Constant-force cable coupler and activity monitor for small unrestrained animals

A mercury commutator with retracting cable coupler and activity monitor is described. A coiled spring maintains stable tension on a spool that takes up slack in a flexible cable attached to the electrode skull cap of a freely moving rat. This vertical movement-compensation assembly swivels on a low-friction horizontal bearing that follows the animal’s circular movements. A sensitive measure of activity is achieved by using a photobeam to detect movement of the cable spool when the animal moves its head. The apparatus can be used with low-ceiling chambers and is suited to long-term continuous recording and stimulation experiments.