Some time cells fired at the same absolute or relative time to the delay onset, but most developed qualitatively different firing patterns under distinct delay periods. This pattern of responses to changing the delay period is reminiscent of the prominent observation of “partial remapping” of place cells when salient spatial cues are altered (e.g., Muller and Kubie, 1987, Breese et al., 1989, Markus et al., 1995, Sharp et al., 1995, Shapiro et al., see more 1997 and Skaggs and McNaughton, 1998). Thus, when salient spatial parameters or task demands are altered in rats traversing open fields, some hippocampal place cells maintain their firing patterns, but others cease firing, begin firing, or fire with qualitatively

different patterns related to the animal’s location even within the same recording session. As animals performed this task, they occupied different locations in each sequential trial period, and they could move during each trial period. These variations in behavior allowed us to determine that, in addition to time and the object that began click here each sequence, both behavioral variation and place contributed to the differences in firing patterns between and within trial periods. In particular, during the delay when there were no differences between overt stimuli and behavior, the activity of most cells was strongly and equivalently influenced by a combination of time and place. As typically

observed in studies on place cells, the activity of time cells was also influenced by behavioral variations reflected in head direction and speed (reviewed in Eichenbaum, 2004). In addition, notably, previous studies have shown that hippocampal neurons encode ongoing behavioral context as well as spatial location, head direction, and speed (e.g., Wood et al., 2000). The present results extend this broad range of contextual variables that affect single-neuron activity to the flow of time in bridging of temporal gaps within sequences of events. Five main findings indicate that hippocampal representation of time during sequential events is quite similar to its representation of space as animals

explore an open field: (1) A large proportion of hippocampal neurons are engaged during performance of through a sequence memory task, just as a large fraction of hippocampal neurons are place cells when rats are engaged in spatial exploration (Thompson and Best, 1989 and Muller, 1996). These findings suggest that place cells and time cells may reflect fundamental mechanisms by which hippocampal neuronal networks parse any spatiotemporal context into quantal units of where and when important events occur. Speculating further, while place cells and time cells are appropriately named for the variables they represent in spatially and temporally defined behavioral paradigms, they may be processing information that is fundamentally neither spatial nor temporal.