Transient activation during block transition

Functional MRI (fMRI) data analysis of blocked-task paradigms typically con siders brain activity present across a temporally extended task block relat ive to a reference block. An open question remains as to whether processes evolving with distinct temporal profiles are also present and can inform us about further functional-anatomic processes underlying task performance. T o explore this question, a meta-analysis of data from these separate studie s was performed. The meta-analysis specifically focused on detecting transi ent activation occurring at the onset and offset of task blocks. The compos ite data set from 39 subjects included four distinct task conditions (from various intentional encoding paradigms) that had equivalent block timing. T ask block activation included a network of regions consistent with prior an alyses of intentional encoding. Activation related to the block transitions included a set of transiently activated regions, consistent across all fou r separate task conditions. The most prominent activation was found in righ t frontal cortex along the dorsal extent of inferior frontal gyrus (near BA 6/44), Importantly, in one condition, this transient activation was presen t in the absence of a response across the task block suggesting dissociatio n between processes in support of ongoing task demands and those associated with transitions between blocks. Other prominent transient activations inc luded posterior superior temporal sulcus, medial occipitoparietal sulcus, a nterior insula, and anterior cingulate sulcus in the right hemisphere. Thes e findings are discussed in relation to models of set shifting and competit ive interactions between brain regions. (C) 2001 Academic Press.