Journal of Neurophysiology, Vol 70, Issue 1 370-386, Copyright © 1993 by APS

ARTICLES

Cochlear nerve fiber responses to amplitude-modulated stimuli: variations with spontaneous rate and other response characteristics

N. P. Cooper, D. Robertson and G. K. Yates
Department of Physiology, University of Western Australia, Nedlands.

1. Single-fiber responses to sinusoidally amplitude-modulated (AM) tones were recorded from the cochlear nerves of anesthetized guinea pigs. Stimuli were presented at the fiber's characteristic frequency (CF) and covered the intensity range between the fiber's minimum rate threshold and 90-100 dB SPL in 5- or 6-dB steps. The amount of modulation in each fiber's response and the average rate of the responses were quantified. The observed response modulation was compared with the modulation to be expected on the assumption that the instantaneous discharge rates varied with intensity in the same way that the average rates did (i.e., as predicted from each fiber's average-rate vs. level function). 2. The difference between the observed and expected response modulation varied widely across fibers. In most fibers' the responses to a limited range of stimulus intensities (typically between 20 and 30 dB above the fiber's rate threshold) were modulated far more than expected on the basis of their average rates, with responses to stimuli either above or below this range differing progressively less from expectation. Little or no response modulation was observed above approximately 70 dB SPL in these fibers. Other fibers exhibited response modulation that exceeded the expected modulation by smaller amounts, but maintained this modulation to much higher sound pressure levels. 3. The discrepancy between the observed and expected responses to AM stimuli also varied with the frequency of modulation (fm) within individual fibers. The discrepancies were least pronounced at low fms (e.g., 10 Hz) but became progressively larger as fm was increased to between 50 and 320 Hz (subject to the inter-fiber variations described in 2, above). 4. The AM response characteristics varied systematically with the fiber's spontaneous rate and other response characteristics (e.g., rate threshold, CF rate vs. level function type, and rapid adaptation characteristics). In particular, the most sensitive, high spontaneous rate fibers had responses that adapted rapidly after the onset of a stimulus, and showed the greatest enhancement of AM-related information at low-to-moderate stimulus intensities. However, these fibers appeared incapable of encoding AM-related information at high intensities, since their response rates "saturated" and their AM response enhancements diminished around 30 dB above threshold. In contrast, the less sensitive (i.e., higher threshold), lower spontaneous rate fibers showed less evidence of rapid adaptation near the onsets of their response, and lesser enhancements of the modulated responses predicted from their average-rate versus level functions.(ABSTRACT TRUNCATED AT 400 WORDS)

This article has been cited by other articles:

				P. C. Nelson and L. H. Carney Neural Rate and Timing Cues for Detection and Discrimination of Amplitude-Modulated Tones in the Awake Rabbit Inferior Colliculus J Neurophysiol, January 1, 2007; 97(1): 522 - 539. [Abstract] [Full Text] [PDF]

				A. Dreyer and B. Delgutte Phase Locking of Auditory-Nerve Fibers to the Envelopes of High-Frequency Sounds: Implications for Sound Localization J Neurophysiol, November 1, 2006; 96(5): 2327 - 2341. [Abstract] [Full Text] [PDF]

				D. H. G. Louage, M. van der Heijden, and P. X. Joris Temporal Properties of Responses to Broadband Noise in the Auditory Nerve J Neurophysiol, May 1, 2004; 91(5): 2051 - 2065. [Abstract] [Full Text] [PDF]

				P. X. JORIS, C. E. SCHREINER, and A. REES Neural Processing of Amplitude-Modulated Sounds Physiol Rev, April 1, 2004; 84(2): 541 - 577. [Abstract] [Full Text] [PDF]

				H. Zhang and J. B. Kelly Glutamatergic and GABAergic Regulation of Neural Responses in Inferior Colliculus to Amplitude-Modulated Sounds J Neurophysiol, July 1, 2003; 90(1): 477 - 490. [Abstract] [Full Text] [PDF]

				B. S. Krishna and M. N. Semple Auditory Temporal Processing: Responses to Sinusoidally Amplitude-Modulated Tones in the Inferior Colliculus J Neurophysiol, July 1, 2000; 84(1): 255 - 273. [Abstract] [Full Text] [PDF]

				P. X. Joris and T. C. T. Yin Envelope Coding in the Lateral Superior Olive. III. Comparison With Afferent Pathways J Neurophysiol, January 1, 1998; 79(1): 253 - 269. [Abstract] [Full Text] [PDF]