Frequency-following Potential Research Articles

On the Origin of the Extracellular Field Potential in the Nucleus Laminaris of the Barn Owl (Tyto alba)

The neurophonic is a sound-evoked, frequency-following potential that can be recorded extracellularly in nucleus laminaris of the barn owl. The origin of the neurophonic, and thus the mechanisms that give rise to its exceptional temporal precision, has not yet been identified. Putative generators of the neurophonic are the activity of afferent axons, synaptic activation of laminaris neurons, or action potentials in laminaris neurons. To identify the generators, we analyzed the neurophonic in the high-frequency (>2.5 kHz) region of nucleus laminaris in response to monaural pure-tone stimulation. The amplitude of the neurophonic is typically in the millivolt range. The signal-to-noise ratio reaches values beyond 30 dB. To assess which generators could give rise to these large, synchronous extracellular potentials, we developed a computational model. Spike trains were produced by an inhomogeneous Poisson process and convolved with a spike waveform. The model explained the dependence of the simulated neurophonic on parameters such as the mean rate, the vector strength of phase locking, the number of statistically independent sources, and why the signal-to-noise ratio is independent of the spike waveform and subsequent filtering of the signal. We found that several hundred sources are needed to reach the observed signal-to-noise ratio. The summed coherent signal from the densely packed afferent axons and activation of their synapses on laminaris neurons are alone sufficient to explain the measured properties of the neurophonic.

Early attention effects in human auditory-evoked potentials

A fundamental question in attention theory concerns the earliest processing stages that can be modulated by selective attention. A series of experiments is reported in which very early attention effects are found under specific conditions in the frequency-following potential (FFP), a brain stem response to low-frequency tone stimuli. In two experiments, stimuli of two different modalities were applied, and attention directed to one of the modalities. In two further experiments, only auditory stimuli were presented. In the first of these last two experiments, a dichotic paradigm with sustained attention to one ear was used, in the second a monotic paired-stimuli paradigm was used, in which the first stimulus served as reference for the second one. Only in the last experiment significant attention effects were found in the latency, but not in the amplitude of the FFP. The results show that a very early attention effect on the latency of the FFP can be demonstrated, but only under highly specific conditions. The size and preconditions of the attention effect suggest that it reflects subtle intramodal tuning mechanisms in the cochlea or in the lower brain stem.

A method to improve the latency estimation of the frequency-following potential (FFP)

If the latency of a noisy frequency-following potential (FFP) is estimated by determining the shift of the (periodical) cross-correlation function (CCF) between the stimulus and the FFP, the result may be unambiguous only within ±1 or ±2 periods of the CCF, because the absolute maximum and adjacent local maxima may not be significantly different. Here we present a method to amplify this difference by applying amplitude modulated stimuli. Using this method we first illustrate the effect of the method by a simulation and then demonstrate its usefulness by measuring real FFPs and estimating their latencies.

Cochlear Initiation Sites of the Frequency Following Potential

This study used a series of non-simultaneous masking stimuli set at subjects' pulsation threshold (PT) level to deduce frequency following potential (FFP) tuning curves for 500-Hz tone bursts at 80 and 100 dB SPL. PT and FFP tuning curves were determined in the presence and absence of continuous high-pass masking noise. Results from study demonstrated differences in FFP tuning curves, depending on the high-pass masking condition. High-frequency end of FFP tuning curve was diminished in the presence of high-pass masking noise. These findings suggested that higher frequency tone bursts were less effective in diminishing FFP amplitude when basal end of cochlea was masked. Findings suggest that cochlear initiation site of FFP extends beyond the apical turn of the cochlea at moderate and high intensity levels. A limited clinical use for FFP is suggested.

Response characteristics of cochlear nucleus neurons to 500-Hz tones and noise: findings relating to frequency-following potentials.

Response characteristics of cochlear nucleus neurons to 500-Hz tones and noise: findings relating to frequency-following potentials.

