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Locomotion Monitor

Project Scope and Requirements

The goal of this project is to design and fabricate a locomotion monitoring system for mouse cognitive research. There are two labs at the UC San Diego Medical School that are sponsoring this project: the Bloodgood lab and the Hnasko lab.

The Bloodgood lab is studying how neurons are fired in sequence to give a sense of displacement when walking. In order to monitor how the mouse encodes displacement, the mouse should be able to travel a relatively large distance without returning to the starting point. A liquid reward such as sugar water must be available to the mouse to make it more cooperative. Accurate position data must also be available to compare with neurological data.

The Hnasko lab is studying how the brain responds to stimuli and how actions such as walking are initiated from the brain. In order to establish links between stimuli, actions, and brain activity, the system must have low friction and inertia so that it is possible to detect the exact moment the mouse starts to walk.  This system must also be able to dispense liquid rewards, but also able to track the amount of sugar water the mouse consumes as well as the times which it does so.  Cues must be delivered to mouse in form of light and sounds.

The needs shared by both labs are summarized below:

The system must

• Have low enough friction to allow the mouse to move the track beneath it while in a head-fixed setup

• Accurately track the distance traveled by the mouse

• Provide behavioral cues to signal the mouse to walk

• Dispense a reward to the mouse for completing certain actions, such as running on cue

Description of Design

Each lab received a locomotion monitoring system. The essential system design is pretty much the same; it's scaled up for the Bloodgood lab and scaled down for the Hnasko lab.

Below is an exploded view of the system. The air chamber is pressurized by an impeller fan, and supports the lightweight foam disk similar to an air hockey table. Thus the disk is free to rotate with very little force required. The position is recorded with the optical encoder, and visual/audio stimuli is delivered to the mouse via the LCD monitor.

The mouse is headfixed to a certain position on the disk, where it can start and stop the disk from rotating by running in place (depicted below).

Results:

In the end, both Locomotion Monitoring systems were able to provide position tracking with a resolution approaching 0.089 mm, with a sample period of 1.3ms. The air flotation system was able to reduce friction in the disk such that the force to start the Bloodgood lab’s disk was 0.000969 N and the force to start motion in the Hnasko lab’s disk to 0.000574 N. The inertia of the larger disk is 0.0016 kg-m^2 and 0.00039 kg-m^2 for the smaller disk. This resulted in the test mice reaching un assisted accelerations equivalent to 85 % of unhindered acceleration for the Bloodgood lab and 91 % for the Hnasko lab. To achieve this disk performance, a motorized impeller with a flow rate of 0.204 m^3/s (433 CFM) for the Bloodgood system and a flow rate of 0.052 m^3/s for the Hnasko system.