Tonotopic mapping of the mouse auditory midbrain with Mn-enhanced MRI

Our previous studies have established the utility of Mn-enhanced MRI (MEMRI) for detecting aggregate sound-evoked activity within the inferior colliculus (IC) of hearing and deaf mice. The present goal was to determine the spatial resolution of MEMRI by examining sound frequency-dependent patterns of activity in the mouse IC. Electrophysiological recordings have established the tonotopic map in mouse IC using threshold stimulation. However, there is little information on the spatial pattern of activity for suprathreshold stimulation. Following Mn injections, mice were exposed to one of the following acoustic stimuli in the free field at 65-89 dB SPL: wideband (1-59 kHz), high frequency band pass (20-50 kHz), or pure tones (16 or 40 kHz). Wideband stimulation elicited a diffuse pattern of IC enhancement covering most of the central nucleus, and this spatial pattern became more confined to the ventral-caudal IC with high frequency band pass stimulation. The spatial pattern resolved into a clear ventral-caudal contour with 40-kHz stimulation.In contrast, the 16 kHz stimulus produced a maximum enhancement midway along the dorsoventral axis. Thus, MEMRI revealed a dorsal-to-ventral pattern of enhancement corresponding to low-to-high frequency sound exposure, consistent with the published tonotopic organization of the mouse IC. Since stimulation was well about threshold, our MEMRI results suggest how louder acoustic events are representated topographically within the IC. These results will form the basis for future analyses of altered tonotopicity in mouse mutants with abnormal midbrain development