The DRT is founded on three main tenets: (1) that there are multiple memory systems, (2) that for some forms of memory the HPC overshadows other systems, and (3) that the overshadowing can be abated with additional learning ( Sutherland et al., 2010). The distributed reinstatement theory (DRT), however, introduced an alternate account to this time-dependent change it states that re-experiencing the learning episode can be key in making a memory become established in other networks and more resistant to HPC damage, whereas time is a mere cofactor of the process ( Sutherland et al., 2010). This phenomenon has been accounted for by long-term systems consolidation theories, which propose gradual strengthening of the representation in neocortical structures resulting in a decreased or differential contribution of the HPC to the recall process ( Squire, 1992 Alvarez and Squire, 1994 Squire and Alvarez, 1995 Nadel and Moscovitch, 1997 Frankland and Bontempi, 2005 Winocur et al., 2010 Sekeres et al., 2018). For instance, there is evidence suggesting that HPC damage is more likely to spare older or remote memories ( Scoville and Milner, 1957 Kim and Fanselow, 1992). Combined, the findings suggest that a visual discrimination memory trace can be strengthened in non-hippocampal systems with overtraining and become independent of the HPC.ĭamage to the hippocampus (HPC) can cause retrograde amnesia for memories that are termed episodic or declarative, yet the vulnerability of these memories to HPC damage is not ubiquitous ( Scoville and Milner, 1957 Rempel-Clower et al., 1996). In Experiment 3, increasing the additional learning beyond criterion by 230 trials, the amount needed in Experiment 2 to train the non-hippocampal systems in absence of competition, successfully prevented the retrograde amnesic effects of post-training hippocampal damage. In Experiment 2, rats received hippocampal damage before learning and required approximately twice as many trials to acquire the visual discrimination as control rats, suggesting that, when the overshadowing or competition is removed, the non-hippocampal memory systems only slowly acquires the discrimination. Regardless of this additional learning, extensive damage to the HPC caused retrograde amnesia for the visual discrimination, suggesting that the memory remained hippocampal-dependent. In Experiment 1, male rats were trained to criterion (80% accuracy on two consecutive days) on a visual discrimination and then given 50 additional trials distributed over 5 days or 10 weeks. In the current study, we examined this overshadowing process for a hippocampal-dependent visual discrimination memory in rats. However, this overshadowing may be mitigated by repeated learning episodes, making a memory resistant to post-training hippocampal damage. The hippocampus (HPC) may compete with other memory systems when establishing a representation, a process termed overshadowing. 2Department of Psychology, Colby College, Waterville, MA, United States.1Department of Psychology, Trent University, Peterborough, ON, Canada.