DEVELOPMENT OF MULTISENSORY NEURONS AND MULTISENSORY INTEGRATION IN CAT SUPERIOR COLLICULUS

The development of multisensory neurons and multisensory integration w as examined in the deep layers of the superior colliculus of kittens r anging in age from 3 to 135 d postnatal (dpn). Despite the high propor tion of multisensory neurons in adult animals, no such neurons were fo und during the first 10 d of postnatal life. Rather, all sensory-respo nsive neurons were unimodal. The first multisensory neurons (somatosen sory-auditory) were found at 12 dpn, and visually responsive multisens ory neurons were not found until 20 dpn. Early multisensory neurons re sponded weakly to sensory stimuli, had long latencies, large receptive fields, and poorly developed response selectivities. Most surprising, however, was their inability to integrate combinations of sensory cue s to produce significant response enhancement (or depression), a chara cteristic feature of the adult. Responses to combinations of sensory c ues differed little from responses to their modality-specific componen ts. At 28 dpn an abrupt physiological change was noted. Some multisens ory neurons now integrated combinations of cross-modality cues and exh ibited significant response enhancements when these cues were spatiall y coincident and response depressions when the cues were spatially dis parate. During the next 2 months the incidence of multisensory neurons , and the proportion of these neurons capable of adult-like multisenso ry integration, gradually increased. Once multisensory integration app eared in a given neuron, its properties changed little with developmen t. Even the youngest integrating neurons showed superadditive enhancem ents and spatial characteristics of multisensory integration that were indistinguishable from the adult. Nevertheless, neonatal and adult mu ltisensory neurons differed in the manner in which they integrated tem porally asynchronous stimuli, a distribution that may reflect the very different behavioral requirements at different ages. The possible mat urational role of corticotectal projections in the abrupt gating of mu ltisensory integration is discussed.