Development And Plasticity Of Binaural Interaction In The Auditory Cortex Of Rats

The auditory system of human and animal integrates and processes the sound information from the two ears,which plays an important role in localizing the sound source,extracting the target sound in noisy environment,and speech comprehension.Previous studies have shown that the auditory system mainly relies on binaural auditory cues such as binaural time difference(ITD)and binaural level difference(ILD)to encode the horizontal spatial locations of acoustic stimuli.The perceptual accuracy of the spatial location of sound source and the sensitivity of binaural cues are gradually improved from infant to adult.Behavioral studies have shown that the primary auditory cortex is necessary for sound localization.However,up to now,the neural mechanisms for the development of binaural information processing in the primary auditory cortex have not been clearly elucidated.During the development of the auditory system the function of the auditory system can be affected by the external acoustic environment and early auditory experience.Noise exposure or auditory injury during hearing development can lead to plastic changes in the encoding of sound information by auditory cortex neurons.However,it is not clear how the abnormal acoustic environment or the abnormal auditory experience during hearing development affects the binaural interaction and the processing of binaural cues in the primary auditory cortex in adulthood.Therefore,we will do the following studies in this thesis: 1)How do the binaural interaction and the processing of binaural cues in primary auditory cortex change with age during hearing development? 2)How do moderate noise exposure and the reversible unilateral conductive hearing loss in early auditory development affect the binaural interaction and the processing of binaural cues in adulthood? Using single cell extracellular recording technique,we studied the development and plasticity of binaural interaction of neurons in the rat primary auditory cortex.In this study,rats were divided into five groups: 1)P14-P18 group(i.e.,Postnatal day 14-18);2)P19-P30 group(i.e.,Postnatal day 19-30);3)Adult group(Postnatal day 56-70,normal hearing development from birth to adulthood);4)Young noise exposure group,i.e.,rats received moderate(70 d B SPL)white noise exposure during P10-P56;5)Young reversible unilateral conductive hearing loss group,i.e.,30%Poloxamer 407 solution was injected into the ear drum chamber of one ear of rats from P14 to form a gelation,which caused hearing impairment.After the gelation was gradually absorbed by the surrounding tissues,the hearing was restored.This method will induce reversible unilateral conductive hearing impairment.Then the rats were raised to adulthood.The first three groups of rats were used to study the development of binaural interaction.The latter two groups and the adult group with normal hearing development(group 3)were used to study the effects of postnatal noise exposure and the reversible monaural conductive hearing impairment during early period of hearing development on the binaural processing in the primary auditory cortex.In a closed-field sound presenting condition,we recorded the auditory responses of neurons in the primary auditory cortex of rats in each group under both monaural and binaural conditions.In the binaural stimulus matrix,the ILD and average binaural level were systematically varied.The ILD changes from-20 d B to +20 d B with a step size of 10 d B,while the average binaural level changes from 20 d B to 70 d B with a step size of 10 d B.According to the responses of 734 auditory cortex neurons to both the monaural and the binaural stimuli,the monaural response types of the 734 neurons were assigned into the four categories: EE if the neuron responds to stimuli at either ear,EO if the neuron responds only to stimuli at the contralateral ear,OE if the neuron only responds to stimuli at the ipsilateral ear,and PB if the neuron does not respond or responds weakly to monaural ear stimulation but responds strongly to binaural stimulation.On the basis of the response of auditory cortex neurons to stimuli in the binaural matrix and to the monaural stimuli at corresponding levels,we classified the binaural interaction properties into the follows: Facilitation(F),Inhibition(I),No interaction(N),and mixed(M).According to the monaural response type and the binaural interaction properties,the binaural interactions of the 734 neurons were classified into nine different types,namely EE/F,EE/I,EE/M,EO/F,EO/I,EO/M,OE/F,OE/M and PB.We analyzed the development and plasticity of binaural processing by neurons in the rat primary auditory cortex based on the following: the monaural response type,the binaural interaction type,the degree of binaural interaction,the selectivity and the sensitivity to ILD.We analyzed the binaural processing characteristics of rat primary auditory cortex neurons in different age groups,including 156 neurons in the P14-P18 group,136 neurons in the P19-P30 group,and 171 neurons in the adult group.The results showed that the monaural response types of the majority neurons in the three groups were EO and EE,and the binaural interaction types included inhibitory interaction,facilitatory interaction,and mixed interaction.The monaural response type,the binaural interaction type,the distribution of the best ILD,and distribution of the ILD preference were nearly mature in early postnatal rats(P14-P18).However,during the early period after the rats acquired hearing,the binaural processing in the primary auditory cortex underwent a developmental refinement.With the increase of postnatal age,there existed an enhancement in the degree of binaural interactions,and an increase in the sensitivity and the selectivity for ILDs in the primary auditory cortex of rat.These results suggest that the ability of binaural information processing by the primary auditory cortical neurons has a developmental refinement process at the early stage of auditory development in rats.We did the data analysis of 126 neurons in the young noise exposure group,and of 138 neurons in youth young reversible unilateral conductive hearing loss group.We found that,compared with the data in the normal adult group,the moderate-level noise exposure at young age did not lead to the change of hearing threshold in both ears,but led to a reduction in the proportion of EO neurons and an increase in the proportion of EE neurons in the primary auditory cortex of rat.Noise exposure had no significant effect on the ILD preference of auditory cortex neurons,but induced a reduction in the degree of binaural interactions,as well as decreases in the selectivity and the sensitivity of auditory neurons to ILDs.Reversible monaural conductive hearing loss during early postnatal hearing development also resulted in a decrease in the proportion of EO neurons and an increase in the proportion of EE neurons in the contralateral primary auditory cortex of rat,but had no significant effect on the distribution of binaural interaction types.Reversible monaural conductive hearing loss during hearing development led to a decrease in the degree of binaural interaction,as well as a decrease in the selectivity and the sensitivity to ILDs in adults after the hearing threshold is restored.In summary,the results in the present study result shows that the binaural interaction and the tuning of ILDs for neurons in the rat primary auditory cortex undergo a developmental refinement process.Moderate-level noise exposure or reversible unilateral conductive hearing loss at early period of hearing development can lead to a reduction in the degree of binaural interaction,and a decrease in the selectivity and the sensitivity to ILDs.This study provides the experimental basis for us to understand the mechanism of the development and plasticity in binaural information processing in the primary auditory cortex.