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Clinical Trial
. May-Jun 2005;12(3):343-51.
doi: 10.1101/lm.90405. Epub 2005 May 16.

Dissociable Contributions Within the Medial Temporal Lobe to Encoding of Object-Location Associations

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Free PMC article
Clinical Trial

Dissociable Contributions Within the Medial Temporal Lobe to Encoding of Object-Location Associations

Tobias Sommer et al. Learn Mem. .
Free PMC article

Abstract

The crucial role of the medial temporal lobe (MTL) in episodic memory is well established. Although there is little doubt that its anatomical subregions-the hippocampus, peri-, entorhinal and parahippocampal cortex (PHC)-contribute differentially to mnemonic processes, their specific functions in episodic memory are under debate. Data from animal, human lesion, and neuroimaging studies suggest somewhat contradictory perspectives on this functional specialization: a general participation in declarative memory, an exclusive involvement in associative mnemonic processes, and a specific contribution to spatial memory are reported for the hippocampus, adjacent cortices, and the PHC. A functional lateralization in humans dependent on the verbalizability of the material is also discussed herein. To further elucidate the differential contributions of the various MTL subregions to encoding, we employed an object-location association memory paradigm. The memory for each of the studied associations was tested twice: by the object, and by the location serving as retrieval cue. The memory accuracy in response to both cue types was also assessed parametrically. Brain activity during encoding which leads to different degrees of subsequent memory accuracy under the two retrieval conditions was compared. We found the bilateral posterior PHC to participate in encoding of both the object associated with a location and the location associated with an object. In contrast, activity in an area in the left anterior PHC and the right anterior MTL was only correlated with the memory for the location associated with an object.

Figures

Figure 1.
Figure 1.
Description of the task. In the encoding phase (left panel), each trial consisted of an orienting cue indicating the position of the next object in an array of 16 black boxes, and then (after a jittered ISI) the picture of a common object appeared. Subjects were asked to make an artificial/natural-judgement. In the retrieval phase (right panel), each object-location association was tested twice in randomly intermixed order: (1) by the object and (2) by the location serving as retrieval cue. When retrieval was cued by the object, one of the studied objects appeared in the center of the screen, followed by the same array of black boxes. The task was to indicate, by selecting one or more locations depending on the accuracy of memory, which object was associated during the encoding phase. When retrieval was cued by the location, the 16 locations appeared on the screen, one with a white question mark, followed by the same 16 objects randomly rearranged in two rows in the middle of the screen. The task was to indicate, by selecting one or more objects depending on the accuracy of memory, which object was associated with the location during the encoding phase.
Figure 2.
Figure 2.
Frequency distribution of the five response categories. The relative proportions of the various response categories (Categories 1-4, number of selected locations/objects; Category 5, the indication of a forgotten location/object) are presented for both retrieval conditions: when retrieval was cued by the object (left) and by the location (right). The dark gray (object cue) and light gray (location cue) bars indicate the frequency of correct responses in a category, the white bars of incorrect responses. The error bars in both panels indicate the standard error of the mean.
Figure 3.
Figure 3.
Activation in the conjunction and interaction analyses in the posterior and anterior MTL. Statistical map: (yellow clusters) result of the conjunction analysis indicating areas where activity during encoding correlated with the subsequent memory performance in response to both cue types; (red clusters) result of the interaction analysis indicating where activity during encoding correlates only with the subsequent memory performance in response to the object but not to the location cue. For display purposes, the statistical maps are threshold at P < 0.005 uncorrected. Graphs: The blue x's reflect the mean activation across group at the peak voxel during encoding for the different response categories when retrieval was cued by the object, the green circles when retrieval was cued by the location. The error bars represent the 90% confidence interval. The blue (object cue) and green (location cue) lines indicate the fit of the applied contrasts of interests in that voxel. (Categories 1-4: number of selected locations/objects; Category 5: the indication of a forgotten location/object.)

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