We’ve all at some point been told that sleep is essential for our memories to be consolidated: when we sleep, our hippocampus takes all of the things we’ve learned during the day and files them away for easy access when we’ll need them next. Known contributors to this phenomenon are Sharp-wave-ripple events, or SWRs, are a brief (lasting only 100-200ms) oscillation of hippocampal neural activity occurring at approximately 200Hz. Interestingly, they occur not only during slow-wave sleep, but also during awake stillness—and although their presence during sleep has frequently been attributed to the memory consolidation process, their function during awake states is unclear. What role might SWRs play while we’re awake, and how does their occurrence affect our behavior?
Dr. A. David Redish at the University of Minnesota is interested in precisely these questions. His lab combines the use of multi-electrode recordings with computational analysis that allow for the study of neural ensemble activity at high temporal resolution. A recent (2016) paper from Dr. Redish’s lab, titled “Interplay between Hippocampal Sharp-Wave-Ripple Events and Vicarious Trial and Error Behaviors in Decision Making”, elucidates a possible role for sharp-wave-ripple events in the context of decision-making.
Redish and his group quantified the presence of Vicarious Trial and Error (VTE) behaviors—a measure of uncertainty and deliberation—in relation to the presence of SWEs captured through neural recordings as rats completed several complex navigation tasks to obtain rewards. These behavioral tasks enabled the experimenters to measure the rats’ learning, deliberating, and decision-making in the context of SWE occurrences. The experimenters were ultimately able to observe an association between SWEs in awake states and VTE behaviors as rats decided which path to take in the tasks.
Part A in the figure above demonstrates the nature of the spatial alternation task. A1 and A2 comprise the first trial, in which the rat had to exit the center arm and move to a side arm to obtain a reward, then return to its starting position in the center arm to obtain a second reward. The second trial, in A3 and A4, shows that the rat needed to travel to the opposite side arm from the first trial (and back to center) in order to obtain the rewards. Part B illustrates a key finding of Dr. Redish’s paper: that the disruption of hippocampal sharp-wave-ripple events (SWR) increased the proportion of VTE behaviors.
The results of Dr. Redish’s study suggest that SWRs and VTE behaviors are inversely correlated. Moreover, because VTE behaviors are associated with increased behavioral uncertainty and variability, they imply that SWRs and associated mechanisms engaged in learning and memory might play a specific role in decision-making once a particular behavior or reward has been learned. Thus, there is clearly a complex interplay of these two processes occurring during awake learning and navigation that demands exploration in greater detail.
To learn more about the work conducted in Dr. Redish’s lab, attend his talk this Tuesday, March 7th, at 4:00pm in the Center for Neural Circuits and Behavior.
Marley Rossa is a first-year Ph.D. student whose research interests encompass the use of computational techniques to analyze neural data and thereby understand the nature of information communicated between neurons and neural ensembles.