Unraveling Cross-Modal Development in Animals: Neural Substrate, Functional Coding and Behavioral Readout

The interaction of every living organism with its environment relies on sensory abilities. Hence, sensory systems need to develop rapidly and early in life to guarantee an individual's survival. Sensors have to emerge that are equipped with receptors that detect a variety of stimuli. These sensors have to be wired in basic interconnected networks that possess the ability to process the uni- as well as multisensory information encoded in the sensory input. Plastic changes to refine and optimize these circuits need to be effected quickly during periods of sensory experience so that uni- and multisensory systems can rapidly achieve the functional maturity needed to support the perceptual and behavioral functions reliant upon them. However, the requirement that sensory abilities mature quickly during periods of enhanced neuroplasticity is at odds with the complexity of sensory networks. Neuronal assemblies within sensory networks must be precisely wired so that processing and coding mechanisms can render relevant stimuli more salient and bind features together appropriately. Focusing on animal research, the first part of this review describes mechanisms of sensory processing that show a high degree of similarity within and between sensory systems and highlight the network complexity in relationship to the temporal and spatial precision that is needed for optimal coding and processing of sensory information. Given the resemblance of most adult intra- and intersensory coding mechanisms, it is likely that their developmental principles are similar. The second part of the review focuses on developmental aspects, summarizing the mechanisms underlying the emergence and refinement of precisely coordinated neuronal and multisensory functioning. For this purpose, we review animal research that elucidates the neural substrate of multisensory development applicable to, the less accessible, human development. Animal studies in this field have not only complemented human studies, but brought new ideas and numerous cutting edge conclusions leading to the discovery of common principles and mechanisms.