The Effects Of Monaural Hearing Loss On The Encoding Of Sound Motion By Neurons In The Rat Auditory Cortex

Humans and animals often get the information of target sounds and environmental sounds by perceiving the sound sources in motion.Normal binaural hearing is helpful for us to discriminate the target sound in motion in the constantly changing acoustic environment.It has been shown that there are neurons in the auditory cortex that are sensitive to sounds in motion,and the auditory cortex is involved in localizing the fixed sound sources and moving sound sources,and processing these information.However,the neural mechanism regarding the effects of hearing loss on the perception of sound motion is not clear until now.In the auditory system,the coding of auditory space depends on the computation of both binaural and monaural acoustic cues.We hypothesized that monaural hearing impairment would lead to abnormal binaural input,and therefore affect the coding of moving sound by auditory cortex neurons.In this study,we investigate the following questions in rats: 1)whether reversible monaural conductive hearing loss at young age affects the coding of sound motion by auditory cortex neurons in adulthood,and 2)how immediate monaural conductive hearing loss in adulthood affects the coding of sound motion in auditory cortex.The rats were divided into two groups: 1)Normal control group: the adult rats,aged from postnatal day 57（P57）day to P77;2)Reversible monaural conductive hearing loss group: On P14,P16 and P18,we injected Poloxam 407 solution into the tympanic cavity.The poloxam 407 solution turned into gel at body temperature,which resulted in monaural conductive hearing loss.Subsequently,the gel was gradully absorbed by surrounding tissues,and hearing was restored before P30.We consider the motion sound as as a sequence of sound in different locations.We simulated moving sound by presenting a sequence of sound to the two ears with varying interaural level differences and interstimulus intervals in a close field acoustic system.The velocities of the moving sound were 66.67 d B/s,50 d B/s,33.33 d B/s,25 d B/s,16.67 d B/s,10 d B/s and 5 d B/s.The average binaural levels of the moving sound were at 50 d B and70 d B.The moving sounds were presented in two directions,i.e.,the direction from the left ear to right ear,and the direction from the right ear to the left ear.Each combination of motion direction,sound level,and motion speed was repeated 30 times.We recorded the auditory responses of auditory cortex neurons to the moving sound under different conditions,and analyzed the effects monaural conductive hearing loss on the encoding of sound motion byf auditory cortex.We recorded and analyzed 270 neurons in the auditory cortex of two groups of rats,including 149 neurons in the primary auditory cortex（AI）of normal control rats and121 AI neurons in the adult rats with reversible monaural conductive hearing loss at young age.We also investigated the response of AI neurons in normal control rats to moving sound under the condition of simulated immediate 20 d B hearing loss in the right ear（Contralateral to the recording side）.We analyzed the sensitivity of auditory cortex neurons to sound motion from two aspects: the preference to the moving direction of the sound,and the shift of motion receptive field determined from two moving directions.The results showed that the proportion of motion-sensitive neurons in AI was higher at higher speed（33.33 d B/s,50 d B/s,66.67 d B/s）than at lower speed（25 d B/s,16.67 d B/s）of motion sounds,and the directional selectivity index was significantly different with varying the velocity of sound motion.The shift of the motion receptive field determined from two motion directions was larger at high velocity than that at low velocity.Apart from the 5 d B/s velocity condition,there were significant differences in the direction index under two average binaural levels.Under the simulated 20 d B monaural hearing loss condition,the proportion of motionsensitive neurons was lower than that under normal hearing conditions.However,the proportion of motion-sensitive neurons with directional selectivity was relatively higher,and the proportion of motion-sensitive neurons with receptive field shift was relatively low,and the degree of motor receptive field shift was relatively small.Reversible unilateral conductive hearing loss at young age did not significantly change the proportion of motion sensitive neurons in the rat of AI during adulthood.Under most of the test conditions,Reversible unilateral conductive hearing loss at young age did not have significant impact on the directional selectivity as well as the motion receptive filed shift of AI neurons during adulthood.The results of the present study are helpful for us to understand the neural mechanism regarding the impacts of different monaural hearing loss on the perception of sound motion.