Large-scale reorganization of the tonotopic map in mouse auditory midbrain revealed by MRI.

The cortex is thought to be the primary site of sensory plasticity, particularly during development. Here, we report that large-scale reorganization of the mouse auditory midbrain tonotopic map is induced by a specific sound-rearing environment consisting of paired low- (16 kHz) and high-frequency (40 kHz) tones. To determine the potential for plasticity in the mouse auditory midbrain, we used manganese-enhanced MRI to analyze the midbrain tonotopic maps of control mice during normal development and mice reared in the two-tone (16 + 40 kHz) environment. We found that the tonotopic map emerged during the third postnatal week in normal mice. Before 3 weeks, a larger percentage of auditory midbrain responded to each of the suprathreshold test frequencies, despite the fact that the primary afferent projections are in place even before hearing onset. By 3 weeks, the midbrain tonotopic map of control mice was established, and manganese-enhanced MRI showed a clear separation between the 16- and 40-kHz responses. Two-tone rearing dramatically altered the appearance of these discrete frequency-specific responses. A significant volume of the auditory midbrain became responsive to both rearing frequencies, resulting in a large-scale reorganization of the tonotopic map. These results indicate that developmental plasticity occurs on a much greater scale than previously appreciated in the mammalian auditory midbrain.