Hypothetical Experiences Encoded by Fast, Regular Firing of Hippocampal Place Cells
Post by Amanda McFarlan
What's the science?
Whether for planning, imagination or decision-making, the ability to construct a hypothetical scenario is an important cognitive process that is fundamental to the brain. Recent studies have identified place cells in the hippocampus (a brain region known to be important for memory and spatial navigation) as a potential neural substrate for thinking about hypotheticals, as place cell firing has been observed to encode hypothetical spatial paths. However, the mechanisms underlying this activity remain unclear. This week in Cell, Kay, and colleagues investigated the role of hippocampal place cells in encoding hypothetical experiences.
How did they do it?
To study the activity of place cells, the authors trained rats to navigate a maze. By design, the maze was extremely simple: it had a single fork where rats had to choose between left or right. The rats were either placed in the centre arm of the maze where they had to move towards the fork (choice imminent group) or they were placed at the fork immediately (choice passed group). As rats ran in this maze, the authors recorded and analyzed the activity of place cells in the moments before the rats chose either the left or right arm of the maze. By doing so, they could determine whether place cells encoded the unchosen arm, and thus, encoded a hypothetical future scenario. The authors examined place cell activity at three levels: single cells, cell pairs, and at the population level. At the population-level, the authors used a decoding algorithm that summarizes the activity of all the cells (approximately dozens to hundreds of cells) recorded in the experiment.
What did they find?
The authors initially found that pairs of place cells encoding either the left or right arm of the maze fired in an alternating pattern at approximately 8 Hz, suggesting that future scenarios (choosing the left or right arm) could be encoded extremely quickly yet also extremely consistently. The authors further found that place cells were also more likely to fire in an alternating pattern when rats were approaching the maze fork (choice imminent group) compared to when they were moving away from the fork (choice passed group). Next, the authors showed that place cells at the population level encoded left and right arms in alternation at 8 Hz, similarly to what was observed in pairs of place cells. In the second stage of their study, they found that place cell activity encoding hypotheticals occurs systematically at specific phases of an 8 Hz neural rhythm called hippocampal theta, indicating that the hypothetical-encoding activity originates from a specific internal brain process. Overall, these findings indicate that hypothetical future scenarios can be neurally encoded both quickly and regularly (at 8 Hz) and that the underlying neural activity can be observed not only at the population level, but even down to the single-cell level.
What’s the impact?
This is the first study to show that neural firing can encode multiple hypothetical future scenarios both quickly and consistently (8 times a second). The authors also found that such neural firing could be seen at the single-cell, cell-pair, and population levels, and was influenced by both behavioral and anatomical factors. Together, these findings provide insight into the neural basis of how the brain can come up with hypothetical scenarios, an ability that is essential to complex cognition.
Kay et al. Constant Sub-second Cycling between Representations of Possible Futures in the Hippocampus. Cell (2020). Access the original scientific publication here.