Frequency-following response: effects of interaural time and intensity differences.

This research investigated whether brainstem neural mechanisms that mediate lateralization of sounds can be extracted from the frequency-following response (FFR). Monaural and binaural FFRs were obtained from normal-hearing subjects to low-frequency (500 Hz) linearly gated tone bursts (4-4-4 msec) at 40, 50, and 60 dB SL and four interaural time differences (ITDs) (0, 333, 500, and 667 microsec). FFRs were also recorded to ITDs and intensity presented in concert and in opposition (lateralization stimuli). The results show that overall intensity and interaural time differentially affect the FFR. The FFRs evoked by ITDs and intensity (in concert and in opposition) are strikingly different. The normalized amplitudes of the binaural interaction component (BIC) are minimally altered by ITDs and intensity. The study presents strong evidence that ITDs of 0, 333, 500, and 667 microsec and lateralization stimuli, easily discriminated perceptually, evoke clearly distinguishable FFR waveforms. These ITDs provide the cues that mammals use to localize sound in a freefield. The BIC is essentially unaffected by overall intensity, ITDs, and lateralization stimuli. Based on the findings of this study, the FFR has the potential to become a tool for identification of normal and abnormal binaural processing at lower brainstem levels.

Real-ear characteristics of the ALGO2 acoustic transducer assembly.

Clinical reports regarding the use of the ALGO2 neonatal automated auditory brainstem response (AABR) screening device have been concerned only with issues of sensitivity and specificity. This study was undertaken to evaluate how couplers and their placement affect the ALGO2 click spectral properties. Although not recommended by the manufacturer, substituting less expensive, uncalibrated couplers for the standard earphone may be a tempting alternative in clinical practice. Click spectra of an ALGO2 were analyzed with a real-ear system so that comparisons could be made between probe tips and the standard coupler. In addition, ALGO2 screening data from neonates using the standard earphone and a substitute probe tip were compared. The results are in agreement with the manufacturer's specifications when the ALGO2 is used as prescribed. However, with probe tip couplers, SPL values were markedly increased, particularly as insertion depth of the probes varied. The neonatal AABR results show that coupler type and placement can also produce inaccurate screening evaluations and erroneous conclusions.

Interaural time effects on the frequency-following response.

Frequency-following responses (FFRs) were recorded to evaluate differences between monaural and binaural waveforms and waveforms evoked by stimuli with interaural time disparities. Eight normal-hearing adult females served as subjects. The stimuli were monaural and binaural 450-Hz tonebursts at 65 and 60 dB SL and interaural time differences of 0 and 660 microseconds, respectively. Normalized amplitudes and periodicities of FFR waveforms within and between subjects were compared. The results showed asymmetric FFR to the various stimuli used in this study. Binaural FFR waveforms were greater than monaural but smaller than summed monaural FFRs. Binaural FFR amplitudes evoked by a zero time difference were greater than amplitudes evoked by a 660-microseconds difference. Additionally, tight phase-locked periodicities were evoked in the FFR monaurally and binaurally. The averaged FFR periodicity to all stimulus conditions from all subjects was 2.29 msec, differing only 6.8 microseconds from the period of the 450-Hz stimulus. In contrast, monaural and binaural neurons in the lower brain stem typically exhibit much less synchroneities to low-frequency tones than the FFR. These data provide evidence that the FFR is not simply a sum of neuronal action potentials. The findings suggest instead the presence of brainstem neuronal networks. Such putative neuronal ensembles apparently maintain a closer correspondence to the period of a low-frequency sound, whether monaural or binaural, than the discharge patterns of single neurons.

Inferior colliculus neuronal responses to masking-level-difference stimuli.

