One-Sided Memories and Sleep

A new study involving memory consolidation in sleep – on one side of the brain only – may help researchers understand how sleep aids memory, and may point towards new means of dealing with various kinds of brain trauma, both emotional and physical.

The study was led by PhD student Ella Bar, who is in the groups of Prof. Rony Paz in the Neurobiology Department at the Weizmann Institute of Science and Prof. Yuval Nir at Tel Aviv University. The research was conceived and performed in the Neurobiology groups of Profs. Yadin Dudai and Noam Sobel, both at the Weizmann Institute. 

Bar knew from previous experiments conducted at the Institute that odors can be a strong tool to reactivate memories in sleep; and she also knew that any memories that are tied to either left or right in relation to the body get processed and ultimately stored in the opposite hemisphere of the brain.

In the first part of the experiment, the subjects were smelling the scent of roses with both nostrils; at the same time they learned the locations of words that were presented on either the left or right sides of a computer screen. Their memory was tested immediately afterwards, and the tired volunteers then laid down to take naps in the lab, with EEG electrodes attached to record their brain activity. While they slept, the odor was again delivered in pulses, but this time to one nostril, only.

Indeed,  during odor delivery in sleep the researchers noted that the two hemispheres of the subjects’ brains exhibited the kind of electrical activity thought to be involved in memory consolidation, with the half that was “smelling the roses” showing the more synchronized activity that is important for the consolidation process. When the subjects woke up and retook the test, they were better at remembering the locations of words that had been presented to the side of the brain that had been stimulated with the smell while they slept.

Since each of our nostrils is a separate smelling organ, with its own connection to the same side of the brain, this experiment showed that the process of memory consolidation during sleep is, indeed affected by memory-related odors and that tracking which takes place in the brain while only one side smells can be a powerful tool for understanding how memories are moved from “short-term storage” to the more permanent memory banks in a different part of the brain.

They were better at remembering the locations of words that had been presented to the side of the brain that had been stimulated with the smell while they slept

“During sleep, information gradually makes a transition from the hippocampus to the neocortex. But how this transition occurs has been a mystery. It appears that smelling memory-related odors during sleep, even if we are unaware of smelling them, is a trigger for that process of memory consolidation. By  introducing a scent to just one nostril in sleep we can reactivate memories for consolidation in a single hemisphere and compare the brain activity between the sides to help us fill in the details of these events,” says Bar.

“The use of this unique cue, a smell delivered to just one side of the brain, tells us that memories might be manipulated in a very specific manner,” says Paz. “They suggest that there is a dialog going on every night between the hippocampus and certain parts of the cortex, and that smell might be used to enhance, or possibly interfere with that dialog.” 

In other words, say the researchers, the new method could eventually go beyond the research lab. For example, the brains of post-traumatic stress disorder patients typically show higher activity in the right hemisphere when recalling the traumatic memories – those which are more related emotional aspects of the memory – and lower activity in the left hemisphere, which more related to the verbal aspects of the memory.  Using this method might help to strengthen the role of the left hemisphere  in the process of creating memory and restore balance. The findings might also point to new avenues in designing treatments for one-sided brain injury, for example, in stroke.

Prof. Rony Paz is Head of the M. Judith Ruth Center for Trauma and Anxiety Research; the Nella and Leon Benoziyo Center for Neurosciences; the Carl and Micaela Einhorn-Dominic Brain Research Institute; the Murray H. and Meyer Grodetsky Center for Research of Higher Brain Functions; and the Gladys Monroy and Larry Marks Center for Brain Disorders. His research is also supported by the Irving B. Harris Fund for New Directions in Brain Research; the Andrea C. and Lawrence I. Marcus Fund for Emotional/Psychological Disorders; the Bernard and Norton Wolf Family Foundation; the Harold L. and Faye B.  Liss Foundation; the Oster Family Foundation; the Leff Family; Mr. and Mrs. Gary Clayman; Rosanne Cohen; and Philip Garoon. Prof. Paz is the incumbent of the Manya Igel Chair of Neurobiology. 

Prof. Yadin Dudai's research is supported by the Braginsky Center for the Interface between Science and the Humanities. 

Prof. Noam Sobel is Head of the Azrieli National Institute for Human Brain Imaging and Research. His research is also supported by the Norman and Helen Asher Center for Human Brain Imaging; the Nadia Jaglom Laboratory for the Research in the Neurobiology of Olfaction; the Rob and Cheryl McEwen Fund for Brain Research; and Sonia T. Marschak. Prof Sobel is the incumbent of the Sara and Michael Sela Professorial Chair of Neurobiology.