Making progress in non-human mental time travel

Humans can remember unique past events and plan for the future and they can imagine themselves at these events when they are not currently occurring, an ability often called mental time travel and thought to be distinctly human (Suddendorf and Corballis, 2007). The behavior of many non-human species indicates that they can also remember unique past events to plan for the future, however it is not known whether they actually imagine themselves outside of the present (Clayton et al., 2003). Corballis (2013a,b) recently commented on new research on the hippocampus (a brain region involved in spatial navigation) showing that rats may be able to imagine themselves in situations other than their current one because when resting they replay particular neurological sequences indicated by the firing of “place cells” associated with familiar and novel spatial trajectories (Gupta et al., 2010). Corballis considers this possible evidence that non-humans imagine themselves in the past and future and, thus, mentally travel in time. I agree that nonhumans likely do imagine themselves in the past and future, however, it is important to note that while both humans and non-humans activate these neurological sequences to engage in goal-directed behavior (Ekstrom et al., 2003), there is no direct evidence yet that these sequences indicate that imagination or planning is involved (Gupta et al., 2012). These studies lack evidence from brain imaging studies that link imagination and planning to the neurological sequences associated with spatial trajectories. Brain imaging studies would also help evaluate Corballis (2013a,b) and Suddendorf ’s (2013) claim that human mental time travel is more complex than that in non-humans because we can imagine not only locations, but also other aspects of particular scenarios including “. . . people, things, places, [and] actions” (Corballis, 2013a, p. 5). It is too early to arrive at this conclusion due to the lack of research investigating imagination in non-humans as well as the absence of nonhuman mental time travel experiments that examine behavior and neurological activity at the same time. To understand what mental processes are involved in mental time travel, we must look at what mental processes are occurring in the brain when performing behavioral experiments. Investigating the question of what nonhumans can imagine requires studying neurological activity across the whole brain, not just the hippocampus, since brain areas outside of the hippocampus are active when humans imagine other individuals, objects, and actions (e.g., Decety, 1996; Hassabis et al., 2013; Schlegel et al., 2013; see Polyn and Sederberg, 2014 for a review). Investigations of whole brain activity combined with creative experimental designs could determine whether non-humans use imagination to mentally travel in time.