Electrophysiological recordings in rats and mice have shown that specific hippocampal neuronal activity patterns are sequentially reactivated during rest periods or sleep. Does the human hippocampus also replay activity sequences, even in a nonspatial task, such as, for example, decision-making? Schuck and Niv studied functional magnetic resonance imaging signals in subjects after they had learned a decision-making task. While people rested, the replay of activity patterns in the hippocampus reflected the order of previous task-state sequences. Thus, sequential hippocampal reactivation might participate in decision-making in humans.
The hippocampus plays an important role in memory and spatial navigation. When rodents navigate a spatial maze, hippocampal neurons called place cells show spatially selective response fields, activating only during visitation of particular places. In this way, during navigation, place cells activate sequentially, reflecting traveled paths. During sleep and wakeful rest, the same sequences of place cells are reactivated from memory, or replayed, although the animal is stationary. Replayed sequences are temporally compressed, occurring on the order of 100 ms, and have been linked to an offline sampling process that is important for memory consolidation. Advances in reinforcement learning, an area of machine learning, suggest that offline experience replay may also serve computational functions underlying nonspatial learning and decision-making.
The study of hippocampal replay in the human brain is challenging because noninvasive neuroimaging techniques have either relatively low spatial or temporal resolution. Nevertheless, we reasoned that fast neuronal replay events may be detectable in blood oxygen level–dependent (BOLD) signals recorded with functional magnetic resonance imaging (fMRI) because the prolonged BOLD response translates short neural events into long-lasting signals. By applying multivariate decoding techniques that can disentangle subtle and spatially overlapp
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