Onset of cross-modal synthesis in the neonatal superior colliculus is gated by the development of cortical influences

Many neurons in the superior colliculus (SC) are able to integrate combinat ions of visual, auditory, and somatosensory stimuli, thereby markedly affec ting the vigor of their responses to external stimuli. However, this capaci ty for multisensory integration is not inborn. Rather, it appears comparati vely late in postnatal development and is not expressed until the SC passes through several distinct developmental stages. As shown here, the final st age in this sequence is one in which a region of association cortex establi shes functional control over the SC, thus enabling the multisensory integra tive capabilities of its target SC neurons. The first example of this corti cotectal input was seen at postnatal day 28. For any individual SC neuron, the onset of corticotectal influences appeared to be abrupt. Because this e vent occurred at very different times for different SC neurons, a period of 3-4 postnatal months was required before the adult-like condition was achi eved. The protracted postnatal period required for the maturation of these corticotectal influences corresponded closely with estimates of the peak pe riod of cortical plasticity, raising the possibility that the genesis of th ese corticotectal influences, and hence the onset of SC multisensory integr ation, occurs only after the cortex is capable of exerting experience-depen dent control over SC neurons.