The Effect Of High Frequency Hearing Loss On The Temporal Resolution Of Low Frequency Region In Guinea Pigs

Objective:To explore the effect of sound signal parameters on auditory temporal resolution in objective measurements and the effect of high frequency hearing loss on temporal resolution in low frequency region of guinea pigs.Methods: The electrodes were implanted in inferior colliculus (IC) and auditory cortex (AC) of guinea pigs. The animals were allowed to rest at least one week. In the gap detection test, noises with four different bandwidths were used to mark gap. We recorded the gap evoked response in awake and anesthetized animals by two doses of pentobarbital anesthesia. The results were compared among the four bandwidths noises and between awake condition and under pentobarbital anesthesia. In another group of guinea pigs, the animals were exposed to tone of 12 kHz for 30 hours continuously to result in high frequency hearing loss. We compared the gap threshold, supra-gap-threshold response latency and amplitude under two different bandwidths before and after acoustic trauma. In modulation detection test, we plotted temporal modulation transfer function with the auditory steady state response amplitude changed with modulation frequency and calculated cut-off frequency. The effect of signal frequency and intensity on the cut-off frequency of IC and AC were evaluated in normal awake animals. In the same group of high frequency hearing loss guinea pigs, we compared the cut-off frequency in low frequency region in IC and AC of guinea pigs before and post acoustic trauma.Results: The gap threshold decreased, the latency of supra-gap-threshold evoked response shortened and the amplitude of supra-gap-threshold evoked response increased with the bandwidth of gap mark signals increased. Within each gap mark signals, the effect of pentobarbital anesthesia on gap evoked response was opposite to that of gap marker bandwidth increase and the anesthesia effect was larger on AC than on IC. In high frequency hearing loss guinea pigs, the gap threshold increased, the latency of supra-gap-threshold evoked response elongated and the amplitude of supra-gap-threshold evoked response decreased under two different bandwidths noises compared to those before tone exposure in IC. The same trends were seen in AC too, but the changes were not statistically significant. In the modulation detection test studied in normal guinea pigs, the effect of signal frequency on cut-off frequency of IC was significant, while the intensity had no effect. The effects of signal frequency and intensity on cut-off frequency of AC were not significant. In guinea pigs with high frequency hearing loss, the cut-off frequencies of IC in low frequency regions decreased compared to those before acoustic trauma, while those of AC changed little.Conclusion: The auditory temporal resolution improved with the increases of bandwidth of gap mark signals, while decreased under pentobarbital anesthesia. The temporal resolution of IC in low frequency region decreased in high frequency hearing loss guinea pigs measured with gap detection and modulation detection, while the changes were not significant in AC.