The hippocampus of the rat.
Memories allow us to record and store information, a central feature of our lives. Since the groundbreaking case of HM, a patient who lost most of his hippocampus, we have known that this brain structure is central to long-term memory formation. But we’re still not sure how the neurons of the hippocampus change on a cellular level to hold those memories in place. A new article published in Science provides a new insight into this: two different types of neurons, with different activity and adaptability, are both needed to handle memories.
The researchers behind the new study tracked the neural firing patterns of rats placed in mazes and allowed to find their way out. The authors were most interested in examining a type of neuron known as a “place cell.” These place cells are hippocampal neurons that are activated when the rat is in a certain place. They play a role in orienting the animal to its environment. Crucially, these cells are central to recalling memories—both positive and negative—related to a location.
The researchers studied these neurons in rats navigating a maze, and while they took a nap after the maze to consolidate their new memories. The authors were interested in a phenomenon called sleep-related hippocampal sharp wave ripples in these rats.
Hippocampal sharp wave ripples are wave-like patterns in voltage oscillations in the hippocampus, usually observed during immobility and sleep. These waves are involved in replaying memories acquired while awake, and sleep-dependent memory consolidation.
In their study of post-maze brain activity, the authors found that ranges of cells fired in a novel environment come from a combination of relatively fast-firing neurons and a distinct set of slow-firing neurons. These firing properties of neurons also tended to predict how flexible they are — slow-firing neurons more easily change their behavior (called “plastic”), while fast-firing neurons are more rigid.
Plasticity is a measure of how changeable a neuron is, how much it is able to respond to new stimuli and how much it contributes to the formation of new memories. In these rats, rigid, predominantly fast-firing neurons had low spatial specificity and underwent very limited changes during sleep consolidation.
In contrast, more plastic, slower firing neurons were more likely to become highly specific to a location during maze exploration. In addition, these slow-firing cells showed increased activity during hippocampal ripples and more bursting (meaning signals fired in rapid succession). Groups of these neurons also saw increased co-activation — meaning small populations all fired at the same time — during post-maze sleep.
So while both rigid and plastic neurons contribute to spatial repetition, their functions are different. Plastic neurons are better at identifying specific sites than stiffer neurons. While the rigid cells are clearly involved in the process, the precision in neural spatial coding is dictated by a small and highly plastic subset of slow-firing neurons.
This finding, while specific to place cells, sheds light on the phenomenon of episodic memory in general. It was previously believed that hippocampal replay networks consisted of similar neurons; this paper shows that different populations of neurons with different properties can work together to form and recall new memories.
Science2016. DOIs: 10.1126/science.aad1935, (About DOIs).