Planum temporale asymmetry and ear advantage in dichotic listening

The planum temporale is a brain structure involved in the language process, for it serves as the foundation for the auditory association cortex. Past research has consistently demonstrated that 60 to 70 percent of the population has leftward asymmetry of the planum temporale. Past research has also suggested that dyslexic individuals tend to have either rightward asymmetry or symmetrical plana. Moreover, many studies have found a positive relationship between the presence of dyslexia and/or language impairment and deficits in the normal right ear advantage found in dichotic listening paradigms. Therefore, this study examined the relationship between planum temporale asymmetry and ear preference in dichotic listening performance. Results revealed no significant difference between ADHD and dyslexic subjects in regard to ear advantage on the free recall dichotic listening task. Further, the free recall dichotic listening task failed to predict ear advantage on the basis of group identification, contrary to expectation. Ancillary findings revealed numerous correlations among brain measurements, dichotic listening variables, intelligence, and visual-spatial variables. In addition, although the directed dichotic listening tasks were not related to degree of planum asymmetry, as predicted, results indicated that subjects who consistently displayed an atypical left-ear advantage tended to have larger right bank areas than those who consistently displayed a typical right-ear advantage. These findings may support the research of Galaburda and colleagues which suggests that some individuals with dyslexia or language deficits tend to have more neurons in the non-dominant plana because of reduced neuronal loss.