Changes in limbic and prefrontal functional interactions in a working memory task for faces

Regional cerebral blood flow, measured with positron emission tomography, was used to identify brain regions that play a special role(s) in a working memory task for faces. Perceptual matching (no retention interval), short-delay (average = 3.5 s retention interval), intermediate-delay (average = 12.5 s), and long-delay (21 s) tasks were considered. From the idea that brain function is the result of neural interactions, the data were analysed using anatomically based, covariance structural equation modeling. In perceptual matching, the dominant functional interactions were observed among the ventral cortical areas, from extrastriate regions, to the anterior temporal, and into the inferior prefrontal cortex. These interactions decreased with longer delay intervals. In the short-delay functional model, interactions along this ventral stream in the right hemisphere appeared to be rerouted through limbic areas with strong interactions among the hippocampal region, the anterior and posterior cingulate, and the inferior prefrontal cortices. For the intermediate-delay model, the hippocampocingulate interactions continued, but showed a shift to more left hemisphere involvement. In the long-delay network, interactions within the right limbic circuit were reduced in favor of strong bilateral inferior prefrontal and frontocingulate interactions. Effects from the prefrontal cortex, especially from the left hemisphere, to temporal and occipito-temporal cortices were particularly strong in the long-delay model, suggesting recruitment of some of the same circuits primarily involved in face perception. The strong corticolimbic interactions at short and intermediate delays may represent maintenance of an iconic representation of the face during the retention interval. However, at longer delays, where image was more difficult to maintain, a frontocingulate-occipital network was used that could represent an expanded encoding strategy resulting in a more resilient memory.