Figure 6.

Distribution of characteristic frequencies and thresholds. A, CFs between 3 and 40 kHz were recorded in CBA/J mice (circles) and between 2.7 and 35 kHz in C57BL/6J mice (triangles). No units were recorded with CF >25 kHz at P12 and >32 kHz at P13. At all postnatal ages, the respective lowest threshold values were found for frequencies between 10 and 20 kHz (see dotted lines, visually delineated audiograms at P12, P13, and P14–P23), which is consistent with the behavioral audiogram of mice [black solid line; based on classically conditioned galvanic skin responses recorded from rear paw pads to shock to front paw pads in 50 mice (Berlin, 1963)]. At P12, minimal thresholds were at 40 dB SPL, and they subsequently lowered to the adult-like values of 0 dB SPL at P14. Already at P28 in C57BL/6J mice but not in CBAJ mice, the high-frequency thresholds showed signs of deterioration. Numbers of units are given as follows: n = #CBA/#C57BL. B, Spontaneous discharge rates at different CFs. In both strains (CBA/J: filled circles, C57BL/6J: open circles), spontaneous discharge rates did not systematically change with the units' CFs.