Researchers develop a computational mannequin of grid cell-mediated episodic encoding

Practically 50 years in the past, neuroscientists found cells throughout the mind’s hippocampus that retailer recollections of particular places. These cells additionally play an necessary function in storing recollections of occasions, referred to as episodic recollections. Whereas the mechanism of how place cells encode spatial reminiscence has been well-characterized, it has remained a puzzle how they encode episodic recollections.

A brand new mannequin developed by MIT researchers explains how these place cells may be recruited to kind episodic recollections, even when there isn’t any spatial part. In line with this mannequin, place cells, together with grid cells discovered within the entorhinal cortex, act as a scaffold that can be utilized to anchor recollections as a linked sequence.

“This mannequin is a first-draft mannequin of the entorhinal-hippocampal episodic reminiscence circuit. It is a basis to construct on to grasp the character of episodic reminiscence. That is the factor I am actually enthusiastic about,” says Ila Fiete, a professor of mind and cognitive sciences at MIT, a member of MIT’s McGovern Institute for Mind Analysis, and the senior writer of the brand new research.

The mannequin precisely replicates a number of options of organic reminiscence methods, together with the massive storage capability, gradual degradation of older recollections, and the flexibility of people that compete in reminiscence competitions to retailer monumental quantities of data in “reminiscence palaces.”

MIT Analysis Scientist Sarthak Chandra and Sugandha Sharma PhD ’24 are the lead authors of the research, which seems in the present day in Nature. Rishidev Chaudhuri, an assistant professor on the College of California at Davis, can also be an writer of the paper.

An index of recollections

To encode spatial reminiscence, place cells within the hippocampus work intently with grid cells – a particular kind of neuron that fires at many various places, organized geometrically in an everyday sample of repeating triangles. Collectively, a inhabitants of grid cells varieties a lattice of triangles representing a bodily house.

Along with serving to us recall locations the place we have been, these hippocampal-entorhinal circuits additionally assist us navigate new places. From human sufferers, it is recognized that these circuits are additionally essential for forming episodic recollections, which could have a spatial part however primarily encompass occasions, equivalent to the way you celebrated your final birthday or what you had for lunch yesterday.

“The identical hippocampal and entorhinal circuits are used not only for spatial reminiscence, but in addition for common episodic reminiscence,” Fiete says. “The query you possibly can ask is what’s the connection between spatial and episodic reminiscence that makes them reside in the identical circuit?”

Two hypotheses have been proposed to account for this overlap in perform. One is that the circuit is specialised to retailer spatial recollections as a result of these forms of recollections – remembering the place meals was situated or the place predators had been seen – are necessary to survival. Beneath this speculation, this circuit encodes episodic recollections as a byproduct of spatial reminiscence.

An alternate speculation means that the circuit is specialised to retailer episodic recollections, but in addition encodes spatial reminiscence as a result of location is one side of many episodic recollections.

On this work, Fiete and her colleagues proposed a 3rd choice: that the peculiar tiling construction of grid cells and their interactions with hippocampus are equally necessary for each forms of reminiscence – episodic and spatial. To develop their new mannequin, they constructed on computational fashions that her lab has been creating over the previous decade, which mimic how grid cells encode spatial info.

“We reached the purpose the place I felt like we understood on some degree the mechanisms of the grid cell circuit, so it felt just like the time to attempt to perceive the interactions between the grid cells and the bigger circuit that features the hippocampus,” Fiete says.

Within the new mannequin, the researchers hypothesized that grid cells interacting with hippocampal cells can act as a scaffold for storing both spatial or episodic reminiscence. Every activation sample throughout the grid defines a “nicely,” and these wells are spaced out at common intervals. The wells do not retailer the content material of a particular reminiscence, however every one acts as a pointer to a particular reminiscence, which is saved within the synapses between the hippocampus and the sensory cortex.

When the reminiscence is triggered later from fragmentary items, grid and hippocampal cell interactions drive the circuit state into the closest nicely, and the state on the backside of the nicely connects to the suitable a part of the sensory cortex to fill within the particulars of the reminiscence. The sensory cortex is way bigger than the hippocampus and may retailer huge quantities of reminiscence.

“Conceptually, we are able to take into consideration the hippocampus as a pointer community. It is like an index that may be pattern-completed from a partial enter, and that index then factors towards sensory cortex, the place these inputs had been skilled within the first place,” Fiete says. “The scaffold does not comprise the content material, it solely comprises this index of summary scaffold states.”

Moreover, occasions that happen in sequence may be linked collectively: Every nicely within the grid cell-hippocampal community effectively shops the data that’s wanted to activate the subsequent nicely, permitting recollections to be recalled in the correct order.

Modeling reminiscence cliffs and palaces

The researchers’ new mannequin replicates a number of memory-related phenomena way more precisely than present fashions which are based mostly on Hopfield networks – a kind of neural community that may retailer and recall patterns.

Whereas Hopfield networks supply perception into how recollections may be fashioned by strengthening connections between neurons, they do not completely mannequin how organic reminiscence works. In Hopfield fashions, each reminiscence is recalled in good element till capability is reached. At that time, no new recollections can kind, and worse, trying so as to add extra recollections erases all prior ones. This “reminiscence cliff” does not precisely mimic what occurs within the organic mind, which tends to step by step overlook the main points of older recollections whereas new ones are regularly added.

The brand new MIT mannequin captures findings from a long time of recordings of grid and hippocampal cells in rodents made because the animals discover and forage in varied environments. It additionally helps to elucidate the underlying mechanisms for a memorization technique referred to as a reminiscence palace. One of many duties in reminiscence competitions is to memorize the shuffled sequence of playing cards in a single or a number of card decks. They often do that by assigning every card to a specific spot in a reminiscence palace – a reminiscence of a childhood house or different setting they know nicely. When they should recall the playing cards, they mentally stroll via the home, visualizing every card in its spot as they go alongside. Counterintuitively, including the reminiscence burden of associating playing cards with places makes recall stronger and extra dependable.

The MIT group’s computational mannequin was in a position to carry out such duties very nicely, suggesting that reminiscence palaces reap the benefits of the reminiscence circuit’s personal technique of associating inputs with a scaffold within the hippocampus, however one degree down: Lengthy-acquired recollections reconstructed within the bigger sensory cortex can now be pressed into service as a scaffold for brand new recollections. This enables for the storage and recall of many extra objects in a sequence than would in any other case be doable.

The researchers now plan to construct on their mannequin to discover how episodic recollections may grow to be transformed to cortical “semantic” reminiscence, or the reminiscence of info dissociated from the precise context through which they had been acquired (for instance, Paris is the capital of France), how episodes are outlined, and the way brain-like reminiscence fashions could possibly be built-in into fashionable machine studying.

The analysis was funded by the U.S. Workplace of Naval Analysis, the Nationwide Science Basis beneath the Strong Intelligence program, the ARO-MURI award, the Simons Basis, and the Okay. Lisa Yang ICoN Heart.