Allen L. Rupert

Action of the Middle Ear Muscles in Normal Cats

Wires have been permanently implanted on the round window of the cochlea in cats. The voltage output of ears responding to sound stimulation has thus been made continuously available in unanesthetized animals for periods up to 4 months. By cutting the middle ear muscles of one ear and comparing its responses with those derived from the normal ear on the opposite side, it has been shown that the muscles: (1) do not appreciably influence absolute sensitivity, (2) contract to intense stimuli within 15 msec of their delivery to either ear, (3) attenuate transmission of tones between at least 500 and 3000 cps, and (4) significantly protect the ear against damage from intense sounds. Spontaneous contractions sporadically and intermittently introduce a transmission attenuation of several decibels in the resting normal cat. The stapedius muscle is much more important than the tensor tympani in producing these effects.

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.

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.

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.

Response characteristics of neurons in the cochlear nuclei to sinusoidal and vowel‐sound stimulation

The response characteristics of neurons in the dorsal (DCN) and ventral (VCN) cochlear nuclei of kangaroo rat (D. spectabilis) to sinusoids and trains of vowel segments are described. Synchronization coefficients, response areas, post‐stimulus‐time histograms, interval histograms, and period histograms are all used to compare functional features of neurons in the cochlear nuclei. Pauser and chopper‐type neurons of DCN exhibited low discharge rates to vowel‐segment sounds. Low‐frequency DCN neurons which had low synchronization coefficients did not follow closely the vowel segment waveforms. The responses of VCN neurons with low best frequencies but high coefficients of synchronization occurred in temporal sequences which were closely related to the waveform within each vowel segment. This paper relates the subnuclear cytoarchitectonics of the cochlear nuclei and stimulus coding. [Supported by Air Force Office of Scientific Research.]

Neuronal response patterns in the cochlear nuclei: effects of vowel position on discharge rate

Single neuronal responses of the kangaroo rat cochlear nuclei to vowel sounds were studied. The vowel sounds used were [aye], [eye], [smcapi], [ɛ], [o], [u] [ɔ], [ə], [ae]. and [ṛ]. Each was approximately 40 msec in duration. Five were linked together to form a 200‐msec stimulus. Eighteen combinations of the ten vowels made up the repertoire of stimuli. The results show that the neutral discharge rate to a vowel sound is appreciably changed as a function of where in the stimulus that vowel sound occurs. This vowel positional effect on the discharge is not the same for all neurons. Furthermore, the discharge rate to a particular vowel was greatly affected by the vowels which preceded it. The cytoarchitectonics and responses of cochlear nuclei neurons are related in an attempt to extract neural coding characteristics of this auditory aggregate. [Supported by Air Force Office of Scientific Research.]