Interaural Level Difference Dependent Gain Control Underlying Binaural Computation Of Mouse Inferior Colliculus Neurons

We live in a rich and colorful world of sound. Sound is an important medium which can help people and animals to understand changes in the external world,communicating with one another. Auditory, as a special sensory system of perception sound provides the basis for sound communication. In the natural environment, the auditory system of human and animals can apperceive the meaningful acoustic information from the complex acoustic environment and accurately judge sound source location, which closely related to the complex structure and pathways of auditory system, and, in this process,the auditory system of the brain on binaural information integration plays an important role.Most people with normal hearing have this experience, when you listen to a fixed voice with two ears, you will feel very clear, and when you cloak one ear, you listen to the voice with another ear, you will find that the strength and clarity of the voice decreased and its position seems to have been changed. The reason for this change is that the binaural information integration mechanism of auditory system plays an important role. The auditory system in the brain, inferior colliculus(IC), as a member of the auditory pathways, as a critical center of which can recieve the binaural information gathering projection of the auditory ascending pathways and intergrate binaural acoustic information.Meanwhile, the IC plays an important role in sound source location.The IC is an important nucleus in the auditory pathways, not only receiving ascending projections from the lower brainstem but also accepting the descending inputs from the higher auditory center. These projections form the complex neural networks of the IC. Many previous studies indicate that the projections can be divided into inhibitory input and excitatory input. The ascending input to the IC mainly orginate from the cochlear nucleus(CN), the superior olive complex(SOC), and the nuclei of lateral lemniscus(NLL), which formed monaural and binaural input pathways. A number of studies in animals have shown that the anteroventral cochlear(AVCN), the posteroventral cochlear nucleus(PVCN), and the dorsal cochlear nucleus(DCN), all project to the comtralateral IC, which provide a direct monaural input for the IC. The ascending binaural input to the IC mainly orginate from the binaural integration nucleus, such as the medial superior olive(MSO), the lateral superior olive(LSO),and the dorsal nucleus of lateral lemniscus(DNLL). The descending projection to the IC orginate from the auditory cortex(AC), the medial geniculate body(MGB), and the superior colliculus(SC). The IC neurons receive directly or indirectly, monaural or binaural, ipsilateral or contralateral, inhibitory or excitatory projections of many auditory neclei and provide the anatomical and physiological basis for binaural information integration of the IC. In the IC binaural neurons, some neurons receive the direct input of the lower binaural nucleus, the input source is relatively simple; and binaural charateristics of some neurons is formed in the IC, the input source is more complex. At present the research of binaural neurons in many, reaction types and input source of binaural neurons have made greater progress. The central nucleus of the inferior colliculus (ICC) is a critical center for binaural processing. In addition to intracollicilar synaptic input,ICC neurons receive ascending input from nearly all auditory brainstem nuclei. Through the integration of contralaterally and ipsilaterally evoked input, ICC neurons can perform distinct functional tasks simultaneously:the processing of sound attributes per se, such as frequency and intensity, and the processing of binaural sound localization cues such as interaural time and level differences. Despite many previous studies, the arithmetic nature of binaural integration,or in another word,the transfer function between monaural and binaural responses, remains not well defined. Most binaural response studies have focused on neural tuning for the spatial location of sound sources,or have varied the acoustic parameters that contribute most to sound localization. In this study, we intend to address the quantitative nature of the monaural-to-binaural response transformation by examining the complete auditory receptive field under purely contralateral, purely ipsilateral, and binaural stimulation conditions.Most ICC neurons are driven strongly by contralateral sound sources, due to the major excitatory projections from the contralateral cochlear nucleus (CN) and the contralateral lateral superior olive(LSO). Other excitatory projections include those from the medial superior olive(MSO), the intermediate nucleus of the lateral lemniscus(INLL), and Ventral nucleus of lateral lemniscus(VNLL) on the same side. Ipsilaterally presented sound can suppress, have no effect on, or in some cases enhance the binaural response relative to the response driven contralaterally alone. These interaural interactions can potentially be described with three arithmetic functions when the generation of binaural frequency tuning is considered:1) a summation or subtraction between contralateral and ipsilateral response;2) a thresholding effect on the contralateral response, with the ipsilateral input serving to increase or decrease the effective spike threshold;3) a multiplicative or divisive normalization (gain modulation) of the contralateral response. These three types of response transformation will have different impacts on auditory processing. Both the summation/subtraction and thresholding effects would change the spectral processing by altering the sharpness of frequency tuning, whereas the gain modulation effect preserves the frequency tuning regardless of changes in response amplitude. In addition, from the transfer function between contralateral and binaural response, we can clearly define the role of ipsilateral input in binaural processing. To determine the transfer function underlying the binaural processing of spectral information, we compared the frequency-intensity tonal receptive fields of spike response driven monaurally and binaurally. We found in both anesthetized and awake mouse that binaural responses resulted from a scaling of contralateral responses, with ipsilateral input serving as a gain contral. In addtion, we provided evidence that the gain value was modulation by interaural level difference (ILD). Interaural level difference actually reflects the space of sound source direction signals. Thus, it can potentially be employed to represent sound source location.