Low Roll-off Research Articles

A model accounting for effects of vibratory amplitude on responses of cutaneous mechanoreceptors in macaque monkey.

1. A mechanoreceptor model, developed in the preceding paper (Freeman & Johnson, 1982), was used to study the effects of vibratory intensity and frequency on the responses of slowly adapting, rapidly adapting and Pacinian afferents in monkey hairless skin. As in the previous paper almost all of the response properties studied here were accounted for by the equivalent circuit model; changes in membrane time constant and amplitude sensitivity accounted for the differences between the three mechanoreceptive fibre types.2. The stimulus-response function of primary concern was the relationship between impulse rate and vibratory amplitude. This relationship had the same general form in each of the three fibre types. Amplitudes, I, less than I(0) produced no impulse on any stimulus cycles. Amplitudes greater than I(1) produced one impulse on every cycle. As I rose from I(0) to I(1) the impulse rate rose monotonically from 0 to 1 impulse/cycle. For each fibre type the form of this ramp depended on the stimulus frequency.3. At stimulus frequencies low in the frequency range of each fibre type the (I(0), I(1)) ramp tended to be steep and sigmoidal in shape. Two or more impulses occurred on some cycles and none on others.4. At intermediate frequencies the (I(0), I(1)) ramps became linear with at most one impulse on each cycle. A short plateau appeared at 0.5 impulses/cycle (i.e. there was a range of intensities yielding one impulse on alternate cycles). All of these response properties at low and intermediate frequencies were explained by the model.5. At higher frequencies the (I(0), I(1)) ramps became shallower and developed discontinuities in slope at impulse rates of 0.5 impulses/cycle. At stimulus frequencies greater than 20 Hz for SAs and RAs, the upper segment of the (I(0), I(1)) slope became steeper. For frequencies greater than 80 Hz, the upper segments of the Pacinian (I(0), I(1)) slopes were shallower than the lower segments. These effects suggested transient periods of hyperexcitability following each action potential, and reductions in sensitivity due to high impulse rates, respectively.6. The model's membrane time constant was adjusted to match the observed reduction in the (I(0), I(1)) slope with increasing stimulus frequency. The time constants required for least-squares fitting were 58, 29 and 4.2 msec for slowly adapting, rapidly adapting and Pacinian afferents, respectively; these values are of the same order as those obtained in the preceding paper.7. Receptor sensitivity varied across the frequency spectrum, slow adaptors being most sensitive at low frequencies, rapidly adapting units at mid-range, and Pacinians at the high frequencies. According to the model, the high frequency roll-off in a receptor's tuning curve is due to the current integrating properties of receptor membrane, and the low frequency roll-off is due to a high pass filter, presumably mechanical, situated in the tissues between the stimulus probe and receptor membrane.8. Impulse phase advances with increasing stimulus intensity in both receptor and model. The ability of the model to fit both the rate-intensity function and phase advance functions in individual receptors is demonstrated.

Auditory structure and function in the mouse middle ear: An evaluation by SEM and capacitive probe

1. The anatomic structures of the mouse middle ear were identified and examined with scanning electron microscopy and the area and lever ratios were measured in a number of ears. 2. The displacement and velocity response at 100 dB for test frequencies between 1.5 and 35.0 kHz were measured with a capacitive probe at three locations: The tip of the long process of the malleus, the transverse portion of the malleus, and the head of the incus. 3. The velocity vs frequency response curve at the tip of the malleus resembled a high-pass filter with a resonant frequency around 12.0 kHz, and a high and low frequency roll-off of about 1.4 dB and 6.7 dB/octave respectively. 4. The displacement response at the tip of the malleus was linear for stimulus intensities between 100 and 130 dB. 5. A comparison of the velocity frequency response curves measured at the tip of the malleus, at the transverse portion of the malleus, and at the head of the incus, provided some indication of the complexity of ossicular motion in the middle ear. 6. The shape of the inverse velocity curve at the transverse portion of the malleus was nearly identical to the threshold sensitivity curve of the mouse above 4.0 kHz.

Coherent ray tracing—measured and predicted shallow‐water frequency spectrum

Experiments have been conducted by the Applied Research Laboratory, The Pennsylvania State University, to study the acoustic signals received from underwater explosive charges under various environmental conditions. This paper describes a computer simulation study incorporating a model of a pressure signature for an underwater explosive charge and ray‐tracing techniques to predict the received energy spectrum in an environment where more than one acoustic path may exist between the source and the receiver. Measured and calculated one‐third‐octave received energy levels are compared for a standard Mk 83 SUS detonated at a depth of 18.3 m at a range of 18–128 km in water 35–79‐m deep. The site of the experiment was approximately 64 km east–northeast of Daytona Beach, Florida. The model shows good agreement with the experimental data in the prediction of a low frequency rolloff and frequency window in the received signal energy spectrum that varies with range separation between source and receiver.

Simultaneous 1.5‐ and 4‐GHz ionospheric scintillation measurement

Analysis of simultaneously recorded 1.5‐ and 4‐GHz scintillations at Tangua, Brazil, has shown that the scintillation index varies approximately as the wavelength between 1.5 and 4 GHz under weak scattering conditions. The amplitude distributions, especially those of the 1.5‐GHz data, are somewhat irregular in shape and cannot be represented by a simple mathematical expression. The power spectra of the amplitude fluctuations on a log‐log plot show a slope of −3 and have low wave number rolloffs near 10−2 m−1. This paper discusses some possible implications of these findings. Half of the 4‐GHz power spectra show the minima which are predicted by the thin‐screen weak scattering theory. The l/e falloff of the autocorrelation corresponds to irregularities between 100 and 200 m. The cross correlation between the 1.5‐ and 4‐GHz scintillations is about 0.3.

Behavioural audiograms from the bat,Rhinolophus ferrumequinum

A conditioned heart rate and muscular activity response was used to measure behavioural audiograms inRhinolophus ferrumequinum. The audiograms are very similar to the evoked potential “on” response threshold curves. They show a low frequency roll off of 30 dB per octave below 15 kHz and three areas of low threshold with SPLs of about 0 dB separated by two insensitivity peaks with SPLs of 25–30 dB. The sharply tuned threshold minima above 80 kHz (Q-values between 45–85) are followed by a high frequency roll off of 160–170 dB/octave.