Environment Diversity Drives Zero-Shot Generalisation of Predictive Spatial Representations

The hippocampus rapidly establishes spatial responses in novel environments. Recurrent networks trained to predict their own sensory inputs develop similar spatial representations, with units tuned to place and head direction, but are typically trained and tested in a single environment. How such networks come to transfer to novel environments, and which parts of their representations carry over, is largely unexplored. Here we varied the diversity of training environments to address both questions. A recurrent network was trained only to predict its next view from egocentric views and self-motion, in 1, 5, 20, or 40 of 50 procedurally generated environments that differed in shape, size, texture, and landmarks. The weights were then fixed and the network tested in the remaining, unseen environments. Spatially and directionally tuned units emerged in every network, but generalisation depended on training diversity. The increase in prediction error from seen to unseen environments fell from 89% after training in one environment to 3% after 40 and, with the total amount of training data matched, was 5% after training in 40 environments against 50% in five. In nearly every network, a ring of directionally tuned units rotated together between environments, as head direction cells in the brain do. Their preferred directions were bunched after training in one or five environments but tiled the circle more evenly after training in 20 or 40, and this evenness predicted generalisation across training conditions. Place units tended to rotate with the ring between environments, more closely than with environment geometry. An evenly spread head direction signal, conserved across species and present early in development, may therefore be a spatial primitive that allows maps to form rapidly in novel environments. We predict that, between differently shaped environments, part of the arrangement of place fields is preserved, rotated with head direction cells.