Abstracts

Rationale: We have previously reported fMRI activations in the temporal lobes during encoding and retrieving of novel verbal, familiar verbal, novel visuoperceptual (VP) or familiar VP material.¹ Novel refers to completely new material — nonsense words and original abstract designs — whereas familiar refers to common abstract nouns and drawings of real objects (considered VP because recognition foils were altered versions of the targets). Based on our findings with TLE patients, we predicted greater activation in the right temporal lobe to novel and VP material, and in the left to familiar and verbal material. We did not find effects of novelty, but there were many differences in side and site of activations within the medial temporal lobes as a function of the verbal / VP dimension. Here we report the roles played by the frontal lobes during encoding and recognition of our different types of stimulus.Methods: Sixteen healthy subjects were scanned while learning and later recognizing abstract designs (novel, VP), drawings of real objects (familiar, VP), nonsense words (novel, verbal), and real words (familiar, verbal). Subjects were scanned during two encoding trials, two recognition trials, and a delayed recognition trial. For the current analyses, data were collapsed across stimulus type and recognition trial.Results: Verbal vs. visuoperceptual. A clear hemispheric difference was observed during encoding of verbal vs. VP material. Whereas VP elicited more activation than verbal material in the right inferior frontal gyrus and middle frontal gyrus (MFG), there were no hemispheric differences in a verbal > VP contrast. There were complementary frontal-lobe differences during recognition: the left MFG was more involved in recognizing verbal material, and the right MFG was more involved in recognizing VP material. Novel vs. familiar. During encoding, there was greater activation to novel than familiar material in the right anterior superior MFG, whereas bilateral frontal activation (left superior frontal gyrus, right MFG) was seen to familiar material. During recognition, the right cingulate was more active for novel material, and familiar material elicited bilateral frontal-lobe activation. Conclusions: Consistent with our initial predictions, we did find a hemispheric difference dependent upon the novelty vs. familiarity of material to be learned, but it was in the frontal lobes. The right frontal lobe is more implicated when unique, original material is encountered, and this remained true during retrieval of that same material. The two frontal lobes were differentially activated also as a function of verbal compared to visuoperceptual material, with greater material specificity seen for VP material and the right hemisphere than for verbal material and the left hemisphere. These results extend to the frontal lobes our previous findings of complex functional asymmetries between the hemispheres in their response to the verbal, visuoperceptual, novel and familiar nature of information. ¹Kennepohl et al., NeuroImage (2007), in press (Funding: Canadian Institutes of Health Research)