Discussion of jpn's comment: This is a creative paper with what seems to be a pretty straightforward, interpretable finding. I especially like that they spent the discussion directly comparing their results to large number of theoretical models, which clearly motivated the study, in order to constrain that space a bit. I thought the experimental design was simple and well motivated. It certainly had flaws, but quite elegantly decoupled sensory and motor streams that are almost always strongly coupled during normal animal behavior. In the sensory space, by running the train backward, while the animal ran 'forward' on the treadmill, propreoceptive (internal reference frame) sensory input provided competing, dissonant information compared to visual, auditory, olfactory (external reference frame) input. Further, by turning the treadmill off while the animal was pulled around by the train, the influence of motor output could also be decoupled from the animal's velocity/location in space, and prevented potential feedback onto hippocampal activity from motor circuit activation.