The spectral centroid, or center of mass of the spectrum, has been shown to play a substantial role in the perception of timbre (McAdams, 2019); however, the neural code for the spectral centroid is not fully understood. Here we used computational models to test spectral-centroid encoding by auditory neurons. Amplitudes of slow fluctuations (slow temporal changes in firing rate) in auditory-nerve (AN) responses vary systematically across neurons tuned to frequencies near the spectral centroid. These changes in fluctuation amplitudes are reflected in the average discharge rates of midbrain neurons that are sensitive to fluctuations (Carney, 2018). We show that the spectral-centroid discrimination thresholds from Allen and Oxenham (2014) can be estimated based on population responses of model midbrain neurons (Carney & McDonough, 2019). Additionally, model midbrain representations of the spectral centroid are influenced by changes in the fundamental frequency (F0) of the stimulus, suggesting a sub-cortical basis for the interaction between pitch and the perception of timbre (Krumhansl & Iverson, 1992; Marozeau & de Cheveigné, 2007, Allen & Oxenham, 2014).