Evidence of Lateralized Epicenters for the Computation of Speech Codes: Rhyming Words and Pseudowords.

Abstract:
The computation of speech codes (i.e. phonology) is an important aspect of word reading. Understanding the neural systems and mechanisms underlying phonological processes is crucial for the investigation of language functions and dysfunction in the brain. In this study, we used high resolution 3D positron emission tomography to investigate neural systems crucial for phonological processes. Six male and female normal volunteers performed visual word and pseudoword rhyming tasks. Word stimuli were presented either in lower case letters or in a mix of lower and upper case letters. The stimuli were kept relatively constant across experimental conditions and presented at a rate of one per second. Brain activation patterns for each rhyming task were compared to two baseline conditions using subtractive methods. Results showed that rhyming words and pseudowords activated the same regions in the left prefrontal cortex and occipital-temporal junction. However, pseudoword rhyming recruited more cortical tissues than word rhyming and also activated supramarginal gyrus. These results suggest that there are strong left lateralized epicenters for the phonological processing of words and pseudowords similar for both males and females. Results also suggest that rhyming pseudowords may require additional neural systems unnecessary for rhyming words. In addition, phonological processing of words did not appear to involve active participation of meaning processes.