Low-frequency Tone Bursts Research Articles

Electromagnetic acoustic wave transducer for the generation of acoustic evanescent waves on membranes and optical and capacitor wave-number selective detectors

A novel electromagnetic acoustic wave transducer (EMAT) capable of generating low-frequency tone bursts on metalized membranes is described. The bursts have a phase velocity that is much less than the speed of sound in the surrounding air so that they are accompanied by a plane evanescent wave adjacent to the membrane. Unlike conventional EMATs that rely on an external magnetic field interacting with an eddy current distribution, this system makes use of an applied current pulse through the membrane itself. A spatially and temporally oscillating Lorentz force is generated on the membrane with the application of a tone burst through a meander-line coil configuration simultaneous with a current pulse through the membrane. Studies were carried out on the frequency selectivity of the EMAT, the steady-state buildup of the tone bursts, and the dependence on gap spacing between the meander-line coil and the membrane. Optical and capacitive probes were used to detect the wave train and verify the wave shape characteristics. Wave-number selective differential optical and capacitive transducers are also described. A property of these transducers is their insensitivity to long wavelengths on the membrane.

Auditory cortical neurons are sensitive to static and continuously changing interaural phase cues

1. The interaural-phase-difference (IPD) sensitivity of single neurons in the primary auditory (AI) cortex of the anesthetized cat was studied at stimulus frequencies ranging from 120 to 2,500 Hz. Best frequencies of the 43 AI cells sensitive to IPD ranged from 190 to 2,400 Hz. 2. A static IPD was produced when a pair of low-frequency tone bursts, differing from one another only in starting phase, were presented dichotically. The resulting IPD-sensitivity curves, which plot the number of discharges evoked by the binaural signal as a function of IPD, were deeply modulated circular functions. IPD functions were analyzed for their mean vector length (r) and mean interaural phase (phi). Phase sensitivity was relatively independent of best frequency (BF) but highly dependent on stimulus frequency. Regardless of BF or stimulus frequency within the excitatory response area the majority of cells fired maximally when the ipsilateral tone lagged the contralateral signal and fired least when this interaural-phase relationship was reversed. 3. Sensitivity to continuously changing IPD was studied by delivering to the two ears 3-s tones that differed slightly in frequency, resulting in a binaural beat. Approximately 26% of the cells that showed a sensitivity to static changes in IPD also showed a sensitivity to dynamically changing IPD created by this binaural tonal combination. The discharges were highly periodic and tightly synchronized to a particular phase of the binaural beat cycle. High synchrony can be attributed to the fact that cortical neurons typically respond to an excitatory stimulus with but a single spike that is often precisely timed to stimulus onset. A period histogram, binned on the binaural beat frequency (fb), produced an equivalent IPD-sensitivity function for dynamically changing interaural phase. For neurons sensitive to both static and continuously changing interaural phase there was good correspondence between their static (phi s) and dynamic (phi d) mean interaural phases. 4. All cells responding to a dynamically changing stimulus exhibited a linear relationship between mean interaural phase and beat frequency. Most cells responded equally well to binaural beats regardless of the initial direction of phase change. For a fixed duration stimulus, and at relatively low fb, the number of spikes evoked increased with increasing fb, reflecting the increasing number of effective stimulus cycles. At higher fb, AI neurons were unable to follow the rate at which the most effective phase repeated itself during the 3 s of stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Spectral frequency and the modulation of the acoustic startle reflex by background noise.

The rat's (Long-Evans) acoustic startle reflex to a high-frequency tone burst (10.5 kHz) was depressed by intense high-frequency band-pass noise (8-16 kHz) but enhanced by low frequency noise (1-2 kHz). However, contrary to the hypothesis that the depression of startle in intense background noise is produced by sensory masking, the reflex to a low-frequency tone burst (at 1 kHz) was depressed by both high- and low-frequency band-pass noise. Two additional hypotheses are offered to supplement sensory masking in order to explain the asymmetry in these data. The first is that the intratympanic reflex, which acts as a high pass filter on acoustic input, is elicited in intense backgrounds. The second is that acoustic startle reflexes elicited by intense low-frequency tones are in part elicited by their high-frequency distortion products and that these distortion products are then masked by high-frequency background noise.

Responses of primary auditory fibers to brief tone bursts.

Responses of primary auditory fibers to short triangularly modulated bursts of tone were obtained in the anesthetized cat. Based on discharge rate alone, the characteristics of the "response areas" obtained with these tone bursts were found to depend on the best frequency of the fiber. For a fiber with low best frequency (below 1 kHz), tones of greater than 10-ms duration had to be presented in order for the frequency resolution of the neuron to be as good as it was for long tones. For fibers with high best frequencies (above 10 kHz), tones of 2 or even 1 ms caused responses that were nearly as frequency selective as those obtained with long tones. A linear minimum-phase model based on the steady-state frequency selectivity of the fibers has been developed and shows generally comparable responses, but with some interesting exceptions. Synchronization of discharges to the waveform of the low-frequency tone bursts was measured and also shown to be generally compatible with the minimum-phase model. Trapezoidally modulated tone bursts were also used.

