THE AUDITORY FEATURES OF VOWEL AND FRICATIVE PHONEMES

The acoustic features of vowels and fricatives are examined in the response patterns of a model of auditory processing. For the vowels, a. few harmonics dominate the peaks of the internal representation, reflecting the formant structure by their spatial locations, and the front cavity tic elements of speech sounds and their relevance to perception and articuation. In recent years, important discoveries in peripheral auditory function (both at the basilar membrane/hair cell level [1-3], and from auditory—nerve recordings [4], have facilitated the construction of cochlear models that can adequately replicate the primary response features in the auditory nerve [5]. With such models, it is relatively easy to analyze the response patterns associated with a. wide variety ofspeech sounds, and under many signal conditions. Specifically, it is now possible to generate internal auditory representations of the acoustic spectra, and hence to. examine closely the expression of such acoustic features as vowel formant locations, amplitudes, and tran— sitions, and fricative spectral shapes. It is important to note, however,‘that beyond the peripheral auditory stages, little is known about the central neural networks and the processing they perform on the cochlear outputs. This adds may be irrelevant for phonemic perception and classification if the central nervous system ignores, or is incapable of processing them. We shall address this point further after first illustrating the response patterns to the stimulus /position/ (Fig.1), as generated by a cochlear model. The peripheral auditory model consists of a linear formulation of basilar membrane mechanics, a fluid-cilia coupling stage which transforms membrane vibrations into hair National Institutes of Health, Bethesda, cell cilia displacements, and a simplified description of the inner hair cell nonlinear transduction of cilia displacements into intracellular electrical potentials. The potentials at each hair cell along the cochlear partition is then taken as a. measure of the probability of firing of the nerve fiber innervating it. Many more details of cochlear function can be incorporated in such models, e.g. adaptation at the hair cell/nerve synapse [6]. active mechanisms of basilar membrane motion [7], the effects of the middle ear muscles and of the efferent system [8]. This simplified model reproduces the major response properties observed experimentally, especially with relatively steady and broad-band stimuli like vowels and fricatives. The outputs of the cochlear model are computed at 128 equally spaced locations along the cochlear partition, and are all displayed together as a ‘2-dimensional spatiotemporal pattern representing the ensamble activity of the tonotopically organized array of auditory nerve fibers [9]. The spatial axis is labeled by the characteristic frequency (CF) of each output channel, i.e. the frequency of the tone which produces its maximum final output at that location (see below for further details of the central processing of the cochlear outputs). The responses to the vowel portions of the stimulus (/I/,/u/) posses a typical structure that is observed in all experimental data [10,11] that is the dominance of the ' entire pattern by a few'stimulus harmonics. These harL ' monics correspond to the largest components located near' the formants of the stimulus spectrum. ’ They excite travelling waves along the basilar membrane which are evident in the fine temporal structure and spatial Spread of the responses. Because of the unique asymmetrical shape of the cochlear filters, the waves decay in amplitude, and begin to as the harmonics become spatially less segregated (less resolved) and begin to interfere (e.g. the responses at the CF’s of the higher harmonics). For each of the vowel responses, the identity of the underlying dominant harmonics can be deduced from him sources: (1) The temporal course of the response (e.g. by measuring the frequency of the synchronized response), or (2) by the location of the above Se 22.1. 1