Masking Effects Of Forward Masker Sound With Different Durations On Acoustical Responses Of Inferior Collicular Neurons To Probe Sound

The temporal features of interaction between excitatory and inhibitory input from both lower and higher auditory nuclei to inferior colliculus(IC) shape their responses to complex sound. In this study, a tone relative to 5 dB above minimum threshold(re MT+5 dB) at the best frequency of recorded neurons was used as forward masker sound under free field stimulation conditions. We analyzed the temporal properties of forward masking in IC through changing the masker duration from 40 ms to 100 ms. In order to study a possible central neural mechanism underlying the forward masking with different masker duration, GABAergic receptor antagonist (bicuculline, Bic) was also applied to recorded neurons.104 IC neurons,were examined under different masker durations. The results showed that: (1) As masker duration increasing ,inhibition in neuronal firing was enhanced (P < 0.0001, One-way ANOVA) and the latency of neurons was lengthened (P < 0.001, One-way ANOVA) ; (2) The inhibition period of neuronal firing were described as early-inhibition, late-inhibition and whole-inhibition. The inhibition period of 28(28%) neurons were altered with the masker duration increasing. Most of them transformed late-inhibition to early-inhibition or whole-inhibition when masker duration increased. (3) When stimulated with single tone, there were three types of rate-intensity function (RIF) of IC neurons, i.e. monotonic (20.43%, 19/93). saturated (47.31%, 44/93) and nonmonotonic (32.26%, 30/93). Among these neurons, 54(58.06%) neurons' RIF changed as the masker duration increasing. The inhibition induced by masker converted most monotonic to saturated, and saturated to nonmonotonic RIF, indicating that the inhibition was enhanced when the intensity of probe tone became higher. (4) The dynamic range (DR) of the most majority (91.40%, 85/93) narrowed with masker sound duration increasing (P < 0.0001, One-way ANOVA), whereas the DR of other (8.6%, 8/93) became wider. (5) After Bic application, the forward masking effects became weak and didn't be enhanced with the masker duration increasing for most neurons (86%, 43/50) . However, the forward masking in a small part of neurons became strong (8%, 4/50,) or was unaffected (6%, 3/50) .These data suggest that the masking effects of forward masker sound with different durations might be correlated with the amplitude and duration of inhibitory input to IC neurons elicited by masker sound. More inhibitory inputs may be activated when masker duration increased, and last longer. GABAergic inhibition plays an important role in masking effects. In some case, the correlation between the forward masking effects and the masker duration seems to be more complicated. In this study, the masker with different duration had profound effects on neuronal firing, inhibition period, RIF and DR, indicating that the masker duration increasing is likely to activate inhibitory inputs with different intensity , duration and transmitters. They integrated to differentially modulate the response property of IC neurons.