Sparse-to-dense coding transformation between hippocampal areas CA3 and CA1

Bats Fifteen adult male Egyptian fruit bats ( Rousettus aegyptiacus ) were included in this study for neural recording experiments (weight, 160–200 g). Information on individual bats is summarized in Extended Data Table 1 . All experimental procedures for the neural recordings were approved by the Institutional Animal Care and Use Committee of the Weizmann Institute of Science. An additional 12 bats were included in a behavioural-control experiment in a smaller setup, without neural recordings (see below). Behavioural setups for neural recordings Four different behavioural setups were used in this study, shown in Extended Data Fig. 1a,b . All of the setups used tunnels with identical cross-section shape, with a width of 2.3 m and maximal height of 2.35 m (ref. 12 ), with uniform illumination (5 lux). Details on individual bats in each setup are found in Extended Data Table 1 . (1) Linear flight sessions in long tunnel: ten of the recorded bats flew back and forth in fixed-size linear tunnels of lengths 200 m or 130 m, shuttling between two landing balls positioned at the two ends of the tunnel (Extended Data Table 1 , bats 1, 3 and 7–14; Extended Data Fig. 1a ). Note that these lengths are rounded: The 200-m tunnel had an effective length of 194 m, and the 130-m tunnel had a length that varied between 129–134 m across sessions. …