Aging reduces excitatory bandwidth, alters spectral tuning curve diversity, and reduces sideband inhibition in L2/3 of primary auditory cortex

Presbycusis, or age-related hearing loss, is caused by changes in both the peripheral and the central auditory system. Many of the peripheral structures that degrade with age have been identified and characterized, but there is still a dearth of information pertaining to what changes occur in the aging central auditory pathway that are independent of peripheral degradation. The primary auditory cortex (A1) of aging mice shows reduced suppressive responses and reduced diversity of temporal responses suggesting alteration of inhibitory processing. To gain a better understanding of how tuning features of the auditory cortex change with age, we performed in vivo 2-photon Ca2+ imaging on L2/3 of the auditory cortex of both adult (n=14, 11-24 weeks old) and aging (n=12, 12-17 months old) mice that retain peripheral hearing in old age. To reveal inhibitory inputs to L2/3 neurons we characterized spectral receptive fields with pure tones and two tone complexes. We find that in contrast to adult mice, L2/3 excitatory neurons from aging mice showed fewer distinct categories of spectral receptive fields, though in a subset of FRA types, we found increased diversity. We also noted a decrease in excitatory bandwidth with age among broadly tuned neurons, but that sideband inhibition became weaker across all FRA types due to a reduced amplitude in inhibitory responses. These results suggest that aging causes changes in circuit organization leading to more homogenous spectrotemporal receptive fields and that the lack of response diversity contributes to a decreased encoding capacity observed in aging A1.