A PET STUDY OF COGNITIVE STRATEGIES IN NORMAL SUBJECTS DURING LANGUAGE TASKS - INFLUENCE OF PHONETIC AMBIGUITY AND SEQUENCE PROCESSING ON PHONEME MONITORING

We previously demonstrated using PET in normal subjects (Demonet et al ., Brain 1992; 115: 1753-68) that, by comparison to a reference task o f monitoring for pure tones, a phoneme monitoring task involving two f actors of complexity (sequence processing and perceptual ambiguity) ac tivated Wernicke's area and Broca's area. In the present experiment we explored the respective influence of these factors on brain activatio n. In addition to the reference task, four phoneme monitoring tasks in non-words were used with stimuli presented binaurally. These included an easy, nonsequential, unambiguous task; a perceptually ambiguous, b ut non-sequential task; a sequential, but perceptually unambiguous tas k; a sequential and ambiguous task identical to the one we used in our previous study. Changes in regional cerebral blood flow were measured in nine right-handed volunteers using PET and infusion of (H2O)-O-15. The sequential unambiguous task was associated with the same pattern of activation as that observed for the equivalent non-sequential task, i.e. activation of Wernicke's area. The two tasks with perceptual amb iguity gave rise to activations in the left fusiform gyrus and in Broc a's area, respectively. Activation of the left fusiform gyrus in the p erceptually ambiguous but non-sequential task suggests a prominent rol e for a strategy involving phoneme-to-grapheme transcoding in subjects , who thereby attempted to visualize non-words that they could not dec ipher auditorily. Activation of Broca's area in the sequential and amb iguous task reproduces our previous result and suggests that subjects resorted to a predominantly verbal rehearsal strategy when performing this task.