David Caplan,1 Gloria Waters,2 Louise Stanczak2 & Nat Alpert3
1 Harvard Medical School, 2 Boston University, 3 Massachusetts General Hospital
We present several studies investigating the functional neuroanatomy of syntactic processing in sentence comprehension. In all studies, PET activity associated with processing syntactically more complex subject object (SO) sentences (e.g., The juice that the child spilled stained the rug) and synonymous syntactically simpler object subject (OS) sentences (e.g., The child spilled the juice that stained the rug) was compared. Previous studies have shown activation in Broca's area in this task in young subjects.
In the first series of studies, we tested young (18-30 year old) and old (70-80 year old) subjects. Both young and elderly participants who were efficient at syntactic processing, as measured by plausibility judgment RTs, replicated the pattern of increased rCBF in Broca's area when making plausibility judgments about the more complex SO sentences compared to the simpler OS sentences. In contrast, both young and elderly participants who were less proficient at the task activated posterior structures in the left parietal lobe.
To investigate the role of working memory in determining these different rCBF patterns, we then carried out an additional study of young participants (n = 9 in each group) who were matched in terms of language processing efficiency, but who differed in terms of working memory capacity as measured by standard tests. High- and low-span participants both activated the same areas in the left and right inferior frontal cortex, as well as midline structures. We then regrouped these subjects to form two groups of subjects (n = 8 in each group) who were matched for working memory capacity but who differed in processing efficiency. High-proficient participants activated the inferior frontal cortex, and low-proficient participants activated posterior structures in the left superior temporal lobe.
The results demonstrate that there are individual differences in the rCBF effects of syntactic processing. These differences do not appear to be related to age or to working memory capacity, as measured by standard tests of working memory. They appear to be related to the proficiency with which subjects process syntactic structures. The determinants of individual differences in processing proficiency and of the rCBF effects seen in these two sets of subjects remain to be determined.