Two years later - Revisiting autobiographical memory representations in vmPFC and hippocampus

Abstract

A long-standing question in memory neuroscience concerns how and where autobiographical memories of personal experiences are represented in the brain. In a previous high resolution multivoxel pattern analysis fMRI study, we examined two week old (recent) and ten year old (remote) autobiographical memories (Bonnici et al., 2012, J. Neurosci. 32:16982-16991). We found that remote memories were particularly well represented in ventromedial prefrontal cortex (vmPFC) compared to recent memories. Moreover, while both types of memory were represented within anterior and posterior hippocampus, remote memories were more easily distinguished in the posterior portion. These findings suggested that a change of some kind had occurred between two weeks and ten years in terms of where autobiographical memories were represented in the brain. In order to examine this further, here participants from the original study returned two years later and recalled the memories again. We found that there was no difference in the detectability of memory representations within vmPFC for the now 2 year old and 12 year old memories, and this was also the case for the posterior hippocampus. Direct comparison of the two week old memories (original study) with themselves two years later (present study) confirmed that their representation within vmPFC had become more evident. Overall, this within-subjects longitudinal fMRI study extends our understanding of autobiographical memory representations by allowing us to narrow the window within which their consolidation is likely to occur. We conclude that after a memory is initially encoded, its representation within vmPFC has stablised by, at most, two years later.

Keywords: Autobiographical memory; Consolidation; Hippocampus; Longitudinal; MVPA; vmPFC.

Fig. 1

The longitudinal experimental design. In study one, participants recalled memories that were 2 weeks old and 10 years old. When they returned for study two, the 2 week old memories were now 2 years old, and the 10 year old memories were now 12 years old.

Fig. 2

MVPA results for recent and remote autobiographical memories in study one. A. Hippocampus (HC), entorhinal and perirhinal cortices (EPC), parahippocampal cortex (PHC), retrosplenial cortex (RSC), temporal pole (TP), lateral temporal cortex (LTC), and ventromedial prefrontal cortex (vmPFC) were examined. Chance=33%. Recent (blue line) and remote (red line) autobiographical memories were represented in medial temporal regions, including the hippocampus, while in vmPFC remote memories were particularly distinguishable; *p<0.05. The difference between recent and remote memories in TP just failed to reach significance. Error bars represent ±1 standard error of the mean. B. Information maps in the hippocampus for recent (shown in blue) and remote (shown in red) autobiographical memories comprised the voxel sets that produced above-chance classification accuracy. The information maps for three example participants are shown superimposed upon 3D images of their right hippocampus. Areas in pink denote where the information maps for recent and remote memories overlapped. C. MVPA results for anterior and posterior subregions of the hippocampus (HC). Above-chance (chance=33%) classification was apparent in anterior and posterior hippocampus for recent (blue line) and remote (red line) memories. Nevertheless, classification accuracies were significantly higher in the posterior hippocampus for remote memories; *p<0.05. Error bars represent ±1 standard error of the mean. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 3

MVPA results for recent and remote autobiographical memories in study two. A. Hippocampus (HC), entorhinal and perirhinal cortices (EPC), parahippocampal cortex (PHC), retrosplenial cortex (RSC), temporal pole (TP), lateral temporal cortex (LTC), and ventromedial prefrontal cortex (vmPFC) were examined. Chance=33%. Two year old (green line) and 12 year old (orange line) autobiographical memories were represented in medial temporal regions, including the hippocampus. In study two, this was also the case for vmPFC. Error bars represent ±1 standard error of the mean. B. Information maps in the hippocampus for 2 year old (shown in green) and 12 year old (shown in orange) autobiographical memories comprised the voxel sets that produced above-chance classification accuracy. The information maps for three example participants are shown superimposed upon 3D images of their right hippocampus. Areas in pink denote where the information maps for the 2 year old and 12 year old memories overlapped. C. MVPA results for anterior and posterior subregions of the hippocampus (HC). No differences between memory types were apparent in either the anterior or posterior hippocampus. Error bars represent ±1 standard error of the mean. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 4

MVPA results for the comparison of the same memories when 2 weeks old and 2 years old. A. Hippocampus (HC), entorhinal and perirhinal cortices (EPC), parahippocampal cortex (PHC), retrosplenial cortex (RSC), temporal pole (TP), lateral temporal cortex (LTC), and ventromedial prefrontal cortex (vmPFC) were examined. Chance=33%. The memories when two weeks old (blue line) and 2 years old (green line) were represented in medial temporal regions, including the hippocampus, while memories were more easily distinguishable when they were 2 years old in vmPFC; *p<0.05. Error bars represent ±1 standard error of the mean. B. Information maps in the hippocampus for the memories when 2 weeks old (shown in blue) and 2 years old (shown in green) comprised the voxel sets that produced above-chance classification accuracy. The information maps for three example participants are shown superimposed upon 3D images of their right hippocampus. Areas in pink denote where the information maps for the memories at the two different time point overlapped. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 5

MVPA results for the comparison of the same memories when 10 years old and 12 years old. A. Hippocampus (HC), entorhinal and perirhinal cortices (EPC), parahippocampal cortex (PHC), retrosplenial cortex (RSC), temporal pole (TP), lateral temporal cortex (LTC), and ventromedial prefrontal cortex (vmPFC) were examined. Chance=33%. When memories were ten years old (red line) and then 12 years old (orange line), they were represented in medial temporal regions, including the hippocampus, and this was also the case for the vmPFC. Error bars represent ±1 standard error of the mean. B. Information maps in the hippocampus for the memories when 10 years old (shown in red) and 12 years old (shown in orange) comprised the voxel sets that produced above-chance classification accuracy. The information maps for three example participants are shown superimposed upon 3D images of their right hippocampus. Areas in pink denote where the information maps for the memories at the two different time point overlapped. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).