Frequency-following potentials in man by lock-in technique

Frequency-following responses (FFR) from continuous tone stimulation were investigated by lock-in analysis. The technique is superior to conventional tone-burst stimulation, because it allows continuous registration of the response parameters with sweeping frequency. The scalp recorded FFR consists of at least two components separable from each other by their phase/frequency relationship. The microphonic component that dominates in ipsilateral records shows latencies of about 0.5 msec with negligible dispersion and must therefore originate in the basal cochlear turn. The neural component exhibits considerable dispersion, reflecting the travelling wave delay up to the region of best frequency. These long cochlear time delays are consistent with an apical FFR origin, and brain stem sources of the continuous tone response appear to be questionable. The present FFR analysis technique opens up a very direct and comprehensive way to assess the integrity of the entire inner ear for high frequency basal and low frequency apical cochlear regions as well.

Cochlear initiation sites of human auditory frequency following potentials

Initiation sites of the frequency following potential (FFP) along the cochlear partition have been thought to be dependent upon stimulus SPL. However, information regarding initiation sites of the FFP have come from simultaneous masking studies, and results from these studies may be confounded by interference from bio‐electric potentials generated by distortion products of the masking stimulus. This study used a series of forward masking tone bursts set at the subjects' pulsation threshold level (PTL) to establish physiologic tuning patterns for a 500‐Hz tone burst at 80 and 100 dB SPL. Tuning patterns for these levels were determined during the presence or absence of continuous high‐pass noise masking. Results from this study revealed selective FFP amplitude reduction by forward masking stimuli of 500 Hz as well as maskers up to 2000 Hz. Tuning patterns were consistently broader for the higher stimulus levels. These findings suggest multiple cochlear initiation sites for the FFP and do not support its use as an audiologic tool for low‐frequency hearing assessment.

Frequenzfolgepotentiale unter dem Aspekt der objektiven Geh�rdiagnostik im Tieftonbereich

Click evoked brainstem potentials have proved to be a reliable tool in testing the integrity of the high frequency channels of hearing (≥2 kHz). Evaluation of low frequency hearing loss (≤1 kHz), however, has shown to be much more difficult. Tackling this task we investigated the value of frequency following potentials (FFP) in case of subjects with known tone audiometric hearing loss. We find no good correlation between FFP threshold and tone audiogram when using 500 Hz transient stimulation and averaging technique. This fact cannot be explained only by recruitment effects. Threshold detection is possibly complicated by interference of different FFP generators. To establish rapid and sensitive on-line FFP analysis and to obtain reliable phase (i.e., latency) information we employed lock-in technique in recording frequency following responses on continuous, sweeping frequency stimulation (100–600 Hz). Results suggest a cochlear microphonic component generated in the basal turn and a neural one from low frequency tuned fibres, whereas inferior colliculus seems not to be involved as FFP generator.

The 500 Hz frequency-following potential in kangaroo rat: an evaluation with noise masking

This study on kangaroo rat has shown that the volume-conducted 500 Hz FFP can be recorded at stimulus levels which are near behavioral threshold. The response is a complex, double-peaked wave form, indicating that multiple brain stem sources are involved in its generation. The FFP wave form changes in a complex manner with intensity. Recordings of the FFP in the presence of broadband noise demonstrate that the response is neural in origin at suprathreshold stimulus levels. The various configurations of the FFP in the presence of noise, high-pass or broadband, are dependent upon the level of the tone, the level of the noise, and the frequency at which the noise high-pass is set. High-pass masking experiments near threshold levels have demonstrated that the FFP is initiated at a restricted region of the apical cochlea. From all of the results, we conclude that the FFP at moderate and low levels (55--65 dB SPL) is generated primarily by neurons with best frequencies below 1.5--2.0 kHz. The onset component of the FFP is similarly affected by noise, indicating that it too is low frequency in origin.

