Fig. 5

Swivel balance cage system. We have built a simple system that prevents tension and torsion on the EEG cable as the rat freely moves in both the horizontal and vertical planes. The mobile rectangular plate that holds the standard swivel can move up and down through the cage’s ceiling as it pivots along a hinge (A double arrows). The cage is placed on the shelves so that the swivel is centered over it. The swivel is counterbalanced by circular weights (B black arrow) placed at the other end of the plate. The circular weights are loosely screwed via non-concentric holes allowing easy balancing by just turning the weights around which alters the torque they exert on the plate to the desired effect. Prior to attaching the animal to the cable, the weights are calibrated so that the counterbalance arm holding the weights is slightly heavier (B). This will provide a minimal continuous upward pull on the headset when the animal is attached, and prevent the swivel from remaining inside the cage when no downward tension is exerted by the animal, as occurs when the rat rises. These simple yet key modifications allow long-term continuous EEG monitoring without premature disconnections of the EEG cable from the rat’s head