SIGNALS FROM THE SUPERFICIAL LAYERS OF THE SUPERIOR COLLICULUS ENABLETHE DEVELOPMENT OF THE AUDITORY SPACE MAP IN THE DEEPER LAYERS

We have examined whether the superficial layers of the superior collic ulus (SC) provide the source of visual signals that guide the developm ent of the auditory space map in the deeper layers. Anatomical tracing experiments with fluorescent microspheres revealed that a retinotopic map is present in the newborn ferret SC. Aspiration of the caudal reg ion of the superficial layers of the right SC on postnatal day 0 did n ot cause a reorganization of this projection. Consequently, recordings made when the animals were mature showed that visual units in the rem aining superficial layers in rostral SC had receptive fields that span ned a restricted region of anterior space. Auditory units recorded ben eath the remaining superficial layers were tuned to corresponding ante rior locations. Both the superficial layer visual map and the deeper l ayer auditory map were normal in the left, unoperated SC. The majority of auditory units recorded throughout the deeper layers ventral to th e superficial layer lesion were also tuned to single sound directions. In this region of the SC, however we observed much greater scatter in the distribution of preferred sound directions and a significant incr ease in the proportion of units with spatia[ly ambiguous responses. Th e auditory representation was degraded, although many of these units w ere also visually responsive. Equivalent lesions of the superficial la yers made in adult ferrets did not alter the topographic order in the auditory representation, suggesting that visual activity in these laye rs may be involved in aligning the different sensory maps in the devel oping SC.