By James Devitt
The information carried by the electrical activity of neurons is a mixture of stored memories, environmental circumstances, and current state of mind, scientists have found in a study of laboratory rats. The findings, which appeared in the journal PLoS Biology, offer new insights into the neurobiological processes that give rise to knowledge and memory recall.
The study was conducted by Eduard Kelemen, a former graduate student and post-doctoral associate at SUNY Downstate Medical Center, and André Fenton, a professor at NYU’s Center for Neural Science and Downstate Medical Center.
Recently, studies have shown that people mistakenly recalled seeing a school bus in a movie if the bus was mentioned after they watched the movie. Other scholarship has shown that a subject’s mindset can also influence the retrieved information. But while the psychological contours of retrieval are well-documented, very little is known about the neural activity that underlies this process.
With this in mind, Fenton and Kelemen centered their study on the neurophysiological processes rats employ as they solve problems that require memory retrieval. To do so, they employed techniques developed during the last two decades that involve monitoring the electrical activity of neurons in the rats’ hippocampus—the part of the brain used to encode new memories and retrieve old ones. By spotting certain types of neuronal activity, researchers have historically been able to perform what amounts to a mind-reading exercise to decode what the rat is thinking and even comprehend the specifics of the rats’ memory retrieval.
In their experiments, Fenton and Kelemen tested the viability of a concept, “cross-episode retrieval”— stimulating the brain activity in a given circumstance that was also activated in a previous, distinctive experience.
To test their hypotheses, rats were placed in a stable, circular arena, then in a rotating, circular arena of the same size, followed by a return to the stable arena. In the rotating arena condition, the surface turned slowly, making it necessary for the rat to think about its location either in terms of the rotating floor or in terms of the stationary room.
Overall, the results showed distinct neural activity between the stable and rotating conditions. However, during the rotating task, the researchers intermittently observed “cross-episode retrieval”—that is, at times, neurons expressed patterns of electrical activity under the rotating-arena condition that were similar to those activity patterns that were used in the stable-arena condition. Notably, cross-episode retrieval occurred more frequently when the angular position of the rotating arena was about to complete a full rotation and return to the same position as in the stable condition, showing that retrieval is influenced by environment.
“These experiments demonstrate novel, key features of constructive human episodic memory in rat hippocampal discharge,” explained Fenton, “and suggest a neurobiological mechanism for how experiences of different events that are separate in time can nonetheless comingle and recombine in the mind to generate new information that can sometimes amount to valuable, creative insight and knowledge.”
The study was funded by the National Science Foundation.