So what does the prefrontal cortex do? People who have suffered lesions to the prefrontal cortex can seem entirely normal at first encounter. Their motor skills are largely intact; they can understand speech and talk normally; yet depending on the precise location and extent of the lesion, they have distinct deficits in the higher-order cognitive functions.

These include alterations in short-term memory, personality, attention, decision-making, and inhibiting socially inappropriate behaviors. While people with prefrontal lesions can follow instructions and perform many tasks normally, they struggle to execute plans that require multiple steps and flexibly adapt as the circumstances change.

The prefrontal cortex also contributes to our ability to make long-term plans, delay gratification, and engage in mental time travel -- so people with prefrontal lesions are not the type to be saving much for retirement. One study used the temporal discounting task to examine how people with lesions to the prefrontal cortex balance immediate and long-term rewards.

Compared to healthy controls and people who suffered lesions to other parts of the brain, prefrontal-cortex patients were significantly more likely to choose smaller short-term rewards in lieu of larger delayed rewards.

Similarly, a number of brain-imaging studies indicate that the degree of activity in parts of the prefrontal cortex is correlated with how long people are willing to delay gratification in temporal discounting tasks.

Brain-imaging studies of healthy humans also suggest that the prefrontal cortex contributes to our ability to engage in mental time travel. For example, when people were asked to imagine a future scenario based on the name of a person and place that they knew, activity in the prefrontal cortex was higher than when they were asked to simply create sentences with those same words.

Furthermore, studies also suggest the prefrontal cortex is more active when people are asked to imagine potential future events compared to when they are recalling past episodes of their lives.

While the prefrontal cortex is important for mental time travel, it would be naive to say that is where mental time travel happens. Attributing any particular task to a specific brain area is a bit like watching a soccer game and asking whose job it is to score goals.

Players on the defense or offense certainly have different roles, but scoring is a team effort and in the end anybody can score. Future-oriented mental time travel is a complicated task that requires the orchestration of a number of different cognitive functions, including accessing past episodic and semantic memories, using these memories to conjure future scenarios, understanding the difference between the past and future, and the ability to judge whether the simulated outcome is desirable or not.

Additionally, it is not sufficient to simply imagine future scenarios: we must remember what we imagined -- in other words we must learn from our mental simulations. If you are planning a camping trip, you want to draw upon your memories of previous trips in order to decide what equipment you should bring. You also want to extrapolate from these memories to simulate novel worst-case scenarios: "What would happen if I were to sprain my ankle or be bitten by a snake?"

Once you have simulated these scenarios -- and assuming you are still going camping, it is important to learn from these simulations and make the appropriate preparations in case one of these scenarios were to transpire.

Given the cognitive complexity of mental time travel, it is to be expected that it relies on a collection of different brain areas working in concert. Indeed, lesion and imaging studies implicate a number of different areas in mental time travel.

As mentioned above, the amnesic patients k.C. Struggled not only to recall past episodes of his life, but to think about what he might do in the future. K.C.'s primary brain lesion was to the temporal lobes, the structure that contains the hippocampus (remember, the temporal in temporal lobe does not refer to time, but the temples or the temporal bone of the skull, which is beside the ears)

One study asked people with medial temporal-lobe lesions to imagine and then describe different potential future scenarios, such as what it would be like to win the lottery. Compared to healthy controls, amnesics with temporal-lobe lesions provided impoverished descriptions and relatively few details about what the experience would be like.

As if often the case, complex cognitive tasks such as mental time travel do not rely on any single brain area, but on a support network of many different areas, each contributing in its own way. In the case of mental time travel the medial temporal lobes may provide access to a foundation of past experiences, whereas the prefrontal cortex might flexibly manipulate these memories to dream up and evaluate novel scenario.

Interestingly, one thing mental time travel may not explicitly require is the ability to tell time.

Much as a calendar represents time but does not actually tell time (it is not a clock), the neural circuits responsible for mental time must represent the past, present, and future but don't necessarily need to actively be able to measure the passage of time.

The ability of animals to predict nature's cycles and anticipate the behavior of predators, prey, and mates alike was a powerful evolutionary adaptation.

Mental time travel was the next step: it made merely anticipating events in the external world an outmoded technology.

Mental time travel allowed humans to go from passively predicting the future to actively creating it. Not enough food? Create a future in which there is an abundance of food through agriculture. Not enough water for agriculture? Create dams, channels, and irrigation systems.

How did our ancestors acquire the ability to mentally project themselves into the past and future? Do we understand the concept of time because we are capable of mental time travel, or do we engage in mental time travel because we graps the concepts of past, present, and future?

Answers to these questions will not be forthcoming from animal studies. Whether we call it mental time travel or not, scrub jays and great apes do have the capacity to guide their present actions towards desirable future outcomes but, compared to humans, there is clearly a vast chasm in their capacity for future-oriented thinking.

If for no other reason, it is difficult to plan for the days, month, and years ahead without a semantic understanding of days, months, and years, or the ability to grasp the concept of time.

Mental time travel is a multidimensional cognitive trait. It is likely a product of numerous converging evolutionary steps, including semantic and episodic memory, language, a number sense, and the spatialization of time into a mental timeline.