First author

Among the brain's many mysterious goings on are theta oscillations. This term describes a brain rhythm observed in the hippocampus — the area responsible for long-term memory and spatial navigation — in many mammals, including rodents and humans. Although the oscillations' function is not fully understood, they are thought to be the 'clock' that controls the timing of hippocampal activity. The prevailing view is that theta oscillations occur synchronously across the hippocampus. But on page 534, Evgueniy Lubenov and Athanassios Siapas of the California Institute of Technology in Pasadena challenge that assumption, demonstrating that theta oscillations do not occur in synchrony, and instead travel across the hippocampus. Lubenov explains why these experiments required considerable patience.

Why did you question the prevailing view?

While recording electrical activity in different regions of the rat hippocampus, we saw differences between electrodes in the phase and amplitude of theta oscillations. Such variations would usually be attributed to the fact that the electrodes were implanted at different depths, because theta phase and amplitude are known to vary according to depth in the hippocampal layers. We wondered whether phase might also depend on other factors, for example, anatomical location across the hippocampus.

How did you tackle the problem?

We started with a series of experiments involving long vertical probes with multiple recording sites, which allowed us to map the depth-dependence of theta oscillations with high resolution. We found a 400-micrometre-thick layer of the hippocampus where the phase of theta oscillations is insensitive to depth. Using custom arrays, we then positioned grids of 28 electrodes in this target region and compared the phases of theta oscillations recorded at different anatomical locations across the hippocampus in freely behaving rats.

What did you find?

That theta oscillations propagate in a consistent direction, and so travel through a series of 'time zones' along the length of the hippocampal axis. We plan to study the mechanisms responsible for these waves. This will help us to reveal how information flows and is processed in the hippocampus.

Was it hard to make these measurements?

Making simultaneous recordings over roughly one-third of the hippocampus in freely behaving animals is not easy. It can take several weeks to gradually position this many electrodes to their targets.