13Hippocampal place cells are spatially tuned neurons that serve as elements of a 14 "cognitive map" in the mammalian brain 1 . To detect the animal's location, place cells 15 are thought to rely upon two interacting mechanisms: sensing the animal's position 16 relative to familiar landmarks 2,3 and measuring the distance and direction that the 17 animal has travelled from previously occupied locations 4-7 . The latter mechanism, 18 known as path integration, requires a finely tuned gain factor that relates the animal's 19 self-movement to the updating of position on the internal cognitive map, with external 20 landmarks necessary to correct positional error that eventually accumulates 8,9 . Path-21
integration-based models of hippocampal place cells and entorhinal grid cells treat 22the path integration gain as a constant 9-14 , but behavioral evidence in humans 23 suggests that the gain is modifiable 15 . Here we show physiological evidence from 24 hippocampal place cells that the path integration gain is indeed a highly plastic 25 variable that can be altered by persistent conflict between self-motion cues and 26 feedback from external landmarks. In a novel, augmented reality system, visual 27 landmarks were moved in proportion to the animal's movement on a circular track, 28 creating continuous conflict with path integration. Sustained exposure to this cue 29 conflict resulted in predictable and prolonged recalibration of the path integration 30 gain, as estimated from the place cells after the landmarks were extinguished. We 31 propose that this rapid plasticity keeps the positional update in register with the 32 60 Figure 1| Dome apparatus, experimental procedure, and sample data. a, Rendering of 61 dome apparatus. The dome shell is rendered semi-transparent for illustrative purposes. b, 62Photo of the apparatus. The dome is raised in the photo to allow visualization of the interior, 63but it is lowered as in (a) for the experiment. c, Illustration of experimental gain G. From the 64 same initial positions of the landmarks and rat, three different gain conditions are shown, in 65 both lab (top) and landmark (bottom) frames of reference. In each case, the rat runs 90° in 66 the lab frame. d, Profile of gain change and epochs during a typical session. An annular ring 67is always projected at the top of the dome (as shown in (a)) for illumination purposes, and is 68 not turned off even in Epoch 4. e, Representative firing rate maps for five different units from 69 five separate gain manipulation sessions, shown in the lab frame (top, middle rows) and 70 landmark frame (bottom row) during Epoch 3 (when the experimental gain was constant). 71 The plots in the top row are color scaled to their own individual maximum firing rates, 72 whereas the middle and bottom row plots are color scaled to the maximum firing rate of the 73 bottom plot of each pair. The difference in spatially averaged firing rates between landmark 74 and lab frames results from the distributed firing of the cells over the entire track in the l...