Seventy-one inferior colliculus neurons, with best frequencies below 1.5 kHz, were studied in a binaural, forward-masking paradigm in chinchilla. Masker and signal frequencies were presented at neuronal best frequency. Masker level was set 10-15 dB above neuronal threshold and varied to include a range of signal-to-masker ratios and overall intensities. Without the masker, 33 of the neurons preferred an in-phase signal (SO), 29 an out-of-phase (S pi) signal, and the remaining 9 had 'no-preference' (NP), responding equally well to SO and S pi. Complete protocols from 53 of the 71 neurons were obtained with and without maskers over a range of levels. With an in-phase masker (NO), some neurons responded better to dichotic (NOS pi) than to diotic (NOSO) sounds. Generally, they maintained a particular phase preference with and without masker. Some neurons, however, altered phase preference and responsivity when binaural maskers were added to signal. Signal-to-noise ratios between 0 and 30 dB were sufficient to differentiate neuronal responsiveness to NOSO and NOS pi. The results suggest that identical neural mechanisms are not involved in processing unmasked (SO or S pi) and masked binaural sounds (NOSO, NOS pi). Furthermore, changes in neuronal sensitivity favor the NOS pi condition upon addition of noise (NO) to the signal (SO or S pi). We conclude that greater neural activity is generated with stimuli which produce masking-level difference than stimuli that do not.

Vowel and vowel sequence processing by cochlear nucleus neurons.

This study examined neuronal discharge rates and temporal patterns to vowels and vowel sequences in chinchilla. The properties of primary-like, chopper, and onset neurons were studied using vowels /i/, /a/, and /u/ individually and paired with separations (0-100 ms), at sound levels above and below thresholds. The interspike interval, period, and post-stimulus-time histograms of all neuronal types to a vowel were modified when in a sequence. Primary-like and chopper discharges were reduced and enhanced depending on vowel sequence parameters; onset neurons exhibited discharge rate reductions only and not enhancements. In addition to rate changes, novel discharge intervals appeared with vowel pairs. An unexpected finding on choppers was that subthreshold levels of the preceding vowel in a paired sequence enhanced discharges to the succeeding one. Reducing levels of preceding or increasing levels of following vowels evoked changes not predictable from single vowel data. Thus the responses to paired vowels in a sequence are interactive. Patterns of discharges and rate functions to vowel sounds from neurons of the same type varied greatly. The cochlear nuclei harbor anatomically and functionally diverse neurons. Because of this heterogeneity, the neural transformations of vowel segments by all cochlear nucleus neuronal types can not be predicted from sinusoidal data.

Asymmetric frequency-following responses.

This study offers evidence of asymmetry in the frequency-following response (FFR), a brainstem evoked potential that mimics stimulus frequency. The FFRs to stimulation of one ear compared to the other are shown to have latency and amplitude dissimilarities, as well as considerable variations in waveform characteristics. The findings were obtained from eight subjects. Tone bursts at 500 Hz were delivered to the right ear, left ear, and binaurally at three sound levels. With few exceptions, subjects had asymmetric FFRs at every sensation level (SL); at some, the sound to the left ear evoked a larger response than the same sound to the right; at other levels, the right ear FFR was larger than the left. The results provide evidence that cortical processing asymmetries must, to some extent, be reflective of asymmetric mechanisms within the lower brain stem.

Rate, frequency, and intensity effects on early auditory evoked potentials and binaural interaction component in humans.

The binaural interaction component (BIC) of the auditory brainstem response (ABR) and BIC of the frequency-following response (FFR) to tonal stimuli were studied in normal-hearing adults. The ABR and BIC latencies from all subjects were consistently shorter to the click-like sound than to the 2.0 kHz tone burst. Increasing stimulus presentation rate produced longer latencies and diminished amplitudes of ABR and BIC waveforms. The consequences of rate changes were independent of sound level. The FFR and BIC latencies to low-frequency tone bursts (0.5 and 1.0 kHz) were minimally affected by rate, but their amplitudes were modified. The results are consistent with and reflective of the functional characteristics of lower brainstem auditory neurons. The results provide evidence that the BIC to tones is differentially sensitive to the rate, frequency, and intensity of sounds other than clicks.

Adaptation of the auditory brainstem response: effects of click intensity, polarity, and position.

