CHANGES INDUCED IN THE REPRESENTATION OF AUDITORY SPACE IN THE SUPERIOR COLLICULUS BY REARING FERRETS WITH BINOCULAR EYELID SUTURE

There have been conflicting reports concerning the importance of visua l experience in the development of auditory localization mechanisms. W e have examined the representation of auditory space in the superior c olliculus of adult ferrets that were visually deprived by binocular ey elid suture from postnatal days 25-28, prior to natural eye opening, u ntil the time of recording. This procedure attenuated the transmission of light by a factor of at least 20-25 and blurred the image so that, as long as the eyelids were still fused, the responses of visual unit s in the superficial layers of the superior colliculus were labile and very poorly tuned. After the eyelids were opened, the representation of the visual field in these layers appeared to be normal. Acousticall y responsive units were, as usual, almost exclusively restricted to th e deeper layers of the superior colliculus. However, unlike normal ani mals, where responses occurring only at stimulus onset predominate, mo st of these units exhibited sustained or multi-peaked discharge patter ns. The degree of spatial tuning of individual units recorded from the normal and deprived groups of animals was not significantly different in either azimuth or elevation. Normally orientated maps of both soun d azimuth and elevation were also found in the visually deprived ferre ts. However, abnormalities were present in the topography and precisio n of these representations and consequently in their alignment with th e overlying visual map. In particular, an increase was observed in the proportion of auditory units with spatially ambiguous receptive field s, in which the maximum response occurred at two distinct locations. T hese results indicate that patterned visual experience is not required for establishing at least a crude map of auditory space in the superi or colliculus, but suggest that it may play a role in refining this re presentation during development.