| Auditory is the important channels of human communication with the outside world. The frequency is one of the most significant characters of the acoustic signal.The acoustic signal of most animals include FM component(frequency modulation;FM). Therefore,it is important to study the FM sound.In this article,we compared the relationship between the middle ear and the inferior colliculus of bats and examine the responds of infancy mice to FM sound.The areas of tympanic membrane and oval window,the lengths of the malleus handle and incus long process of the 21 kinds of bats,including 13 kinds of FM bats and 8 kinds of CF-FM bats were measured.The relationships between the structure of the middle ear and the bat echolocation frequency were analyzed.The results showed that:(1) The lengths of the malleus handle were significantly positive correlated with the lengths of the incus long process(P＜0.001;r=0.978;n=17),and the positive correlation between the areas of the membrane and the areas of the oval window were also very strong(P＜0.001;r=0.906;n=10);(2) The transformer ratios of the middle ear were positive correlated with the area ratios of the tympanic membrane and the oval window (P＜0.001;r = 0.993;n = 8).The relationships between the lengths of the malleus handle and the lengths of the incus long process were not significant(P＞0.05;r=-0.29; n = 9);(3) The negative correlation was found between the dominant frequency and the area of tympanic membrane(P＜0.01;r=0.978;n=21),between the dominant frequency and the area ratio of the tympanic membrane and the oval window(P＜0.01;r =0.978;n = 21),and between the dominant frequency and the transformer ratios of the middle ear(P＜0.001;r = -0.895;n = 10),but there was not significant relationship between the dominant frequency and the length ratio of the malleus handle and the incus long process(P＞0.05;r=0.328;n=17).The second part of this article,in a free sound field conditions,we examined the response of the inferior colliculus neurons of 15 infancy mice(3 to 4 weeks) to FM sounds by extracellular recording.A total of 112 inferior colliculus neurons were recorded.The results showed that:(1) The proportion of band-pass duration selective neurons in the infancy mice was higher than that of the adult mice.Most of IC neurons (75%) displayed duration selectivity,including band-pass(36%),short-pass(32%), long-pass(3%).(2) The modulation range and the duration of FM sound affected the direction selectivity of IC neurons to FM sound.When the duration of FM sound was fixed at 32 ms,the proportion of direction selective neurons was gradually increased with the modulation range increasing(16%in 2 kHz modulation range to 52%in 32 kHz modulation range).When the modulation range was fixed at 32 kHz,the proportion of direction selective neurons was gradually increased with the duration increasing(21%in 4 ms duration to 50%in 32 ms duration)(3) The type of rate selectivity were determined by the duration tuning features,but it had no correlation with the modulation range(MR) of FM.Among different modulation rang,the selectivity for the modulation rate of many neurons were same(79%),including long-pass neurons(35%),band-pass neurons(30%,), short-pass(1%),all-pass(12%),but the selectivity for the modulation rate of some neurons changed(21%).Comparing the relationship of the structure of the middle ear and the dominant frequency of bats,we found that there was close relationship between the tympanic membrane,ossicles of the bats and the dominant frequency.In addition,there were a high percentage of neurons which selected to direction and modulation rate of FM sound. Therefore,we suggested that the middle ear and the inferior colliculus of the animals played an important role in analyzing the frequency of the sound.
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