Orthogonal spectral and temporal envelope representation in auditory cortex

Speech perception relies on two fundamental acoustic components: spectral and temporal. While spectral information is known to be represented in the auditory cortex through tonotopy, how temporal features are organized has remained unclear. Here, by varying rise-ramp steepness and frequencies, we reveal that the steepness of the temporal envelope--a critical cue for phonemes discrimination and sound source perception--is systematically mapped in the mouse auditory cortex. Using widefield calcium imaging, we discovered that the envelope steepness is represented orthogonally to the tonotopic axis, forming a two-dimensional cortical map that mirrors the dual structure of sounds. This organization was observed in primary-like auditory regions but not in higher-order-like areas, indicating distinct auditory processing streams. These findings uncover a principle of cortical organization, suggesting that the auditory cortex encodes sound along two independent axes and thereby provides a neural basis for parallel processing for complex sounds such as speech and natural acoustic environments.