Summation of pain produced in different anatomical regions*

Unescapable shock was administered to rats in a spatial preference task through pairs of electrodes implanted subcutaneously near the neck and tail. The animals were permitted to choose between shock through both pairs of electrodes or through one pair by crossing from one side of a cage to the other. Rats preferred shock in two locations to an equal amount in one location. The results indicate that summation of pain is greater when stimuli are added in the same rather than in different places.