In his main work Ungar made rats choose to enter either a ligthed or a dark box. Normally nocturnal animals, upon entering the dark, rats were given an electric shock, and the rats were quickly trained to enter the lighted box. After a prolonged training, an extract was prepared from their brains, which was injected into mice which were tested in the same lighted/dark setup. By measuring time spent by the mice in the boxes, it was found that the mice injected with an extract from the treated rats could be distinguished from the ones injected with the extract from the untreated rats.