Greater working memory load results in greater medial temporal activity at retrieval - PubMed (original) (raw)

Greater working memory load results in greater medial temporal activity at retrieval

Karin Schon et al. Cereb Cortex. 2009 Nov.

Abstract

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.

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Figures

Figure 1.

Figure 1.

Sternberg task. Subjects performed 72 LOAD2 trials (top) and 72 LOAD4 trials (bottom) in 8 runs. Each trial consisted of 4 phases—1) encoding: During encoding, 2 or 4 sample scenes were viewed sequentially for ∼1600 ms each. After each scene, the screen turned black for ∼400 ms. 2) Maintenance: Subjects were instructed to remember the scenes from the sample period across a variable-length delay period. 3) Retrieval: Subjects indicated via button press whether the scene matched one of those seen during the encoding period (50% of trials were Match trials). 4) ITI: Each trial ended with a variable-length fixation/ITI.

Figure 2.

Figure 2.

Load-dependent activations during encoding and retrieval (LOAD4 > LOAD2). SPMs are displayed on anatomical images derived from an average obtained from the normalized structural images of all subjects using a statistical threshold of _p_FDR <0.05 with SVC for ROIs outside PFC. Time courses display percent signal change with respect to the overall ROI mean and are separated into 3 plots: 1) Peri-sample onset time courses (left), 2) Postsample time courses (center), and 3) Peri-probe onset time courses (right) for Nonmatch trials. Peri-sample onset time courses illustrate activity during encoding with zero corresponding to the onset of the first sample scene during the encoding phase, postsample time courses illustrate activity during the maintenance phase with zero corresponding to the onset of the last sample scene during the encoding phase (i.e., scene #2 for LOAD2 trials and scene #4 for LOAD4 trials), and peri-probe onset time courses illustrate activity during retrieval with zero corresponding to the onset of the probe during the retrieval period. Blue lines indicate activity during LOAD4 trials, and red lines indicate activity during LOAD2 trials. (A) WM load effect during encoding in the right and left anterior VLPFC and the right DLPFC, (B) WM load effect during retrieval in the left anterior hippocampal formation, in the left and right retrosplenial area, and in the left amygdala. R, right; L, left.

Figure 3.

Figure 3.

Load-independent activations during encoding, maintenance (delay), and retrieval. SPMs are displayed on anatomical images derived from an average obtained from the normalized structural images of all subjects using a statistical threshold of _p_FDR <0.00005 with whole-brain correction for encoding >fixation/ITI and for retrieval >fixation/ITI and _p_FDR <0.05 with whole-brain correction for delay >fixation/ITI. Time courses display percent signal change with respect to the overall ROI mean and are separated into 3 plots: 1) Peri-sample onset time courses (left), 2) Postsample time courses (center), and 3) Peri-probe onset time courses (right). Peri-sample onset time courses illustrate activity during encoding with zero corresponding to the onset of the first sample scene during the encoding phase, postsample time courses illustrate activity during the maintenance phase with zero corresponding to the onset of the last sample scene during the encoding phase (i.e., scene #2 for LOAD2 trials and scene #4 for LOAD4 trials), and peri-probe onset time courses illustrate activity during retrieval with zero corresponding to the onset of the probe during the retrieval period. Blue lines indicate activity during LOAD4 trials, and red lines indicate activity during LOAD2 trials. (A) Activity during encoding versus fixation/ITI in the right hippocampal body/tail, the right fusiform gyrus, and the right VLPFC, (B) Activity during maintenance (delay) versus fixation/ITI in the right anterior DLPFC, (C) Activity during retrieval versus fixation/ITI in the right hippocampal body/tail, the right fusiform gyrus, the left VLPFC, and the right anterior DLPFC, (D) Activity during retrieval that is greater than activity during encoding (retrieval > encoding) in the right hippocampal body/tail and in the right anterior DLPFC. R, right; L, left.

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