PHYSIOLOGY: Standard extracellular techniques were used to record 44 on and on-sustained units from the contralateral IC of 1-4 month-old CBA mice. Stimulus generation and manipulation were controlled via a Pentium computer using a digital signal processing platform (Tucker-Davis AP2). Gap stimuli (65 dB) were 150-ms noise bursts with silent gaps (varied from 0 to 96 ms) occurring after the first 100 ms (“NB1”). The NB1 response was the number of spikes produced during the first 25 ms of NB1. The NB2 response was the number of spikes produced during the first 25 ms of the response to the noise burst following the gap (“NB2”). To control for latency, which differs from unit to unit, an NB2 measurement window was set in the 48-ms gap PSTH (where the NB2 response was well established) and slid backwards or forwards by the difference in gap widths for the other PSTHs. Minimum gap threshold (MGT) was the shortest gap producing an NB2 response that differed statistically from the 0 ms gap response (Barsz, Benson, & Walton, 1998).

As pictured below (left), gap stimuli were presented in quiet and CBN. A unit was considered to be specialized if the MGT did not change in CBN (below middle) by more than one log step. Often, nonspecialized units did not respond even to very long gaps, as shown in the example below (right) for a 96 ms-long gap.

Neural MGT plotted 44 on and on-sustained cells. By definition, the MGT of the 23 specialized cells in -6 dB CBN (S -6) was within 1 log step of the quiet value (S Q) in CBN (Median= 2.53 in Q, 3.15 in CBN). The 21 nonspecialized neurons had some longer MGTs even in quiet (NS Q), but the median was still 2.85 ms. In CBN the median MGT of nonspecialized cells (NS -6) grew remarkably to 72.00 ms. The 10 nonspecialized neurons that did not respond to the gap were assigned an MGT of 100 ms.

Work supported by the National Institute on Aging - NIH Grant: AG09524, R03 AG15314 and RICHS, Rochester, NY.

Behavioral and Single-Unit IC Responses of CBA Mice to Partially Filled Gaps in NoiseKathy Barsz, James R. Ison, Paul D. Allen, and Joseph P. Walton

University of Rochester, Rochester, New York, 14642-8629, USA

REFERENCES

Barsz K, Benson PK, Walton JP (1998) Gap encoding by inferior collicular neurons is altered by minimal changes in signal envelope. Hear. Res. 115:13-26. Green DM, Forrest TG (1989) Temporal gaps in noise and sinusoids. J Acoust Soc Am. 86(3):961-70. Forrest TG, Green DM (1987) Detection of partially filled gaps in noise and the temporal modulation transfer function. J Acoust Soc Am. 82(6):1933-43. Koch, M. (1999) The neurobiology of startle. Prog Neurobiol. 59(2):107-28. Walton JP, Wilson WW (1996) Properties of neurons specialized for encoding silent gaps in the presence of background noise in the inferior colliculus of the CBA mouse. Assoc. Res. Otolaryngol. Abstr. 19.Young JS, Fechter LD (1983) Reflex inhibition procedures for animal audiometry: a technique for assessing ototoxicity. J Acoust Soc Am. 73(5):1686-93.

BACKGROUND

The temporal information contained in speech is illustrated below. If the phrase were spoken in a continuous background noise (CBN), the silent gaps (open arrows) before the ‘t’ in ‘wait’ and the ‘x’ in ‘six’ would be partially filled. It has been shown that the minimum gap threshold (MGT) of human listeners is impervious to the presence of CBN which is more than 6 dB below the level of the gap stimulus, although the psychometric function relating gap width to detection performance becomes shallower (Forrest & Green, 1987; Green & Forrest 1989). The present study sought to explore the neurophysiological basis for this result by measuring the psychometric function of single units in the inferior colliculus (IC) when gaps are presented in quiet and CBN to CBA mice, which are an excellent model of human audition. In order to compare the gap detection of the CBA mouse to that observed in humans, behavioral data are also presented.

