Abstract
There is a long-standing debate as to whether recollection is a continuous/graded process or a threshold/all-or-none process. In the current spatial memory functional magnetic resonance imaging (fMRI) study, we examined the hippocampal activity distributions—the magnitude of activity as a function of memory strength—to determine the nature of processing in this region. During encoding, participants viewed abstract shapes in the left or right visual field. During retrieval, old shapes were presented at fixation and participants classified each shape as previously in the “left” or “right” visual field followed by an “unsure”–“sure”–“very sure” confidence rating. The contrast of left-hits and left-misses produced two activations in the hippocampus. The hippocampal activity distributions for left shapes and right shapes were completely overlapping. Critically, the magnitude of activity associated with right-miss-very sure responses was significantly greater than zero. These results support the continuous model of recollection, which predicts overlapping activity distributions, and contradict the threshold model of recollection, which predicts a threshold above which only one distribution exists. Receiver operating characteristic analysis did not distinguish between models. The present results demonstrate that the hippocampus operates in a continuous manner during recollection and highlight the utility of analyzing activity distributions to determine the nature of neural processing.
Highlights
Long-term memory can be based on non-detailed familiarity or detailed recollection
Recollection-based ROCs that do not include forgotten items in the analysis are adequately fit by the continuous model but not the threshold model [7,17,18]
As neither model adequately fit the behavioral ROC, the chi-square analysis results did not distinguish between the continuous model of recollection or the threshold model of recollection
Summary
Long-term memory can be based on non-detailed familiarity or detailed recollection. Familiarity is widely believed to be a continuous process, ranging in strength from weak to intermediate to strong.the nature of recollection has been a topic of debate. Long-term memory can be based on non-detailed familiarity or detailed recollection. Familiarity is widely believed to be a continuous process, ranging in strength from weak to intermediate to strong. Until about a decade ago, recollection was widely thought to be an all-or-none threshold process, where memories are either completely remembered or forgotten [1,2,3]. A growing body of recent behavioral evidence indicates that recollection is a continuous process [4,5,6]. The two models of recollection are formally referred to as the continuous unequal variance model and the two-high threshold model [7]. The same items could be presented at fixation in gray and participants would make a confidence rating ranging from “very sure green” to “very sure red”
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