Medial temporal lobe activation during context-dependent relational processes in episodic retrieval: an fMRI study. Functional magnetic resonance imaging

Abstract

Previous studies have reported that the medial temporal lobe (MTL) structures contribute to the processing of relations among multiple stimuli in episodic encoding. There have been few studies, however, on the episodic retrieval requiring processing of relations among multiple components that was involved in our events. We used functional magnetic resonance imaging (fMRI) to investigate neural activities during the retrieval of relations within an organized episode and the recognition of an episodic component. Healthy, normal participants memorized 50 four-scene comic strips before fMRI scanning. In the retrieval phase with fMRI scanning, participants were engaged in three tasks: a visual identification (VI) task, a story recall (SR) task, and a picture recognition (PRe) task. In the VI task, participants were asked to judge whether they could identify at least one female character in the two scenes presented vertically. In the SR task, participants were shown the first and last scenes from strips memorized previously and asked to judge whether or not the two scenes were from the same strip. In the PRe task, participants were shown two scenes and asked to judge whether they both belonged to the memorized scenes. The two contrasts of SR with VI and PRe with VI demonstrated some commonly activated areas, such as the bilateral middle frontal gyrus and cerebellum. More importantly, the SR task differentially activated the bilateral parahippocampal gyrus, whereas the PRe task differentially activated right prefrontal areas, including the inferior frontal and precentral gyri. The results suggest that the activity of the MTL structures may be strongly associated with episodic memory retrieval requiring context-dependent relational processing.

Figure 1

Examples of encoding stimuli. The title of this four‐scene comic is Aimu‐Yacchi (My name is Yacchi). © Aska Gotho, reproduced with permission.

Figure 2

Examples of stimuli presented in the retrieval phase with fMRI scanning. Examples of (A) the SR task, (B) the PRe task, and (C) the VI task.

Figure 3

The behavioral data of the three task conditions. The means of (A) hit responses, and (B) false‐alarm responses. C: Reaction time data. Triangles, VI; diamonds, SR; squares, PRe.

Figure 4

An activated image and the percent signal changes of voxels of interest (VOIs) in the bilateral parahippocampal gyrus. The left side of this figure is the actual left side of brain. To provide a means of descriptively rendering the magnitudes of activations in those areas, two VOIs of 4‐mm radius were conducted in the bilateral parahippocampal gyri with the activated peak voxels centered. The mean percent signal changes of these VOIs were statistically compared among three tasks of SR, Pre, and VI (*P < 0.05; **P < 0.01).

Figure 5

Activated images and percent signal changes of VOIs in the right inferior frontal gyri. The bottom of this figure is the actual right side of brain. Two VOIs of 4‐mm radius were conducted in the right prefrontal areas with the activated peak voxels centered. The mean percent signal changes of these VOIs were statistically compared among three tasks of SR, Pre, and VI (*P < 0.05; ** P < 0.01).