Reversed theta sequences of hippocampal cell assemblies during backward travel

3 years ago



Rats were made to 'walk' left while moving right (by putting them on a small treadmill mounted on a model train), while recording place cells. Does theta phase continue to correlate with absolute position within the place field? No. Phase precession is 'reversed'. Likewise, theta sequences start from where the rat came from, and extend to where he is going, regardless of the direction he faces while running. Interestingly contradicts most network models of theta sequences.



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.

Re: Novelty


Theta sequences flips while head direction signal remains forward - very nice demonstration that theta sequence direction isn't a head-direction biased attractor network sequence.

Re: Rigor



I have two criticisms, one major and one minor. Starting small, the use of rainbow color maps is highly misleading due to the non-uniform sensitivity of human vision across the visible spectrum, the non linear progression of luminescence across colors, and artificial 'bands' that it introduces into data. This document outlines the problem and solutions: The major criticisms i have is that the the trained moved at a constant velocity regardless of the effort that the rat put forth on the treadmill. I think that coupling the velocity of the treadmill to the velocity of the train (via a direct or inverted mapping) would be a major improvement because it would link the animal's motor commands to its physical location in terms of the 'allocentric' frame of reference. Perhaps hippocampal circuitry is 'disregarding' the allocentric frame of reference because it 'realizes' that the mutual information between motor commands and allocentrially-referenced position is ~0. If the train's movement was coupled to motor output, forward or backward, this would not be the case.

Re: Rigor