Ultrastructure of the tympanic membrane in the Rhesus monkey

The tympanic membranes of 8 Rhesus monkeys (macaca mulatta) were investigated by transmission electron microscopy. The interface of the epidermis and the fibrous framework appeared rather smooth and the contact between the basal cell layer and the lamina propria was secured by a dermo-epidermal junction. Cross-sectioned fibrils varied in shape and thickness, the thin fibrils being quadrangular. The findings of this study indicate a great similarity between the tympanic membrane of man and that of the Rhesus monkey. A corresponding similarity has been documented concerning the function of the Eustachian tube--middle-ear system, and it is concluded that the Rhesus monkey is highly suitable for studies of the tympanic membrane.     

Effect of vestibulo-cerebellar lesions on asymmetry of vertical optokinetic functions in the squirrel monkey

The role of the cerebellar uvula and nodulus in vertical optokinetic after-nystagmus (OKAN) was studied in 4 squirrel monkeys. Aspiration ablation of the uvula and nodulus resulted in no significant change in the initial or peak gain of vertical optokinetic nystagmus (OKN) during the 24-week post-operative observation. However, the asymmetry of vertical OKAN was significantly altered. Using a protracted upward OK stimulus, slow phase-down OKAN-II, which was not seen pre-operatively, was significantly increased. In contrast, a downward OK stimulus produced little change in slow phase-up OKAN-II. Thus, the asymmetric degree of vertical OKAN-II was decreased after uvulonodulectomy. In addition, there was a post-operative reduction in the vertical oculomotor stability. When slow-phase eye velocity of OKAN was plotted along the time scale, the amplitude and frequency of the sinusoidal pattern was increased. OKAN-III and OKAN-IV were found in 50% of the monkeys after uvulonodulectomy. It is therefore thought that inhibition and directional control from the uvula and nodulus influence the stability and asymmetrical behaviour of the leaky integrator in the second order output system.     

Modulation transfer functions: a comparison of the results of three methods

Modulation transfer functions (MTFs) were measured with three different psychoacoustical paradigms in the same normal-hearing subjects. In the temporal-probe method, the threshold of a 4-ms probe tone (frequencies of 1000 and 4000 Hz) was measured at various envelope phases within a 100% sinusoidally amplitude-modulated (SAM) noise at modulation frequencies from 2 to 256 Hz. For the derived-MTF method, the threshold of a 500-ms tone at 1000 and 4000 Hz was measured in the same noise at the same modulation frequencies. For the modulation-detection paradigm, modulation thresholds were measured as a function of modulation frequency for bandpass filtered SAM noise centered at 1000 and 4000 Hz. MTFs with lowpass shapes were observed with all three methods. Differences were observed in the cutoff frequencies and/or attenuation rates when the data were fitted with lowpass filter transfer functions. Factors influencing those differences are discussed.     

Temporal modulation transfer functions in normal-hearing and hearing-impaired listeners

Modulation thresholds for sinusoidally amplitude-modulated broadband noise were obtained from normal-hearing and sensorineural hearing-impaired listeners as a function of modulation frequency. The resulting temporal modulation transfer functions (TMTFs) indicated that the impaired listeners were generally less sensitive than the normals to amplitude modulation and, unlike previously published data from normal-hearing listeners, TMTFs in the impaired listeners were level dependent: sensitivity to modulation, particularly for modulation frequencies greater than 100 Hz, decreased with decreases in level. TMTFs were also obtained with band-limited noise from the normal-hearing listeners: the noise was low-pass filtered at 1.6 kHz after modulation and was generally presented with a 1.6-kHz high-pass masker. The TMTFs in the low-pass condition were similar to the TMTFs obtained with broadband noise from the impaired listeners, suggesting that the impaired temporal processing in the hearing-impaired listeners is a result of a narrower effective, 'internal' bandwidth. Increment thresholds for continuous broadband and low-pass noise were obtained in conditions similar to those in which TMTFs were obtained. In general, a similar power-law relationship between modulation threshold and increment threshold was found to exist for both the normal-hearing and the hearing-impaired listeners.     

Intracochlear acoustic pressure measurements: transfer functions of the middle ear and cochlear mechanics

Direct intracochlear acoustic pressure recordings (from 20 to 20,000 Hz) are used to measure the middle-ear transfer functions (forward and reverse) and to better understand the cochlear mechanics in the guinea pig. In the forward direction, the middle-ear transfer function is strongly dependent on the frequency and presents a maximum of +30 dB at 1,000 Hz (bulla open). In the reverse direction, the middle-ear transfer function looks like an ideal reverse middle-ear pressure transformer with -35 dB gain and 0 degrees phase lag from 20 to 8,000 Hz (bulla open, closed ear canal). Passive cochlear mechanics is studied with the help of intracochlear pressure measurements and differential cochlear microphonic potential recordings in the different turns.     

