Hearing without understanding

Unlike the visual system, where ganglion neurons leaving the retina project directly to the thalamus, auditory information leaving the cochlea via auditory neurons imbues the auditory pathways, which comprise a set of  relays; their functions are only very partially elucidated. We are just beginning to understand how the neuronal networks of the auditory cortex are organized, and how their activity is dynamically modulated by the sensory and behavioral context. For example, this involves understanding how a public's sensitivity to a sound is heightened when that sound has physiological relevance or can trigger a specific behavior. Another example is to understand how auditory perception modulates the emotional state of the public (particularly at stake in musical perception), or to interpret the emotional state of a speaker through the modulations of his voice. In language perception, the aim is to understand how the meaning of words is extracted from the sound cues detected by the ear, and how ambiguities due to the use of words with multiple meanings can be resolved by taking into account the sound context in a hierarchical processing. At a lower level of processing, how does the auditory system enable attention to be focused on particular sounds within a complex auditory scene comprising multiple interfering sources ? The typical situation in which these questions are often considered is that of the "  coktailparty ", already mentioned in previous lectures. Finally, there is the question of the mechanisms by which the integration and interpretation of meaningful sound cues leads to decision-making at the behavioral level, a decision likely to ensure survival in certain cases. From the detection of sounds by the cochlea to the triggering of a behavior dictated by the interpretation of sound cues and their integration into meaningful objects, it is the full anatomical and functional complexity of the auditory system that is most often brought into play, to produce a motor decision.

The aim of this series of lectures has therefore been to see how a meaningful auditory representation is constructed, from the periphery to the primary auditory cortical areas and their associated areas. Our understanding of these integration mechanisms, which form the core of the auditory system's functioning, is far from satisfactory today, even for the simplest integration mechanisms. Thus, even at the peripheral level, by far the best-known part of the auditory system, questions remain about many fundamental aspects of the mechanical response of the auditory sensory epithelium. That's why, in the first lecture, we discussed recent work highlighting the importance of the reticular lamina (formed by the abutting apical surfaces of auditory sensory cells and their supporting cells) in detecting the envelope of the sound waveform. The second lecture focused on one of the most fundamental mechanisms of multisensory integration, bimodal audio-visual integration, illuminated by recent studies integrating experimental data and statistical modeling. The third lecture focused on the efferent system ; its role in fine control of cochlear sensitivity has been extensively studied. On the other hand, the massive downward innervation it projects to virtually all the relays and nuclei of the auditory pathways suggests that it controls auditory perception at all levels of its elaboration. This is an emerging field of research, thanks to the modulation of neural network activity made possible by optogenetic techniques, the first results of which lay the foundations for reflex control by corticofugal pathways projecting onto the inferior colliculus. Finally, the fourth lecture was devoted to the disorders that arise when sound integration mechanisms are lacking, and in particular to the peripheral and central causes of language disorders.