Successful verbal encoding into episodic memory engages the posterior hippocampus: a parametrically analyzed functional magnetic resonance imaging study

Abstract

The medial temporal lobe (MTL) is essential for episodic memory encoding, as evidenced by memory deficits in patients with MTL damage. However, previous functional neuroimaging studies have either failed to show MTL activation during encoding or they did not differentiate between two MTL related processes: novelty assessment and episodic memory encoding. Furthermore, there is evidence that the MTL can be subdivided into subcomponents serving different memory processes, but the extent of this functional subdivision remains unknown. The aim of the present functional magnetic resonance imaging (fMRI) study was to investigate the role of the MTL in episodic encoding and to determine whether this function might be restricted to anatomical subdivisions of the MTL. Thirteen healthy volunteers performed a word list learning paradigm with free recall after distraction. Functional images acquired during encoding were analyzed separately for each participant by a voxel-wise correlation (Kendall's tau) between the time series of the T2*-signal intensity and the number of subsequently recalled words encoded during each particular scan. Of the 13 participants, 11 showed voxel clusters with statistically significant, positive correlations in the posterior part of the hippocampus. Across participants, an ANOVA on the number of voxels with significant, positive correlations within individually defined volumes of interest confirmed a statistically significant difference in activation for anterior versus posterior regions of the hippocampus. However, no differences between left and right hippocampal activation were revealed. Thus, these findings demonstrate that successful encoding into episodic memory engages neural circuits in the posterior part of the hippocampus.

Figures

Fig. 1.

A, Depiction of one experimental block. Encoding task with the sequential presentation of 15 words per list (duration of each presentation, 500 msec; interstimulus interval, 2500 msec). Each set of MRI scans was acquired during the presentation of five words (15 sec).Distraction task: a 15 sec same/different decision task (duration of each presentation, 150 msec; interstimulus interval, 850 msec). Free recall task for 45 sec. This sequence was performed for a total of 20 blocks in each participant.B, One example of changing hemodynamic response pattern by shifting the assignment between word list and each particular scan by zero, one, or two words, respectively, for 0, 3, or 6 sec. Brain regions showing a significant, positive correlation between their fMRI signals and the number of successfully encoded words. Colored voxels exceeded the statistical threshold (<0.05) and were overlaid on structural images. The left side of each scan is the right side of the brain.

Fig. 2.

Brain regions showing a significant, positive correlation between their fMRI signals and the number of successfully encoded words. Colored voxels exceeded the statistical threshold (<0.05) and were overlaid on structural images. In thetop row the red bar depicts the approximate location of structural and functional images in the column below. The bottom three rows depict adjacent slices from three subjects (A–C), with one row corresponding to one individual. The left side of each scan is the right side of the brain. The arrows indicate voxels with significant, positive correlations in the posterior part of the hippocampus.