Research Focus
Reading the hippocampal code by theta phase-locking

https://doi.org/10.1016/j.tics.2005.10.003Get rights and content

Both the prefrontal cortex and the hippocampus are crucial for memory encoding and recall. However, it remains unclear how these brain regions communicate to exchange information. Recent findings using simultaneous recordings from the hippocampus and prefrontal cortex of the behaving rat have demonstrated that prefrontal cells' firing is phase-locked to the hippocampal theta rhythm. This suggests that phase synchronization clocked by the theta rhythm could be crucial for the communication between hippocampal and prefrontal regions.

Section snippets

Simultaneous measurements of prefrontal and hippocampal cells

In their study Siapas et al. implanted tetrodes [4] in the medial PFC and the CA1 subfield of four rats. This allowed the researchers to measure spiking activity from 411 hippocampal and 316 prefrontal cells in total. They also recorded local field potentials (LFPs), which allow the detection of population activity like the theta rhythm. Although a large fraction of hippocampal cells have been shown to exhibit place-specific firing (hence ‘place cells’) [5], much less is known about cells in

A role for theta phase coding?

Although the results of Siapas et al. provide evidence of information flowing from the hippocampus to PFC, the findings do not directly address the nature of the neuronal code being exchanged. The relationship between place cells and the hippocampal theta rhythm has been the subject of many studies. It is well established that as a rat enters a place field, the respective place cell fires late in the theta cycle. As the rat progresses through the place field, the firing advances to earlier and

Open questions

The experimental findings by Siapas et al. lead to several new questions that deserve to be addressed experimentally. For example, the phase locking of PFC neuronal firing to the hippocampal theta rhythm was found in rats performing a variety of spatial tasks. It would be interesting to know if specific parameters of the tasks predict the degree of phase locking: for instance, is the phase locking more prevalent during recall than during encoding? Another important question pertains to the

Acknowledgements

This research was (in part) supported within the framework of the NWO Innovative Research Incentive Schemes. I thank Laura L. Colgin for valuable comments on the manuscript.

References (14)

There are more references available in the full text version of this article.

Cited by (0)

View full text