Functional differentiation of medial temporal and frontal regions involvedin processing novel and familiar words: an fMRI study

Results of recent functional magnetic resonance imaging (fMRI) studies of m emory are not entirely consistent with lesion studies, Furthermore, althoug h imaging probes have identified neural systems associated with processing novel visual episodic information, auditory verbal memory using a novel/fam iliar paradigm has not yet been examined, To address this gap, fMRI was use d to compare the haemodynamic response when listening to recently learned a nd novel words. Sixteen healthy adults (6 male, 10 female) learned a 10-ite m word list to 100% criterion, similar to 1 h before functional scanning. D uring echo-planar imaging, subjects passively listened to a string of words presented at 6-s intervals, Previously learned words were interspersed pse udo-randomly between novel words. Mean scans corresponding to each word typ e were analysed with a random-effects model using statistical parametric ma pping (SPM96), Familiar (learned) words activated the right prefrontal cort ex, posterior left parahippocampal gyrus, left medial parietal cortex and r ight superior temporal gyrus. Novel words activated the anterior left hippo campal region, The results for the familiar words were similar to those fou nd in other functional imaging studies of recognition and retrieval and imp licate the right dorsolateral prefrontal and left posterior medial temporal lobe (MTL) regions. The results for novel words require replication, but a re consistent with the substantial lesion and PET literature implicating th e anterior MTL as a critical site for processing novel episodic information , presumably to permit encoding. Together, these results provide evidence f or an anterior-posterior functional differentiation within the MTL in proce ssing novel and familiar verbal information. The differentiation of MTL fun ctions that was obtained is consistent with a large body of PET activation studies but is unique among fMRI studies, which to date have differed from results with PET. Further, the finding of left MTL lateralization is consis tent with lesion-based material-specific models of memory.