| Objectives. The aims of this dissertation are: (1) To examine the neural correlates of automatic discrimination, as indexed by mismatch negativity (MMN), of phonetically-similar, short-duration vowels ([I]-[epsilon]) in typically-developing school-age children and adults (Experiments 1 and 2). (2) To examine the relationship between MMN and behavioral performance (Experiments 1 and 2). (3) To determine whether MMN is an index of acoustic or phonetic processing (Experiment 1). (4) To examine the effect of attention on brain discrimination (Experiment 2). (5) To determine whether complexity of the paradigm affects the brain's discriminative processes (Experiment 1 vs. 2).; Design. Vowels ([I]-[epsilon]) were presented using oddball paradigms in two experiments. In Experiment 1, two conditions were presented in which two deviants fell across- and one within the phonetic-category from the standard. In Experiment 2, a one deviant oddball paradigm was used. In one condition (Attend), the subjects actively discriminated a non-target tonal deviant embedded in the auditory stream. In a second condition (Passive), the subjects ignored the stimuli and watched a silent video. Identification and discrimination abilities were also examined in both experiments.; Results. In both experiments, essentially similar behavioral perceptual abilities were found for the children and adults, yet the neurophysiological indices of discrimination differed across groups. Significantly greater variability of the amplitude of the difference waveforms were found for the children compared to the adults in both experiments. In Experiment 2, attention to the auditory stream slightly modified the MMN in both the children and adults, although the effect differed across groups.; Conclusions. These findings support the claim that the brain's discriminative processes continue to mature during the school-age years and that under certain test conditions automatic discriminative mechanisms and attention-dependent behavioral processes are not related. Due to limited research, little is known about MMN maturation and its elicitation in individual subjects. Even less is known about the relationship between MMN and auditory processing or language development. Response variability across subjects and poor signal-to-noise ratios limit the clinical use of MMN at this time. Future studies will need to be conducted to address these issues before MMN can be considered clinically useful. |
