Effects of excitatory and non-excitatory suppressor tones on two-tone rate suppression in auditory nerve fibers.

Recordings were obtained from individual auditory nerve fibers in anesthetized chinchillas. Rate versus level functions were obtained for best frequency (BF) tones alone and for simultaneously-gated tone pairs comprising a BF tone and a second tone at a fixed intensity that produced evidence of two-tone rate suppression. Care was taken in selecting a range of suppressor tone levels that included excitatory (i.e., the suppressor tone evoked a rate change by itself) and non-excitatory (i.e., no suppressor tone-evoked rate increase) suppressor tone levels. Addition of a suppressor tone produced a shift of the dynamic range portion of the BF rate versus level function to higher test intensities. A parallel shift of the dynamic range portion of the rate versus level function was associated with a non-excitatory suppressor tone. The shift produced by an excitatory suppressor tone was characterized by a decrease in slope. Results indicated that the magnitude of shift increased monotonically as suppressor tone intensity was raised and that there was a gradual transition from a non-excitatory response to an excitatory response. The rate of shift (i.e., dB of shift per dB change in suppressor tone intensity) did not differ for non-excitatory versus excitatory responses, but was substantially greater for below-BF suppressor tones (1.38 dB/dB) than for above-BF suppressor tones (0.54 dB/dB). The rate of shift did not, however, appear to be related systematically to suppressor tone frequency separation from BF. Above- and below-BF suppression was noted for fibers over the range of best frequencies tested (110 Hz to 16.4 kHz).

Representation of tones in noise in the responses of auditory nerve fibers in cats. I. Comparison with detection thresholds.

Rate and temporal responses evoked by 1-kHz or 8-kHz tones in continuous broadband noise are described for large populations of auditory nerve fibers in anesthetized cats. The signal-to-noise ratios (S/Ns) of the tone and noise stimuli were above behavioral detection thresholds. Stimulus combinations were presented (1) over a range of moderate to high noise intensities at a constant S/N and (2) using high intensity noise and varying S/N. Responses of low (less than 1 spike/sec) and medium (1 to 19 spikes/sec) spontaneous rate (SR) fibers were compared with those of high SR (greater than 19 spikes/sec) fibers. Low and medium SR fibers with best frequencies in the region of the test tone frequency exhibited tone-evoked rate changes at all sound levels tested. High SR fibers, in contrast, exhibited much weaker tone-evoked rate changes at the lowest noise level tested. In the presence of high intensity noise, high SR fibers did not exhibit tone-evoked rate changes due to saturation by the noise. Fibers with best frequencies in the region of 1 kHz also exhibited strong phase-locking to the 1 kHz tone which increased as the tone level increased but which did not differ for the various SR groups. Results suggest that information in the rate responses of low and medium SR fibers can account for the encoding of information about tones in noise by the nervous system.

Similarity of dynamic range adjustment in auditory nerve and cochlear nuclei.

Rate versus level functions were recorded for responses to best-frequency (BF) tones of 116 cochlear nucleus units and 53 auditory-nerve fibers in the presence of interrupted tone backgrounds and continuous noise backgrounds of various intensities. The backgrounds shifted the dynamic ranges of rate-level functions to higher test intensities, so in the presence of backgrounds, rate saturation occurred at higher intensities than in quiet. The shift in saturation intensity evoked by each background was measured by comparing the rate-level function recorded with the background to one recorded without. The relation between change in saturation intensity and background intensity could be approximated by the formula (formula: see text) delta Isat is the shift in saturation intensity, I is the background intensity, theta is the threshold for evoking shift, and A is the ratio of shift to background intensity re theta. In the appendix, it is shown that A is a measure of a unit's ability to avoid saturation by the background stimulus. The optimal value of A is unity, at which point a unit's operating range is infinite. The value of A depended on BF for interrupted tone backgrounds, but not for continuous noise backgrounds. For BF less than 10 kHz, the mean value of A for tone backgrounds was 0.33 in the auditory nerve, 0.37 in the ventral cochlear nuclei (VCN), and 0.47 in the dorsal cochlear nucleus (DCN). The difference between auditory nerve and VCN was not statistically significant. For BF greater than 10 kHz, the mean A was 0.16 in auditory nerve and 0.30 in VCN. The mean value of A for noise backgrounds was 0.79 in auditory nerve, 0.86 in VCN, 0.86 in DCN units of response types II and III, and 1.04 in DCN type IV units. Only the differences between DCN type IV and the non-DCN unit groups were statistically significant. The qualitative changes produced in rate-level functions by tone and noise backgrounds were similar in auditory nerve and cochlear nuclei except for DCN type IV units. The shifts in rate functions produced by interrupted tone backgrounds did not prevent saturation of the rate response at background intensities above the dynamic range of the unit as recorded in quiet. However, the rate response to test tones was preserved in the presence of all noise background levels used (up to a 30-dB spectrum level). The shift in rate function produced by the noise was almost sufficient to allow the unit to encode test intensity relative to noise background intensity.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of continuous noise backgrounds on rate response of auditory nerve fibers in cat.

