A Publication of a Czech Neurophysiologist in the Journal Nature
How is the relevant memory activated in the brain? An answer is provided by a just published work in the specialised journal Nature (Theta-paced flickering between place-cell maps in the hippocampus). Doctor Karel Ježek from the Institute of Physiology of the ASCR in cooperation with a team of researchers from the Norwegian University of Science and Technology in Trondheim (NTNU) and the International Institute for Advanced Studies in Trieste (SISSA), led by Prof. Edvard Moser, discovered a method allowing the detailed description of the process of the activation of memory on the level of individual neurons.
Memory traces – memories – can be presented as different models of the activity of many neurons in the brain centres responsible for the memory function. Various groups of neurons then encode different memories. The authors observed the spatial memory in the hippocampus, which is a key part of the brain for memory functions. Hippocampal neurons (neurons of place) are active in diverse places of the environment which we know well. Their collection comprises so-called spatial maps (Fig. 1). In their experiments, the scientists monitored laboratory sewer rats – in box A the group of neurons comprising map A was active, in box B the other group (Map B, Fig. 2). The boxes differed only in the lighting, so that it was possible to change the identity of box A into box B suddenly, and vice versa. By the simple switch of the lighting, the sewer rat was hence ‘teleported’ between both places in a moment.
The scientists evaluated the currently active spatial map in high time resolution – several times a second, they determined which spatial memory is aroused in the brain at the moment. The change of the boxes caused also an exchange of the maps – the original was suppressed and the new activated. A detailed analysis showed that in a short time (even several seconds) after the ‘teleportation’ both memory traces compete with each other and the brain once activates the correct map to return immediately to the map encoding the previous box (Fig. 3).
It was this essentially curious situation that first helped make the fundamental discovery lying in the revelation of the principle of the temporal organisation of the memory processes in the hippocampus. The team discovered that the neuron population processes information from the senses (it seeks a suitable memory for it) in short, repeating cycles, which correspond to a 6–11 Hz rhythm. This rhythm, called ‘theta’, is one of the dominant EEG rhythms of the brain. The cyclic, constantly repeating process of the activation of the memory allows the brain to adjust several times a second the potential inaccuracies in the recollection of the memory that best corresponds to the information which the hippocampus receives from other parts of the brain. The memory machinery in the hippocampus is in this way structured in time by the theta rhythm in the chain of some kind of quanta that represent the elements in the recollection of the memory trace.
‘It is purely basic research and the fact that the work is coming out in Nature proves its importance. It takes us much further in the understanding of how memory actually functions,’ Doctor Karel Ježek noted on the scientific contribution of the discovery.
Fig. 1
Fig. 2
Fig. 3
For more information: MUDr. Karel Ježek, PhD., Institute of Physiology of the ASCR
Prepared by: Department of Media Communication (Head Office of the ASCR) and Institute of Physiology of the ASCR
13 Oct 2011
