The effect of divided attention on encoding and retrieval in episodic memory revealed by positron emission tomography

The effects of divided attention (DA) on episodic memory encoding and retri eval were investigated in 12 normal young subjects by positron emission tom ography (PET). Cerebral blood flow was measured while subjects were concurr ently performing a memory task (encoding and retrieval of visually presente d word pairs) and an auditory tone-discrimination task. The PET data were a nalyzed using multivariate Partial Least Squares (PLS), and the results rev ealed three sets of neural correlates related to specific task contrasts. B rain activity, relatively greater under conditions of full attention (FA) t han DA, was identified in the occipital-temporal, medial, and ventral-front al areas, whereas areas showing relatively more activity under DA than FA w ere found in the cerebellum, temporo-parietal, left anterior-cingulate gyru s, and bilateral dorsolateral-prefrontal areas. Regions more active during encoding than during retrieval were located in the hippocampus, temporal an d the prefrontal cortex of the left hemisphere, and regions more active dur ing retrieval than during encoding included areas in the medial and right-p refrontal cortex, basal ganglia, thalamus, and cuneus. DA at encoding was a ssociated with specific decreases in rCBF in the left-prefrontal areas, whe reas DA at retrieval was associated with decreased rCBF in a relatively sma ll region in the right-prefrontal cortex. These different patterns of activ ity are related to the behavioral results, which showed a substantial decre ase in memory performance when the DA task was performed at encoding, but n o change in memory levels when the DA task was performed at retrieval.