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PubMed1985

Thresholds for primary auditory fibers using statistically defined criteria.

The discharge behavior of auditory-nerve fibers near "threshold" was investigated in anesthetized cats using low-intensity sinusoidal stimuli presented at the respective characteristic frequencies. Particular attention was paid to fibers with "low" and "medium" rates. Estimates of threshold derived from statistically significant increases in discharge rate indicate that the average threshold values for low-spontaneous fibers are only slightly higher (ca. 5 dB) than the averages for the corresponding high-spontaneous fibers, with the medium-spontaneous fibers having intermediate averages. The difference between these average values is considerably less than the more than 20-dB difference obtained using threshold criteria based on an absolute increment in discharge rate [e.g., M. C. Liberman, J. Acoust. Soc. Am. 63, 442-455 (1978)]. The main reason for the difference between the results of the two techniques is the fact that the slopes of the rate-intensity functions for the high-spontaneous fibers are considerably steeper near "threshold" than those for fibers of the other two classes. The results are taken as supportive of a recent model of primary-fiber discharge [C. D. Geisler, Brain Res. 212, 198-201 (1981)].

Animals↗

PubMed1985

Changes in the phase of excitor-tone responses in cat auditory-nerve fibers by suppressor tones and fatigue.

The responses of single auditory-nerve fibers in anesthetized cats to two-tone stimuli were studied. One of the two tones, F1, was near, above, or below characteristic frequency (CF). The second tone, F2, was located above CF. With sufficient care, F2 was made purely suppressive, eliciting no synchrony responses by itself. The vector phases of the associated period histogram calculated for F1 were carefully studied. For 78% of the fibers under study, a statistically significant increase in phase lag was consistently observed when a suppression of rate discharge occurred. The phase-intensity curve did not approximate a horizontally shifted version of the unsuppressed curve, as is seen for the related rate- and synchrony-intensity curves; rather, the amount of phase shift at any one stimulus condition tended to be monotonically related to the amount of rate suppression generated (vertical shift). Using two different measures, a significant correlation was found between the added phase lag and the discharge-rate reduction caused by F2. The amount of phase lag, along with the corresponding rate reduction, increases with the increasing intensity of F2 within the suppression area, and decreases as F2 moves away from it. These phase-lag effects were found to be uncorrelated with a fiber's CF, with its spontaneous rate, with its threshold, or with its Q value. By contrast, a reduction of discharge rate due to adaptation was not accompanied by any significant phase shift. Fatigue of the fiber due to lengthy sound exposure was found to have strong effects on the shift of response phase to single-tone stimuli.

Acoustic Stimulation↗