BEHAVIOR: The subjects were 23 2-6 month-old CBA mice. An accelerometer measured the force of a baseline acoustic startle response (bottom trace; ASR; Koch, 1999) that was produced to a 110-dB 20 ms noise burst (black rectangle) embedded in 70-dB continuous background noise burst (CBN) (the ASRbaseline). ASR is inhibited when preceded by a detectable stimulus. Thus, startle responses were also measured (the ASRstimcond) when gaps (intensity decrements in the 70-dB noise to create noise floors of -40, -30, -20, -10, and -6 dB) varied from 0 to 15 ms that preceded the startle stimulus by 60.

The response: Rd’ = (ASRstimcond - ASRbaseline) SDallcondition. The minimum gap threshold (MGT) was defined as the shortest gap duration that provided at least 50% of the asymptotic inhibition (Young & Fechter, 1983).

amplitude

Shortgap

Longgap

Long rise-timeRapid AM Slow AM

Short rise-time

This figure shows the behavioral MGT values for the 23 mice. The median gap thresholds were 3, 3, 3, 4, and 15 ms for gaps in CBN of -40, -30, -20, -10, and -6 dB. It can be seen that the CBN had little effect on gap detection until the noise floor was within 6 dB of the gap stimulus.

Overall the inhibitory effect of the gaps increased rapidly to reach near maximum Rd' values at 4 to 6 ms, with maximum values greater for the gaps with CBN of -20 to -40 dB. In these conditions, the Rd' values approached a similar asymptote at about 1.5. In -10 dB CBN the asymptote was about 0.8. In – 6 dB Rd' was never significantly different from 0 over this range of gap durations.

Average response to NB2 plotted as a function of gap duration in specialized (S; red) and non-specialized cells (NS; blue). The horizontal lines show average NB1 response. S cells had a higher driven response to both NB1 and NB2 than did the NS cells, and recovered faster with increasing gap duration. The response of NS cells was suppressed as CBN increased. CBN did not change the NB2 response of S cells until it was within 6 dB of the gap stimulus; because the CBN suppressed the NB1 response but had less effect on the NB2 response, S cells responded more to NB2 than to NB1 (facilitation; green hatching).

#331

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-0.20.00.20.40.60.81.01.21.41.61.82.0

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N = 23 (mean age = 4 mo)

Gap Duration (ms)

Rd'

GAP STIMULI top = quietbottom = CBN

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SUMMARY & CONCLUSIONS•Average behavioral MGT was 3 ms in -40 dB CBN. This result is consistent with the average neural MGTs, which were 2.53 (specialized cells) and 2.85 (nonspecialized cells) ms in quiet.•Average behavioral MGT was 15 ms in -6 dB CBN, although some mice had low thresholds. The group average MGT seems poor, compared to the specialized neural MGT average of 3.15 ms, but was better than the non-specialized neural MGT of 72 ms in -6 dB CBN). Perhaps the critical neural counterpart for behavior is not the MGT, but the amplitude of the NB2 onset reaction, which varied little from quiet to – 12 dB, but was reduced by at least 50% in – 6 dB noise.•The behavioral gap functions were shallower in CBN levels of -10 dB or greater. In this respect, they are similar to the gap functions of the nonspecialized cells.•Some specialized cells showed facilitation in that the NB2 spike count exceeded the NB1 count at moderate CBN levels. This is due, in part, to the fact that increases in CBN resulted in decreases in NB1 response, perhaps due to suppression.•The gap functions of the specialized cells are consistent with the finding of Green & Forrest (1989), who report that, for the “ideal listener” gap functions were not flat in -6 dB CBN.•Forrest & Green (87) report that the MGT of “ideal listener” increases systematically as the gap is filled. However, in -6 db CBN, the MGT is still under 5 ms. The 3.15 ms MGT of the specialized cells in -6 dB CBN is consistent with this result.•In this view, the ASR-generated behavioral MGTs for the mice cannot be said to represent “ideal listener”, in that the mice do not appear to be capitalizing on the information available from the specialized cells in the IC.

BEHAVIOR

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Quiet

NB2 NSNB2 S

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S Q S -6 NS Q NS -6

MG

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ms)

2346

20304060

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PHYSIOLOGY

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-6 dB Noise

NB2 NS

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