overstimulation of the guinea pig cochlea with a 16 kHz pure tone induces a loss in threshold sensitivity that can be either temporary or permanent depending on the duration of the trauma. When a second tone of lower frequency (10, 5 or 2 kHz) is presented to the same cochlea simultaneously with the first tone, then the resultant threshold loss produced by the 16 kHz tone is significantly less. This applies to both temporary and permanent threshold losses. The reduced threshold loss disappears as the intensity of the second tone decreases. This type of nonlinear cochlea behaviour is similar to other acoustically evoked nonlinearities generally grouped under the term, two-tone suppression or inhibition. The results disagree with the "equal energy hypothesis' as a method to establish damage risk criteria in noise-induced hearing loss.

Animals↗
PubMed1982

Measurement of basilar membrane motion in the guinea pig using the Mössbauer technique.

Basilar membrane motion was measured at the 16-19 kHz place of the guinea pig cochlea using the Mössbauer technique. The threshold of the gross cochlear action potential (CAP) evoked by pure-tone bursts was used as an indication of neural threshold. CAP threshold deteriorated progressively after the cochlea was opened and the Mössbauer source placed on the basilar membrane. A close relationship was found between the amplitude of basilar membrane motion at the source place frequency and CAP threshold. Basilar membrane velocity at CAP threshold SPL was about 0.04 mm/s over a 60-dB range of CAP threshold. Intensity functions for basilar membrane motion were linear for frequencies more than an octave below the source place frequency but demonstrated progressive saturation for frequencies greater than an octave below the CF. This nonlinear behavior was eliminated as the CAP threshold became less sensitive and was absent post mortem. Isovelocity curves at the 0.04 mm/s criterion were remarkably similar to frequency threshold curves from primary afferent fibers innervating a similar place on the basilar membrane. The isovelocity curve was a better fit than the isoamplitude curve suggesting that inner hair cells respond to basilar membrane velocity. As the CAP threshold deteriorated, the isovelocit curves lost sensitivity around the best frequency, whereas sensitivity to frequencies below 10 kHz remained constant even after the animal was killed. We suggested that most of the frequency response and nonlinear behavior of inner hair cells and afferent fibers may be found in basilar motion.

Animals↗
PubMed1981

Primary auditory neurons: nonlinear responses altered without changes in sharp tuning.

Two-tone suppression as well as distortion product responses in mammalian auditory neurons were altered by exposure to short, high intensity tones in the frequency region of the suppressing or primary tones. These changes occurred without significant alteration of the single neurons' tuning curves. The findings support the notion that two-tone suppression and distortion product responses are a function of the integrity of cochlear regions remote from the final transduction site.

Acoustic Stimulation↗
PubMed1981

Acoustic trauma: single neuron basis for the "half-octave shift".

Exposure to an intense pure tone can induce a loss of hearing sensitivity. If this loss recovers, then the desensitization is regarded as a temporary threshold shift (TTS). At the single auditory neuron level this TTS was monitored as a loss of sensitivity at the neuron's most sensitive or characteristic frequency (CF). When pure-tone exposures were presented at frequency intervals measured from the neuron CF, then a frequency half an octave below the CF was the most effective for inducing a CF TTS. All exposure frequencies higher than half an octave below the CF produce a marked reduction in TTS growth with intensity, when compared to lower exposure frequencies. This behavior is such that, with increasing exposure frequency higher than the -1/2-octave point, the intensity needed to produce a given TTS grew faster than the neuron sensitivity. However, below the -1/2-octave point all exposure frequencies were similarly behaved. Strong similarities exist between the frequency-specific requirements for TTS and the mechanical and neural nonlinearities found in other studies. This suggests that the half-octave shift may well be a direct result of basilar membrane nonlinearities.

Acoustics↗
PubMed1980

Single auditory neuron response during acute acoustic trauma.

By recording from single auditory neurons in the spiral ganglion of the guinea pig cochlea it was possible to monitor threshold changes to acoustic stimuli in the same neuron during acute exposures to continuous pure tones (100 dB SPL) delivered for periods of up to 180 min. Initial changes in the tuning curve were sensitivity losses at and above the characteristic frequency (CF). This asymmetric loss resulted in a shift in the CF to lower frequencies. Further exposure (up to 20 min) produced a complete loss of the sharply tuned portion of the tuning curve and the most sensitive area moved to lower frequencies by up to 0.75 of an actave when compared to the original CF. Sensitization of the low frequency section of the tuning curve could take place for short exposures (up to 20 min) but disappeared with further exposure, whether continuous or interrupted. Long term exposures (60--80 min) produced losses in the low frequency portion of the tuning curve until a point was reached when the unit no longer responded to stimuli at 110 dB SPL.

Acoustic Stimulation↗
PubMed1980

Effects of loud tones on the inner ear: a combined electrophysiological and ultrastructural study.

