Verbal asymmetries and levels of processing in an interactive dual-task paradigm

This study examined how the dual-task laterality paradigm produces patterns of cerebral asymmetry different from those obtained in studies of comparable single-laterality tasks. We systematically examined at what level of processing a concurrent verbal memory-load influences accuracy of recognition of laterally presented nouns relative to that in a control condition. Four groups of 20 subjects each were tested in an interactive dual-task paradigm requiring them to compare lateralized nouns directly with one of three nouns concurrently held in memory prior to accuracy of recognition. The groups differed according to the types of comparisons being made: physical identity, phonetic orthographic similarity, phonetic similarity using non-orthographic words, and category membership. Subjects in each group carried out three tasks. They were required to determine if a specified match had occurred between a memory-load word and the lateralized word, identify the lateralized word, and report the memory-load words. Another 20 subjects were assigned to a control laterality condition in which no concurrent memory-load was used. Control subjects were required to respond to a laterally presented word by pressing a reaction-time key to indicate recognition of the word and then to report the word. No significant visual-field differences occurred in accuracy of recognition for either the control or matching-task groups. However, reaction-time latencies and errors increased as a function of the level of processing. The accuracy of recognition did not support the predictions of Kinsbourne's attentional model but the reaction times provided some support for the concept of selective left-hemisphere interference proposed by Hellige. A limited-capacity approach was suggested as a possible alternative explanation of laterality effects.