Spatial Memory Activity Distributions Indicate the Hippocampus Operates in a Continuous Manner

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.