The Effect Of Postnatal Exposure To Noise On Sound Temporal Gap Processing By Rat Auditory Cortex Neurons In Adulthood

Environmental noise is considered to be one of the important factors that influence health. Previous studies in human and animals have shown that long-term exposure to noise can affect the normal physiological functions (including auditory function). The ability of auditory system processing the transient change of temporal information in sound stimuli plays an important role in animal communication and human speech. The present study investigated the impact of moderate-level noise exposure in different postnatal period on the sound temporal information processing of neurons in the adult auditory cortex. Most of previous studies regarding sound temporal information processing acuity were conducted in behavior experiments using a gap detection paradigm. In contrast, using neurophysiological method, the present study tried to explore the neural mechanism of sound gap detection by recording the response of rat auditory cortex neurons to sound stimuli with a short gap-Rats were divided into three groups:the critical period noise exposure group (CPN group), the adult noise exposure group (AN group), and the control group (CON group). In this study, we have collected and analyzed the data of gap detection thresholds and the average first-spike latencies of221neurons in the rat primary auditory cortex. The results are as follows:1.The effect of noise exposure on the response threshold of neurons in the rat primary auditory cortexThe average minimum thresholds of neurons among the three groups as follows:65±8.81dB SPL (CPN group),52.31±9.91dB SPL (AN group), and51.14±7.76dB SPL (CON group). Statistical analysis showed that the average minimum thresholds of neurons in the CPN group was significantly higher than that in the CON and the AN groups. However, no significant difference was found in the minimum threshold between the CON group and the AN group. 2. The effect of moderate-level noise exposure during different postnatal periods on the gap detection thresholds of auditory cortex neuronsThe gap detection threshold of cortical neurons in the three groups of rats determined under different sound levels are as follows:For the control group, the percentage of neurons whose the gap detection threshold that were less than5ms were44%at60dB SPL,44.3%at70dB SPL, and46.75%at80dB SPL. For the CPN group, the percentage of neurons whose the gap detection threshold that were less than5ms determined at60,70, and80dB SPL were12.5%,15.09%, and23.44%, respectively. In contrast, for the AN group, the percentages were44.62%at60dB SPL,41.03%at70dB SPL, and48.68%at80dB SPL. The results indicate that the percentage of neurons in the rat auditory cortex that can detect short gap duration was smaller in CPN group of rats than that in the other two groups of rats. In contrast, the percentage of neurons whose gap detection thresholds were great than50ms were greater in CPN group than that in the other two groups. The detail data are as follows: for the control group, the percentages were25.33%at60dB SPL,21.52%at70dB SPL, and22.08%at80dB SPL; for the CPN group, the percentage was50%at60dB SPL,49.06%at70dB SPL, and39.06%at80dB SPL; for the AN group, the percentage was20%at60dB SPL,12.82%at70dB SPL, and3.95%at80dB SPL Statistical analysis showed that the gap detection threshold of neurons in the CPN group was higher than that in the AN group and CON group, however, no significant differences were found in the gap detection threshold of neurons between the AN group and the CON group. Moreover, the sound stimulus level also influences the gap detection threshold; the gap detection threshold was lower at a higher sound level.3. The effect of noise exposure on the average first-spike latency of neurons in the rat primary auditory cortexWe compared the average first-spike latencies of response to noise2in the sound sequence in three groups of rats, and we found that the average first-spike latencies in the CPN group was significantly longer than that in the CON and the AN groups. However, no significant difference was found in latency between the CON group and the AN group. We also found that the response latency was related to the sound stimulus level. The response latencies were shorter at a higher sound level and longer at a lower sound level.The results in the present study have shown that moderate-level noise exposure in the critical period of auditory development have significant impact on the sound temporal information processing of neurons in the rat auditory cortex, whereas the same noise exposure during adulthood did not have that effects. The results suggest that it is important to avoid long-term noise exposure during critical period of auditory development in order to have a normal auditory function.