The Temporal Property Of Masker-elicited Forward Masking In Inferior Collicular Neurons

Inferior collicular (IC) neurons of the midbrain accept excitatory convergence and inhibitory inputs from both lower and higher auditory nuclei. The interaction between these two opposing inputs shapes different response properties of IC neurons. In this study, we examined this interaction between excitatory and inhibitory inputs to IC neurons of mouse using a masker and a probe at the best frequency (BF) of neurons. We analyzed the temporal property of forward masking through changing the masker duration from 5 ms to 60 ms and increasing the interval from 50 ms to 200 ms. In order to study a possible neural mechanism underlying the forward masking, an antagonist, bicuculline (Bic), of GABAergic receptor was applied by ionophoresis to recorded neurons.85 IC neurons were recorded and these results showed the different types of responses to the probe under the action of masker: (1) the discharge rate and latency of 85.3% of neurons decreased and increased with masker duration increasing, 4.4% of neurons with opposite properties of response, and 10.3% of neurons with nonmonotonic rate-intensity functions. (2) Number of impulses in neurons of 67.6% increased and their latencies in theses neurons became shorter with interval between masker and probe and other neurons showed complicated patterns of responses. (3) Through analyzed the recovery time from forward masking under the action of different masker duration, we found that there was typical temporal property in forward masking and the recovery time needed by IC neurons increased with masker duration changing at range from 5 ms to 30 ms and then became saturationat about 120 ms. (4) The responses of these neurons to the probe appeared weaker with the masker increasing, but we did not observe the dependence on the absolute intensities of two tones, respectively. (5) The forward masking became weak or disappear during Bic application by iontophoresis to recorded neurons because it could disinhibited GABAergic inhibition by Bic.These results demonstrated that the response properties of neurons to probe under the action of masker were determined by the time and intensity of excitatory and inhibitory inputs to IC neurons. The amplitude of masker-elicited an inhibitory input to IC were related with duration and intensity of masker, but the durations of forward masking in these neurons were just relation with masker duration. When the inhibitory inputs to IC elicited by masker encounter the excitatory inputs elicited by probe, it weakened the probe response and produced forward masking. Variation in duration and intensity of masker and interval between masker and probe, it actually changed the quantity of inhibitory inputs to IC and affected the response of neurons to probe. Altogether, forward masking is a result of convergence of excitatory and inhibitory inputs to IC, but in forward masking, there were dependences on the time and intensity of masker and some neural inhibition activated by masker. Because the forward masking could be weaken by iontophoresis of bicuculline, this inhibition input was conducted by GABA and this is the reason why forward masking happened in auditory center was not explained completely by the theory of random vibration in cochlear.