| Deficient neural encoding, and subsequent perception, of the acoustic structure of speech has been described as contributing to language-based learning problems. These experiments used auditory evoked potentials to examine the effects of rapid presentation of speech signals, and the masking of these signals by noise, on brainstem and cortical processing of speech in normal children and children with language-based learning problems. Additionally, these effects were studied in a guinea pig model. The synchronous representation of repeated speech, presented in noise, was abnormally degraded in children with learning problems, as indicated by measures of cortical inter-response correlation. In spite of this deficit in response timing, the overall amplitude of the responses was no different between groups. Brainstem encoding of stimulus onset was abnormally prolonged, and representation of a critical periodic component of the signal, the first formant, was diminished, in the children with learning problems, while processing of another periodic component, the fundamental frequency, appeared normal. In spite of these abnormal features, some overall measures of response magnitude were similar across all children. Cortical responses from guinea pigs suggested that the abnormal processing of slower rates of stimulation demonstrated by children with learning problems could be modeled by normal processing of faster rates of stimulation, emphasizing the abnormal processing of rapid information that is proposed as contributing to learning problems. Responses from guinea pig inferior colliculus indicated that the first formant component of the response, on which children with learning problems demonstrated abnormal brainstem representation, was sensitive to rapid rates of stimulation, while the fundamental frequency component of the response, on which children with learning problems appeared normal, were less sensitive to rapid rates of stimulation. These findings also support deficits in the encoding of rapid acoustic information as contributing to language-based learning problems. A noise-induced enhancement of the amplitude of the fundamental frequency component of the response was demonstrated in the guinea pig inferior colliculus and thalamus, but this did not appear to relate to enhanced synchronization of stimulus encoding at subcortical or cortical levels. |
