Age-related decline in temporal sound processing: insights from envelope steepness map in the mouse auditory cortex.

Introduction: Spectral information of speech sounds is spatially arranged from low to high frequencies in the auditory cortex, known as tonotopic maps. Recently, we identified an envelope steepness map encoding the rise-ramp steepness of sounds, which is located orthogonally to the tonotopic map. Temporal components, particularly the sound envelope defined by the rise-ramp steepness, play a crucial role in speech perception. We hypothesized that the ability to sense temporal components of sounds, such as envelope steepness, would be diminished in aged mice. Methods: Responses to variations in rise-ramp time were investigated using transcranial macroscale calcium imaging of the auditory cortices of GCaMP6f-expressing mice. Normalized distance relative to the rise-ramp time of 0.01 ms was plotted for each rise-ramp time (0.1, 1, 10, and 100 ms) in the logarithmic scale graph: the steeper the slope of the fitted regression lines, the greater the distance between the peaks of the rise-ramp time of 0.01 ms and 100 ms. The slope of this regression line was compared between the different age groups: 1/3 months and 6/9/12 months after birth. Results: The slope of the fitted regression line for 5 kHz in the bilateral anterior auditory field and 20 kHz in the bilateral primary auditory field was significantly less steep in mice at 6/9/12 months after birth than at 1/3 months after birth. Conclusion: The shorter distance from the peak of 0.01 ms to 100 ms in animals 6/9/12 months after birth suggests difficulty in the separation of the rise-ramp time in aged animals. These findings may support the cortical mechanisms underlying age-related decline in speech perception.