Although hippocampal theta oscillations are believed to be essential for the formation and retrieval of episodic memories, it is not clear how the information is handled in the hippocampal formation during theta states. We simultaneously recorded the activity of many (~100) neurons and local field potentials (LFP) from multiple layers of the hippocampus and entorhinal cortex of rats performing spatial tasks. Analysis of the temporal delays between population activities in successive anatomical stages suggests that the temporal windows set by the theta cycles allow for local circuit interactions and thus a considerable degree of computational independence in subdivisions of the hippocampus-entorhinal cortex loop.

To uncover the mechanism of local circuit computation, it is essential to understand how the individual neurons in a given region participate in information processing. The hippocampal CA1 region is one of the most extensively studied regions of the brain and most studies tacitly assume that pyramidal neurons in this region represent a homogeneous cell population. However, using high spatial resolution silicon probes, we found that deep and superficial pyramidal cells were strikingly different in terms of firing rate, burst propensity, spatial representation, and temporal relationship to various LFP oscillations. Thus, CA1 pyramidal cells in adjacent sub-layers can address their targets jointly or differentially, depending on brain states and oscillations, thereby form functionally distinct streams.

We also addressed a long-standing “neuromyth” (SFN Neuromyths, “We use only 10% of our brain”). The answers are both complex and interesting. Firing rates, spike transfer strengths and magnitude of population synchrony in the hippocampus formation showed lognormal-like distribution mostly irrespective of brain states and environments. Our finding suggests that the brain is largely a preconfigured network where a minority of highly active neurons does most of the work all the time.