Is the Hippocampus Activated above Baseline during WM Maintenance?
The main hypothesis of this study concerned the identity and number of spatial patterns of delay period brain activity associated with WM load. However, there is another, perhaps simpler, hypothesis we were able to incidentally test: whether hippocampus is engaged above baseline levels during the delay period. When the estimated fMRI signal intensities (relative to the inter-trial interval baseline) associated with the delay period from the voxels most strongly expressing the second component were examined, they tended to be close to zero averaged across WM loads (while following an inverted U-shape with respect to WM load) in both hemispheres. Similar results were obtained in hippocampal ROIs. This is consistent with the hippocampus not generally reported as being activated during WM processing (Courtney et al., 1996, 1997; Smith et al., 1996; Braver et al., 1997; Cohen et al., 1997; Rypma et al., 1999). However, Ranganath and D’Esposito (2001) reported delay period fMRI signal increa
