A study of short-term remembering in the possum: Using a Delayed-Matching-To-Sample Procedure.

Abstract

In Experiment one 7 Brushtail possums were trained, using food, to perform a Delayed-Matching-To-Sample procedure using still and flickering light stimuli, over a 0, 1 and 2 second delay. A criterion of 80 percent correct for 5 consecutive days was set for the requirement to probe test. Probe session delays were 0, 0.5, 1, 2, 4, and 8 seconds. Performance was analysed using the measure of discrimination log d. Log d = 1 at the possums training delay and dropped back to log d = 0 at the longer inexperienced delays. The possums performance at this task was only adequate, so in case the stimuli were causing discrimination difficulties they were changed for the following experiment to horizontal and vertical light stimuli. Experiment two used the horizontal and vertical light stimuli to further test DMTS in possums. The possum's responding stayed at 50 percent correct, or chance, so the task was changed to a Simultaneous-Choice procedure. However, this did not have any effect on improving their results with responding accuracy remaining at 50 percent correct. Experiment three involved a conditional discrimination procedure, which was implemented to find out whether the possums could in fact discriminate between a horizontal and vertical light. All the possums in this experiment reached 80 percent correct overall and at responding to either a horizontal or vertical light. These results are enough to conclude that it was not the stimuli used in the previous two experiments which that had been causing the difficulties but the task itself. It is not known exactly why the possums were so unsuccessful in performing a DMTS or MTS task. More research into DMTS in possums in greatly needed and would offer a better understanding of the results of this study.