Orthogonal spectral and temporal envelope representation in auditory cortex

Speech sounds are acoustically composed of two main components: spectral and temporal. The spectral components or carrier frequency is important to detect pitch and perceived through activity at specific locations on the tonotopic map in the auditory cortex. The temporal envelope—the dynamic changes in sound wave amplitude over time—plays an essential role in speech perception by mediating phoneme recognition. However, how the steepness of the envelope, which shapes the temporal structure, is represented in the auditory cortex remains poorly understood. This study investigates how the steepness of the envelope of sound waves is represented as a functional organization in the mouse auditory cortex, in relation to spectral tonotopic organization. Using macroscale calcium imaging in GCaMP6f-expressing mice, we mapped the auditory cortex’s responses to tones with systematically varying rise-ramp steepness and frequencies. Our findings reveal that, in two primary-like regions of the auditory cortex, the steepness of the rise ramp is orderly mapped orthogonally to the tonotopic gradient, forming a two-dimensional representation on the cortical surface. Conversely, a significant representation of rise-ramp steepness was not detected in higher-order-like auditory regions. These findings suggest that a dichotomous functional organization independently processes temporal envelopes and spectral features in the mammalian auditory cortex, with cortical functional structures mirroring the two acoustic structures.