Evaluating knowledge of what, when, and where in rats
Babb, Stephanie J.
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Episodic memory was originally defined by Tulving (1972) as the recall of a personal, past event, and where and when the event occurred. A series of experiments using an 8-arm radial maze were designed to identify whether rats could discriminate what, when, and where. In each experiment, four forced locations were chosen to provide food (randomly chosen each trial; study phase). In Experiments 1, 2, and 3, three of these locations provided regular pellets, and one was randomly selected to provide a unique food (chocolate pellets). In Experiment 4, two locations provided regular pellets, and two randomly chosen locations each provided 3 pellets of a different unique food type (e.g., grape or raspberry pellets). The rats were then removed from the radial maze. After a short or long retention interval, the rats were returned to the maze, all 8 locations were available, and the locations that were not available in the study phase provided regular pellets (test phase). After the long retention interval, the unique location(s) also provided food. The rats made more revisits to the unique locations after the long retention interval than after the short retention interval. In Experiment 1, chocolate was paired with lithium chloride (LiCl) outside of regular testing. In subsequent trials using long retention intervals, the rats avoided the location that provided the devalued chocolate. This discrimination of what (food type), when (long vs. short retention interval), and where (location of the randomly chosen arm for each trial) could have been based on (a) time of day, (b) failure to encode the location of the unique food, (c) an inherent strategy to revisit the chocolate location more after a long than after a short retention interval, (d) switching from a win-stay to a win-shift strategy after the long retention interval, and/or (e) failure to specifically encode the content of the unique location. These possible alternative explanations were tested by controlling for time of day, administering the LiCl during the retention interval (i.e., after encoding its location in the study phase), comparing revisits to the unique location(s) after the retention intervals during initial and terminal blocks of training, and using two unique flavors and selectively devaluing one unique food type while leaving the other unaffected. These alternative explanations were rejected in subsequent experiments. The data support the hypothesis that rats have detailed knowledge of what, when, and where, and provide evidence of episodic-like memory.