Effects Of Postnatal Noise Exposure On The Repetition Rates Of Pure And Frequency-modulated Tones Encoded By Neurons In Rat Auditory Cortex

Previous studies have shown that neurons in the auditory centers(e.g.,inferior colliculus,auditory cortex)of normally developing adult animals selectively respond to different parameters and to different types of acoustic stimuli.The central auditory neurons have the ability to follow the repeated sound stimulation.In a certain range of repetition rate,the auditory neurons can follow the repeated sound stimulation.However,when the repetition rate is high,the auditory neurons' ability to follow the repeated sound stimulation decreases or disappears.It has been demonstrated that the structure and function of the auditory system can be affected by auditory experience in the early postnatal period.Moderate to high level noise exposure can lead to deficits in the encoding of the frequency,spatial,temporal and other information of sound stimuli.However,it is not clear how postnatal moderate level noise exposure affects the selectivity of auditory cortex neurons to periodic pure tone and frequency-modulated(FM)sound in different sweep directions,and how postnatal moderate level noise exposure affects the ability of neurons to follow these periodic sound stimuli.In the present study,we exposed rats to moderate level(70 d B SPL)broadband noise environment for 24 hours per day from the postnatal day 10 to postnatal day 56,and used the extracellular single-channel electrophysiological recording technique to record the responses of auditory cortex neurons to sound stimuli within two weeks immediately after the cessation of noise exposure.We determined the responses of neurons in the primary auditory cortex(A1)of both the control group of rats and the noise exposure group of rats to periodic sounds including pure tones,upsweep FM sounds,and downsweep FM sounds.The results showed that the ability of A1 neurons in the normal control group to follow periodic upsweep FM sound stimuli was significantly higher than that to follow the periodic pure tone sound stimuli,whereas the ability of A1 neurons to follow periodic downsweep FM sound stimuli is the worst.The results indicated the strongest selectivity for periodic upsweep FM sound stimuli for rat A1 neurons.The selectivity of rat A1 neurons in the noise exposure group to these three types of periodic sound stimuli was similar to that in the control group.Compared with that in the control group,postnatal moderate level noise exposure did not significantly change the minimum threshold and the response latency of A1 neurons,but increased the bandwidth of frequency tuning of A1 neurons,and induced a significant decrease in the ability of A1 neurons to follow periodic pure tone and periodic upsweep FM sound stimuli.The results of this study provide experimental evidence for us to further understand the selectivity of auditory neurons to different periodic acoustic stimuli,and how the noise exposure during the development of auditory function affects the ability of auditory centers to process sound temporal information.