OLFACTORY DISCRIMINATION LEARNING IN THE VIRGINIA OPOSSUM

Abstract

As in other marsupials, the brain of the Virginia opossum (Didelphis virginiana) has no corpus callosum connecting the cerebral hemispheres, making it suitable for splitbrain experiments of interhemispheric communication (Herring, Mason, Doolittle, & Scarrett, 1966). In previous split-brain experiments done on rats, cats, and monkeys, the sensory modalities of touch and vision were studied. In the opossum, however, neither of these modalities was suitable due to the bilateral cortical representarion of touch stimuli and the inaccessibility of the optic chiasm which must be sectioned in vision srudies (Lende, 1963). T he sensory modality of olfaction was chosen for study because of the accessibility of the olfactory bulbs and the availability of data on techniques of electrophysiological recording from the olfactory bulbs of opossums (Phillips, 1962). To test the practicality of using olfaction to study interhemispheric communication, in the present study we attempted to teach opossums an olfactory discrimination. Method.-Four adult Virginia opossums, one male and three females, were Ss. They were housed individually in 24-in. x 24-in. X 36-in. stainless steel cages with water continuously available. Ss were given pelleted dog food for 1 hr. each day for 3 days prior to the beginning of maze trials. The apparatus was a cross-maze used as a T-maze in this experiment. The start box and goal boxes were 12 in. X 12 in. x 30 in. (inside dimensions) and the choice point connecting these was 12 in. X 12 in. X 12 in. The maze had black plastic walls, a clear plastic top, and a grid floor. The guillotine doors of the maze were electronically controlled from a monitor in a room adjacent to the maze room. The maze room was 8 ft. x 10 fc. x 8 ft. and was dark during the running of Ss, with the exception of light from photocells located 6 in. from the end of each arm of the maze. A 5-in. X 4-in. X 3-in. glass dish was placed in each goal box near the choice point and a 5-in. X 4-in. glass plate was placed at the end of each goal box away from the choice point. The olfactory stimulus was two drops of oil of wintergreen in one of the dishes and one drop in the corresponding plate. The glassware was washed out and a fresh stimulus applied at the beginning of each day's trials. A trial required placing the glassware with the olfactory stimulus randomly in one or the other goal box and allowing S to run from the start box into one of the goal boxes where a photocell switch was activated closing the guillotine doors. On the glass plate in the goal box containing the olfactory stimulus was the reinforcement, an M&M candy which is palatable and easy to use.