| The inferior colliculus (IC) occupies a strategic position in the central auditory system. Evidence from lots of studies indicates that it is an interface between lower brainstem auditory pathways, the auditory cortex and motor systems. The IC receives ascending input from a number of auditory nuclei in the lower brainstem. Moreover, it receives crossed input from the opposite IC and descending input from auditory cortex. For a long time, many experiments were performed on IC to study the encoding of sound frequency, intensity and duration. Many other aspects such as sound location, binaural properties, processing of pulse repetition rate, and corticofugal modulation also have studied extensively.As a good kind of model animal, mouse (Mus musculus) was extensively studied in numerous auditory researches which used pure tone as sound stimulus. However, relative very few studies has done about how did the IC neurons of mouse respond to frequency-modulated (FM) sweeps. The FM sound not only exists in the human language, but also constitutes a major part in animal sound communication. Some researches have performed on the IC of many animals such as echolocating bats and rats and these researches show that some IC neurons can encode this complex sound efficiently. This research observed the response of IC neurons to FM stimuli with different modulation ranges in the free-field conditions.15 healthy adult mice with normal hearing were used in this experiment, and 90 IC neurons which responded to sounds were obtained. The results showed that: (1) the modulation range - spike function of the IC neurons included four different types: the spikes of most neurons decreased as the modulation rate increased and this type termed short-pass (in up-sweep: 54/90, 60%; in down-sweep: 57/90, 63.33%). The second types were band-pass (in up-sweep: 17/90, 18.89%; in down-sweep: 12/90, 13.33%) and all-pass (in up-sweep: 15/90, 16.67%; in down-sweep: 17/90, 18.89%). The remaining were long-pass (in up-sweep: 4/90, 4.44%; in down-sweep: 4/90, 4.44%). (2) The study also observed the latency and response period of the neurons in short-pass neurons stimulated with FM sounds which included different modulation ranges. The results showed that the latencies of most short-pass neurons increased as the modulation ranges increase (in up-sweeps: 36/54, 66. 67%;in down-sweeps: 36/57, 63.16%). The change was significant either use a one-way ANOVA test or the paired t-test (P < 0.01). Likewise, the periods of most short-pass neurons decreased as the modulation ranges increase (in up-sweeps: 42/54, 77.78%; in down-sweeps: 45/57, 80.70%), and the change was also significant (P < 0.01). (3) Some IC neurons showed direction selective to FM stimuli. When the modulation range is 15 kHz, 34 of the 81 IC neurons showed the directional selectivity, in which 18 neurons were selective to up-sweep and 16 neurons were selective to down-sweep. (4) The modulation rang can also effect the directional selectivity of the IC neurons to FM. The total ratio of the direction selective neurons experienced the progress of firstly ascend and then descend. When the modulation range was 15 kHz, there had the most direction selective neurons. With the up-selective neurons, they also experienced the progress of firstly ascend and then descend. But when there had the most up-selective neurons, the modulation range is 10 kHz. With the down-selective neurons, the case is much complex, they experience the progress of firstly ascend and then descend and then ascend. When the modulation ranges were 15 kHz and 25 kHz, there had the most direction selective neurons. The ratio of up-selective neurons to down-selective neurons was 3 to 1 when the modulation range is 2 kHz, when the modulation range increased, the ratio decreased to about 1.3 to 1.The results indicated that the IC neurons of kunming mouse can process FM stimuli efficiently. Most of the neurons had the maximum spikes when the modulation ranges were narrowest. This phenomenon could contribute to the inhibitory input from the inhibitory band near the excitatory frequency tuning curves and/or the inhibitory post-synapse potential caused by the CF sound stimulus. In additional, the modulation range can cause effect on the directional selectivity of the IC neurons to FM sounds and the effect on up- and down-selective neurons was different. The neural mechanism need to be further studied.
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