ABSTRACT
C57BL/6J mice show a progressive high-frequency hearing loss beginning about 2 to 3 months of age (Chapters 13, 24, 28; Mikaelian, 1979). Because mammalian high-frequency hearing (i.e., above the10-kHz limit of other vertebrates) evolved under strong selective pressure for sound localization, such a hearing loss should affect not only the ability of mice to detect sound, but also to localize it (R. Heffner and Heffner, 1992a; H. Heffner and Heffner, 1998). Specifically, high frequencies are necessary for two of the three basic locus cues: the binaural intensity difference of a sound at the two ears and the monaural pinna cues that arise from the directionality of the pinnae (the third cue being the binaural time difference, primarily a low-frequency cue). The two high-frequency cues require sounds that are effectively shadowed by the head and/or pinnae because low frequencies bend around small obstacles with little attenuation. Just how high an animal must hear to be able to use these cues depends on the size of its head and pinnae — the smaller the animal, the higher it must hear, which is why mice can hear above 80 kHz (H. Heffner and Masterton, 1980; Markl and Ehret, 1973). Thus, the loss of high-frequency hearing should have a detrimental effect on the ability of mice to localize sound, especially because they may not be able to compensate by relying on binaural time cues because the maximum interaural delay their small heads generate is so small (about 60
µ
s). We performed a study to observe the effect of age-related, high-frequency hearing loss in
C57BL/6J mice by determining their sound-localization acuity at two different ages. An additional goal was to assess the ability of mice to localize brief sounds because previous research had only been able to demonstrate an ability to home in on sounds that were continuously repeated (Ehret and Dryer, 1984). Sound-localization thresholds and the ability to localize filtered noise bursts were determined for three C57BL/6J mice using an avoidance procedure involving suppression of drinking (for details, see H. Heffner and Heffner, 1995). The animals were then retested later, at which time their absolute thresholds for 16-and 32-kHz tones were also determined. The animals had free access to food in their home cages and received water during daily test sessions.
