Laura Colgin, PhD
Associate Professor
Department of Neuroscience
University of Texas at Austin
(April 24, 2018)
Gamma Oscillations in the Hippocampal Network
The formation and retrieval of memories is too complex for single neurons. The processes take a network of neurons functioning together. Networks of neurons firing in coordinated effort present as rhythms at different frequencies depending on the behavior. Gamma rhythms (at a frequency of roughly 8–100 Hz) are thought to reflect information flow in the hippocampus. Doctor Colgin discussed her work examining different gamma rhythm frequencies. Her research shows that fast and slow gamma rhythms may serve different purposes for memory encoding and retrieval.
How the brain stores and retrieves memories is one of the key questions in neuroscience. Isolated activity within individual brain cells, or “neurons”, cannot support complex cognitive operations such as learning and memory. Instead, groups of neurons must coordinate their activity to form functional networks that are capable of carrying out such high-level tasks. How is coordination of distributed neurons achieved in the brain? Brain rhythms reflect synchronized activity across large ensembles of neurons and are thought to be key for coordinating neurons during many cognitive tasks, including learning and memory.
The hippocampus is a brain area that is essential for creating and storing memories of events and experiences. Accordingly, rhythms in the hippocampal network are thought to play a role in learning and memory. Three major classes of rhythms are observed in the hippocampus: sharp wave-ripples (~200 Hz ripples occurring on ~0.1-1 Hz sharp waves), theta rhythms (~8 Hz), and gamma rhythms (~25-100 Hz). Sharp wave-ripples occur during quiet wakefulness and slow-wave sleep and are thought to play a role in “offline” memory consolidation. In contrast, theta rhythms occur during active behaviors. Gamma rhythms occur during all behavioral states but are largest when they are nested within theta oscillations. The late John Lisman, a Brandeis faculty member, put forward several highly influential hypotheses about the function of theta-nested gamma rhythms. For example, he suggested that sequences of gamma cycles nested within a theta cycle serve to organize sequences of hippocampal “place cells”, neurons that fire selectively in specific spatial locations.
Much evidence now suggests that the class of oscillations traditionally known as gamma should be subdivided into distinct subtypes that exhibit different frequencies and reflect distinct streams of information flow in the hippocampal network. “Slow gamma” rhythms (~25-55 Hz) couple activity in hippocampal subfield CA1 to inputs from CA3, a neighboring subfield that is believed to be essential for retrieval of previously stored memories. In contrast, “fast gamma” rhythms (~65-100 Hz) link CA1 to inputs from superficial layers of medial entorhinal cortex that transmit current spatial information. The Colgin lab hypothesized that if slow and fast gamma rhythms constitute different oscillatory states of the hippocampal network, then hippocampal place cells should represent spatial information differently depending on whether slow or fast gamma rhythms are present. In line with this hypothesis, they found that sequences of place cells represented relatively long paths in a temporally compressed manner during theta-nested slow gamma rhythms and accurately tracked ongoing trajectories in real-time during theta-nested fast gamma rhythms. They also obtained preliminary evidence suggesting that these different types of place cell ensemble firing patterns are differentially related to behavioral performance on a spatial memory task. The results suggest that distinct slow and fast gamma rhythms may prevent interference between memory encoding and retrieval operations in the hippocampal network.