Studies On The Amplitude-temporal Property And Frequency Tuning Of Inferior Collicular Neurons With Auditory Behavior-related Response Patterns In Hipposideros Pratti

The inferior colliculus(IC)of mammals' midbrain is an important relay station and subcortical processing and integrating center for auditory information in the central auditory pathway.Our previous studies showed the IC neurons of constant frequency-frequency modulation(CF-FM)bat discharged as single-on(SO)or double-on(DO)neurons when stimulated with behavior-related CF-FM sounds,also called neurons with behavior-related response patterns.During echolocation,the amplitude,duration and frequency of bat's echolocation pulses varied,and these varied parameters contrain rich environmental information.The central auditory neuron can extract the required environmental information from these variations.However,the underlying mechanism is not entirely clear.Therefore,we studied the amplitude-temporal property,frequency tuning property,and temporal or/and spectral integration during frequency tuning of two types of neurons in the IC of Pratt's roundleaf bat(Hipposideros pratti)using free-field stimulus and in vivo intracellular recording,and obtained the results as following.1.SO and DO neurons have different amplitude-temporal properties to behavior-related CF-FM sounds.66.9%of SO neurons(N=81/121)displayed a response pattern invariance to CF-FM sound stimuli regardless of amplitude and duration.This property is speculated to make the bat use CF component-elicited response to effectively acquire the information about Doppler shift and wingbeats of insects.33.1%of DO neurons(N==40/121)displayed obvious amplitude-temporal property in firing rates,discharge patterns and occurrences of DO responses,and there exists complementarity between the amplitude and duration;this property is speculated to make the bat compensate for the deficiency in low echo duration induced by the increase of PRR through the increase in echo amplitude,thus ensuring the responses of DO neurons to CF and FM components respectively,and possibly effectively acquiring the information about target distance and texture.2.SO and DO neurons have different frequency tuning(FT)properties to behavior-related CF-FM sounds.54.4%of SO neurons(N=43/79)have longer post-spike hyperpolarizations(PSHs)followed behind action potentials(AP)elicited by CF and CF components of CF-FM sound stimuli,and the mean band width(BW)of impulse-frequency tuning curve(FTC)to CF and CF components of CF-FM sound stimuli in SO neurons with PSHs is significantly broader than that of SO neurons without PSHs(N=36/79,45.6%)(p<0.05).29.7%of DO neurons(N=11/37)have shorter PSHs,and 70.3%of them(N=26/37)have no PSHs,but no significant difference exists between the mean BW of them(p>0.05).46.8%of SO neurons(N=37/79)have lateral inhibitory area(LIA)at one side or both sides of FT,but only 5.4%of DO neurons(N=2/37)have LIA,and the mean BW of impulse-FTC to CF and the CF components of DO neurons is significantly broader than that of SO neurons(p<0.05).The mean BW of impulse-FTC to CF component of behavior-related sound stimulus is significantly narrower than that of CF sound stimulus alone(p<0.001).Basing on the above resulsts,we speculate that the frequency processing ability of SO neurons to CF and CF components of behavior-related sound signals is higher than that of DO neurons.The FM component can influence the frequency selectivity of SO and DO neurons to CF components,enhancing the processing ability of these two types of neurons to CF components.3.Because the spectral and temporal integration at the behavior level is based on a single neuron,it is helpful to reveal the mechanism by studying the spectral and temporal integration of individual neurons.The obtained results showed that four different types of response participate in the temporal or/and spectral integration of FT.The spike,PSH,facilitatory responses and inhibitory responses.Among them,the spike and facilitatory responses can generate amplitude and frequency dependent"broadening effect" through temporal or/and spectral integration;while the PSH and inhibitory inputs can generate frequency dependent"sharpening effect"through temporal or/and spectral integration.These two types of integration are possibly based on temporal and spatial summation of different convergent inputs.The spectral and temporal integration between different frequencies is possibly mainly due to spatial summation supplemented by temporal summation,while the temporal integration between same frequencies is possibly mainly due to temporal summation.4.The present study examined the temporal integration of IC neurons without inhibitory inputs at both low and high frequency sides and PSHs followed behind spikes,it is observed that there still exist sharpening frequency tuning effects under two-tone stimulus condition.It is speculated that the inhibitory inputs which sharpen FT may occur in the nuclei below IC,and the IC succeeded it.On the basis of these results,different amplitude-temporal and spectral properties of SO and DO neurons possibly match their functions in echolocation.The temporal or/and spectral integration during the FT of IC neurons makes us clealy understand the relation beween different types of excitation,different types of inhibition,excitation and inhibition,the sharpening effect of inhibition and the broadening effect of facilitation,and the integration and summation.