On the locus of the bilateral lexicality priming effect

Abstract

A long tradition in neuropsychological research has involved the delineation of relative specializations of the left and right cerebral hemispheres. Recently, emphasis in this field has changed to the nature of the interactions between hemispheres in cognitive processing. One example is the study of bilateral lexicality priming, which has been touted as the result of interhemispheric processing in word recognition (Iacaboni & Zaidel, 1996). In this task, two letter strings are presented bilaterally, and participants are required to determine if the cued stimulus is a word or a nonword. Lexicality priming is observed when the lexical status (word/nonword) of the unattended stimulus influences the response to the cued stimulus. Under the assumption that the left visual field stimulus is processed in the right hemisphere and the right visual field stimulus is processed in the left hemisphere (direct access) lexicality priming reflects an interhemispheric interaction. However, it is possible that, even under conditions of bilateral presentation, the left visual field stimulus is relayed to the left hemisphere for processing (callosal relay), and thus lexicality priming is not an interhemispheric process but one that occurs entirely within the left hemisphere.

The three experiments presented in this thesis were designed to determine whether lexicality priming could be observed in the presence of independent evidence for direct access, allowing for the interpretation of lexicality priming as an interhemispheric effect. Following the logic of Zaidel (1983) direct access can be inferred from an interaction between visual field and some stimulus variable, or from an interaction between visual field and response hand. In Experiment 1, participants performed a bilateral lexical decision task in which they were exogenously cued to respond to one stimulus. Stimulus imageability was manipulated in order to examine the visual field by imageability interaction. While a robust lexicality priming was observed, the necessary visual field by imageability interaction was not observed, suggesting that lexicality priming occurred under a callosal relay pattern of processing. In Experiment 2, a response-hand manipulation was used to test direct access. Lexicality priming and a visual field by response hand interaction were both observed, suggesting lexicality priming under direct access. However, both these effects interacted with sex, such that only women demonstrated the visual field by response hand interaction, and only men demonstrated lexicality priming. Therefore lexicality priming was observed only in the presence of evidence for a callosal relay pattern. Experiment 3 also used a response hand manipulation with slightly different stimulus parameters. All subjects displayed lexicality priming, but no evidence for direct access. The findings from all three experiments are consistent with the hypothesis that lexicality priming exists only under a callosal relay pattern of processing, and is an intrahemispheric rather than interhemispheric effect.