Hippocampal Ensembles Support Memory for Nonspatial Events in Time
Post by Megan McCullough
The takeaway
The firing patterns of neurons in the hippocampus code for sequences of nonspatial events which suggests that the hippocampus is critical for organizing our memories in time.
What's the science?
Previous research has implicated the hippocampus in playing an integral role in the relationship between memory and behavior. More specifically, across species, the hippocampus is known to be necessary for the ability to remember when events occurred and the ability to then use this information to predict future events. Although many studies have shown this relationship, it remains unclear how hippocampal neurons support this complex function. This week in Nature Communications, Shahbaba and colleagues aimed to uncover the neural mechanisms for the involvement of the hippocampus in the temporal organization of past experiences using electrophysiological techniques and statistical machine learning methods.
How did they do it?
Since previous research shows that the involvement of the hippocampus in memory is consistent across species, the following experiment was conducted with rats. The authors trained the animals in a nonspatial sequence memory task, which involved the presentation of sequences of odors. The rats were rewarded when they correctly identified whether each odor was presented in the correct position in the sequence. Over the course of a few weeks, neural activity was recorded from the pyramidal cell layer in the dorsal CA1 area of the hippocampus of each animal as they performed the task. The firing patterns of the neurons in the CA1 region during the task were then examined for patterns related to the time component of the memories of the odor stimuli. Machine learning was then used to uncover how sequential memories were represented by the firing pattern of the neurons in the hippocampus.
What did they find?
The authors found that the firing pattern of hippocampal ensembles encoded information about time during the presentation of each odor and for the full sequence of odors. The neuronal activity also captured the sequential relationship among the odors in the sequence. This illustrates the crucial role the hippocampus plays in supporting the temporal organization of memories.
What's the impact?
This study provides key evidence that hippocampal neurons code for temporal and sequential information of events and that this activity is important to correctly recall the order of events. These results broaden our understanding of the neural mechanisms behind how memories are organized with regard to time and how we use these memories to inform future events. This study joins a growing body of research into the mechanisms of human learning, memory, and decision making.