This study describes the effects of broadband background noise on the average discharge rate to best-frequency (BF) tones of auditory nerve fibers in the cat. The effects of exposure to long-term continuous noise are compared to the effects of noise gated on and off simultaneously with test tones. Addition of background noise causes a shift of the dynamic portion of tone-evoked rate versus level functions to higher tone intensities. The shift occurs at a mean rate of 0.61 dB of shift for each 1-dB increment in noise level. The rate of shift is independent of best frequency and spontaneous discharge rate. The noise level at which the shift begins is frequency dependent and is consistent with the frequency-dependent bandwidths of auditory nerve fiber tuning curves. The adjustment of the dynamic range shows many similarities to two-tone suppression. Therefore, it is most likely that it is caused by suppression of the response to the BF test tone by energy present in the noise at surrounding frequencies. At high noise levels, the ability of auditory nerve fibers to respond to test tones is limited by the rate response to the noise. As noise level increases, the discharge rate it evokes approaches a fiber's saturation rate and ultimately eliminates the fiber's ability to respond to test tones. Low spontaneous rate fibers, which have been shown to have higher thresholds and wider dynamic range (17,29), are significantly more resistant to saturation by high noise levels. Exposure to broadband noise prior to onset of test tones produces an overall decrement in response rate. This phenomenon is similar to the effects of short-term adaptation (32) and seems to develop independently of the shift of dynamic range. At high noise levels, previous exposure to the noise produces a small dynamic range shift. This effect is similar to that produced by suppression but is smaller. The effect is occluded in continuous noise backgrounds by the adjustment of sensitivity produced by suppression.

Superior temporal lesions and delayed auditory matching in monkeys.

This study considers whether or not unilateral removal of superior temporal cortex in monkeys disrupts performance of a nonspatial delayed auditory matching task. Preliminary evidence of such an impairment has been suggested. Monkeys were evaluated before and after unilateral and serial bilateral removal of superior temporal cortex. No significant postoperative impairment was noted.

Broadband masking noise and behavioral pure tone thresholds in cats.

The threshold for detection of pure tones in broadband noise was determined for three cats using an auditory reaction time procedure. Critical ratio is defined as the ratio of the signal power at the masked threshold for detection to the spectrum level of the noise. Critical ratios were obtained for 250-, 500-Hz, and 1-, 2-, 4-, 8-, and 16-kHz tones over a wide range of noise intensities. Results indicate that critical ratios increase with the frequency of the tone stimulus. At frequencies below 4 kHz, critical ratios remain constant at moderate and high noise intensities. For frequencies above 4 kHz, critical ratios increase as the level of the masking noise is raised from moderate to high levels. The difference between low- and high-frequency behavior of the level dependence of critical ratios is considered in terms of two possible mechanisms: (1) different mechanisms may be involved in the encoding of low- and high-frequency information by the nervous system or (2) the difference in level dependence may be due to attenuation by the action of the middle ear muscles at high sound levels.

Temporal integration of pure tones in the cat.

A behavioral method for assessing auditory threshold in cats using food reward conditioning is described. The method requires that the cat press its nose against a touch panel in order to receive a stimulus and that it remove its nose when the stimulus is detected. Behavioral thresholds for pure tone stimuli of various durations are described for three laboratory-raised cats. For all frequencies tested, detection threshold varies as an exponential function of stimulus duration. For frequencies from 125 Hz to 8 kHz, integration time constant and stimulus frequency are inversely related. Response latencies are also affected by the duration of the stimulus. Results are considered in the light of similar measures obtained from human subjects.