Selectivity for the rate of frequency-modulated sweeps in the mouse auditory cortex.

Frequency-modulated (FM) sweeps are common components of vocalizations, including human speech. Both sweep direction and rate influence discrimination of vocalizations. Across species, relatively less is known about FM rate selectivity compared with direction selectivity. In this study, FM rate selectivity was studied in the auditory cortex of anesthetized 1- to 3-mo-old C57bl/6 mice. Neurons were classified as fast pass, band pass, slow pass, or all pass depending on their selectivity for rates between 0.08 and 20 kHz/ms. Multiunit recordings were used to map FM rate selectivity at depths between 250 and 450 μm across both primary auditory cortex (A1) and the anterior auditory field (AAF). In terms of functional organization of rate selectivity, three patterns were found. First, in both A1 and AAF, neurons clustered according to rate selectivity. Second, most (∼60%) AAF neurons were either fast-pass or band-pass selective. Most A1 neurons (∼72%) were slow-pass selective. This distribution supports the hypothesis that AAF is specialized for faster temporal processing than A1. Single-unit recordings (n = 223) from A1 and AAF show that the mouse auditory cortex is best poised to detect and discriminate a narrow range of sweep rates between 0.5 and 3 kHz/ms. Third, based on recordings obtained at different depths, neurons in the infragranular layers were less rate selective than neurons in the granular layers, suggesting FM processing undergoes changes within the cortical column. On average, there was very little direction selectivity in the mouse auditory cortex. There was also no correlation between characteristic frequency and direction selectivity. The narrow range of rate selectivity in the mouse cortex indicates that FM rate processing is a useful physiological marker for studying contributions of genetic and environmental factors in auditory system development, aging, and disease.