Regional Brain Activations Predicting Subsequent Memory Success: An Event-Related Fmri Study of the Influence of Encoding Tasks

Abstract

We determined the brain regions that were differentially sensitive to two, randomly inter-mixed tasks: Deep Encoding, in which subjects processed items according to their meaning (is the word pleasant or unpleasant?) and Shallow Encoding, in which items were processed according to two underlined letters in the word (are the letters in alphabetical order?). The former task was associated with activations in a set of brain regions including left lateral prefrontal cortex (PFC) and left medial temporal cortex. The latter showed relatively greater activation in right PFC. Both findings are consistent with predictions made on the basis of previous functional neuroimaging work. Following scanning, each subject underwent a recognition memory task. The results of these provided the basis for a further sub-division of encoding events, according to whether they were predictive of subsequent recognition success or not. Unsurprisingly, recognition performance was greater for words that had been deeply encoded. For both encoding conditions, words that were subsequently recognised were associated with greater activation in a sub-set of regions identified by the deep versus shallow contrast. These included left PFC and medial temporal regions. In left PFC this performance-predicting activation was significantly greater for the deep encoding condition. Our results support previous studies suggesting a role for left PFC and medial temporal cortex in episodic memory encoding. They provide more evidence, too, for a less consistent finding: the interaction between the encoding task and the success of subsequent recognition.