Mechanism Of Effect Of Weak Noise On The Dynamic Range Of Mouse Inferior Collicular Neurons

The ability to extract a signal from complex auditory scene is a focus of interest in neuroscience. In natural environment, the signal always masked by interfering noise, that the critical problem is auditory system how to catch the signal under the noise conditions and what neural mechanism underlying it. In order to study the effect of weak noise on the sound signal extraction of mouse (Mus musculus Km) inferior collicular (IC) neurons from environment, we examined the change in neuronal rate intensity function (RIF) resulted from a weak noise relative to 5 dB below minimum threshold (reMT-5 dB) under free field stimulation conditions.A total of 182 IC neurons were recorded in present study and RIFs of 86 IC neurons were measured. We found that the RIFs (79%, n = 68/86) inhibited by noise was very significant (P < 0.001, n = 68), the other two types induced by weak noise were facilitated RIFs (10.5%, n=9/86) and basically unaffected RIFs (10.5%, n = 9/86), respectively, but there was no significant change (P > 0.05) in these two types. The degree of noise inhibition at minimum threshold (MT) was the strongest in the whole intensity domain, and the inhibition decreased with the signal intensity increasing {P < 0.0001). Also, after calculated the dynamic range (DR) of RIF inhibited by noise, we found DRs of neurons (72%, n = 49/68) became the smaller (P < 0.001, n = 49). And bicuculline (an antagonist of GABAergic receptor A) application increased the number of impulses, entirely or partially cancelled the effects of noise on the RIF, increased DRs of neuron response, and weaken the inhibitory effect of weak noise on RIF and the sensitivity of neuron to intensity change. It showed that activated inhibition by weak noise could be GABAergic inhibition.From above the findings, we could give a hypothesis that the input of random oscillation induced by noise in the cochlea to central auditory system could be integrated in the central auditory nucleus and the response of sound-sensitive neuron to sound stimuli could be adjusted to an optimum state for signal intensity coding. In present study, we obtained large numbers of phasic discharge pattern neurons, which play important role in analysis and perception of sound signal from environment with background noise. Suggesting that the sensitivity of IC neuron to intensity change was weaker under the quiet conditions than the noise environment. It makes benefit to catch the rapid intensity fluctuations, which are thought to be important information-bearing parameters for signal processing, in human speech of "cocktail party effect" and specific communication signals of other species. This is probably why human and animal extract signal of changing intensity more easy than the constant intensity.