ausblenden:
Schlagwörter:
-
Zusammenfassung:
Auditory brainstem evoked responses (ABR) usually lead to 5 waves. The peaks of the waves reflect the summed synchronized excitatory postsynaptic potentials (EPSPs) to sound onsets from the cochlea (peak 1, P1), the cochlear nucleus (P2), the superior olivary complex (P3), the superior olive plus lateral lemniscus plus inferior colliculus (P4), and the lateral lemniscus plus inferior colliculus (P5). The better the EPSPs are synchronized, the higher are the wave amplitudes and the narrower are the wave widths, which means less jitter of wave latencies. Therefore, the degree of variation of wave latencies can be
taken as measure for the precision of temporal coding of sounds of certain properties in the auditory brainstem centers. Here, we report data of ABR recordings from 7 ketamin-xylazin anesthetized adult NMRI mice per experiment.
Silver wire skin electrodes were placed over the contralateral colliculus inferior and the bulla ipsilateral to the loudspeaker emitting the following sounds freefield. Experiment 1: Series of four 50 kHz tones (duration 10 – 150 ms including 1 ms rise/fall time, 200 ms interval between the tone bursts, 75 dB SPL), mimicking mouse pup ultrasounds. Experiment 2: Pairs of a noise burst (7 ms including 1 ms rise/fall, 50 dB SPL) followed by a 50 kHz tone (50 ms including 1 ms rise/fall, 75 dB SPL) with variable intervals (0 – 100 ms) between the two sounds, mimicking a phoneme of a stop consonant and a vowel. Experiment 3: Series of four harmonic complexes (frequencies of 3.8 + 7.6 + 11.4 kHz, 100 ms duration including 2 ms rise/fall times, 60.5 dB SPL of each harmonic) with variable intervals (10 – 700 ms) between the
complexes, mimicking series of mouse pup wriggling calls. The series or pairs of the sounds were repeated 500 times with 1 s interval between the repetitions. Among our results are the following: (1) Standard deviations (SDs) of average peak latencies to 50 kHz series are 10 – 15% of the peak latencies for all waves. Average latency differences between the wave peaks are about 0.7 – 1.2 ms with SDs of only 2 – 5%. (2) SDs of average peak latencies to noise bursts are only 1 – 2% of the peak latencies for all waves. SDs of average latencies to the 50 kHz tone in the
noise-tone pair are about 10 – 30% of the peak latencies for all waves and independent of the interval length. Average latency differences between the wave peaks are about 0.8 – 1.2 ms with SDs of 1 – 6% for the noise and 10 – 30% for the 50 kHz tone. (3) Average latencies of P2 – P5 in response to series of harmonic complexes may significantly be prolonged by about 10% depending on the interval between the sounds for intervals of 50 ms and shorter. This latency adaptation neither influences the average precision (SD) with which the peaks occur nor the precision of the latency differences between the wave
peaks. We conclude that the absolute and relative temporal precision of synaptic transfer in auditory brainstem
centers is high, especially in response to noise bursts. SDs of the average latencies of P1 and P2 and the latency differences between both are near 10 μs or 1%. The latencies may be influenced by preceding sounds which, depending on the interval length between and the kind of sounds, may reduce the precision of coding or delay the response.
