On Theta Waves and LTP

Theta waves are low-frequency large-amplitude oscillations (6-7/s) recorded in and around the hippocampus in a variety of mammals, notably rodents, rabbits, and marsupials. Theta rhythms appear to correlate with motor activity, such that their timing coincides with whisker movement and sniffing. These waves are typically investigated with respect to their ostensible involvement in memory, as their power correlates with impairments in memory-related tasks (e.g. spatial alternation, spatial reversal) in lesion studies of the fornix and septum.

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Notably, theta phases appear to reset their timing in response to stimulus presentation, suggesting potential involvement in the encoding of memories. Their ability to shift phases in response to relevant stimuli was first shown in the 1990s (Givens, 1996). McCartney et al. (2004) were one of the first groups to report convincing evidence that this shift may serve to improve long-term potentiation (LTP). They recorded electrical activity in the perforant pathway of rats in a delayed nonmatching to position task, typically used to assess working memory. After collecting data on the theta wave phase reset, these researchers electrically stimulated the hippocampus of rats either at the peak of the new theta phase, or at its lowest point (the "through"). High-frequency stimulation potentiated excitatory postsynaptic potentials (EPSPs) only when performed at the peaks of theta waves (see graph below; on the y-axis - % change in EPSP slope; on the x-axis - stimulation timing [T:through, P:peak]).

McCartney et at. (2004)

All of this strongly points towards theta synchronization as a mechanism for optimizing long-term potentiation. 

References

Hasselmo, M. E. (2005). What is the Function of Hippocampal Theta Rhythm?–– Linking BehavioralData to Phasic Properties of Field Potential and Unit Recording Data. Hippocampus, 15, 936–949. doi:10.1002/hipo.20116

McCartney, H., Johnson, A. D., Weil, Z. M., & Givens, B. (2004). Theta reset producesoptimal conditions for long-term potentiation. Hippocampus, 14(6), 684–7. doi:10.1002/hipo.20019