Material-specific lateralization in the medial temporal lobe and prefrontal cortex during memory encoding

Numerous observations in patients with unilateral lesions of the medial tem poral lobe (MTL) and the prefrontal cortex indicate that memory processes a re lateralized according to content. Left-sided lesions interfere with verb al memory processes, whereas right-sided lesions interfere with visuospatia l (non-verbal) memory processes. However, functional imaging studies have r esulted in contradictory data, some studies showing lateralization in the p refrontal cortex determined by stage of processing (encoding versus retriev al) and others suggesting that lateralization is dependent on the type of m aterial. Few studies have examined this issue in the MTL. In order to test the hypothesis that the lateralization of encoding processes in the MTL and frontal regions is dependent on the verbalizability of the material, we pe rformed behavioural and functional imaging studies. We demonstrated differi ng verbalizabilities of three classes of non-verbal stimuli (scenes > faces > abstract patterns) using a dual-task verbal interference behavioural par adigm. A functional neuroimaging study of encoding was carried out using th ese three types of stimuli, plus words. During whole-brain functional MRI a t 1.5 T, eight normal right-handed adults were presented with alternating b locks of novel and repeated stimuli under intentional memory encoding condi tions. Verbal encoding resulted in left-lateralized activation of the infer ior prefrontal cortex and the MTL. Pattern encoding activated the right inf erior prefrontal cortex and the right MTL. Scenes and faces resulted in app roximately symmetrical activation in both regions. The data indicate that t he lateralization of encoding processes is determined by the verbalizabilit y of stimuli.