Two-tone inhibition in auditory-nerve fibers

Dynamic properties of excitation and inhibition in the cochlear nucleus

The dynamic properties of inhibition and excitation of single units in the cochlear nucleus were studied using tones that were amplitude modulated either sinusoidally or with pseudorandom noise. The cross-correlation analysis of the unit discharge rate and the pseudorandom noise modulation of the stimuli showed that the dynamic properties determined from the responses to a single modulated excitatory tone (at CF), to a modulated excitatory tone together with an unmodulated inhibitory tone (above CF), and to a modulated inhibitory tone together with an an unmodulated excitatory tone were almost identical with regard to latency as well as to delay of the peak of the cross-covariance function. On the basis hereof it is inferred that the inhibition is either a cochlear phenomenon or that it is transmitted over pathways with identical temporal properties as those of the excitation. When 2 tones were presented simultaneously the modulation of the excitatory tone usually gave a higher degree of modulation of the discharge rate than did modulation of an inhibitory tone. Addition of an unmodulated inhibitory tone to a modulated tone at CF resulted in an extension of the range of intensities over which the maximal gain was relatively constant. In many units this range extended from about 10dB above the unit's threshold to 60 dB or more above.

Diode pump cochlear audition theory

A model of the cochlea is developed whereby the motion of the basilar membrane due to sound pressure in the scala media creates, through the pivotal motion of the pillars of Corti, modulated pressure on the fluid in the inner sulcus. This modulated pressure causes fluid to flow through the tectorial gap at selected longitudinal positions along the basilar membrane causing excitation or inhibition of the hair cell activity depending upon the direction of fluid flow. Specific functional relationships are developed for a two component model composed of a resonant and a nonresonant region. Relevant experimental data show good agreement with the predictions of the model.

Phase opposition between inner and outer hair cells and auditory sound analysis

Recordings of single-unit responses in the auditory nerve of normal and kanamycin-treated Mongolian gerbils indicate that inner and outer hair cells of the cochlea interact in phase opposition. After kanamycin treatment, the firing rate in some fibers is increased during the basilar membrane motion toward scala vestibuli, in others, during its motion towards scala tympani. Because of statistical correlation with anatomical changes and characteristic time patterns, the first response polarity is associated with inner hair cells, the second, with outer hair cells. It is shown that normal responses can be reconstructed from the two kinds of responses seen after kanamycin treatment. The phase opposition between inner and outer hair cells, in connection with the expected effect of spiral fibers, provides an explanation for neural sharpening of mechanical filter action in the cochlea.

Measurement of vibration of the basilar membrane in the squirrel monkey

The Mössbauer technique, which can be used to measure very small velocities, on the order of 0.2 mm/sec, has been used to measure the response of the basilar membrane to tones and clicks in squirrel monkeys. The results verify that there is a mechanical frequency analysis performed in the cochlea and that the traveling wave theory holds true. The resonance curves indicate that the tuning of the basilar membrane is greater than was thought. The basilar membrane in the 7–8 kHz region of the cochlea vibrates nonlinearly at frequencies near the “resonant frequency.” The click response shows that the “tail” of the decaying oscillatory response does not decrease in proportion to click amplitude while the early displacements of the basilar membrane have a nearly linear relationship with click amplitude. These results are in good agreement with the results of the measurements using tones as stimuli. Experiments examining postmortem behavior of the basilar membrane indicate a rapid decrease in the sensitivity of vibration along with a decrease of up to one octave in the “resonant” frequency within a six hour period after the animal's death. The shift in resonant frequency is accompanied by a corresponding shift in the phase characteristic. The low frequency slope of the resonance curve becomes 6 dB/octave exactly as Békésy found while the high frequency slope decreases slightly. Comparison of the mechanical resonance curves with the neural data for single auditory nerve fibers in the squirrel monkey indicates that the exquisite tuning exhibited in the nerve cannot be explained solely on the basis of the mechanical behavior of the basilar membrane.

The frequency response and other properties of single fibres in the guinea-pig cochlear nerve

