Spectral characteristics of the responses of primary auditory-nerve fibers to frequency-modulated signals.
The spectral responses of cat single primary auditory nerve fibers to sinusoidal frequency-modulated acoustic signals applied to the ear are examined. Period histograms were constructed from the neural spike-train data, and the frequency spectrum was determined by Fourier transforming these histograms. Several clusters of spectral components were present. The lowest-frequency cluster consists of components at DC, at the modulation frequency, and at its harmonics. In the next cluster, components surround the carrier frequency and are separated from it by the modulation frequency and its harmonics. Higher-frequency clusters surround frequencies that are twice and three times the carrier frequency. The components in each cluster are separated from the multiples of the carrier frequency by the modulation frequency and its harmonics. The magnitudes of the spectral components were investigated for carrier frequencies located below, at, and above the unit characteristic frequency, and for different signal levels, modulation frequencies, and modulation indices. The components at the modulation frequency and its harmonics were strong and present over a wide range of signal levels, carrier frequencies, modulation frequencies, and nerve-fiber characteristics. The presence of components at the modulation frequency indicates that a demodulation process is occurring. This process may be significant for speech recognition.
Khanna
S M
SM
Fowler Memorial Laboratory, Department of Otolaryngology, Columbia College of Physicians and Surgeons, New York, New York 10032.
Teich
M C
MC
D000161:Acoustic Stimulation; D000200:Action Potentials; D000818:Animals; D002415:Cats; D005583:Fourier Analysis; D009475:Neurons, Afferent / Q000502:physiology; D015398:Signal Transduction; D013018:Sound Spectrography; D000159:Vestibulocochlear Nerve / Q000502:physiology*