Audition differently activates the visual system in neonatally enucleated mice compared with anophthalmic mutants.

The occipital cortex, normally visual, can be activated by auditory or somatosensory tasks in the blind. This cross-modal compensation appears after early or late onset of blindness with differences in activation between early and late blind. This could support the hypothesis of a reorganization of sensory pathways in the early blind that does not occur in later onset blindness. Using immunohistochemistry of the c-Fos protein following a white noise stimulus and injections of the anterograde tracer dextran-biotin in the inferior colliculus, we studied how the occurrence of blindness influences cross-modal compensation in the mutant anophthalmic mouse strain and in C57BL/6 mice enucleated at birth. We observed, in mutant mice, immunolabeled nuclei in the visual thalamus - the dorsal lateral geniculate nucleus - in the primary visual area (V1) and a few labeled nuclei in the secondary visual area (V2). In enucleated mice, we observed auditory activity mainly in V2 but also sparsely in V1. No labeled cells could be found in the visual thalamus. Tracing studies confirmed the difference between anophthalmic and birth-enucleated mice: whereas the first group showed inferior colliculus projections entering both the dorsal lateral geniculate and the latero-posterior nuclei, in the second, auditory fibers were found only within the latero-posterior thalamic nucleus. None was found in controls with intact eyes. We suggest that the prenatal period of spontaneous retinal activity shapes the differences of the sensory reorganization in mice.