The memory coding system is more versatile than previously thought

Aug. 10, 2020

A multidisciplinary team of researchers from the Institute of Neurosciences of Alicante, a joint centre of the Spanish National Research Council (CSIC) and the University Miguel Hernández, the Institute of Cross-disciplinary Physics and Complex Systems, of the CSIC and the University of the Balearic Islands, the Cajal Institute, of the CSIC, and the University of La Laguna, has managed to identify that the theta rhythm of the hippocampus is formed by three main generators and to demonstrate, through the use of optogenetic tools, that these generators are independent, that their synchrony changes according to different behaviors and that specific bands of fast gamma waves control the slow theta oscillation according to cognitive needs. The work, carried out through the analysis of the extracellular potentials registered in rats that freely explored known or novel environments, is published in the eLife magazine.

The activity of the hippocampus is organized into sequences of slow waves, called theta, controlled by internal memory processes and external sensory signals, but little is known about how these processes are coordinated. Although theta-wave activity is considered a single coherent oscillation, it is actually the result of complex interactions between the different areas of the hippocampus that generate them. Highly synchronized theta rhythms occur preferably during tasks that require coordination between representations from internal memory and sensory information from the outside. Additionally, researchers have found that rapid gamma oscillations in specific frequency bands are associated with theta generators mainly when they are synchronized.

The work developed by this multidisciplinary team has led the researchers to propose the existence in the hippocampus of a mechanism to segregate or integrate processes based on the coexistence of different patterns of slow and fast waves, which are coupled or decoupled in a flexible way. When cognitive needs require it, information processing is carried out in parallel, without interference between processes, and when it is necessary to contrast the information stored in the memory with that coming from the outside, the rhythms are synchronized and integrated. In this way, memory would be updated when there is new information in the environment and it would be preserved in already known contexts.

In the hippocampus, a key brain structure for such important functions as orientation or memory, two types of simultaneous oscillations or waves are recorded, called theta and gamma. These waves are caused by the electrical activity generated by communication between neurons. Theta waves oscillate slowly (8 Hz) and are found in various brain structures, while gamma waves are much faster (between 30 and 120 Hz, depending on the region of the hippocampus) and only appear in short periods of time at a specific phase of the theta oscillation.

The two types of waves are closely related, since while gamma waves contain information packets that are transmitted from one region of the hippocampus to another, slow theta waves act like a clock that coordinates the sequences of activity between the different regions. Like radio or television signals, gamma oscillations would play the role of forming different "channels" that would use different frequencies and phases of the rhythm to communicate without interfering with other channels.

The researchers looked at whether slow-wave generators in the hippocampus worked together to produce a common theta oscillation or whether they could work independently, with several theta oscillations coexisting at the same time. This second scenario, which they have validated with their work, allows for multiple frames of theta-gamma interaction, providing a much more versatile information coding system.

Although the conditions that trigger coordination between the structures of the hippocampus are not yet understood in detail, since the decoupling between theta and gamma components seems to represent an early electrophysiological signature present in cases of Alzheimer's disease, schizophrenia and other psychiatric disorders, this study raises new mechanisms that may be crucial to understand these pathologies.

Víctor J. López-Madrona, Elena Pérez-Montoyo, Efrén Álvarez-Salvado, David Moratal, Óscar Herreras, Ernesto Pereda, Claudio R. Mirasso y Santiago Canals. Different theta frameworks coexist in the rat hippocampus and are coordinated during memory-guided and novelty tasks. eLife. DOI: 10.7554/eLife.57313


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