Neural representation of spectral and temporal information in speech

Abstract

Speech is the most interesting and one of the most complex sounds dealt with by the auditory system. The neural representation of speech needs to capture those features of the signal on which the brain depends in language communication. Here we describe the representation of speech in the auditory nerve and in a few sites in the central nervous system from the perspective of the neural coding of important aspects of the signal. The representation is tonotopic, meaning that the speech signal is decomposed by frequency and different frequency components are represented in different populations of neurons. Essential to the representation are the properties of frequency tuning and nonlinear suppression. Tuning creates the decomposition of the signal by frequency, and nonlinear suppression is essential for maintaining the representation across sound levels. The representation changes in central auditory neurons by becoming more robust against changes in stimulus intensity and more transient. However, it is probable that the form of the representation at the auditory cortex is fundamentally different from that at lower levels, in that stimulus features other than the distribution of energy across frequency are analysed.

Basic auditory processes involved in the analysis of speech sounds

“…The masking pattern should be parallel to the excitation pattern of the masker, but shifted vertically by a small amount. In practice, the situation is not so straightforward since the shape of the masking pattern is influenced by factors such as offfrequency listening, the detection of beats and combination tones Alcántara et al 2000) and by the physiological process of suppression (Delgutte 1990; see also Young 2008). Moore & Glasberg (1983b) have described a way of deriving the shapes of excitation patterns using the concept of the auditory filter.…”

“…These filters are called the 'auditory filters'. Fletcher thought that the basilar membrane within the cochlea (see Young 2008) provided the basis for the auditory filters. Each location on the basilar membrane responds to a limited range of frequencies, so each different point corresponds to a filter with a different centre frequency.…”

“…(b) Psychophysical tuning curves One method of measuring the shape of the auditory filter involves a procedure which is analogous in many ways to the determination of a neural tuning curve (see Young 2008), and the resulting function is called a psychophysical tuning curve (PTC). To determine a PTC, the signal is fixed in level, usually at a very low level, say, 10 dB above absolute threshold (called 10 dB sensation level, SL).…”

“…One way of interpreting the masking pattern is as a crude indicator of the excitation pattern of the masker (Zwicker & Fastl 1999). The excitation pattern is a representation of the effective amount of excitation produced by a stimulus as a function of characteristic frequency (CF) on the basilar membrane (see Young 2008) and is plotted as effective level (in decibels) against CF. In the case of a masking sound, the excitation pattern can be thought of as representing the relative amount of vibration produced by the masker at different places along the basilar membrane.…”

Introduction. The perception of speech: from sound to meaning

“…The paper by Young (2008) describes the representation of speech sounds in the auditory nerve and at higher levels in the central nervous system, focusing especially on vowel sounds. The experimental data are derived mainly from animal models (especially the cat), so some caution is needed in interpreting the results in terms of the human auditory system.…”