The presence of grid structure was quantified by calculating, for each cell, a grid score based on rotational symmetry in the cell’s spatial autocorrelogram IWR-1 chemical structure (Sargolini et al., 2006 and Langston et al., 2010). Cells were classified as grid cells if they had grid scores and spatial information scores that each exceeded the 95th percentile of grid scores and spatial information scores, respectively, from a shuffled distribution

for the respective age group (Figure 4B). Two out of 128 cells (1.6%) passed this dual criterion in the P16–P18 group (Figure 4C). The fraction was slightly but significantly larger than in the shuffled data, where 0.2% of the cells passed both criteria (Z = 3.3, p = 0.001). In the P19–P21 group, seven out of 185 cells (3.8%) passed the dual criterion (chance level: 0.2%–0.3%; Z = 8.1, p < 0.001). At subsequent ages, the percentage of grid cells increased slowly (all p < 0.001). The percentage of cells that passed the grid cell criterion was significantly larger in the adult group than in the entire group of young animals (P16–P36; Z = 9.02, p < 0.001). Cells that passed the criterion for grid cells showed a significant increase in grid scores

check details across age blocks (Figure 4D; F(7, 82) = 3.858, p = 0.001). The stability of the grid fields increased significantly with age (Figures 4E and 4F; within trials: F(7, 82) = 6.1, p < 0.001; between trials: F(7, 82) = 11.1, p < 0.001); as did the spatial discreteness of the firing fields (ANOVA for spatial coherence: F(7, 82) = 2.9, p < 0.01; spatial information: F(7, 82) = 2.3, p < 0.05). Head direction cells were present in all age groups, in agreement with previous studies (Langston et al., 2010 and Wills et al., 2010). Directional

modulation was expressed by the mean vector length of the cell’s firing rate. Cells were classified as head direction cells if the mean vector length exceeded the 95th percentiles of shuffled distributions for both directional information and mean vector length. Fifty-five out of 128 cells (43.0%) passed the criterion for head direction cells in the P16–P18 group. This fraction is significantly larger than in the shuffled data, where 0.9% either of the cells passed both criteria (Z = 49.0, p < 0.001). The percentage of head direction cells did not increase with age (P19–P21: 40.5%; P22–P24: 34.5%; P25–P27: 29.6%; P28–P30: 25.3%; P31–P33: 34.1%; P34–P36: 35.0%, and adult: 48.8%). Cells that passed the criterion for head direction cells showed a significant increase in mean vector length across age blocks (F(7, 424) = 4.3, p < 0.001). The stability of directional tuning increased significantly (within trials: F(7, 421) = 3.8, p < 0.001; between trials: F(7, 406) = 3.6, p = 0.001). The key finding of this study is that entorhinal border cells are already present when rat pups make their first navigational experiences. When rat pups leave the nest at the age of 2.