Understanding Speech Understanding: towards a Unified Theory of Speech Perception

Ever since Helmholtz, the perceptual basis of speech has been associated with the energy distribution across frequency. However, there is now accumulating evidence that speech understanding does not require a detailed spectral portraiture of the signal. As a consequence, a new theoretical perspective, focused on time, is beginning to emerge. This framework emphasizes the temporal evolution of coarse spectral patterns as the primary carrier of information within the speech signal, and provides an efficient and effective means of shielding linguistic information against the potentially hostile forces of the natural soundscape, such as reverberation and background acoustic interference. The auditory system may extract this relational information through computation of the low-frequency modulation spectrum in the auditory cortex, and this representation provides a principled basis for segmentation of the speech signal into syllabic units. Because of the systematic relationship between the syllable and higher-level lexicogrammatical organization it is possible, in principle, to gain direct access to the lexicon and grammar through such an auditory analysis of speech.