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A fundamental question in comparative cognition concerns the evolution of consciousness. It is unlikely that consciousness appeared in humans without any precursors in other animals. Yet, the concepts that dominate our understanding of consciousness are inherently human centered, focusing on subjective experiences with a rich use of language. This understanding of consciousness is likely empirically intractable in studies of nonhumans. An alternative approach focuses on adopting a functional perspective. What function does consciousness serve? What can an animal capable of such a function do via its behavior? In this connection, I review the development of animal models of episodic memory. Episodic memory involves recalling the past and in humans is described as the phenomenological conscious experience of projecting oneself (autonoesis) in time (chronesthesia). Because there are no agreed upon empirical approaches to investigate subjective experiences in nonhumans, efforts to develop animal models of episodic memory have focused on the contents of episodic memory. I review experiments using rats that suggest that, at the moment of a memory assessment, the animal remembers back in time to an earlier event or episode. I conclude by evaluating implications of episodic memory in rats as a functional window into the evolution of consciousness. 

I outline the perspective that searching the contents of memory is a form of mental time travel in nonhumans that is relatively tractable because it focuses on the contents of memory. I propose that an animal model of mental time travel requires three elements: (1) the animal remembers multiple events using episodic memory, (2) the order of events in time is included in the representation, and (3) the sequence of events can be searched to find a target that occurred at a particular time. I review experiments suggesting that rats represent multiple items in episodic memory (Element 1) in order of occurrence (Element 2) and engage in memory replay to search representations in episodic memory in sequential order to find information at particular points in the sequence (Element 3). The cognitive building blocks needed for mental time travel may be quite old in the evolutionary timescale. 

A fundamental question in the development of animal models of episodic memory concerns the role of temporal processes in episodic memory. Gallistel (1990) developed a framework in which animals remember specific features about an event, including the time of occurrence of the event and its location in space. Gallistel proposed that timing is based on a series of biological oscillators, spanning a wide range of periods. Accordingly, a snapshot of the phases of multiple oscillators provides a representation of the time of occurrence of the event. I review research on basic timing mechanisms that may support memory for times of occurrence. These studies suggest that animals use biological oscillators to represent time. Next, I describe recently developed animal models of episodic memory that highlight the importance of temporal representations in memory. One line of research suggests that an oscillator representation of time supports episodic memory. A second line of research highlights the flow of events in time in episodic memory. Investigations that integrate time and memory may advance the development of animal models of episodic memory. 

Vivid episodic memories in humans have been described as the replay of the flow of past events in sequential order. Recently, Panoz-Brown et al. (2018) developed an olfactory memory task in which rats were presented with a list of trial-unique odors in an encoding context; next, in a distinctive memory assessment context, the rats were rewarded for choosing the second to last item from the list while avoiding other items from the list. In a different memory assessment context, the fourth to last item was rewarded. According to the episodic memory replay hypothesis, the rat remembers the list items and searches these items to find the item at the targeted locations in the list. However, events presented sequentially differ in memory trace strength, allowing a rat to use the relative familiarity of the memory traces, instead of episodic memory replay, to solve the task. Here, we directly manipulated memory trace strength by manipulating the odor intensity of target odors in both the list presentation and memory assessment. The rats relied on episodic memory replay to solve the memory assessment in conditions in which reliance on memory trace strength is ruled out. We conclude that rats are able to replay episodic memories. 

The publication of the centennial year of the Journal of Comparative Psychology is an occasion to reflect on the state of our discipline. In this article, I focus on one aspect of comparative psychology, namely comparative cognition. This focus stems from my long-standing interest in comparative cognition. The trends and challenges in comparative cognition share many of the trends and challenges in the broader field of comparative psychology. In the first part of this article, I outline my perspective on the field. Next, I consider challenges. I end with a section on prospects for the future.