Medial temporal lobe contributions to episodic sequence encoding

Abstract

Memory for the order of events within an episode requires mechanisms capable of bridging gaps between those events and laying down memory traces that will support the subsequent retrieval of episodic sequence information. It has been proposed that the hippocampus and surrounding medial temporal lobe (MTL) play a critical role in these processes. Here, using functional magnetic resonance imaging, we examined MTL activation during the encoding of sequentially presented noun triplets. Using performance on a retrieval test, encoding activations predictive of subsequent triplet recognition ("hits" vs. "misses") as well as activations differentiating triplets that were subsequently recognized with (ordered) and without (misordered) memory for the original sequence of presentation were identified. Within the MTL, activations in bilateral hippocampal and parahippocampal cortical regions predicted subsequent order memory, with greater activations observed during encoding of triplets subsequently correctly ordered than those subsequently misordered. Interestingly, activation in these regions did not correlate with old/new triplet identification, suggesting that these MTL regions contributed specifically to encoding of episodic details supporting subsequent recovery of sequence information.

Figure 1.

Schematic of behavioral tasks. During each 7500 ms, scanned encoding trial participants saw 3 nouns presented sequentially after which they had 3500 ms to create a mental image of a scene incorporating the 3 words (asterisk) and 750 ms to report their degree of success at creating the mental image (question mark). Each unscanned retrieval trial consisted of a probe for participants’ memory of whether a noun triplet was presented during encoding (“old or new?”) followed by a prompt to reconstruct the order of presentation of the words in a noun triplet during encoding (“order?”). See Stimuli and behavioral procedures for more details.

Figure 2.

Hippocampal regions showing greater BOLD activation during subsequently ordered relative to subsequently misordered trials. (A) Right anterior and mid-hippocampus and left hippocampus/subiculum displayed on mean anatomical brain. (B) Estimates of mean activation across the subset of participants included in the old/new analyses (see Materials and methods) in each hippocampal region for subsequently forgotten, recognized but misordered, and recognized and ordered trials. Lines indicate paired t-test results in the pairwise comparisons (* denotes 2-tailed P < 0.05).

Figure 3.

Parahippocampal regions showing greater BOLD activation during subsequently ordered relative to subsequently misordered trials. (A) Left and right parahippocampal regions of interest displayed on mean anatomical brain. (B) Estimates of mean activation across the subset of participants included in the old/new analyses (see Materials and methods) in each parahippocampal region for subsequently forgotten, recognized but misordered, and recognized and ordered trials. Lines indicate paired t-test results in the pairwise comparisons (* denotes 2-tailed P < 0.05).