Different Forward Masking Patterns Of Sustained Noise Burst And Segmental Noise Burst In The Inferior Collicular Neurons

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.In order to explore the response properties of the IC neurons in echolocating bats and mice, and the different forward masking patterns of sustained noise burst and segmental noise bursts in the IC, the present study was consist of three sections. In the first section, to study the acoustic response properties of the IC in the pipistrellus abramus, single unit extracellular recording with microelectrode was used in free field conditions. In the 65 recorded neurons, the results showed: Characteristic frequency (CF) , minimum threshold (MT) and response latency was between 18.9 and 76.7 kHz (42.94 ± 11.29), 29 and 80dB SPL (58.65 ± 12.62), 3.1 and 13.4 ms (6.10 ± 1.47), respectively; CFs increased with the recording depth, but CF and MT were not correlated; There were three different types of discharge patterns in the IC, including phasic pattern (73.9%), chopper (15.4%) and tonic (10.7%); The types of frequency tuning curves (FTC) were all V-shaped, most of which were wide type and few were narrow type. In addition, the high-frequency slope was often steeper than the low-frequency slope.Very few study was performed on the IC using frequency modulated (FM) stimuli though much of the response properties were studied using pure tone in the mouse (Mus musculus, Km). For this purpose, in the second section, we observed the response of IC neurons to FM especially rapid FM stimuli in the free-field conditions. According to the MT difference between the pure tone and FM stimuli, the 99 IC neurons recorded were classified into three types. Type Ⅰ neurons (57/59, 57.6%) were characterized by the lower FM stimuli MT then pure tone stimuli while type Ⅲneurons (30/99, 30.3%) with lower MT when presented with pure tone stimuli. In type Ⅱ neurons (12/99, 12.1%), the MT was equal when the kind of tone were presented. Through analyzing the different firing rate between up-sweep and down-sweep, we found that 36 IC neurons (36/99, 36.4%) were direction selective and among them 22 (22/99, 22.2%) were up-selective and 14 (14/99, 14.2%) were down-selective. Up-selective neurons had a wider distribution range than down-selective neurons in type ⅠⅡⅢneurons. We also found that CFs of both type Ⅰ and direction selective neurons had very densest distribution inthe range of 10~20 kHz (77.2% and 83.3% respectively). In addition, we observed the responses of 24 IC neurons to FMs stimuli of three different modulation rate and we found that most of the neurons (15/24, 62.5%) were much sensitive to rapid FMs stimuli. Further more, the proportion of the direction selective neurons had decrease decline when the modulation rate was increasing (45.8% vs 41.7% vs 33.3%). The results suggested that the IC of mouse could process FMs stimuli effectively and FMs with directions made significant roles in sound communication in the mouse.Although there has been a growing body of literature showing the neural correlation of forward masking caused by a pure tone masker in the auditory neurons, relative few studies have addressed the description of how the forward masking caused by a noise burst, especially a sequence of noise bursts, is transformed into neuronal representation in the central auditory system. In the third section, using a noise forward masking paradigm under free field stimuli conditions, the current in vivo study was devoted to exploring it on the inferior collicular (IC) neurons in the mouse. A total of 96 IC neurons were recorded. Rate-intensity functions (RIFs) with and without the presentation of masker, sustained noise burst (SNB) or segmental noise burst (SGNB), were measured in 51 neurons. We found that the relative masker intensities were distributed over a wide range between 21 dB below the minimum threshold (MT) and 19 dB above the MT of the corresponding probe tone. The masking effect of the SGNB on firing rate in nearly half a number of neurons (type I, 45.1%,) was stronger than that of the SNB (P < 0.001), whereas in a smaller fraction of neurons (type III, 17.7%,) it was weaker than the SNB (P < 0.001). There was no significant difference of masking effect between the SNB and SGNB in type II neurons (37.2%, P > 0.05). Irrespective of type I or type III neurons, ihe inhibitory effects of both kinds of maskers were all greater at lower probe intensities but decreased significantly with the increase of probe intensity (P < 0.001). Interestingly, as the probe intensity increased, the difference of masking effect between the SNB and SGNB was disappeared (P > 0.05). In addition, we observed that temporal masking pattern could be transformed when the masker was changed from the SNB to SGNB. The main type of this transformation was from early-inhibition to equivalent-inhibition pattern (53.9%, 7/13). Our data provide the evidence that the inhibitory effect of these two maskers has differential weights over time and intensity domains of the IC neurons responding to a pure tone. This suggests that the forward masking of noise is by no means the source of simply suppression in neuronal firing rate. There might be a few of active neural modulating ways in which the coding of temporal acoustical information can be operated.