Recent studies have revealed the existence of hippocampal neurons that fire at successive moments in temporally structured experiences. Several studies have shown that such temporal coding is not attributable to external events, specific behaviours or spatial dimensions of an experience. Instead, these cells represent the flow of time in specific memories and have therefore been dubbed 'time cells'. The firing properties of time cells parallel those of hippocampal place cells; time cells thus provide an additional dimension that is integrated with spatial mapping. The robust representation of both time and space in the hippocampus suggests a fundamental mechanism for organizing the elements of experience into coherent memories.
At a glance
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This study revealed the existence of time cells in animals performing a non-spatial task in which paired stimuli were separated by a delay. The authors showed that temporally specific firing patterns during the delay are not explained by variations in location or behaviour but are controlled by the critical temporal parameter of delay duration.
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This study uses multivoxel pattern analysis of functional MRI scans in humans to show that the similarity of representations across a series of events predicts the likelihood of remembering the order of those events, similar to findings in rodents based on neural ensemble firing patterns in reference 12.
- Supplementary information S1 (movie) (5.20 MB)
Time cells in rats running on the treadmill. The firing patterns of three CA1 neurons are shown. For each, the time and location of the rat when individual spikes occur are plotted as dots (in a different colour for each neuron) on the rat's head. Note that even though the rat's head is approximately in the same location during the run, the neurons fire in sequence (pink then green then blue). Also note that each neuron additionally fires at a location on the maze outside the treadmill. The treadmill is on when a red triangle appears in bottom left corner.