Hemispheric specialization in prefrontal cortex: effects of verbalizability, imageability and meaning

What determines the hemispheric laterality of information processing in the human prefrontal cortex (PFC)? Functional neuroimaging studies of declarative memory have explored two proposals: process-specificity and material-specificity. Early studies reported left-lateralized activation in prefrontal regions during memory encoding but right-lateralized activation during memory retrieval, for both verbal and nonverbal stimuli. Subsequent studies show that in addition to process-specific lateralization in some PFC regions, activation in other prefrontal regions varies according to the type of stimulus materials used. Verbal materials have been shown to preferentially activate regions of the left PFC and nonverbal materials homologous regions of the right PFC, during both encoding and retrieval. Findings are interpreted as consistent with the material-specificity hypothesis, which posits different lateralization of verbal and nonverbal memory processes [Hemispheric specialization and interaction, 1974]. Whereas initial fMRI and PET studies of material-specificity compared activation across material types (i.e. words vs. pictures) recent studies have begun to investigate hemispheric effects of stimulus manipulations within each material type. Based on these results and on findings in the behavioral and neuropsychological literature, it is hypothesized that activity in certain regions of left and right PFC may vary along two distinct, continuous dimensions: verbalizability and imageability. Recent experimental data will be evaluated according to the predictions of this proposal, and theoretical implications will be discussed. q 2003 Elsevier Science Ltd. All rights reserved.