Interpreting PET and fMRI measures of functional neural activity: The effects of synaptic inhibition on cortical activation in human imaging studies

Human brain imaging methods such as postiron emission tomography and functi onal magnetic resonance imaging have recently achieved widespread use in th e study of both normal cognitive processes and neurological disorders, Whil e many of these studies have begun to yield important insights into human b rain function, the relationship between these measurements and the underlyi ng neuronal activity is still not well understood. One open question is how neuronal inhibition is reflected in these imaging results. In this paper, we describe how large-scale modeling can be used to address this question. Specifically, we identify three factors that may play a role in how inhibit ion affects imaging results: (1) local connectivity; (2) context; and (3) t ype of inhibitory connection. Simulation results are presented that show ho w the interaction among these three factors can explain seemingly contradic tory experimental results. The modeling suggests that neuronal inhibition c an raise brain imaging measures if there is either low local excitatory rec urrence or if the region is not otherwise being driven by excitation. Conve rsely, with high recurrence or actively driven excitation, inhibition can l ower observed values. (C) 2001 Elsevier Science Inc.