A Rapid, Hippocampus-Dependent, Item-Memory Signal that Initiates Context Memory in Humans

Faraneh Vargha-Khadem,Aidan J. Horner,David G. Gadian,Sebastian Jentschke,Emrah Duzel,Lluis Fuentemilla

doi:10.1016/j.cub.2012.10.055

Abstract

SummaryThe hippocampus, a structure located in the temporal lobes of the brain, is critical for the ability to recollect contextual details of past episodes. It is still debated whether the hippocampus also enables recognition memory for previously encountered context-free items. Brain imaging [1, 2] and neuropsychological patient studies [3, 4] have both individually provided conflicting answers to this question. We overcame the individual limitations of imaging and behavioral patient studies by combining them and observed a novel relationship between item memory and the hippocampus. We show that interindividual variability of hippocampal volumes in a large patient population with graded levels of hippocampal volume loss and controls correlates with context, but not item-memory performance. Nevertheless, concurrent measures of brain activity using magnetoencephalography reveal an early (350 ms) but sustained hippocampus-dependent signal that evolves from an item signal into a context memory signal. This is temporally distinct from an item-memory signal that is not hippocampus dependent. Thus, we provide evidence for a hippocampus-dependent item-memory process that initiates context retrieval without making a substantial contribution to item recognition performance. Our results reconcile contradictory evidence concerning hippocampal involvement in item memory and show that hippocampus-dependent mnemonic processes are more rapid than previously believed.

Highlights

An important goal in memory research is to map the functional organization of a cognitive process onto anatomy. This has proven controversial for the hippocampus, a structure located within the medial temporal lobes (MTL), and its relationship to the functional components of recognition memory [5, 6]
Lesion-behavior studies, suffer from a difficulty in unambiguously dissociating between memory processes, making it difficult to conclude that a given process is functionally preserved
In our MEG data, we looked across all sensors and time points for significant differences in event-related fields (ERFs) between item-memory hits and correct rejections (CRs: new items identified as new)

Summary

Methods

The experiment consisted of six study-test cycles. Participants were presented with word-scene pairs and required to perform a living/ nonliving judgment on the word. Old/new words were presented, and participants performed an old/new item recognition judgment (item memory) and a further confidence judgment. If they responded ‘‘old’’ they performed a three-alternative forced-choice source-memory test, choosing which scene was originally paired with the word (context memory) (see Figure 1A and Supplemental Information for details of trial sequence)

Results

Discussion

Conclusion