1/ We have established new methods to perform the large scale sampling of auditory system activity and process the large datasets. We have then acquired complete datasets for the main auditory networks in the mammalian brain: the auditory cortex, thalamus, colliculus, the superior olivary complex and the cochlear nucleus. In addition, we have obtained, through a collaboration with J. Bourien of the University of Montpellier, detailed simulations of the main input to the auditory system: the cochlea.
2/ These four datasets have allowed us to identify systematically some important transformations of auditory information across these stages of the auditory system that had not been systematically described before. First, we have observed that the temporal information in sounds is transformed into spatial information without loss of temporal resolution over the scale of few 100ms. This transformation had been hypothesized in specific models of auditory processing but never clearly demonstrated through a systematic comparison of the emerging properties of the auditory code across regions of the auditory system. This transformation goes together a complexification of sound offset responses which carry more and more information about recent acoustic cues. Another novel computation that we have discovered is the transformation of sound amplitude information into a spatial code with neurons that are specific both of sound identity and sound loudness. This transformation emerges early in the auditory system and is refined up to the cortex. We have also observed that neurons become more and more specific of feature combinations from the beginning to the end of the the auditory system. Responses to multicomponent sounds that are not anymore the sum of single components in the cortex, while the early representations are more linear.
3/ We compared these transformations were compared to transformations operated in deep neural networks used to classify sounds. We observed that the classification of multiple sound feature categories generates a structure in deep network representations that resemble the auditory system in many respects. This includes all the transformations we identified in the auditory system. This indicates that the auditory system transforms sound representations to perform multipurpose categorization.
4/ We have shown that the transformation observed across the auditory system can to a great part be explained by linear-non-linear operations across stages of the auditory system with a stronger non-linear step between the thalamus and the cortex. This allowed us to model step by step the transformations of the auditory system.
5/ We have established perturbation methods based on optogenetic tools, and have shown that, in the mouse, the region of the auditory system which are involved in perception depend on the difficulty of the perceptual task through which we measured it. We have also used these perturbation methods to demonstrate that targeted activation of the final stage of the auditory system can influence auditory perception directly.
6/ Following these studies have also established optogenetic tools to design spatio-temporal activity patterns in the auditory cortex. These tools have allowed us to establish causally the role of some of the transformation performed by the auditory system, in particular the emergence of spatial codes for temporal cues. We have observed that temporal information that exits the auditory system cannot be associated to discriminative behavioral decisions, which provides a fundamental reason why it must be transformed to spatial information. We are further developing these tools in two follow-up projects: one for the design of cortical implant for auditory rehabilitation (EIC project Hearlight 2021-25), one for high resolution stimulation within neural network.
7/ We have shown that the transformation of auditory information within the auditory system is highly sensitive to anesthesia, during which representations are strongly disrupted. In sleep, in the contrary transformations are preserved.
These results are published in scientific journals, preprint server or their publication is under preparation.
