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Cerebral Cortex Advance Access published online on February 18, 2009
Cerebral Cortex, doi:10.1093/cercor/bhp006
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© The Author 2009. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Greater Working Memory Load Results in Greater Medial Temporal Activity at Retrieval

Karin Schon1,2, Yakeel T. Quiroz1,2, Michael E. Hasselmo1,2 and Chantal E. Stern1,2,3

1 Department of Psychology and Center for Memory and Brain, 2 Center of Excellence for Learning in Education, Science, and Technology, Boston University, Boston, MA 02215, USA, 3 Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA 02129, USA

Address correspondence to Karin Schon, Center for Memory and Brain, Boston University, 2 Cummington Street, Suite 109, Boston, MA 02215, USA. Email: kschon{at}bu.edu.

Most functional magnetic resonance imaging (fMRI) studies examining working memory (WM) load have focused on the prefrontal cortex (PFC) and have demonstrated increased prefrontal activity with increased load. Here we examined WM load effects in the medial temporal lobe (MTL) using an fMRI Sternberg task with novel complex visual scenes. Trials consisted of 3 sequential events: 1) sample presentation (encoding), 2) delay period (maintenance), and 3) probe period (retrieval). During sample encoding, subjects saw either 2 or 4 pictures consecutively. During retrieval, subjects indicated whether the probe picture matched one of the sample pictures. Results revealed that activity in the left anterior hippocampal formation, bilateral retrosplenial area, and left amygdala was greater at retrieval for trials with larger memory load, whereas activity in the PFC was greater at encoding for trials with larger memory load. There was no load effect during the delay. When encoding, maintenance, and retrieval periods were compared with fixation, activity was present in the hippocampal body/tail and fusiform gyrus bilaterally during encoding and retrieval, but not maintenance. Bilateral dorsolateral prefrontal activity was present during maintenance, but not during encoding or retrieval. The results support models of WM predicting that activity in the MTL should be modulated by WM load.

Key Words: delayed match to sample • episodic memory • hippocampus • neuroimaging • subiculum


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