The Study Of Selectivity On The Direction And Rate Of Frequency-Modulated Sweeps In Inferior Colliculus Of Aged Mice

Frequency-modulated(FM) sound is prevalent in the vocalization of most animals and human speech.The FM sweeps direction and modulation rate(MR) play an important role in characterizing different types of acoustic signals.Many studies in different auditory structures have shown that neurons have direction and rate selectivity. In this experiment,the aged KM mice were used to study the selectivity for the direction and rate of FM sweeps by changing the duration and modulated range.In free field stimulation conditions,we examined responses of the inferior collicular (IC) neurons to FM sounds in 16 aged mice(14 to 18 months) by using extracellular recording.92 acoustical sensitive IC neurons were recorded.The results showed as follows:(1) Best frequency(BF) and minimum threshold(MT) of aged mice IC neurons were higher than that of adult mice.Nearly half of IC neurons(46.0%,39/85) exhibited duration selectivity under pure tune stimulation condition.Among these neurons,33.0% were short-pass(28/85),11.8%(10/85) were band-pass,1.2%(1/85) was long-pass.A small fraction of neurons showed band-rejected features(4.7%,4/85).The remains were non-selectivity for duration(including 4 non-regular neurons).The proportion of all-pass neurons(44.6%38/85) was higher than that of adult mice.(2) At given duration,the number of direction selective neurons and downward selective neurons were gradually increased with widening the MR,We fount that the direction selectivity of IC neurons was affected by the sweep duration at certain MRs,but the proportion of downward selective neurons was always higher than that of upward selective neurons.These data suggested that aged mice preferred the downward FM sweeps.(3) Among duration selective neurons,the duration tuning characteristics was the dominant factor for rate selectivity.Under different MRs,the FM rate selectivity of 38.5%(15/39) duration selective neurons showed the same tuning characteristics.This percentage was lower than in the adult mice.On the other hand,the proportion of the real FM rate selective neurons was very small.(4) In the duration selective neurons,we observed that few neurons(12.8%,5/39) displayed different FM rate selectivity features under the stimulation condition of different MRs.For these non-duration selective neurons,52.2% (24/46) of them showed FM rate selectivity.The majority of them(83.3%,20/24) showed the rate selectivity only when the MR was close to or beyond the excitatory frequency band,suggesting that MR was an important factor in FM rate selectivity, especially in the non-duration selective neurons.Our results indicate that MR is one of the most important factors in shaping the FM direction selectivity.For duration tuning neurons,their FM rate selectivity is highly correlated with the duration tuning characteristics.It hints that the neural mechanism of rate selectivity of IC neurons may be the same as the mechanism underlying the duration selectivity.On the other hand,MR is also involved in determining the FM rate selectivity in some IC neurons,especially in the non-duration selective neurons.Furthermore,we noticed that there were some differences of response to FM sweeps between aged and adult mice IC neurons.These data suggest that the selectivity to FM sounds parameters in the IC neurons may be correlated to development and aging of auditory system.