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The remains of the day

A study shows the stages of sleep in which the brain stores experiences and recollections

Jaime PradesMemories resonate through regions of the brain until they are converted into genuine learning or recollections. First, the information is lodged temporarily in the hippocampus, a region whose name comes from its shape, which is similar to that of a seahorse; they then migrate to a more external layer, the cortex, in a consolidation process that takes place during sleep. The phenomenon of memory propagation has been known since the 50s, but for the first time ever scientists have been able to investigate in detail the brain activities involved in this process. An article whose main author is the scientific director of the Edmond and Lily Safra International Neuroscience Institute in Natal (IINN-ELS), 36-year old neuroscientist Sidarta Ribeiro, from Brasilia, indicates that the construction of memory takes place in a specific sequence of the sleep cycle: the slow wave stage, in which one sleeps deeply, and in the subsequent REM (Rapid Eye Movement) stage, when dreaming is intense.

Sidarta analyzed 28 sleep cycles of 15 laboratory rats submitted for 20 minutes to contact with objects they had never seen before. During the experiment, the researcher monitored the activity of hundred of neurons in the hippocampus and in two areas of the cortex. During the slow-wave stage, he observed a stronger sort of echo of the electrical impulse patterns observed in the early contacts with the objects. This phenomenon of memory reverberation goes hand in hand with the activation of the neuron network that stores the representation of that experience. This reverberation, the study shows, lasts for hours in the cortex, but is far faster in the hippocampus, providing electrophysiological evidence of how the  memory travels within the brain. During the REM stage, an increase in the expression of the genes Arc and Zif-268 in the cortex was recorded, but not in the hippocampus. These genes are connected with the consolidation of memories. The study, which was led by Sidarta, is being published in the November issue of the journal Frontiers in Neuroscience. “The activation of the genes during REM sleep is equivalent to an order for the cortex to store the information that has just been reverberated, consolidating the memory. With each sleep cycle, the memory becomes more strongly anchored in the cortex”, says Sidarta. “We are showing, for the first time, molecular and electrophysiological evidence of how specific sleep stages participate in the process of migration in the memory”.

Sidarta Ribeiro’s interest in the role of sleep in consolidating memory resulted from an accident of fate. In 1995, having graduated from the University of Brasilia and completed his master’s degree in biophysics from the Federal University of Rio de Janeiro, he went to New York to do a doctorate in molecular cognitive neurobiology at Rockefeller University. He faced a cultural shock he had not expected. “I was six months behind my class and realized that I lacked the theoretical basis to keep up with it. I decided to use all the time I could in the lab and upgrade my knowledge, but I felt very sleepy and would end up going home to sleep. I occasionally slept as much as 16 hours a day. This went on for a couple of months and then I managed to adapt, keep up with the class and move on”, he recalls. Intuitively, he concluded that sleep had a role in his troubled adaptation, but when he started researching the issue, he found it had not been studied much. “There is a reference book called Principles of Neuroscience according to which little was known about the cognitive function of sleep and of dreams. This aroused my curiosity”.

During his stay in New York, from 1995 to 2000, Sidarta published a series of articles on the verbal communication of birds, as this was his group’s line of research, meanwhile also investing in his studies on the consolidation of memories. In a cover feature article published in 1999 by the journal Learning and Memory, Sidarta and his co-workers from Rockefeller University described the discovery of the expression of a gene connected with memory consolidation during REM sleep. In 2000, Sidarta decided to orient his career toward the study of sleep as he transferred to Duke University for his post-doctoral work, under the guidance of Brazilian Miguel Nicolelis, and where he perfected his knowledge of the use of electrodes to monitor precisely and simultaneously the workings of hundreds of neurons (it was thanks to this technology that Miguel Nicolelis was able to make monkeys move a mechanical arm  with only impulses transmitted from their brains).

In 2004, in another cover feature of the journal Learning and Memory, Sidarta formulated a theory that explained why the hippocampus is the temporary abode of memories, whereas the cortex is their permanent warehouse. The molecular and electrophysiological phenomenon triggered by sleep produce a short lasting reverberation in the hippocampus, while the cortex continues to undergo waves of neural plasticity. The article that is being published in Frontiers in Neuroscience puts this theory to the test for the first time.

Sidarta Ribeiro, now one of the world’s best know researchers in this field, also proposes a review of the disregarded view that scientists take of psychoanalysis (although concepts such as the symbology of dreams are still awaiting scientific corroboration), by showing that at least two of the ideas Freudian and Jungian theories advocate make sense. One of them is the evidence that dreams almost always are connected with experiences that took place the day before. The other is the recovery, during sleep, only of the day’s most striking episodes. The reverberation of learning in the construction of memories helps to explain both.

Two years ago, Sidarta moved from Duke University in Durham, North Carolina, to Natal, in the state of Rio Grande do Norte, where IINN-ELS was created under the leadership of Miguel Nicolelis. The institute’s idea is to develop cutting-edge science linked to educational and social projects for underprivileged students. Sidarta, who helped to design the project, became the institute’s scientific director. “We managed to bring together the conditions required for carrying out advanced research in Natal. The work is more pleasurable here than in the States, from all points of view, because there is more freedom”, he states.