Temporal Features Of The Effect Of Preceding Noise On Spatial Response Areas Of Neurons In Rat Primary Auditory Cortex

Sound localization is one of the primary functions of the auditory system. Previous studies on this issue mainly focused on the neurons' spatial response features to single tone or noise signals.Natural acoustic environment is often complex,and the acoustic signals were continous complex sounds from different directions in the acoustic space.Thus,it is important to study the neural mechanism of acoustic information processing in the brain under complex acoustic environment.Using conventional extracellular electrophysiological technique,we determined the spatial response areas of neurons in the rat primary auditory cortex(AI) and the features of the spatial response areas.The effects of preceding noise on AI neuron's spatial response area under different noise-probe interval were studied to determine how the noise-probe interval affects spatial response areas of AI neurons.This study has three parts:PartⅠ.The set up of free field acoustic space and the design of device for spatial acoustic stimuli.Two kinds of acoustic field are often used in sound localization studies:(1)virtual acoustic field,which is used for emulating spatial acoustic stimulus;(2)free field, which is realized by one or many speakers located in real space.The free acoustic field can emulate the natural acoustic environment much better than virtual acoustic field.The free field acoustic device designed in this study has two major parts: mechanical moving parts and driven-control system.The features of this acoustic device are:only one speaker is used for transmitting acoustic stimulus;the speaker can be freely located at any desired location in the animal's acoustic space;the speaker moves in the spherical surface,and the spheric radius can be easily changed as desired.This acoustic device will benefit a lot in studies on sound localization of auditory system.PartⅡ.The features of spatial response area of rat AI neurons.Sixty-five neurons in 33 healthy adult Sprague-Dawley(SD) rats' primary auditory cortices were studied with the conventional electrophysiological technique. Pure tone at a neuron's charatersitic frequency(CF) and 10～20 dB above threshold was used as probing acoustic stimulus,and the frontal hemispherical spatial response areas of AI neurons were determined.The spatial origin was defined as the exact front of the animal's head,and the range of spatial response area were within horizontially -90～+90 degrees(positive for contralatercal space relative to the recording cortex), and vertically -90～+90 degrees(positive for above horizontal area).The testing directions were set every 30 degrees in both horizontally and vertically,and 37 spatial testing directions in the frontal spatial response area were used.On each testing direction acoustic stimulus were repeated 30 times.The firing rate and first spike latency in each of the 65 neurons' spatial response areas were analysed.In spatial response areas that using firing rate as index,all the neurons were divided into five types according to the position of geometrical center of neuron's spatial preferred region in frontal space:contralateral hemisphere preference neuron(Contra,44/65, 67.7%),ipsilateral hemisphere preference neuron(Ipsi,11/65,16.9%),midline preference neuron(Mid,3/65,4.6%),omnidirectional neuron(Omni,4/65,6.2%),and complex neuron(Comp,3/65,4.6%).According to the position of geometrical center of neuron's shortest first spike latency region,all the neurons were divided into five types:L-contralateral neuron(L-contra,37/65,56.9%),L-ipsilateral neuron(L-ipsi, 10/65,15.4%),L-midline neuron(L-mid,7/65,10.8%),L-omnidirectional neuron (L-omni,2/65,3.1%),L-complex neuron(L-comp,9/65,13.8%).Spatial response areas of all the neurons analysed by index of both firing rate and first spike latency showed typical hierarchic structures.The firing rates in neuron's preferred regions were greater than that in non-preferred regions,and the latency in preferred regions were shorter than that in non-preferred regions.The firing rates and first spike latencies were significatly negative correlated for 71%of neurons(p＜0.05) across their whole Spatial response areas.The spatial distribution of neurons' geometrical centers of spatial preferred region was similar to the spatial distribution of neurons' geometrical centers of short first spike latency regions.These results indicated that, rat AI neurons had spatial selectivity,and most of the neurons preferred contralateral space;there was a significant correlation between first spike latency and firing rate for majority of the neurons;the neuron's spatial preference across latency was similar to the spatial preference across firing rate.PartⅢ.Temporal features of the effect of preceding nois e on the spatial response areas of neurons in rat primary auditory cortex.In this study,broadband noise was used to emulate the complex acoustic background in natural acoustic environment.The temporal features of the changes of spatial response areas of neurons were studied.Broadband noise was used as preceding conditional stimulus,pure tone at the CF of the AI neuron was used as probe,the levels of probe and noise were 10～20 dB above the neuron's thresholds. The inter-stimulus interval(ISI) between preceding noise and probe tone were changed,and changes of the neuron,s tone spatial response area were determined.The spatial response areas to pure tone stimuli under preceding noise condition were compared with the spatial response areas in quiet condition.The results showed that, the shorter ISI was,the stronger the preceding noise inhibited the neuron's tone response.At the condition of preceding noise,the firinng rate to pure tone stimuli,the area of spatial response area were shrinked,and the neuron's first spike latencies in many directions were prolonged.While ISI became longer,neurons' responses to probe tone were recovered.The firing rate,area of spatial response area and first spike latency were all recovered.The effect on responses in neuron's preferred directions by the changes of ISI was smaller than the effect on non-preferred directions.While ISI was short,the neurons maintained their response to preferred regions.As ISI increased longer,the neurons recovered their response in the preferred directions quicker than that in non-preferred directions.These results indicated that,preceding noise inhibited the spatial response areas of rat AI neurons;the inhibition increased when ISI became shorter,and the inhibition decreased when ISI became longer.In the whole spatial response area of an AI neuron,both the inhibitory effect of preceding noise and the recovery of neuron's response under noise environment were non-linear.