Evaluation of frequency-following potentials in man: Masking and clinical studies

The frequency-following potential (FFP) can have applicability in the assessment of hearing-impaired subjects only if it can be shown that its generation is initiated by neurons which have low best frequencies (2.0 kHz or lower). This study presents results from five subjects with high frequency hearing losses and three subjects with normal hearing. Using 500 Hz tone bursts in the presence of continuous noise of various configurations, it has attempted to determine how in normal hearing subjects, the amplitude and latency of the FFP may be affected. Recordings of the FFP in the presence of noise and wave forms from hearing impaired subjects provide evidence that the FFP is initiated in the cochlea largely by neurons whose best frequencies are 2.0 kHz or lower. Hearing impaired subjects may exhibit ‘deviant’ responses to tone bursts. These FFP responses may be related to peculiarities of the hearing loss and provide, therefore, a potential means for assessing the temporal viability of the low frequency channels of the auditory neuraxis.

Components of the frequency-following potential in man

The scalp recorded frequency-following potentials (FFP) are a composite of several FFP's which may be distinguished by comparing simultaneously recorded waveforms from vertical and horizontal derivations in response to tones of very low frequency (below 350 Hz). The two most prominent FFP's were designated FFP 1 and FFP 2. FFP 1 was recorded equally well in vertical and horizontal derivations and at a high stimulus intensities tended to be the predominant FFP. FFP 2 followed FFP 1 usually by about 1.7 msec and was optimally recorded in the vertical derivation. FFP 2 threshold was about 10 dB lower than threshold for FFP 1 and in several subjects, FFP 2 was observed at 25 dB SL. Two other FFP's, a far-field recorded cochlear microphonic potential and a low-amplitude FFP, the latter presumably of neural origin, were also studied.

Evaluation of FFP in kangaroo rat: Apical or basal?

Davis and Hirsh [Audiology 15, 181–195 (1976)] argue that the intensity required (40 dB SL) to evoke the 500-Hz frequency-following potential (FFP) in human subjects is sufficient to stimulate the basal cochlea, driving high-frequency neurons at the tails of their tuning curves. We investigated this question by comparing the threshold of the vertex FFP with the phase-locking thresholds of low and high best frequency neurons to 500-Hz tone bursts in auditory nerve and cochlear nuclei of kangaroo rats. The effects of broadband and high-pass noise on the FFP and phase-locked neuronal responses were also compared. Only neurons with best frequencies around 500 Hz locked at FFP threshold levels. Neurons with best frequencies up to 1.5 kHz locked when the sound was increased 20 dB, and a 40-dB increase produced locking in 2.0–2.5-kHz neurons. The masking data are in agreement with the conclusions that within 10 dB of FFP threshold only a restricted region of the cochlea is activated, and a greater region is involved as intensity is increased. However, with appropriate high-pass masking, the 500-Hz region of the cochlea can be selectively evaluated. [Supported by NIH and AFOSR.]

Optimization of magnetically coupled drive of the ear

The ear level hearing aid represents the ultimate in basic hearing aid configuration. This paper presents a feasible electromagnetic drive approach that virtually eliminates acoustic feedback problems of ear level aids as well as providing precise low distortion performance heretofore unavailable in a miniature aid. New magnetic materials of the platinum-cobalt and rare earth cobalt class make possible the realization of the goal of 100 dB re 0 dB = 0.0002 dyn/cm2 sensation level for 1 mW input as needed for low battery drain. These acoustic levels are measured via frequency following potentials (FFP) as well as by the microphonic cochlear potentials recorded at the round window. Suggested additional features are covered as well as the culminating role of electret microphones, lithium batteries, and modern IC fabrication techniques to make possible a programmable aid of unprecedented quality. [Supported by NIH grant.]

Some effects of stimulus intensity on response of auditory nerve fibers in the squirrel monkey.

Some effects of stimulus intensity on response of auditory nerve fibers in the squirrel monkey.