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.
In this study the response characteristics of inferior colliculus neurons were investigated. Three types of binaural, high-frequency stimuli were used: (1) two-tone complexes, (2) noise bands, and (3) transients. For two-tone complexes, frequency and depth of modulation were varied. Continuous, binaural, low-pass, uncorrelated noise was used to mask the difference tone. For high-frequency transients the low-pass masker had either a 3 or 5-kHz upper cutoff frequency. Neurons were found which responded differentially to interaural-time differences for one or all three of the stimuli employed. This study therefore provides a possible neural correlate of the psychophysical findings that humans can lateralize continuous, complex, high-frequency stimuli on the basis of interaural time. The data also imply that lateralization of transients may be performed by high-frequency neurons. [Supported by NIH.]
It is well known that auditory medullary neurons preserve the temporal characteristics of low-frequency sounds by discharging at integral multiples of the stimulus period. Monaural recordings from kangaroo rat cochlear nuclei and binaural superior olive neurons show that the variability in synchrony of neural discharges decreases as the frequency of the sound increases. Variability in synchrony is apparent from period histogram displays and may be expressed by vector strength and interquartile range. These results provide a possible neural basis for the finding that the difference limen for pitch and the minimum audible angle, in absolute time, decreases as the frequency of the sound increases. Evidence is also presented that relates the shortest intervals of neural discharge to the loss of synchrony as frequency of sound stimulation increases. [Supported by AFOSR and NINDS.]
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