Recognition without awareness in humans and its implications for animal models of episodic memory

Animal models of episodic memory-the ability to recall events from the past along with relevant spatial, temporal and other contextual detail-have been criticized on the grounds that it is difficult to determine if animals engage in the "mental time travel" that accompanies episodic recall in humans. It is possible that animals rely on mechanisms different from those operative in humans. We have added a new dimension to these concerns by showing that a test widely considered to measure episodic memory in humans could be constructed such that it measures nonconscious (implicit) memory under certain test parameters. The same parameters are common in tests frequently used to study recognition in nonhuman animals. In particular, recognition was driven by implicit memory when (1) recognition decisions could not be guided by semantic information, and (2) decisions were made with relatively high automaticity. These findings suggest that nonhuman animals may rely on implicit memory processing in many episodic-memory testing circumstances, as semantic content is minimal and automaticity is likely after the extended practice usually required. Identifying factors that promote and discourage recognition based on implicit memory is therefore necessary for creating accurate cognitive and neurobiological models of memory processing in humans and other animals.