Ontogeny of object versus location recognition in the rat: acquisition and retention effects

Abstract

Novel object and location recognition tasks harness the rat's natural tendency to explore novelty (Berlyne, 1950) to study incidental learning. The present study examined the ontogenetic profile of these two tasks and retention of spatial learning between postnatal day (PD) 17 and 31. Experiment 1 showed that rats ages PD17, 21, and 26 recognize novel objects, but only PD21 and PD26 rats recognize a novel location of a familiar object. These results suggest that novel object recognition develops before PD17, while object location recognition emerges between PD17 and PD21. Experiment 2 studied the ontogenetic profile of object location memory retention in PD21, 26, and 31 rats. PD26 and PD31 rats retained the object location memory for both 10-min and 24-hr delays. PD21 rats failed to retain the object location memory for the 24-hr delay, suggesting differential development of short- versus long-term memory in the ontogeny of object location memory.

Keywords: Long-Evans; hippocampus; incidental learning; long-term retention; perirhinal cortex; short-term retention.

FIGURE 1

A: A schematic diagram of the novel object recognition (OR) task. The objects here are shown in Configuration 1. In the sample phase, the rat is placed in a chamber with two identical objects (fake apple) for 5 min. Following the sample phase, the rat is removed from the arena for a 5-min delay. When the rat is returned to the chamber for the test phase, one object from the sample phase has been replaced with a novel object (glass jar). B: A schematic diagram of the object location recognition (OL) task. Objects are shown in Configuration 1. The procedure is the same as that for the OR task except the test phase. In the test phase, one of the familiar objects is moved to a novel location in the arena.

FIGURE 2

Total exploration time (s) for each age group during the study phase of Experiment 1A. Mean exploration times are shown with error bars representing SEM. (*p <.01). Rats in the PD17 age group explored objects for significantly less time than rats in either the PD21 or PD26 groups, which did not differ.

FIGURE 3

Results of Experiment 1A shown as novelty exploration ratios by age group in the object recognition task (left panel) and the object location recognition task (right panel). Exploration ratios were calculated as (time exploring novel)/(total exploration time of both objects). Rats in each condition exhibited significant novelty preference, except for the PD17 rats in the OL task which did not significantly differ from chance performance (.5). (#p <.05; *p <.01)

FIGURE 4

Results of Experiment 1B shown as novelty exploration ratios by weaning group in the OR task (left panel) and OL task (right panel). The data for the weaned PD26 group is the same as that reported in Experiment 1A. Exploration ratios were calculated as (time exploring novel)/(total exploration time of both objects). Rats in each condition exhibited significant novelty preference. (#p <.05; *p <.01).

FIGURE 5

Total exploration time (s) for each age group during the study phase of Experiment 1C. The PD17 group is the same as that reported from Experiment 1A. The PD21 and PD26 groups shown are only from the Restricted group in Exp. 1C. Mean exploration times are shown with error bars representing SEM. (#p <.05). Rats in the Restricted PD26 age group explored objects for significantly more time than rats in either the PD17 or Restricted PD21 groups, which did not differ. The restricted time manipulation made the study phase exploration times of the PD21 rats comparable to that of the PD17 rats from Experiment 1A.

FIGURE 6

Results of Experiment 1C shown as novelty exploration ratios by age group. The PD17 group is the same as that reported from Exp. 1A. The PD21 and PD26 groups shown are only from the Restricted group in Exp. 1C. Exploration ratios were calculated as (time exploring novel)/(total exploration time of both objects). Rats exhibited significant preference for the displaced object only at PD21 and 26 ages. (#p <.05).

FIGURE 7

Results of Experiment 2A shown as novelty exploration ratios by retention interval condition. Exploration ratios were calculated as (time exploring novel)/(total exploration time of both objects). Rats exhibited significant preference for the displaced object after all four retention interval lengths. Rats that had not previously studied the objects (Test only) showed no object preference. (#p <.05; *p <.01).

FIGURE 8

Results of Experiment 2B shown as novelty exploration ratios by retention interval and age groups. Exploration ratios were calculated as (time exploring novel)/(total exploration time of both objects). PD26 rats exhibited significant preference for the displaced object after both the short and long retention intervals. PD21 rats showed significant preference after the 10-min delay, but failed to differ significantly from chance performance (.5) after a 24-hr retention interval. (#p <.05; *p <.01)