Mechanism Of Effect Of Weak Noise Forward Masking On The Frequency Tuning In Inferior Collicular Neurons

Human or animals often search sound signal from noise environment in the nature, when the gap between two sounds is short enough, the preceding noise can bring forward masking to the following sound signal. Previous research showed that certain intensity weak noise masker can increase sensitivity of signal intensity on mouse inferior collicular neurons, benefit to the sense of hearing system to catch the signal sound in noise environment. To study the change of frequency tuning in inferior collicular neurons when added difference duration weak noise forward masking, we examined the changes in neuronal frequency tuning resulted from a weak white noise as forward masking relative to 5dB above minimum threshold (reMT+5) and different duration (40 60 80 100 ms) under free field stimulation conditions. Study the mechanism of effect of weak noise forward masking on the frequency tuning in inferior collicular neurons.169 inferior collicular (IC) neurons were obtained in present experiment and frequency tuning curves (FTCs) of 69 IC neurons among these neurons were measured. When gap between offset of masker sound and onset of probe sound is 40 ms, weak noise forward masking sharpened frequency tuning curves of most neurons (P<0.01, n=25), and this effect increased with masker duration increasing (P<0.05). Accounted the inverse-slope (IS), we found the inhibitory rate to high frequency was clearly higher than low frequency. Most IS_high decreased, this effect increased with masker duration increasing (P<0.01); however, there was no regular changes in IS_low. Masker sound also increased the minimum threshold (MT) to probe sound (P<0.001, n=31), and this effect also increased with masker duration increasing.Regulated the gap between masking sound offset and probe sounde onset, and made the inhibitory rate of weak noise to sound signal fixed to 50%, measured the FTCs of neurons to probe sound. Weak noise forward masking also sharpened FTCsof most neurons (P<0.01,n=22), and this effect increased with masker duration increasing (P<0.05); accounted IS of FTCs, the inhibitory rate to high frequency was also clearly higher than low frequency, and the inhibitory rate to high frequency increased with masker duration increasing (P<0.01).After micro-ionophoresis channelled BIC to 22 IC neurons which inhibited by weak noise, the inhibitory rate of weak noise decreased. When added 40 ms masker, Q₁0 increase percentage declined from 26.15 ± 18.52% to 13.21 ± 8.85%, and Q₃0 increase percentage declined from 39.85 ± 24.27% to 17.89±13.78%. When added 80 ms masker Q₁0 increase percentage declined from 33.57 ± 16.24% to 15.62 ± 7.51%, and Q₃0 increase percentage declined from 52.64 ± 23.61% to 26.98 ± 12.77%. We can see the remove of GABAergic inhibition can partially weaken the masking effect caused by weak noise.Calculate according to the above phenomenon, the effect of weak noise forward masking sharpened FTCs were related to the inhibition in central auditory system, such as GABAergic inhibition. The weak noise caused stochastic resonance in cochlear may activated inhibitory neural circuit of GABAergic inhibition, it gave a kind of inhibitory importation to two side of FTCs, and made the frequency analysis near best frequency become more delicacy and precision.IC is the importation relay station of hearing system, which accept not only excitatory importation but also inhibitory importation. Forward masking decreased MT, possibly because the inhibitory importation reached before excitatory importation, lowered the nerve excitability, caused the masking effect to MT. The effect of weak noise forward masking increased with the masker duration lengthening, and the release of inhibitory transmitter in inhibitory neural circuit increased, so the effect of forward masking increased with the masker duration lengthening in certain range.