Cortex rules: The neural mechanisms differentiating multisensory integration and unisensory integration in the midbrain

Multisensory neurons in the superior colliculus (SC) integrate cues from different senses and the responses to multisensory stimuli often equal or exceed the summed influences of the modality‐specific stimulus components. This capacity is unique to multisensory information as we have shown that such rarely occurs for pairs of within‐modal stimuli (e.g., two visual stimuli)(Alvarado et al., 2007). That the same neuron integrates excitatory inputs differently depending on their source is fortuitous from a functional perspective but the architecture underlying this dual capacity is unclear. A likely candidate is a projection to the SC from the anterior ectosylvian cortex (AES)(Wallace and Stein, 1994; Jiang et al., 2001; Alvarado et al., 2007). We examined the degree to which the AES‐derived cortico‐collicular projection is multisensory‐specific by evaluating the impact of cortical inactivation on both unisensory and multisensory integration in the same multisensory neurons. Cortical inactivation nearly abolished multisensory enhancement but had no impact on unisensory integration. These findings suggest that this cortico‐collicular circuit has evolved for the purpose of combining information across multiple senses and highlights an essential distinction between brain mechanisms for integrating stimuli within and across senses. This research was supported by NIH grants EY016716 and NS36916.