Brainstem Level Auditory Function in Specific Dyslexics and Normal Readers.
Grant, Dorothy Elizabeth Fraser
1980
Abstract
The purpose of this investigation was to compare the results of psychophysical and electrophysiological auditory tests of brainstem function in two groups of adolescent male subjects. The experimental group was comprised of 12 young men defined medically and /or educationally as having 'specific' dyslexia. The control group was comprised of 12 normal readers who were age-matched to the experimental subjects. All subjects demonstrated normal pure-tone sensitivity and normal speech discrimination on a routine audiometric assessment battery prior to participating in the experimental sessions. The tonal masking level difference (MLD) task was the psychophysical test employed. Responses to two antiphasic conditions (signal out of phase and noise out of phase) were assessed utilizing a Bekesy threshold tracking response technique. The electrophysiological measures obtained were scalp-recorded brainstem auditory-evoked responses (BAER), elicited by both click and tonal stimuli in both ears. Positive-peak and interpeak response latencies were measured for each of two presentation rates, 11/second and 31/second. Frequency-following responses (FFR) to brief tones were measured in terms of onset latency and period of the waveform. Statistical analysis for the signal out of phase condition of the MLD test showed that the mean score of the dyslexic group was significantly smaller than that of the normal reading group. For the noise out of phase condition, mean score difference approached, but did not reach, significance. The findings for the click-evoked BAER positive-peak and interpeak latencies did not differ significantly between groups for either ear or for either rate of stimulus presentation. The tone-evoked FFR, however, was found to have a significantly shorter onset latency for the normal readers. The FFR waveform, as measured by differences in period from that of the stimulus period, was also significantly different between groups. Additionally, visual inspection of the FFR suggested considerable variability from trial to trial among the experimental subjects. The implications drawn from the findings obtained were that brainstem assessment procedures seem to be viable avenues of exploration in specific dyslexia. It appears that the addition of psychophysical measures of phase processing to the audiological assessment of children and young adults with specific dyslexia can potentially add information to both diagnostic and remediation processes. Some of the electrophysiological measures of brainstem auditory-evoked activity used in this study do not appear to differentiate sufficiently nor consistently enough to allow for definitive interpretations and therefore must be used with considerable caution. Brainstem auditory-evoked responses to click stimuli were no different in dyslexics compared with normals in terms of positive-peak latencies. Other parameters are suggested which may be worthy of future investigation. The frequency-following response to tonal stimuli appeared to be quite different between specific dyslexics and normal readers even though such parameters as binaural interaction, decussation and hypothesized multiple generators of the response have yet to be determined in individuals with normal brainstem auditory processing. These findings in no way provide evidence that brainstem auditory disorders are etiological factors in specific dyslexia. However, they do show that the two conditions may exist together. Further investigation by the neuroaudiologist into the auditory brainstem function of specific dyslexics seems indicated.Types
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