Development of multisensory integration: Transforming sensory input into motor output

By pooling and integrating signals from different sensory channels, special ized populations of "multisensory" neurons not only help to maximize the br ain's ability to detect and identify external events, but also help to init iate reactions to them. Although multisensory neurons are found in many are as of the brain, those in the midbrain (i.e., superior colliculus, SC) have been studied most extensively and have served as a model for understanding some of the neural operating principles of multisensory integration, as we ll as the impact of these processes on overt attentive and orientation beha viors. However, this capability is not hard-wired at birth. Very young SC n eurons are responsive only to unimodal inputs; it is not until many days la ter that some of them begin to respond to inputs from more than a single se nsory modality, and even then they are not yet capable of integrating these inputs to produce the synthesized multisensory signals that characterize t he normal adult. The mast significant occurrence to precede this maturation al change is the appearance of influences from association regions of the n eocortex. These influences appear abruptly on any given individual neuron, but because different neurons are targeted at different times during develo pment, it takes many weeks before the mature complement of such neurons is achieved. It is likely that the maturational timing of the interplay betwee n the cortex and SC determines not only the kinds of multisensory informati on that can be integrated in SC neurons, but the kinds of multisensory beha viors that SC neurons are able to mediate at different stages of developmen t. (C) 1999 Wiley-Liss, Inc.