High-resolution 7T fMRI of Human Hippocampal Subfields during Associative Learning

Abstract

Examining the function of individual human hippocampal subfields remains challenging because of their small sizes and convoluted structures. Previous human fMRI studies at 3 T have successfully detected differences in activation between hippocampal cornu ammonis (CA) field CA1, combined CA2, CA3, and dentate gyrus (DG) region (CA23DG), and the subiculum during associative memory tasks. In this study, we investigated hippocampal subfield activity in healthy participants using an associative memory paradigm during high-resolution fMRI scanning at 7 T. We were able to localize fMRI activity to anterior CA2 and CA3 during learning and to the posterior CA2 field, the CA1, and the posterior subiculum during retrieval of novel associations. These results provide insight into more specific human hippocampal subfield functions underlying learning and memory and a unique opportunity for future investigations of hippocampal subfield function in healthy individuals as well as those suffering from neurodegenerative diseases.

Fig. 1. Behavioral Task

(A) Subjects learned (encoding) and recalled (retrieval) unrelated-pairs of words (example shown is schnell-Haus [fast-house]), objects, and face-names. During the encoding block, subjects saw twelve pairs of unrelated items and were instructed to learn the pairs. During the baseline (ctl) block, subjects were instructed to fixate on a symbol (“+” or “o”) and press a button when the symbol changed. During the retrieval block, subjects saw the first item of the pairs and were asked to remember the second paired item. (B) The task consisted of six blocks of alternating encoding (Enc) and retrieval (Ret) separated by the baseline control (Ctl) task.

Fig. 2. Orientation of Image Acquisition

Shown is the area where high-resolution structural and zoomed EPI images are acquired in the coronal oblique plane perpendicular to the long axis of the hippocampus.

Fig. 3. Generation of Flat Maps

Shown is for one example subject. (A) The gray matter (green) is flattened into a (C) 2D map (right side shown) showing regions CA4DG (CA4, dentate gyrus), CA2, CA3, CA1, subiculum (sub), entorhinal cortex (ERC), perirhinal cortex (PRC), parahippocampal cortex (PHC), and fusiform gyrus. (B) Boundaries between regions are demarcated and projected into 2D space after gray matter is unfolded into a flat map. Shown are the boundaries between dentate gyrus and CA3 (red), CA3 and CA2 (orange), CA2 and CA1 (yellow), CA1 and subiculum (light blue), subiculum and PHC (green), PHC and fusiform (pink). For boundaries between anterior regions (ERC and PRC) see figure 4.

Fig. 4. Unfolding method

Each subjects’ gray matter (green, C, F) is created by segmenting the lateral and dorsal white matter and CSF areas (blue) and medial non-MTL gray matter and cerebral spinal fluid (orange). The gray matter is grown in layers in between (B, E) from blue to orange voxels. (A, D) Boundaries between regions are demarcated on each slice for each individual subject in 3D space. (A) Shown are the anterior MTL boundaries between dentate gyrus and CA3 (red), CA3 and CA2 (orange), CA2 and CA1 (yellow), CA1 and subiculum (light blue), subiculum and ERC (green), ERC and PRC (dark blue), PRC and fusiform (pink). (D) Additional posterior boundaries are shown including the boundary between subiculum and PHC (green) and between PHC and fusiform (white).

Fig. 5. Anatomical Regions of Interest

Voxels in 2D space are projected into 3D space to create anatomical regions of interests showing anterior (A–D) and posterior regions (E–H): CA4 and dentate gyrus (CA4DG [red]), CA3 including fimbria (orange), CA2 (yellow), CA1 (light blue), subiculum (green), entorhinal cortex (blue), perirhinal cortex (brown) and parahippocampal cortex (pink).

Fig. 6. Behavioral Performance

The task consisted of 6 blocks of encoding (learn) and retrieval (recall). Shown is the average percent correct across subjects (N=14) during blocks 1 thru 6 of the paired associates task.

Fig. 7. Group Voxel-wise Analysis

Group voxel based mixed-effects unfolded t-test maps (statistical maps of significantly activated and deactivated regions; recall > learn: 2.3 ≥ t ≥ 10 and learn > recall: 2.3 < t < 10, p < 0.05 cluster corrected). Shown are significant increases and decreases within the left MTL regions during learning and recall of associative pairs. Regions shown include CA4 and dentate gyrus, CA3, CA2, CA1, subiculum (sub), entorhinal cortex (ERC), perirhinal cortex (PRC), and parahippocampal cortex.

Fig. 8. fMRI example data

An example subject’s activation map superimposed on an oblique-coronal high-resolution fMRI image during the learning and recall of paired associates compared to baseline. Shown are clusters of significantly activated regions (statistical maps of significantly activated regions; z ≥ 2.4, p < 0.05 corrected).

Fig. 9. Hippocampal Region of Interest Analysis

Average percent (%) signal changes during learning and recall separately compared to baseline during learning blocks 1–3 in CA4 and dentate gyrus (CA4DG), CA3, CA2, CA1, and subiculum. Error bars correspond to the standard error across subjects.

Fig. 10. CA3 and Subiculum activity during each task block

Average percent (%) signal changes during learning and recall separately compared to baseline in CA3 and subiculum for each of the six task blocks. Error bars correspond to the standard error across subjects.

Fig. 11. Extra-hippocampal Region of Interest Analysis

Average percent (%) signal changes during learning and recall separately compared to baseline in entorhinal cortex (ERC), perirhinal cortex (PRC), and parahippocampal cortex (PHC). Error bars correspond to the standard error across subjects.