The Superior Colliculus A Model For Understanding The Neural Bases Of Multisensory Integration

Perhaps the brain's best studied multisensory neurons are those in the superior colliculus (SC). This structure, which is located on the surface of the midbrain (Fig. 3) plays its primary role in attentive and orientation behaviors. It uses visual, auditory, and somatosensory information individually or in concert to direct the peripheral sensory organs toward an object or event of interest. Its best known role is in the initiation and control of eye and head movements (collectively referred to as gaze) in order to fixate a target; however, in species with highly mobile ears, it has also been shown to be involved in generating coordinated and directed ear movements. Recent evidence has also implicated the SC in the control of some limb movements. The importance of the SC in the current context is that it has been used as a general model for understanding the neural principles of multisensory integration and relating them to overt behavior.

The SC receives its inputs from many sources, both sensory and motor. Most of the sensory structures that project to the SC contain neurons responsive only to a single sensory modality—visual, auditory, or soma-tosensory. Thus, rather than reflecting multisensory information that is preprocessed elsewhere, the SC is a primary site of cross-modal convergence. This is underscored by the finding that although association cortex, one of the major sources of input to the SC, contains both modality-specific and multisensory neurons, only its modality-specific neurons project to the SC. Despite the counterintuitive nature of this finding (it would seem fitting to have multisensory neurons communicate directly with other multisensory neurons), the independence of this cortical circuit has been an experimental asset. It has allowed investigators to test whether the principles of multisensory integration found in the SC also apply to this independent multisensory circuit and, by extension, to other multisensory regions of the brain.

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