The Effects Of Noise Exposure On The Auditory Stream Segregation Of Neurons In The Rat Auditory Cortex

In complex acoustic environments,humans and animals need to segregate,combine and encode sounds in order to better process sound information from different sources.If a sequence of sound is perceived as one or more coherent wholes,each “coherent whole” is considered as a “stream”.The process of perceiving a sequence of sound as multiple streams is called auditory stream segregation.Auditory stream segregation can be tested by presenting a sequence of pure tones in an alternating pattern or a synchronous pattern.Human psychophysical studies have demonstrated that there is a “primitive” mechanism for auditory stream segregation,which is bottom-up and not controlled by attention.The auditory system uses the basic characteristics of alternating acoustic stimuli,such as frequency,presentation rate,space and other cues to segregate auditory stream.However,the neural mechanism of auditory stream segregation has not been fully elucidated.Noise is ubiquitous in our life.Previous studies have shown that high level noise can cause damage to multiple structures along the auditory pathway.Moderate level noise exposure in the early postnatal period has profound effects on the auditory cortex neurons,which can lead to a decline in the ability of auditory cortex neurons to encode the frequency,time,space and other information of acoustic stimulation.Since the frequency separation of acoustic stimuli in a sequence of sounds is an important factor affecting the auditory stream segregation,and the frequency tuning characteristics of auditory cortex neurons can be affected by early postnatal moderate noise exposure,we hypothesize that moderate noise exposure at different periods after birth may affect the auditory stream segregation.To test this hypothesis,we exposed rats of different ages to continuous white noise(70 d B SPL)for a period of time.The young noise exposure group of rats was exposed to the noise from postnatal day 10 to postnatal day 56,and the adult noise exposure group of rats was exposed to the noise from postnatal day 57 to postnatal day 103.The rats in the control group were adult rats raised in normal environment without any noise exposure.We recorded the responses of primary auditory cortex neurons of rats to sound sequences with different frequency separation and presentation rates of acoustic stimuli,and further analyzed the degree of segregation of auditory stream by auditory cortex neurons among the three groups of rats to various sound sequences.We found that the frequency separation,the presentation rate,and the sound sequence pattern significantly affect the degree of auditory stream segregation of auditory cortex neurons in the three groups of rats.Further analysis showed that,compared with the control group,moderate level noise exposure in early postnatal period resulted in a significant decrease in the ability of auditory stream segregation of primary auditory cortex neurons,whereas moderate level noise exposure in adult period had no significant effects on this ability of primary auditory cortex neurons.This study provides experimental evidence for us to further understand the neural mechanism of auditory stream segregation,and the mechanism of acoustical environment-dependent plasticity.