Study identifies previously unknown function in the brain involving memory.

It is known that the hippocampus is central to episodic memory and spatial navigation both of which rely on sequences of spatiotemporal information.  Spatiotemporal trajectories are coded by theta sequences, ordered series of hippocampal place cell spikes that reflect the order of behavioural experiences. Theta sequences are thought to be organized by co-occurring gamma rhythms. However, how sequences of locations are represented during distinct slow and fast gamma subtypes remains poorly understood.

Now, a study from researchers at The University of Texas at Austin has identified a mechanism which compresses information needed for memory retrieval, imagination or planning and encodes it on a brain wave frequency that’s separate from the one used for recording real-time experiences. The team state that the breakthrough in understanding a previously unknown function in the brain has implications for research into schizophrenia, autism spectrum disorder (ASD), Alzheimer’s disease and other disorders where real experiences and ones that exist only in the mind can become distorted.  The opensource study is published in the journal Neuron.

Previous studies show brain cells share different kinds of information with one another using a variety of different brain waves, analogous to the way radio stations broadcast on different frequencies.  Theta sequences provide temporally compressed representations of spatial paths, which activate within individual cycles of the extracellular theta oscillation and may be important for encoding or retrieval of spatial memories.  A second type of oscillation in the hippocampus, known as gamma, is thought to interact with theta to temporally organize theta sequence.  In the brain, fast gamma rhythms encode memories about things that are happening right now; these waves come rapidly one after another as the brain processes high-resolution information in real time.  However, the question of whether longer sequences are represented during fast gamma and shorter sequences during slow gamma has not yet been investigated.   The current study found that one of these frequencies allows us to play back memories, or envision future activities, in fast forward.

The current study recorded the local field potential (LFP) and spiking activity from 604 place cells in the CA1 hippocampal area of seven rats traversing a linear track with the spatial path represented by place cell ensemble activity during individual theta cycles.  Results show that slow gamma rhythms, used to retrieve memories of the past, as well as imagine and plan for the future, store more information on their longer waves, contributing to the fast-forward effect as the mind processes many data points with each wave.

The lab go on to hypothesize that this mechanism can help explain how a person can imagine a sequence of events they’re about to do in a time-compressed manner; the person can plan out those events and think about the sequences of actions they’ll do.

The team surmise that their research could also explain why people with schizophrenia who are experiencing disrupted gamma rhythms have a hard time distinguishing between imagined and real experiences.  They go on to hypothesise that maybe the schizoid personality is transmitting their own imagined thoughts on the wrong frequency, the one usually reserved for things that are really happening.  For the future, the researchers plan to use neurological animal models similar to ASD, and Alzheimer’s disease in humans to better understand what role this mechanism plays and ways to counteract it.

Source: The University of Texas at Austin


Schematic showing location of all 56 recording sites in this study (red dots). Spatial Sequence Coding Differs during Slow and Fast Gamma Rhythms in the Hippocampus. Colgin et al 2015.
Schematic showing location of all 56 recording sites in this study (red dots). Spatial Sequence Coding Differs during Slow and Fast Gamma Rhythms in the Hippocampus. Colgin et al 2015.

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