The brain's response to incidental intruded words during focal text processing

The functional neuroanatomy associated with processing single words incidentally, outside focal attention, was investigated. We asked subjects (n = 15) to listen, focus on, and comprehend a story narrative, and then single, unrelated but meaningful words were intruded into the ongoing narrative. We also manipulated the type of intruded word, using either neutral or emotionally valent words, to evaluate the extent of semantic processing and a potential encoding advantage for one type of material. Analyses emphasized the areas of activation unique to the intruded words as distinguished from the narrative text. Subjects were normal, healthy adults (n = 15). Compared to narrative text, the intruded words were associated with activation in the right middle temporal gyrus (BA 39) and posterior cingulate/precuneus regions (BA 30, 23). We conclude that the intruded words did make contact with word-level lexical but not necessarily semantic structures in the middle temporal region. The data suggested that the intruded words were processed by a "nonexecutive" monitoring system implemented by a pairing of activation in posterior, medial structures such as the posterior cingulate with deactivation in brain stem structures. This pattern induced a shift to more passive, less effortful, nonstrategic monitoring of the words. Thus, attention processing, not semantic processing, changes best characterized the brain activation unique to the intruded words. This posterior, medial region is discussed as a substrate dedicated to processing a second, incidental stream of information and thereby providing a crucial mechanism for implementing dual processing of the kind examined here.