The aim of this research was to study the effects of several stimulus parameters on adaptation characteristics of the auditory brainstem response (ABR). Recordings were obtained from normal hearing adults to separate trains of 5 rarefaction and 5 condensation clicks with interclick intervals of 10 msec and intertrain intervals of 500 msec at three intensities. Absolute latencies for waves I, III, and V were essentially unchanged by polarity; latency shifts, however, were induced by parametric manipulations of click intensity, polarity, and position in the train. Furthermore, variations in ABR wave morphology appeared with changes in intensity and polarity. Adaptation, as measured by amplitude, was considerable; the measures of adaptation, however, were not related in a simple manner to the latency shifts. The findings indicate that adaptation of the ABR is a consequence of an interplay of central and peripheral processes to click polarity, sequence, and intensity. Finally, the results provide evidence that when fast stimulus presentation rates are used to estimate thresholds rapidly, one must be aware that certain stimulus parameters can alter the ABR waveforms.

A developmental study of bone conduction auditory brain stem response in infants.

Two studies, vibrator placement and masking, were performed to evaluate the developmental aspect of bone conduction auditory brain stem response (ABR) in human infants. Subject groups included newborns, 1-yr-olds, and adults. In the vibrator studies, ABRs were obtained from placements of the bone conduction vibrator on the frontal, occipital, and temporal bones. Results showed that temporal placements in neonates and 1-yr-olds produce significantly shorter wave V latencies of ABR than frontal or occipital placements. In adults, differences of wave V latencies from various vibrator placements were comparatively small. In the masking studies, ABRs were acquired from vibrator placements at the temporal bone in the presence of ipsilateral air conducted masking noise from the experimental groups. Results showed that interaural attenuations of bone conduction click stimuli are the largest in neonates, somewhat smaller from 1-yr-olds, and the smallest in adults. The findings of this research strongly suggest that temporal placements for bone conduction ABR should be used, in some instances, when testing infants and 1-yr-olds. The results of this study support the proposition that bone conduction ABR is a feasible and reliable diagnostic tool in testing infants.

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 imparied 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.

Phase locking in monaural and binaural medullary neurons: implications for binaural phenomena.

The synchrony of neural impulses in response to low-frequency sinusoids is described for auditory medullary neurons. The results are summarized as follows: (1) In general, neural synchrony is found to improve with increases in intensity and frequency of stimulation for both monaural and binaural neurons when measurements are make in absolute time. (2) An analysis of our population of neurons implies that two separate mechanisms are responsible for the decrease in synchrony found in many neurons as compared to primarylike neurons with high-locking ability. The two mechanisms are convergence of mistimed impulses and electrontonic changes which occur in dendrites. (3) An analysis of binaural vector strength data provides an explanation for physiological differences between cyclic and noncyclic vector strengths as a function of interaural time and reveals the effects of mistimed convergence upon neural synchrony.(4) In contrast to the inferior colliculus, where the neurons discharge best with contralateral leads in time, superior olivary neurons exhibited no such preference. Some discharge best to ipsilateral while others to contralateral leads. This comparison reveals a striking difference in the coding characteristics of medullary and inferior colliculus neurons. (5) Finally, the results are compared with the psychophysically determined difference limens.

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 frequently (below 350 Hz). The two most prominent FFP's were designated FFP1 and FFP2. FFP1 was recorded equally well in vertical and horizontal derivations and at a high stimulus intensities tended to be the predominant FFP. FFP2 followed FFP1 usually by about 1.7 msec and was optimally recorded in the vertical derivation. FFP2 threshold was about 10 dB lower than threshold for FFP1 and in several subjects, FFP2 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.

Response characteristics of cochlear nucleus neurons to vowel sounds.

