Dynamic modulations on intensity sensitivity evoked by weak noise in the inferior collicular neurons

In order to explore the possible mechanisms by which ethologically relevant sounds can be extracted from complex auditory environments, this study examined the effects of weak noise on the rate-intensity functions (RIFs) of neurons responding to tone burst in the inferior colliculus (IC) of nine mice (Mus musculus Km). Under free field stimuli conditions, a total of 112 IC neurons were recorded. RIFs with and without simultaneous presentation of weak noise, of which the intensity was relative to 5 dB below minimum threshold of tone burst, were measured in 44 IC neurons. By means of evaluating the changes of dynamic range (DR), slope of RIFs, and percent inhibition at different tone burst intensities evoked by the weak noise, three types of variations in RIFs were observed, i. e., inhibition (39/44, 88.6%), facilitation (2/44, 4.6%), and no effectiveness (3/44, 6.8%). Statistical analysis indicated that only inhibitory effect of weak noise was significant (P< 0.001, n = 39). The inhibitory effect of weak noise was greater at lower stimulus intensity of tone burst but decreased significantly with increased stimulus intensity (P< 0.0001, n = 39). In addition, the DR and slope of RIFs became narrower and steeper with weak noise presentation, respectively (P< 0.01, n = 31). The results from the present study suggest that weak noise exerts a dynamic modulatory action on acoustical intensity sensitivity of IC neurons, which possibly leads to a better understanding of neural mechanisms underlying the extraction of sound signals from natural auditory scenes.