Tonotopic organization in the basal region of the ventromedial nucleus of the thalamus revealed by fiber photometry recording

Significance: Although the transmission of auditory information in the brain has been extensively studied, the mechanism underlying fine auditory discrimination remains incompletely understood. The basal region of the ventromedial nucleus of the thalamus (bVM) has recently been found to convey frequency-specific auditory information to the primary auditory cortex (A1). Inhibition of the bVM significantly impairs fine auditory discrimination in mice. These findings indicate that the bVM plays an important role in frequency information processing. However, direct functional evidence for tonotopic organization within the bVM is still lacking. Aim: functional mapping strategy. Approach: To characterize the response properties of bVM neurons projecting to A1, we combined Cre-dependent retrograde viral labeling with fiber photometry in awake mice. Using a "one recording site per animal" strategy, we systematically recorded from 26 distinct locations and successfully reconstructed the tonotopic map of the bVM. Results: We identified a mediolateral tonotopic gradient within the bVM, with best frequencies progressing from low in medial regions to high in lateral regions. Conclusions: Our findings provide direct functional evidence of tonotopic organization within the bVM, supporting its role as an auditory relay and its contribution to fine auditory discrimination.