The Effects Of Monaural Hearing Loss On The Processing Of The Gap Duration Of Sounds By Neurons In The Rat Primary Auditory Cortex

The perception of speech is closely related to the processing of the silent gap durations between sound stimuli by the auditory system.The shortest silent gap duration between two adjacent acoustic stimuli that humans and animals can detect is defined as the gap detection threshold.The gap detection threshold is often used to analyze the auditory system’s ability to distinguish the temporal information of acoustic stimuli.The processing of the sound gap durations by the central auditory system has been investigated under normal hearing conditions.In the hearing impaired populations,different degrees of monaural hearing loss at young age and in adulthood are caused by various reasons.The monaural hearing loss can affect the perception of speech.However,it is not clear how monaural hearing loss affects the sound temporal information processing in the auditory centers.In this study,we investigated the effects of both immediate monaural conductive hearing loss in adulthood and the reversible monaural conductive hearing loss at young age on the ability of auditory cortex neurons to detect the gap durations between two adjacent acoustic stimuli when determined in adulthood.In order to study the effects of monaural hearing loss on the auditory cortex neurons of SD rats,the responses of primary auditory cortex neurons to sequential acoustic stimuli with different gap durations were recorded by extracellular single-cell recording method,and the gap detection thresholds of cortical neurons were measured.The animals were divided into two groups: 1)The normal hearing development group,without any hearing loss surgery from birth to adulthood,which were used to study the effect of immediate monaural hearing loss on the gap detection thresholds of auditory cortex neurons in adult rats.According to the sound levels of the binaural inputs,three hearing conditions were included: Normal hearing condition;Simulated immediate 20 d B monaural conductive hearing loss condition(20 d B MCHL,the sound level was attenuated 20 d B in the ear contralateral to the recording auditory cortex relative to the normal hearing condition);3)Simulated immediate 40 d B monaural conductive hearing loss condition(40 d B MCHL,the sound level was attenuated40 d B in the ear contralateral to the recording auditory cortex relative to the normal hearing condition).These adult rats were given sequential acoustic stimulation with different silent gap durations in a closed acoustic field.2)In the young reversible monaural conductive hearing loss group(YRUCHL group),30% Poloxamer 407 solution was injected into the middle ear cavity from the pars flaccid of the tympanic membrane in the right ear on the 14 th,16th and 18 th days after birth.The solution was liquid at low temperature(4℃)and turned into gels at body temperature after entering the middle ear cavity.The gels blocked the normal vibration of the eardrum and led to conductive hearing loss.After the gel is gradually absorbed by surrounding tissues in the following several days,the hearing threshold of the injected ear gradually returned to normal levels.Adult rats with reversible monaural conductive hearing loss at young age were presented to sequential acoustic stimulation with various silent gap durations in a free acoustic field.Our results showed that under the condition of close acoustic field,the gap detection thresholds of neurons in the primary auditory cortex were increased under immediate monaural conductive hearing loss conditions compared with that under normal hearing conditions.Furthermore,the gap detection thresholds increased with increasing degree of monaural hearing loss.The 20 d B MCHL had little impacts on the proportion of auditory cortex neurons with gap detection thresholds not more than 5 ms,however,the 40 d B MCHL significantly decreased the proportion of auditory cortex neurons with gap detection thresholds not more than 5 ms.Compared with the gap detection thresholds determined from the auditory cortex of control rats,the reversible conductive hearing loss at young age had no significant effects on the gap detection thresholds of cortical neurons during adulthood.These results suggest that transient monaural conductive hearing loss at young age has no significant effect on the auditory temporal information processing in the auditory cortex in adulthood.However,immediate monaural conductive hearing loss in adulthood may lead to decreased ability in the temporal information processing in the auditory cortex.The results of the present study help us to understand how monaural hearing loss affects the sound temporal information processing in the auditory system.