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.

Effective Auditory–Verbal Encoding Activates the Left Prefrontal and the Medial Temporal Lobes: A Generalization to Illiterate Subjects

“…One possible way of interpreting these data is that the suggested age-related rCBF changes observed with PET indicate an age-related encoding failure. In this PET study of upper middleaged/older illiterate women, effective episodic encoding correlated with the activity in the PFC and the MTL regions similar to the observations in young study populations (Alkire et al, 1998;Brewer et al, 1998;Fernandez et al, 1998;Wagner et al, 1998c). In our previous PET study of the literate group (Petersson et al, 1998a) we found no obvious differences in the activation pattern during either encoding or retrieval compared to the activation patterns described in younger populations (Buckner, 1996;Cabeza and Nyberg, 1997a;Fletcher et al, 1997).…”

“…Both these studies used well-educated college students. Similar results, using young subjects (mean age 25), in a metabolic FDG-PET study (Alkire et al, 1998) and a blocked FMRI study (Fernandez et al, 1998) also indicate that effective encoding is related to increased MTL activation. Since long-term memory processes are thought to be general purpose mechanisms, it is important that these results are replicated in other study populations.…”

“…Brewer, personal communication). Similar results, using young subjects (mean age 25) in a blocked FMRI study, also indicate that effective encoding is related to increased MTL activation (Fernandez et al, 1998). The within-subject design in these studies makes it important to replicate these results in other study populations.…”

“…In contrast, few FMRI studies have examined relational encoding, one exception being the study of Rombouts et al (1997), which observes anterior MTL activations. All previous studies investigating effective encoding (Alkire et al, 1998;Brewer et al, 1998;Fernandez et al, 1998;Wagner et al, 1998c) required item encoding with no explicit relational demand and these studies observed posterior MTL activation associated with subsequent memory. In this PET study we used an encoding task that demands interitem relational or associative processing and observed a significant correlation between anterior MTL activity bilaterally and the subsequent memory performance.…”

Functional Inferences Vary with the Method of Analysis in fMRI

“…With declarative memory paradigms that activate the medial temporal lobe limbic structures, interest has focused on the significance of anterior vs. posterior, right vs. left-sided, and hippocampal vs. parahippocampal activation patterns (Dolan and Fletcher, 1999;Fernandez et al, 1998;Grady et al, 1995;Haxby et al, 1996;Kelley et al, 1998;Rombouts et al, 1997;Stern et al, 1996;Wagner et al, 1998). Task specific differences in the anatomic distribution of fMRI activation, when averaged across subjects, have been cited as proof of functional specialization of different brain areas.…”