1. Micro-electrode recordings were obtained from over 100 single fibres in the cochlear nerve of the pentobarbitone or urethane anaesthetized guinea-pig. The acoustic system was calibrated at the tympanic membrane and threshold sound level measurements so corrected.2. The minimum thresholds of the fibres approached with 10-20 dB of the behavioural thresholds reported in the literature. Exceptions to this were fibres from preparations where there was evidence of malfunction of the cochlea either from abnormally low perfusion or local damage, and a few high frequency fibres. With these high threshold fibres excepted, the range of thresholds at a given frequency in any one animal was less than 20 dB.3. The slopes of the low and high frequency cut-offs of the frequency-threshold curves (;tuning curves') within 25 dB of minimum threshold, ranged from 10 to 60 and from 20 to 125 dB/octave respectively for fibres with characteristic frequencies below 2 kHz, increasing to 90-180 and 200-600 dB/octave respectively for fibres with characteristic frequencies at about 8 kHz. These slopes represent the minimum values for the high-frequency cut-offs, which increase towards 1000 dB per octave in some cases at higher levels above threshold. At 30-50 dB above threshold, the low frequency cut-offs become suddenly less steep and approximate to 5 dB per octave.4. The relative sharpness of the frequency-threshold curves, measured as the ;Q(10 dB)', i.e. the ratio of characteristic frequency to the band width at 10 dB above minimum threshold, ranged from 1 to 4 for fibres with characteristic frequencies below 2 kHz, to 3-15 for fibres with characteristic frequencies near 10 kHz.5. The slopes and ;Q(10 dB)' measures of the frequency-threshold curves of most of the abnormally high threshold fibres approximated to, or were lower than those of analogous measurements of the guinea-pig basilar membrane vibration patterns.6. Four fifths of the cochlear nerve fibres had spontaneous discharge rates greater than 1/sec. No consistent relationship was observed between the rate of this activity and response properties, with the exception that nearly half of the high threshold fibres were silent. In these and other respects the response properties to tonal and click stimuli resembled those of cochlear nerve fibres in the cat. In no case was inhibition of the spontaneous discharge by single tones observed.7. It is concluded that, contrary to earlier reports, the cochlear nerve fibres of the guinea-pig are substantially more frequency selective than the existing measurements of the guinea-pig basilar membrane displacement. In terms of band width, this discrepancy approaches a factor of ten. The finding of a considerable range of band widths within optimal preparations, and frequency-threshold curves approximating to the mechanical functions in fibres from pathological cochleas, provides circumstantial evidence for a physiologically vulnerable sharpening mechanism occurring within the cochlea subsequent to the displacement pattern of the basilar membrane.

Mapping of interactions in the pitch memory store

A technique obtaining a precise mapping of interactive effects in the pitch memory store is described. Subjects were required to compare two tones for pitch when these were separated by a 5-second interval during which six other tones were played. In the second serial position of the intervening sequence there was placed a tone whose pitch bore a critical relationship to the pitch of the first test tone. When the critical intervening tone was identical in pitch to the first test tone, memory facilitation was produced. As the separation in pitch between these two tones increased, errors rose progressively, peaked at a separation of 2/3 tone, and declined roughly to baseline at a whole tone separation. It is concluded that the pitch memory store is arranged logarithmically in a highly ordered and specific fashion.

Coding of sounds in lower levels of the auditory system

The great number of investigations and advanced developments in neurophysiology and psychoacoustics during recent years have extensively increased our knowledge about the frequency analysis of simple sounds in the peripheral auditory system.

New methods have facilitated quantitative measurements of the amplitude of the submicroscopic vibration of a narrow segment of the basilar membrane in anaesthetized animals at physiological sound intensities. The results of these studies have quantitatively confirmed the results of past studies by showing that the basilar membrane has a selectivity with regard to tone frequency. In addition to this, the recent studies have increased our knowledge about the finer details of vibration of the basilar membrane. At the lowest levels used in the recent investigations, i.e. about 70 dB SPL, the selectivity in the 7 kHz region of the basilar membrane was found to be greater than expected on the basis of extrapolation of older data. Moreover, the high frequency slope of the tuning curves of the basilar membrane was found to be particularly steep. The results of these recent studies, furthermore, showed that the basilar membrane vibrates in a non-linear way at intensities within the physiological range. This non-linearity results in a broadening of the selectivity curves of a narrow segment of the basilar membrane when the sound intensity is increased.

Little is known as to how the motion of the basilar membrane is transformed to excitation of the cochlear sensory cells, i.e. the haircells. The excitation may be related to displacement, spatial differentiation or other transformations of the basilar membrane motion. Recording from the interior of mammalian haircells has so far been unsuccessful, and the neural excitatory process within the haircells in the cochlea is as yet practically unknown. Studies of the haircells in the lateral line organ of fish have provided fundamental knowledge about their excitation; since they in many respects resemble those in the mammalian cochlea, the results very probably can be applied to the excitatory process in the mammalian cochlea.

Properties of 'two-tone inhibition' in primary auditory neurones

1. Properties of two-tone inhibition in primary auditory neurones of cats were studied with phase-locked sound stimuli. One sound was a continuous tone at the best frequency of a given neurone, and the other, a tone burst which was changed in amplitude, frequency, and phase relative to the continuous tone.2. The tone burst which caused two-tone inhibition had either an excitatory or no effect when it was delivered alone. Inhibitory areas commonly appeared on both sides of the excitatory area when the best frequency was higher than a few kc/s.3. Two-tone inhibition began and ceased within a few milliseconds of the onset and termination of the excitation caused by a tone burst. The degree of inhibition was greatest at the beginning of the tone burst and reached a plateau within 500 msec. The discharge rate during inhibition could be lower than the rate for either tone alone or for spontaneous activity. At the termination of inhibition, prominent rebound in the discharge rate was found.4. With an increase in amplitude of a tone burst, for either a fixed or equally increased continuous tone, the discharge rate during inhibition decreased to a minimum and then began to increase. That is, the degree of inhibition was non-monotonically related to the sound level.5. Compound period histograms of discharges during inhibition showed that single neurones usually carried information about the combined wave form of the two tones. The information about each tone was, however, modified by the inhibitory phenomenon in both amplitude and phase from that indicated by the compound period histograms for the individual tones.6. Possible mechanisms and functional significance of two-tone inhibition are discussed.