As important as memory is to our daily functions, the ability to extract fundamental features and commonalities from various episodic experiences and to then generalize them into abstract concepts is even more crucial for both humans and animals to adapt to novel and complex situations. Here, we report the neural correlates of the abstract concept of nests or beds in mice. Specifically, we find hippocampal neurons that selectively fire or cease to fire when the mouse perceives nests or beds, regardless of their locations and environments. Parametric analyses show that responses of nest cells remain invariant over changes in the nests' physical shape, style, color, odor, or construction materials; rather, their responses are driven by conscious awareness and physical determination of the categorical features that would functionally define nests. Such functionality-based abstraction and generalization of conceptual knowledge, emerging from episodic experiences, suggests that the hippocampus is an intrinsic part of the hierarchical structure for generating concepts and knowledge in the brain.Here is one figure and one movie from the paper:
Legend - Invariant responses over the geometric shapes, physical appearances, colors, construction materials, etc. (A) Invariant responses of Cell #1 to the geometric shapes of nests. As shown by both the perievent spike rasters and perievent spike histograms, Cell #1 exhibited a significant firing increase in response to a new circular nest made out of the top part of a cardboard coffee cup with a wall height of 2.5 cm and a diameter of 7.5 cm (Left), a square cardboard nest (Center), and a triangular cardboard nest (Right). (B) Invariant responses of Cell #1 to nests made from different materials. A metal nest (Left), plastic nest (Center), and porcelain nest (Right) were tested. (C) The cell also increased its firing when the mouse encountered natural cotton nests (Left and Center) but not to five cotton balls that were simply lumped together (Right). The bin width in the perievent spike histogram is 250 ms.
Click to download Movie 1. Description of movie: "Transient-on" type of nest cell responses and its functionality-based encoding of conceptual knowledge of nests. The first movie segment shows that Cell #1 of mouse-A exhibited "transient-on"-type responses to the home nest but not to another similarly shaped, smaller circular object (water cup). The second movie segment shows that Cell #1 did not respond to the plastic nest that was placed in an inverted manner (so that it would function as a small stage). However, once the plastic nest reverted back to its normal nest position, the cell exhibited robust firing up to 40 Hz. Because it is the same object and placed at the same location, this inversion experiment demonstrates that the nest cell encodes the functionality of the nest rather than merely physical appearances, materials, or spatial location, etc.