Planar-curve analysis of auditory brain stem responses: Preliminary observations

When auditory brain stem responses (ABRs and/or FFRs) are collected from three orthogonally placed electrode pairs and voltage is plotted in three-dimensional space, a three-dimensional Lissajous figure is derived. Segments of the Lissajous figure (roughly corresponding to the beginning and ending points of individual waves in the classically recorded ABR or to the beginning and ending points of the plateau region in the scalp-recorded FFR) form planar-curves. ABRs elicited by click stimuli and FFRs elicited by low frequency tone bursts were collected from a group of adult animal (cat) and human subjects to investigate (in a preliminary manner) some of the consequences of this recording and analysis technique. Among the results were the following: (1) The formation of planes appears to be a consequence of a systematic change in the focus of synchronous input, as a function of time, and of the fact that auditory nuclei are for the most part geometrically ordered. (2) Planar-curve position appears to be determined by spectral content of stimuli, and by the dimensions and orientations of the electrical output of ABR generators. (3) Planar-curve size increases (usually in only one preferred direction) as stimulus intensity is increased, reflecting presumably patterns of excitation within generators as stimulus intensity is increased. These results suggest that: (1) Placement of electrode pairs orthogonally around the head allows for efficient recording of dipole activity in any direction, irregardless of the orientation of the dipole. (2) Information in these different waveforms plotted three-dimensionally constitutes extraction of spatial information that could not be visualized in ABR or FFR waveforms recorded using only single electrode pair techniques. [Work supported by Hearing Research, Inc.]

Electrocochleographic Study of Meniere's Disease

Electrocochleography (ECOG) reflects the functional state of the cochlea, and can be used as a tool of clinical otology. It is thus valuable when making a diagnosis of the different pathological conditions which affect the cochlea.In the present series, 16 ears in patients with Meniere's disease were examined by ECOG. As a control, 6 normal hearing ears and 10 high tone loss ears also served as subjects. Short tone bursts were used as stimuli, and the trans-tympanic recording method was used.Main findings in the case of Meniere's disease are summarized as follows.(1) The N1 thresholds were similar to the subjective ones.(2) The waveforms of the AP-SP potentials recorded in Meniere's cases were broad.(3) Both in Meniere's ears and high tone loss ears, a positive summating potential (SP+) was recorded when using high frequency tone bursts.(4) In the ears of those with Meniere's disease, the large negative summating potential (SP-) was obvious for low frequency tone bursts.(5) In the fluctuant stage of hearing loss, the negative summating potential (SP-) amplitude related to the attack of Meniere's disease, but not to the degree of hearing loss.(6) In those with Meniere's disease, the input-output curves of N1 amplitude were rather steep compared to normal ears, and a steep slope thus was indicative of a positive recruitment.(7) In the late stage of Meniere's disease, the difference between those with the disease and those with a high tone loss was not evident using ECOG.

Phase-locked responses to low frequency tones in the medial geniculate body

Over 2500 extracellular single unit recordings were obtained from the medial geniculate body (MGB) of cats anaesthetized with nitrous oxide. Three out of four units were activated by tone bursts, most of them presenting a transient ‘on’ response. Only about 10% of the units showed a sustained excitatory ‘through’ response. Some of these neurons, when activated by low-frequency tone bursts, had discharge synchronized with the phase of the tonal stimulus. Such phase-locked units were principally found in the pars lateralls, but also in the pars magnocellularis and pars ovoidea of the MGB. Significant phase-locking (vector strength R ⩾ 0.5) was observed up to 1000 Hz. About 20% of all units, responding in a sustained fashion to tone bursts below that frequency, were phase-locked. Monaural stimulation led to a shift of the mean phase angle compared to that measured when the tone bursts were delivered simultaneously to both ears. Implications of these experimental results on the synaptic jitter and the information processing through the auditory pathways are discussed.

Scalp-recorded frequency-following responses in neonates.

Low-frequency tone bursts (250, 500 and 1 000 Hz) were used to elicit frequency-following responses (FFRs) from full-term, healthy neonates. Easily identifiable FFRs were recorded from all well babies with stimulation at low-to-moderate levels (30-45 dB HLn). The amplitude and threshold values of neonatal FFRs were not significantly different from those of normally hearing adults. As in the adult, neonatal FFRs elicited by tone burst stimuli of 250 and 500 Hz were of greater amplitude and were observed at lower stimulus levels than FFRs recorded at higher frequency stimuli. At stimulus levels of 65 dB HLn, latency to the first peak in the FFR decreased as a function of increasing stimulus frequency. Neonatal FFRs evoked by tone bursts of 250 and 500 Hz exhibited significantly longer latencies compared to FFR recordings obtained from adults. These results suggest that: (1) Low-frequency basilar membrane sensitivity in neonates can be assessed as early as the first day of life. (2) The response properties of the FFR recorded from neonates were similar to the response properties of the FFR recorded from the adult. (3) For infants who fail conventional brainstem-evoked response screening procedures and/or for infants who are born "at risk", the FFR holds great promise as an assessment technique to determine the integrity of the low-frequency-sensitive regions of the cochlea and auditory brainstem.

