We just posted the preprint for a method that allows 2-photon imaging while mice freely rotate horizontally and run around a real (or virtual) 2-D environment. The system allows attaching other instruments (ephys, opto, etc.) to the headpost. We think that this approach is useful not only for studies of 2-D navigation, but more generally will allow studies of natural and computationally complex behaviors.
The mice run around on an air-floated maze (similar to Kislin et al. 2014 and Nashaat et al. 2016). Horizontal rotation has been demonstrated to work well for behaviour in VR in rats using a harness (Aronov and Tank 2014) and more recently with a head-fixation system in mice (Chen et al. 2018), and seems to not only make the animals more comfortable, but also seems to preserve head-direction encoding and grid-cell activity.
— Jakob Voigts (@jvoigts) March 2, 2018
Our system is well tolerated by mice with minimal habituation, and we get stable 2-photon imaging even during fast head rotations and locomotion (see video).
Jakob Voigts, Mark Harnett: An animal-actuated rotational head-fixation system for 2-photon imaging during 2-d navigation
The main feature of our approach is that the rotation is active – we measure the torque applied by the mouse and move the headpost with a motor which has enough torque to quickly accelerate/decelerate the heavy rotating headpost, making it appear to have low friction and inertia. This means that the weight of the headpost doesn’t matter much, so we could make the system mechanically stable (and you can attach whatever instruments to the headpost – neuropixel probes anyone?).
Also, we modified the usual flat air maze approach (Kislin et al. 2014 and Nashaat et al. 2016) to be rotationally restricted: The maze can translate but not rotate, which is important in order for the torque applied by the animals to go completely to the headpost, where it is measured and actively compensated instead of spinning the maze.
The system right now depends on a fair bit of strategically applied epoxy, but we’re in the process of turning it into a (somewhat) easily replicated add-on to existing systems.