George Moushegian

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.

Components of the frequency-following potential in man☆

The scalp recorded frequency-following potentials (FFP) are a composite of several FFPs 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 FFPs 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 FFPs, a far-field recorded cochlear microphonic potential and a low-amplitude FFP, the latter presumably of neural origin, were also studied.

Role of Interaural Time and Intensity Differences in the Lateralization of Low‐Frequency Tones

There has recently been a renewal of interest in the roles of intensity and time in the lateralization of sounds. The technique generally employed is to offset the effect of an interaural time difference in one direction with a difference of level in the opposite. In the present series of experiments a different method was used, that of having the subject adjust the interaural time relation for a noise until it appears to be in the same lateral position as the stimulus tone. Using this procedure, we have obtained results which support the findings of other recent workers that increasing the intensity of the stimulus to an ear will cause it to transmit earlier in time. We have found additional evidence, however, which shows that the central nervous system, too, responds to interaural intensity differences, and that its response is different from that of the peripheral system. When time and intensity are opposed, time has less effect on it than when time and intensity both favor the same side.

Neuronal coding of vowel sounds in the cochlear nuclei.

Abstract Response patterns of ventral and dorsal cochlear nucleus neurons of the kangaroo rat to vowel and tonal stimuli were compared. Neurons from the ventral caudal portions of the ventral cochlear nucleus display a high degree of frequency information transfer of the vowel sounds. Their discharge patterns to vowels indicate that they are able to code differentially the subtleties of the vowels which drive them. Neurons located in the central region and the fusiform cell layer of the dorsal cochlear nucleus, on the other hand, rarely show synchronous discharges to any of the major frequency components of a vowel sound. Instead, their responses are most often characterized by long intervals not related to the usual parameters which define these stimuli, with the exception of fundamental frequency.

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.

Coding of interaural time differences by some high-frequency neurons of the inferior colliculus: Responses to noise bands and two-tone complexes

Abstract The results of this experiment show that some high-frequency neurons of the central nucleus of the inferior colliculus are differentially responsive to interaural time differences when stimulated by noise bands and two-tone complexes. Since the stimuli were presented at low to moderate levels and in the presence of low-pass maskers, the interaural time sensitivity of the neurons is not attributable to low-frequency spectral components. The functional characteristics of these neurons provide a possible neurophysiological basis for the ability of human subjects to lateralize complex, high-frequency sounds by means of temporal cues.

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.

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.

Inhibition in the auditory nerve

Some recent findings from auditory nerve studies are considered as evidence for primary afferent inhibition.

Frequency following response in subjects with complete unilateral hearing loss: evidence for two generators

The use of auditory evoked response averaging in assessing brainstem integrity depends principally on analysis of the early components, waves I‐V of Jewett, and the frequency‐following response (FFR). This experiment examines effects of complete unilateral hearing loss on FFR. We recorded Cz‐Al and Cz‐A2 responses evoked by monaurally and binaurally presented tone bursts (500 Hz, 14 msec in duration) from subjects with normal hearing in each ear and from subjects with complete unilateral hearing loss. Monaural stimulation of either ear in normal subjects, and of the unimpaired ear in hearing‐loss subjects evoked larger responses ipsilaterally than contralaterally. Stimulation of the impaired ear in hearing‐loss subjects evoked no response. Binaural stimulation in normal subjects evoked Cz‐Al and Cz‐A2 responses of equal amplitude, each larger than the ipsilateral monaural response. Binaural stimulation in hearing‐loss subjects evoked responses equivalent to those obtained monaurally. The results support t...