Response to noise of auditory nerve fibers in the squirrel monkey

The response of auditory nerve fibers to noise was investigated in the squirrel monkey. The time structure of responses to white noise was analyzed by means of interspike interval histograms and autocorrelograms. The histograms invariably revealed the presence of oscillations in the excitability of neurons with best frequencies between 100 Hz and 2 kHz. The autocorrelogram oscillations had fundamental frequencies similar or identical to the best frequencies measured with tone bursts. The number of effective cycles of oscillation ranged from three to eight and was positively correlated with both best frequency and the sharpness of the threshold tuning curve. The time structure of responses to a BF centered, 200 Hz wide band of noise was investigated in low frequency neurons. Several features of the responses could be mimicked by a simple model, based on the assumption that action potentials are initiated by unidirectional elevations of the basilar membrane. The rate of discharge was studied as a function of the intensity of a narrow band of noise centered at the best frequency of each neuron. The rate intensity functions were similar to those obtained by stimulation with best frequency tones of the same intensity, but the curves for noise often had lower plateaus and smaller slopes. The rate of discharge was also studied as a function of the width of bands of noise with constant spectrum level. Increases in the bandwidth, within a certain range, led to increases in the rate of the discharge. A doubling of the bandwidth was approximately equivalent to a 3 dB increase in the spectrum level of a band of noise of constant width, as it would be expected from a summation of spectral components. As the bandwidth was increased beyond a certain value, the stimulus either became progressively less effective than those with narrower bandwidths or a plateau was reached.