Objects as enrichment: Effects of object exposure time and delay interval on object recognition memory of the domestic pig

A modified spontaneous object recognition test was used to examine object recognition memory in the domestic pig. This test uses preference for a novel object over a previously encountered sample object as indicating recognition of the sample object, and no preference as indicating no recognition. Two factors hypothesized to affect object recognition are duration of exposure to the sample stimulus and delay interval before re-exposure. Both of these factors could be manipulated in a rotational object enrichment program for pigs. Reducing exposure time and increasing the delay interval before re-exposure should decrease object recognition and prolong novelty-induced object exploration. We exposed 5-week-old pigs to different sample objects in their home pens for 10 min and 2 days, respectively.)Wc tested for object recognition memory at various delay intervals after initial exposure by placing littermate pairs in a test pen for 10 min and recording snout contact with a sample object and a completely novel object. At a 1-h delay, half the pairs were tested with the 2-day sample object; the other half received the 10-min sample object. At a 3-h delay, pairs were tested with the opposite sample object. Pairs were also tested with the 2-day sample at a 5-day delay and the 10-min sample at a 6-day delay. We predicted that pigs would show a preference for the novel versus the 2-day sample object at all three delays, but would only prefer the novel object over the 10-min sample object at the 1-h and 3-h delays. Pigs did not show novelty preference in the presence of the 10-min sample object at any delay. Novelty preference in the presence of the 2-day sample object occurred at the 3-h (P < 0.05) and 5-day delays (P < 0.001), but not the 1-h delay. The lack of novelty preference when pigs were tested with the 10-min sample object may have been due to failure to habituate to the sample object. Testing in a different location from the initial sample object exposure and retroactive interference from exposure to the 10-min sample object may have contributed to a temporary lack of novelty preference when pigs were tested with the 2-day sample object at the 1-h delay. The finding that pigs retained a memory for the 2-day sample object for at least 5 days suggests that restricting object exposure to less than 2 days may help to preserve the exploratory value of objects rotated among pens. (C) 2006 Elsevier B.V. All rights reserved.