The developmental emergence of a map of auditory space in the superior colliculus of the guinea pig.

We describe the developmental emergence of a topographically ordered map of auditory space in the deep layers of the guinea pig superior colliculus. The central auditory system of this precocial mammal is functional in utero. A map of auditory space in the superior colliculus is not, however, electrophysiologically demonstrable until 32 days after birth (DAB). The map develops in 4 postnatal phases. In very young animals (1-15 DAB) vigorous responses to auditory stimuli were recordable from collicular sites. Apart from a preference for stimuli orginating in the contralateral hemifield of auditory space, the responses in these young animals, however, showed little evidence of directional tuning. In animals aged between 16 and 30 DAB, an increasing proportion of responses displayed directional tuning but such responses were not arranged so as to constitute a topographic map. A relatively short third developmental phase (31-32 DAB) involved the ordering of individually tuned responses so as to form a topographic map of the contralateral auditory hemifield. As the animals further mature the individual responses display a higher degree of spatial tuning reflecting an ongoing process of refinement.