Neural Representation of Frequency Resolution in the Mouse Auditory Midbrain

Abstract

The theory of frequency analysis of complex sounds in the auditory system is based on the concept of a bank of internal bandpass filters with continuously variable center frequencies. These filters, called critical bands, determine the spectral resolution power of the ear, i.e. the ability to detect and perceive peaks in a sound spectrum separately. Originally, critical bands have been determined in experiments on the perception of tones in noise (Fletcher 1940); therefore they are psychophysical measures. Critical bandwidths (CBs) have been studied in psychophysical tests in humans (e.g. Zwicker and Feldtkeller 1967; Scharf 1970; Moore 1982) and behavioral tests in animals (e.g. Pickles 1975; Ehret 1976; Nienhuys and Clark 1979; Dooling 1980). In these tests, two fundamental properties of critical bands have been established: 1) species-specific dependence of the CBs on their center frequency and 2) intensity independence of the CBs through a wide range of sound intensities. The starting point of the frequency resolution of the whole auditory system is the frequency-place transformation in the cochlea. For mammals with non-specialized cochleae, CBs cover about equal distances along the basilar membrane (Greenwood 1961, 1990). Hence, the sizes of the CBs of a given species are strongly related to the species cochlear tonotopy. The neural coding of CBs has its first correlate in the frequency tuning of the cochlear nerve fibers without realizing, however, the intensity independence of the CBs (Ehret 1995). How are CBs coded in higher auditory brain centers? Only for the cat, experimental data on CBs are available for the auditory nerve (Pickles and Comis 1976), the central nucleus of the inferior colliculus (Ehret and Merzenich 1985; 1988) and the primary auditory cortex (Ehret and Schreiner 1997). Compared with behavioral data (Pickles 1975; Nienhuys and Clark 1979) obtained in tests with comparable methods of sound presentation (narrow-band masking), the two main perceptual properties of CBs mentioned above are found in neurons at the midbrain and cortical levels. The midbrain