Cochlear distribution of frequency-following response initiation. A high-pass masking noise study.

The cochlear initiation of the frequency-following response (FFR) to 500-Hz tone bursts was assessed in 6 normal-hearing subjects by measuring FFR amplitude as a function of the low-frequency cutoff of high-pass masking noise (3 550, 1 800, 900, and 450 Hz). The major fall off of FFR amplitude occurred when the masking noise began to mask the apical portion of the cochlea. This effect was especially apparent at low intensities. The results are interpreted as supporting the view that at low intensities, low-frequency tone bursts evoke the FFR primarily through the apical portion of the cochlea.

Effects of high‐pass masking on the scalp‐recorded FFR in humans and cats

Low frequency tone burst stimuli were used to elicit the scalp‐recorded frequency following response (FFR) in the presence of high‐pass maskers (with variable low frequency cutoffs) from normal hearing human subjects and anesthetized cats. Threshold shift and binaural loudness balance procedures, conducted with similar masking stimuli, were administered to compare psychophysically derived masking curves with the FFR curves. Among the results were the following: (1) At moderate to high probe‐tone levels, the amplitude of the FFR masked with a 2‐kHz cutoff ranged from a mean value of 75% for humans to a value of 55%–75% for cats. (2) At probe‐tone stimulus levels near FFR threshold, the amplitude of the FFR masked with cutoffs of 1.5, 1.8, and 2.0 kHz was equivalent to the amplitude of the unmasked FFR. (3) The FFR masking curves did not correspond with the psychophysically derived curves except at low probe‐tone stimulus levels. Although the area of stimulation (along the basilar membrane) for the FRR is broad, the most powerful input arises from the apical aspect of the cochlea (below 2 kHz). Except for lower stimulus levels, the responding area was apparently wider than the critical band around a low frequency signal. [Research supported by the Coleman fund and Hearing Research, Inc. San Francisco, CA.]

Studies of peripheral gating in the auditory system of cats

Cochlear potentials were recorded from the round window in unanesthetized cats with unilaterally tenotomized middle ear muscles. The effects of repetitive stimulation and conditioning procedures on the cochlear microphonic (CM) and auditory-nerve action potential (AP) were examined. Experimental procedures provided control over arousal effects and acoustic input variability and enabled precise quantification and statistical analysis of the data. Repetitive stimulation parameters were selected to provide optimal conditions for the occurrence of olivocochlear bundle (OCB) effects. Responses to high and low frequency tone bursts, noise bursts, and near-threshold click stimulation were investigated. In no case was there any systematic change in either CM or AP as a function of repetitive stimulation. The effects of aversive conditioning procedures on cochlear potentials were examined by pairing a train of clicks with infra-orbital shock. Appropriate interstimulus and inter-trial intervals were utilized. There were no significant changes in either CM or AP as a consequence of these procedures. Simultaneous behavioral observations indicated that initial shock presentation resulted in sensitization of the eye-blink response to the acoustic CS, and that this sensitization effect dissipated as a function of repeated pairing. The stability of CM and AP under these conditions is contrary to the view that cochlear potential amplitude varies as a function of procedures assumed to change the significance of the acoustic stimulus. It is concluded that the OCB does not function as a peripheral gating mechanism in the auditory system under these conditions.

The role of middle ear muscle contractions in contralateral remote masking

Middle ear muscle contractions are shown to produce a small but significant increase in the contralateral remote masking of a low-frequency tone burst when its threshold shift is determined as a function of its time delay with respect to a high-frequency masking stimulus (either octave band noise or tone). For low levels of masking stimulus (below 85 dB re 000021 μ bar) there is a smooth fall in masking with increasing delay. However, when the masking noise exceeds 85 dB or the masking tone exceeds 95 dB, there is an increase in masking amounting to an average (over seven subjects) of 3·2 and 1·4 dB, respectively, for masking stimuli of up to 115 dB. These increases occur at delay times of between 200 and 100 msec depending upon subject and upon masking stimulus level. The behaviour of these masking increases, with stimulus level, agrees with the behaviour of the impedance changes produced in the acoustic reflex bridge when the middle ear muscle reflexes are elicited. The assumption is therefore made that the attenuation of a low-frequency tone in contralateral remote masking is not only due to central masking but a small yet significant contribution is provided by middle ear muscle contractions.