Temporal Features of Speech in the Auditory System: Normal and Dyslexic Children and an Animal Model

An amazing characteristic of the speech signal is that it contains a variety of temporal features that occur simultaneously in the signal, and each of these features provides unique and essential information for speech perception. An equally astonishing fact is that, in most cases, the human auditory system is able to efficiently extract these temporal acoustic features from the speech signal as a precursor to higher-order linguistic, cognitive and mnemonic processes associated with speech reception. A clinical population that has shown abnormal processing of rapid temporal features in speech is reading-impaired individuals (RI), and it has been proposed that auditory-temporal impairments preclude normal development of phonological systems necessary for reading acquisition. The primary goals of this work are to describe central mechanisms responsible for encoding temporal features in speech in the unimpaired human auditory system, and to examine the extent to which these mechanisms may be impaired in RI. We have pursued these goals by investigating both brainstem and cortical representations of speech-sound stimuli in unimpaired (control) and RI children using auditory evoked-potentials. Results are the first to show right-hemisphere cortical asymmetry in the representation of the speech envelope, the slow temporal cue that provides syllable pattern information in speech. This result supports the hypothesis that a neural mechanism for temporal encoding in the human auditory system is the asymmetrical routing of this acoustic information between the cerebral hemispheres. We also provide the first neurophysiological evidence that reading-impaired (RI) individuals have impaired speech envelope representation, a finding that challenges an influential hypothesis stipulating that temporal impairments are specific to rapid features of speech. Additionally, we describe functional relationships in the ascending human auditory system: we show that temporal acuity in the human auditory brainstem predicts cerebral asymmetry for rapid acoustic processing but is not related to the slow temporal features of the speech envelope. We also examined near-field auditory responses in an animal model to explore mechanisms for auditory temporal processing in more localized neuronal populations than those afforded by the far-field potentials measured in humans. Results show that a non-primary auditory pathway may be specifically tuned to encode the slow temporal features in acoustic signals, and suggest that non-primary pathways may be important for processing the speech envelope in humans. Results from the animal model also show how ensembles of auditory cortical neurons can simultaneously represent the fundamental frequency and speech envelope in speech signal. Taken together, we have made new discoveries of how the unimpaired human auditory system encodes perceptually-important temporal features inherent to the speech signal, and the abnormal function of these mechanisms in RI. We have also proposed a role for non-primary auditory pathways in the coding of slow temporal information. These results are considered with respect to existing hypotheses addressing temporal information processing in the central nervous system, hierarchical models of speech perception and theories of dyslexia.