**3. The cochlea**

Comparative studies (Webster & Plassmann, 1992) show that the low-frequency hearing in gerbils is associated with adaptations of the middle ear (e.g. large middle ear cavities that facilitate the transmission of low frequencies to the inner ear) and of the basilar membrane (e.g. increased width compared to other small rodents).

#### **3.1 Activity of single auditory nerve fibres**

Analysis of auditory nerve fibre activity provides information about sound processing in the cochlea. A reference species, in which auditory nerve fibre function has been studied in much detail, is the cat (Kiang, 1965). In a comparative analysis of gerbil auditory nerve fibre activity, Schmiedt (1989) demonstrated a good correspondence with data from the cat and suggested that this "implies the presence of fundamental mechanisms that are common to

human subjects. Gerbils have been suggested as a particularly suitable model for research on diverse aspects of ageing, including audition (Cheal, 1986). Here we will review the

The human audiogram (filled circles, thick continuous line; Zwicker & Fastl, 1990) shows the lowest thresholds at low frequencies. The gerbil audiogram (filled triangle, black continuous line; Ryan, 1976) is similar to the human audiogram, but the hearing range of gerbils extends to frequencies above 20 kHz. Compared to human and gerbil, thresholds of rat (open circles, thick dotted line; Kelly & Masterton, 1977) and mouse (open triangles, thin

Comparative studies (Webster & Plassmann, 1992) show that the low-frequency hearing in gerbils is associated with adaptations of the middle ear (e.g. large middle ear cavities that facilitate the transmission of low frequencies to the inner ear) and of the basilar membrane

Analysis of auditory nerve fibre activity provides information about sound processing in the cochlea. A reference species, in which auditory nerve fibre function has been studied in much detail, is the cat (Kiang, 1965). In a comparative analysis of gerbil auditory nerve fibre activity, Schmiedt (1989) demonstrated a good correspondence with data from the cat and suggested that this "implies the presence of fundamental mechanisms that are common to

dotted line; Radziwon et al., 2009) are much higher for frequencies below 4 kHz.

studies of age-dependent changes in the auditory system of gerbils.

Fig. 1. Audiograms from human, gerbil, rat and mouse

(e.g. increased width compared to other small rodents).

**3.1 Activity of single auditory nerve fibres** 

**3. The cochlea** 

mammalian auditory systems", making the gerbil a useful model for hearing loss in ageing studies.

In addition to normative data gathered using young gerbils, several studies have also analysed auditory nerve fibre activity in gerbils older than 1 year. In auditory nerve fibres of quiet aged three-year-old gerbils, Schmiedt et al. (1990) found that thresholds were elevated by 20-30 dB at the tip (characteristic frequency, CF) of the tuning curves, while the low frequency tails were much less affected, resulting in a reduced tip-to-tail ratio. Measures of frequency selectivity, like Q10dB and Q40dB (Hellstrom & Schmiedt, 1996), were similar for young and old gerbils in fibres with CFs below 4 kHz, while auditory nerve fibres with higher CFs were on average less sharply tuned in old gerbils. A comparison of rate-level functions (the discharge rate of auditory nerve fibres plotted as a function of stimulus level) at CF in old and young gerbils showed that these functions in old gerbils were shifted to higher levels, consistent with elevated thresholds of auditory nerve fibres. However, the slopes of functions in the dynamic range region between threshold and saturation of old gerbils were at least as steep as those from young gerbils. The distributions of spontaneous rates in large samples of auditory nerve fibres from young and old gerbils were similar for fibres with CFs below 6 kHz, while the proportion of low spontaneous rate fibres with CFs above 6 kHz was only 30% in old gerbils, compared to 60% in young gerbils (Schmiedt et al., 1996). Fibres with low spontaneous activity typically have higher thresholds and larger dynamic ranges compared to fibres with high spontaneous rates (Winter et al., 1990). Thus, a loss of the contribution of auditory nerve fibres with low spontaneous rate may affect processing of supra-threshold signals and contribute to a decreased ability to understand speech in noise.