Guinea pigs were exposed to a 10 kHz pure tone and damage to the cochlea was investigated immediately after exposure, or after a recovery period ranging from 18-25 days. Structural damage was assessed using scanning electron microscopy and functional damage estimated using the N1 threshold audiogram. Exposure at 106 dB for 1 h caused obvious abnormalities of outer hair cell stereocilia. The location and extent of this damage was related to the immediate or long term deficits in the N1 threshold audiogram.

Acoustic Stimulation↗
PubMed1980

Acoustic trauma in the guinea pig cochlea: early changes in ultrastructure and neural threshold.

Scanning microscopy was used to examine guinea pig cochleas for structural damage immediately after exposure to a pure tone ranging from 96 to 129 dB SPL. Functional changes to the cochlear neural sensitivity were assessed using the N1 audiogram. principal findings were: (1) The order of damage to receptor cells with increasing sound intensity was OHC1, then IHC, then OHC2 and OHC3. (2) The spatial distribution of damage to OHC1 and IHC differed with IHC tending to show damage mainly in the vicinity of the exposure frequently location and OHC1 damage spreading basal-ward of this point. (3) N1 threshold losses spread progressively to lower frequencies as exposure intensity increased. This was accompanied by an apical spread of damage to the receptor cells.

Animals↗
PubMed1980

Nonlinear mechanical behaviour of the basilar membrane in the basal turn of the guinea pig cochlea.

A capacitance probe is used to measure the vibrational displacement of the basilar membrane in the basal turn of the guinea pig. Nonlinear behaviour is exhibited in the region of the mechanical cut-off frequency similar to that previously described only for the squirrel monkey. The degree of nonlinearity appears to be directly correlated with the single unit threshold as reflected in the N1 action potential. Loss of neural sensitivity indicates loss of nonlinear behaviour in the mechanics which is undetectable shortly after death.

Action Potentials↗
PubMed1980

Response properties of spiral ganglion neurons in cochleas damaged by direct mechanical trauma.

Electrical activity of single neurons in the spiral ganglion of the guinea pig cochlea was correlated with receptor hair cell damage produced by local mechanical lesions of the organ of Corti. Damage to or loss of hair cells was assessed using scanning electronmicroscopy. Principal finds were (1) neurons emanating from damaged regions showed changes in their tuning curves such as loss of sensitivity, broadening of tuning curves; (2) in some cases, the loss of sharp tuning tip resulted in an apparent shift of the tuning curve to lower frequencies; and (3) apparently pure outer hair cell lesions, of limited spatial extent, could result in the same effects as mixed inner and outer hair cell lesions.

Animals↗
PubMed1979

Effect of divalent cations on spontaneous and evoked activity of single mammalian auditory neurones.

Recordings were obtained from single primary auditory neurones in the guinea pig cochlea during perfusion of the scala tympani with solutions containing elevated concentrations of Mg2+, Mn2+ and Co2+, or with lowered concentrations of Ca2+. Such perfusions caused a reversible depression of spontaneous firing rates. This is consistent with the notion that spontaneous firing is the result of background release of excitatory transmitter from cochlear hair cells, in the absence of acoustic stimulation. The above ion modifications also produced varying changes in single neurone response curves to acoustic stimuli. In one half of the 14 neurons studied these changes were also compatible with a classical blockage of synaptic transmission. The other half however, showed little or no change in sensitivity at low frequencies while large threshold elevations occurred at high frequencies.

Action Potentials↗
PubMed1979

Bone conduction mechanisms: Mössbauer measurements on the role of ossicular inertia.

The Mössbauer technique was used to measure displacements of the stapes footplate and adjacent temporal bone during bone conduction stimulation at frequencies from 250 to 400 Hz in anaesthetized guinea pigs. The stapes was found not to be driven at amplitudes or phases that differed significantly from those of the temporal bone. Measurement of stapes displacements during air conduction stimulation, and of temporal bone displacements during bone conduction stimulation producing matching cochlear microphonic amplitude, enabled calculation of limiting values of amplitude and phase difference necessary to produce the required relative displacement. The obtained values (less than 1 dB for amplitude and 1--4 degrees for phase) were beyond the resolution of the measurement system employed for reasonable nuclear counting times. The results provide quantitative estimates of the magnitude of inertial effects, but do not establish whether ossicular inertia is an important factor in bone conduction stimulation.

Acoustic Stimulation↗
PubMed1979

Cochlear action potential threshold and single unit thresholds.

There is a close correlation between the sound pressure of tone burst required to affect a primary auditory neuron at its characteristic frequency and that which will produce a detectable N1 response at the same frequency. Units with thresholds from 80--0 db SPL (recorded from damaged and undamaged cochleas) were 0--20 dB , respectively, more sensitive than the action potential response.

Action Potentials↗