Frequency-importance and transfer functions for recorded CID W-22 word lists

Frequency-importance and transfer functions for the Technisonic Studios' recordings of the CID W-22 word test are reported. These functions may be used to calculate Articulation Index (AI) values or to predict scores on the W-22 test. The functions were derived from the word recognition scores of 8 normal-hearing listeners who were tested under 308 conditions of filtering and masking. The importance function for the W-22 test has a broader frequency range and a different shape than the importance function used in the current ANSI standard on the Articulation Index (ANSI, 1969). The transfer function is similar in slope to the ANSI transfer function for 256 PB-words, but is shifted to the right of that function by 0.05 AI.     

Auditory neurophonic responses to amplitude-modulated tones: transfer functions and forward masking

Two auditory neurophonic responses - one recorded from the scalp (frequency following response or FFR) and one from the auditory nerve (auditory nerve neurophonic or ANN) - were obtained following stimulation of the cat cochlea with amplitude-modulated (AM) high-frequency tones. The carrier frequencies varied between 2 and 30 kHz. The modulation frequencies varied between 400 and 3000 Hz. The AM responses were compared with pure-tone neurophonic responses. The AM response waveforms were found to have a similar spectral composition, similar rates of adaptation, and similar rates of recovery from forward masking as the comparable pure-tone responses. As with the pure-tone neurophonics, an unmodulated masking stimulus can produce prolonged depression of the probe response. The amount and duration of this depression is dependent upon the level and frequency of the masker. The frequency dependence of the depression is demonstrated by forward masked tuning curves which indicate that the AM responses arise from fiber populations which have restricted characteristic frequency distributions centered on the carrier frequency. Response amplitude as a function of stimulus level (I/O) functions, response amplitude as a function of carrier frequency (carrier transfer functions or CTF) and response amplitude as a function of modulation frequency (modulation transfer functions or MTF) were also measured. It was found that the I/O functions were saturating monotonic functions of stimulus intensity, CTFs were flat for carrier frequencies from 6 to 30 kHz, and MTFs were flat for modulation frequencies from 100 to 1500 Hz. These results are compared with similar data for single units and compound action potentials.     

Temporal modulation transfer functions measured from auditory-nerve responses following sensorineural hearing loss

The ability of auditory-nerve (AN) fibers to encode modulation frequencies, as characterized by temporal modulation transfer functions (TMTFs), generally shows a low-pass shape with a cut-off frequency that increases with fiber characteristic frequency (CF). Because AN-fiber bandwidth increases with CF, this result has been interpreted to suggest that peripheral filtering has a significant effect on limiting the encoding of higher modulation frequencies. Sensorineural hearing loss (SNHL), which is typically associated with broadened tuning, is thus predicted to increase the range of modulation frequencies encoded; however, perceptual studies have generally not supported this prediction. The present study sought to determine whether the range of modulation frequencies encoded by AN fibers is affected by SNHL, and whether the effects of SNHL on envelope coding are similar at all modulation frequencies within the TMTF passband. Modulation response gain for sinusoidally amplitude modulated (SAM) tones was measured as a function of modulation frequency, with the carrier frequency placed at fiber CF. TMTFs were compared between normal-hearing chinchillas and chinchillas with a noise-induced hearing loss for which AN fibers had significantly broadened tuning. Synchrony and phase responses for individual SAM tone components were quantified to explore a variety of factors that can influence modulation coding. Modulation gain was found to be higher than normal in noise-exposed fibers across the entire range of modulation frequencies encoded by AN fibers. The range of modulation frequencies encoded by noise-exposed AN fibers was not affected by SNHL, as quantified by TMTF 3- and 10-dB cut-off frequencies. These results suggest that physiological factors other than peripheral filtering may have a significant role in determining the range of modulation frequencies encoded in AN fibers. Furthermore, these neural data may help to explain the lack of a consistent association between perceptual measures of temporal resolution and degraded frequency selectivity.     