Most studies in auditory neurophysiology have utilized tonal stimuli to determine the coding properties of neurons in the cochlear nuclei. In this investigation of the kangaroo rat, cochlear nuclei, neuronal responses to vowel sounds, as well as tones, were studied. The vowel sounds, each about 40 msec in duration were: see article. Five were linked together to form a 200 msec stimulus and various combinations of five vowel sounds provided us with 18 different stimuli. The results show that neurons in the cochlear nuclei are remarkably sensitive and selective to vowel sounds. Furthermore, the responses of these neurons to pure tones do not provide a complete basis to predict the types of responses to the vowel sounds. More significant is the finding that the neural discharge rate and pattern of discharge to a particular vowel may depend on where the vowel appears in the stimulus and what other vowel precedes it. This vowel positional effect is not the same for every neuron. We have called this phenomenon a neural "set".

The frequency-following response in subjects with profound unilateral hearing loss.

Frequency-following responses (FFRs) evoked by 500 c/sec tone bursts 14 msec in duration, presented at 50 dB SL were recorded from Cz--A1 and Cz--A2 electrode derivations using eight subjects with normal bilateral hearing, and eight subjects with profound unilateral hearing loss. Monaural stimulation of either ear in normal subjects, and of the unimpaired ear in hearing-loss subjects, evoked larger responses from the ipsilateral electrode derivation than from the contralateral electrode derivation. Stimulation of the impaired ear in hearing-loss subjects evoked no response. Binaural stimulation in normal-hearing subjects evoked Cz--A1 and Cz--A2 responses of equal magnitude, each larger than either the ipsilateral or contralateral monaural response, but each slightly smaller than the sum of the ipsilateral and contralateral monaural responses. Binaural stimulation in hearing-loss subjects evoked responses equivalent to those obtained monaurally. The results provide evidence of binaural interaction in normal-hearing subjects and indicate that FFR arises from at least two separate symmetric neural sources, possibly by iterative activation of brainstem evoked response (BER) generators.

Early tone-evoked responses in normal and hearing-impaired subjects.

Early evoked responses to 500-Hz tone bursts were recorded from normal and hearing-impaired children and adults. The threshold values of the early evoked responses provide useful estimates of auditory functioning, even among difficult-to-test populations, such as deaf-blind children. Latency measures indicate that the early response is generated at the brain stem. Latency measures from hearing-impaired subjects show that the response can identify recruitment. Several subjects having a history of nonspecific communication disorders, e.g., dyslexia, exhibited aberrant early evoked response waveforms. The early-evoked response measures, therefore, amy be useful in detecting and assessing communication disorders which are believed to be of cortical origin, but now should be considered to have a basis in brain stem dysfunction.

Functional characteristics of superior olivary neurons to binaural stimuli.

This investigation was undertaken to study the timing properties of low-frequency binaural neurons located in the medulla of kangaroo rat (Dipodomys spectabilis). The results show that the response variables, vector strength (VS) and discharge rate (DR), are not necessarily related responses; each may be conveying a different parameter of acoustic stimuli. The results also lead to the conclusion that binaural low-frequency neurons, whether they are excitatory-excitatory (EE) or excitatory-inhibitory (EI), in essence, function similarly. Finally, this investigation presents findings which suggest that a clock, which may be part of a mechanism for pitch as well as for spatial localization, is activated by sounds, providing thereby a reference signal for neural discharges.

Human frequency-following responses to monaural and binaural stimuli.

Frequency-following responses, with latencies circa 6 msec, were recorded from five normal-hearing human subjects to brief 500 c/sec tone bursts presented monaurally. The frequency-following responses appear as peaks occurring at 2 msec intervals superimposed on a slow wave (pedestal-like) component. Comparisons were made between the frequency-following responses evoked by binaural and monaural stimuli. The results show that the binaural responses may be interpreted as the sum of two monaural responses. It is concluded, therefore, that there are two independent populations of neurons, each capable of generating a frequency-following response is not a microphonic-like response but rather that the individual waves in the frequency-following response are evoked by the collective activity of phase-locked single units. Finally, on the basis of the distinctness of the individual waves in the frequency-following response, it is concluded that the neural generators of the response must be spatially compact.