Distinct integration of spectrally complex sounds in mouse primary auditory cortices

The standard hierarchical signal transmission along the lemniscal auditory pathway in mice changes in the cortex, where two tonotopically organized auditory regions receive thalamic inputs in parallel: the primary auditory cortex (A1) and the anterior auditory field (AAF). These fields show distinct properties of sound-evoked responses, where A1 responds robustly to sound onset and AAF exhibits faster and more transient responses following both sound onset and offset. Previous reports showed a strong involvement of AAF in temporal processing, revealing its particular role in encoding temporal information. A more regular tonotopy, narrower frequency response areas, and more robust direction selectivity to frequency modulated sweeps led to the speculation that A1 codes better the spectral composition of a sound than AAF. However, potential mechanisms explaining why A1 favors spectral processing have not been previously described. Using in vivo electrophysiological recordings in the mouse auditory cortex, we found that A1 neurons, unlike AAF neurons, respond stronger and faster to spectrally complex tones than to pure tones. Next, we show that both regular and putative fast-spiking neurons in A1, but not in AAF, display larger responses to spectrally complex tones than to pure tones. Finally, we use a laminar analysis to demonstrate that A1 neurons in layer 2/3 respond stronger to spectrally complex tones than neurons in layer 4, indicating an important transformation of the neural representation of spectral complexity between thalamo-recipient and supragranular layers in A1, but not in AAF. Our study reveals circuit features contributing to distinct processing of spectrally simple and complex sounds in the two primary auditory cortices and supports a dual-stream processing in the core auditory cortex.