Exchange Rates of Gases Across the Tympanic Membrane in Rhesus Monkeys

The exchange rates of CO₂ and He across the tympanic membrane were estimated in 5 monkeys. For these experiments, the monkey was anesthetized and one arm of a polyethylene ?T? tube was introduced into the external canal of the test ear and sealed to the ambient environment with wax. One arm of the T tube was attached to a pressure transducer and the other to an argon gas source via a valve. Silica tubing sealed within the probe provided periodic gas samples for composition analysis by an online mass spectrometer. Prior to each experiment, the probe was washed with Argon. In 5 experiments the probe was sealed within the external canal of animals with physiological middle ear gas compositions, and in 5 experiments the probe was sealed within the external canal of animals whose middle ears were partially washed with He. The gas in the probe was sampled and analyzed at 10-min intervals for up to 4 h. The results documented a significant increase in the percentage composition of CO₂ but not He in the experiments conducted with physiological middle ear gas compositions, and increases in both He and CO₂ in the experiments conducted after the middle ear was washed with He. Estimated, average exchange constants for He and CO₂ were 0.0005     

Temporal modulation transfer functions for single neurons in the auditory midbrain of the leopard frog. Intensity and carrier-frequency dependence

The sensitivity for amplitude modulation was investigated for 77 neurons from the auditory midbrain of the leopard frog. The results show that tuning to modulation frequencies occurs in about one-third of the units but is quite varied in its appearance. Two slightly differing characterizations for this tuning have been used; the overall response or rate-Modulation Transfer Function and the synchronized response or temporal-MTF (tMTF). The relation between the two characterizations is given by the vector-strength. Only one-third of the units showed a vector-strength that was significantly different from zero. Another synchronization measure, the synchronization factor which is based on the auto-coincidence function, was significantly different from zero in about 3/4 of the units. The Best Modulation Frequency (BMF) and tuning band-width were found to be independent of both stimulus intensity and carrier frequency, although the average BMF for band-pass units was slightly higher for the amphibian papilla range of carrier frequencies than for the basilar papilla range (66 Hz vs. 49 Hz). The most frequent BMF for band-pass units was around 55 Hz, this does not correspond with the dominant modulation frequency of the mating call which is around 20 Hz. The synchronization measures were negatively correlated with intensity and independent of carrier frequency. The phase response of the tMTF was used to calculate the group delay. In contrast to the latency of the units the group delay was independent of stimulus intensity.     

Temporal modulation transfer functions for band-limited noise in subjects with cochlear hearing loss.

The modulation depth required for the detection of sinusoidal amplitude modulation was measured as a function of modulation rate, giving temporal modulation transfer functions (TMTFs). The carrier was a one-octave wide noise centred at 2 kHz, and it was presented in an unmodulated background noise lowpass filtered at 5 kHz. Three subjects with unilateral cochlear hearing loss were tested. For each subject, the normal ear was tested both at the same sound pressure level (SPL) and at the same sensation level (SL) as the impaired ear. The TMTFs were essentially the same for the normal and impaired ears, both at equal SPL and at equal SL. The better ears of three subjects with bilateral cochlear losses were also tested. Again, TMTFs were essentially the same as obtained for normal ears. These results suggest that temporal resolution is not necessarily adversely affected by cochlear hearing loss, at least as measured by this task.     

Temporal modulation transfer functions in cochlear implantees using a method that limits overall loudness cues

Temporal modulation transfer functions (TMTFs) were measured for six users of cochlear implants, using different carrier rates and levels. Unlike most previous studies investigating modulation detection, the experimental design limited potential effects of overall loudness cues. Psychometric functions (percent correct discrimination of modulated from unmodulated stimuli versus modulation depth) were obtained. For each modulation depth, each modulated stimulus was loudness balanced to the unmodulated reference stimulus, and level jitter was applied in the discrimination task. The loudness-balance data showed that the modulated stimuli were louder than the unmodulated reference stimuli with the same average current, thus confirming the need to limit loudness cues when measuring modulation detection. TMTFs measured in this way had a low-pass characteristic, with a cut-off frequency (at comfortably loud levels) similar to that for normal-hearing listeners. A reduction in level caused degradation in modulation detection efficiency and a lower-cut-off frequency (i.e. poorer temporal resolution). An increase in carrier rate also led to a degradation in modulation detection efficiency, but only at lower levels or higher modulation frequencies. When detection thresholds were expressed as a proportion of dynamic range, there was no effect of carrier rate for the lowest modulation frequency (50 Hz) at either level.     

Pure tone thresholds for the rhesus monkey

Auditory thresholds were measured for 18 ears from 13 rhesus monkeys using a simple reaction-time procedure. The threshold contour was a smooth W-shaped function with rises at the extreme frequencies and around 4 kHz and was comparable in shape with previously reported thresholds for this animal. Standard deviations averaged 5.3 dB.