Whenever I drink hot chocolate I’m always reminded of the first time I rode a bike. Simply recalling the moment my dad let go of my bike as I was sent down my neighbor’s steep driveway unearths the snapshots of my mom making me hot chocolate in celebration and my dad taking off my training wheels. Our brain’s ability to encode and store memories is a popular subject of neuroscience research but little is known about how the brain is able to link these memories in time. A current theory called the memory allocation process hypothesizes that learning triggers a temporary increase in excitability, which biases the representation of one memory to share the neuronal ensemble representing a subsequent memory, allowing the recall of one memory to increase the likelihood of recalling another memory.
Dr. Denise Cai is an Assistant Professor of Neuroscience at Icahn School of Medicine at Mount Sinai studying temporal linking as well as memory capacity and sleep’s influence on memory. Studies in her lab involve how groups of neurons, or neuronal ensembles, in the brain represent contextual memories and how information shared between these neuronal ensembles can provide insight into how the brain sews together the snapshots of our lives to provide a temporal sequence for our memory.
A study published by Denise Cai et. al. titled “A shared neural ensemble links distinct contextual memories encoded close in time” aims to provide evidence for the memory allocation process and describe the mechanisms by which the brain links two distinct memories in time. Cai used in vivo calcium imaging with mini-microscopes of the CA1 region of the mouse hippocampus, a brain region known to be involved in memory, during exploration of three novel environments (Figure 1a-c). These environments (A, B, and C) represent three different contextual memories. Calcium imaging allows the group to observe which neurons are active during exploration of each distinct environment and if these environments are represented by different or similar neuronal ensembles. The mice were exposed to two of the contexts either 5 hours apart (B and C) or 7 days apart (A and C). The group observed that a larger number of neurons are shared between the representations of environments explored closer in time, environments A and B, than the contexts that were explored seven days apart, A and C (Fig 1d,f). Importantly, the group saw no significant difference in the number of active cells in each context, accounting for the possibility that the shared ensembles were influenced by a difference in the levels of activity in different environments (Fig 1e).
Cai’s research provides evidence for the memory allocation process by showing that neurons activated by one context are also activated in a subsequent context, thereby supporting the notion that the activation of one neuronal ensemble can increase the likelihood of activation of another neuronal ensemble. The neurons that are shared between contexts could serve as the link between two memories shared close in time and provide a code for the sequencing of our memories that connect my first bike ride to a mug of hot chocolate.
To hear more about the work being done in Dr. Denise Cai’s lab, please join us at 4:00pm, Tuesday 03/10/2020 in the Marilyn G. Farquhar Seminar Room at the Center for Neural Circuits and Behavior.
____________________________________________________________________________________________ Kim Gagnon is a 1st year Neurosciences Ph.D. student currently rotating in Dr. Matthew Shtrahman’s lab studying the role of adult neurogenesis in memory formation and retrieval.