Neurons in the hippocampal CA regions respond selectively to the location of the animal. These place cells activate in a restricted location called the place field (O'Keefe and Dostrovsky 1971). Later studies revealed that the ongoing theta oscillation in the local field potential (LFP) signal of the hippocampus who modulates the spiking activity of place cells (O'Keefe and Recce 1993). When the animal enters the field of a place cell, the cell fires spikes at the late phase of the theta oscillation and, as the animal proceeds through the field, spikes occur at progressively earlier phases. This negative correlation of the firing phase of place cells to their field location is called phase precession and is a robust phenomenon.
The sequential activity during the rats' exploration is also modulated by the theta oscillation (Skaggs et al. 1996). In each theta cycle, cells activate in a sequential order from the cells with fields located slightly behind the current location of the animal to the ones ahead of the animal. The sequential nature of these cell activity events is measured by calculating the correlation between the spiking time and the order of the cells according to their place field location (Foster and Wilson 2007). The causal relationship between phase precession and theta sequences remains unclear. One possibility is that phase precession leads to sequential ordering within theta cycles. Alternatively, phase precession might be the result of the directional activation of a group of cells with overlapping place fields. Recent studies, however, suggest that phase precession and theta sequences might be independent. While it has been shown that theta phase precession is present from the first encounter to a novel environment, theta sequences develop later with experience (Feng et al. 2015). The length of theta sequences has also been observed to be determined by the goal location and not by the extent of phase precession (Wikenheiser
and Redish 2015). In this project, we will examine an abstract mathematical model of a population of phase precessing
units to see how the quality of theta sequences are related to that of phase precession. These relationships will be tested in a more biologically plausible network of spiking neurons.
O'Keefe, J., Dostrovsky, J. (1971). The hippocampus as a spatial map. Preliminary evidence from unit
activity in the freely-moving rat. Brain Research 34(1):171-175
O'Keefe, J., Recce, M., (1993). Phase relationship between hippocampal place units and the EEG theta
rhythm. Hippocampus 3(3):317-330
Skaggs, William E., McNaughton, Bruce L., Wilson, Matthew A., Barnes, Carol A. (1996). Theta phase
precession in hippocampal neuronal populations and the compression of temporal sequences.
Foster, David J., Wilson, Matthew A. (2007). Hippocampal theta sequences. Hippocampus
Feng, T., Silva, D., Foster, D. J. (2015). Dissociation between the Experience-Dependent Development
of Hippocampal Theta Sequences and Single-Trial Phase Precession. Journal of Neuroscience
Wikenheiser, Andrew M., Redish, David A. (2015). Hippocampal theta sequences reflect current goals.
Nature neuroscience 18(2):289-94
Dr. Amir Azizi and Prof. Dr. Sen Cheng