Postnatal Development Of Binaural Interactions Of Neurons In The Rat Inferior Colliculus

Normal binaural hearing can help human and animals to perceive target sound in a complex acoustic environment.Neurons in the central auditory system process and integrate binaural information from the two ears.The binaural interaction of auditory neurons has been reported at many stations in the central auditory system.Previous studies on binaural interaction were mainly conducted in adult animals.However,it is not clear whether there is a postnatal development of binaural interactions for neurons in the central auditory system.Previous studies have shown that the binaural interaction first occurs in the superior olivary complex,and the integrated binaural information are further sent to auditory centers(i.e.,lateral lemniscus,inferior colliculus,medial geniculate body,and auditory cortex)along the ascending auditory pathway to be processed.In the central auditory pathway,the inferior colliculus receives information from almost all the auditory centers in the brainstem,and also receives descending modulation from auditory cortex and medial geniculate body.Therefore,the inferior colliculus is an important relay station and information processing center.In the present study,we recorded the responses of neurons in the rat inferior colliculus to monaural stimuli and binaural stimuli,and analyzed the postnatal developmental changes in the binaural information processing of auditory neurons in the inferior colliculus.Three groups of rats were used in the present study: Group 1(Postnatal day(P)13-16,juvenile rats),Group 2(P17-30,juvenile rats),Group 3(P57-77,adult rats).Under close-field acoustic conditions,we presented monaural and binaural sound stimuli to rats,and the frequency of the sound stimuli was the characteristic frequency of the recorded inferior collicular neurons.The sound levels of the monaural stimuli were from 0 to 80 dB SPL with 10 dB steps.The sound levels of the binaural stimuli were presented by varying the interaural level differences(from-20 dB to +20 dB with 10 dB steps)and average binaural level(from 20 to 70).We recorded the responses of inferior collicular neurons to monaural stimuli and binaural stimuli,and analyzed the binaural input types and the binaural interaction types of inferior collicular neurons in each group of rats.The data of the present study were recorded from 420 neurons in the inferior colliculus of three groups of rats,including 126 neurons from P13-16 rats,141 neurons from P17-30 rats,and 153 neurons from P57-77 rats.The binaural input types of these neurons were classified into three categories: 1)EO type,i.e.,the neuron was excited by stimuli from contralateral ear but did not respond to stimuli from the ipsilateral ear;2)EE type,i.e.,the neuron was excited by stimuli from either ear;3)OE type,i.e.,the neuron was excited by stimuli from ipsilateral ear but did not respond to stimuli from the contralateral ear.The binaural interaction types of the inferior collicular neurons included the inhibitory binaural interaction,facilitatory binaural interaction,mixed binaural interaction(showing both inhibitory and excitatory interaction),and no binaural interaction.For the three groups of rats,the great majority of the inferior collicular neurons exhibited the binaural input types of EO and EE,and OE type occurred rarely.The EO type of neurons occupied the greatest proportion in each group of rats.We did not find significant differences in the proportion of neurons regarding the binaural input types among the neurons in the three groups of rats.For the binaural interaction types of neurons in the three groups,majority of the neurons show inhibitory binaural interaction and mixed binaural interaction.However,the proportion of neurons with inhibitory binaural interaction was increased with increasing postnatal age,and the proportion of neurons with mixed binaural interaction type was decreased with increasing postnatal age.Moreover,both the degree of binaural interaction and the strength of inhibitory binaural interaction were greater in the neurons in the adult rats(P57-77)than those in the juvenile rats(P13-16 rats and P17-30 rats).The selectivity and sensitivity of inferior collicular neurons to interaural level differences were greater in adult rats than in juvenile rats.Our results demonstrated that,the binaural input type of inferior collicular neurons did not change with postnatal age from at least P13,however,the binaural interaction,the selectivity and sensitivity of inferior collicular neurons exhibited postnatal developmental changes.The results in the present study help us to understand the functional developmental mechanism in the binaural information processing by central auditory neurons.