Chronesthesia

Chronesthesia, or mental time travel, is a mental ability first hypothesized by Endel Tulving in the 80s. This refers to the ability to be aware of one's past or future. While many may describe it as uniquely human, others now argue that this ability can transcend to include non-human animals as well as birds. The mechanisms of mental time travel are not yet fully understood since there is a level of obscurity and complexity when trying to measure if or when someone underwent mental time travel or not. However, studies have been conducted to map out areas of the brain that may be responsible for mental time travel.

Overview
Chronesthesia is defined as a hypothetical ability that allows humans to be constantly aware of the past and the future. Endel Tulving, one of the pioneers in this field, explained that humans adapted chronesthesia as a way to advance their survival.

Some people may go further as to say that it is a crucial ability for humans. There seems to be some confusion about the definitions of episodic memory, memory for the future, and mental time travel. Episodic memory involves projecting oneself back in time and recollecting many aspects of previous experiences. Mental time travel is more robust when planning for the future than for re-experiencing past events. This makes sense since it is the present and the future, and not how the past is represented, that matter. Therefore, mental time travel involves both past and future thinking, while episodic memory only deals with mentally traveling to the past.

With regards to memory for the future, this is actually a subcategory of mental time travel.

Brain regions involved
Scientists have mapped regions of the brain that could possibly be involved in mental time travel. While these types of studies are not numerous, they help advance understanding of this complex phenomenon.

fMRI mapping of brain regions
Addis D. et al. conducted an fMRI study to examine neural regions mediating construction and elaboration of past and future events. The left hippocampus and posterior visuospatial regions are involved in past and future event construction, neural differentiation. The right hippocampus, right frontopolar cortex, and the left ventrolateral prefrontal cortex are involved in future event construction.

The elaboration phase, unlike the construction phase, has overlap in the cortical areas comprising the autobiographical memory retrieval network. In this study, it was also found that the left hippocampus and the right middle occipital gyrus were significantly activated during past and future event construction, while the right hippocampus was significantly deactivated during past event construction. It was only activated during the creation of future events.

Episodic future thinking involves multiple component processes: retrieval and integration of relevant information from memory, processing of subjective time, and self-referential processing. D'Argembeau et al.'s study found that the ventral medial prefrontal cortex and posterior cingulate cortex are the most activated areas when imagining future events that are relevant to one's personal goals than to unrelated ones. This shows that these brain regions play a role in personal goal processing, which is a critical feature of episodic future thinking.

Brain regions involved in the 'what' and 'where' of an event
Cabeza R. et al. conducted a positron emission tomography (PET) scan study on a group of human test subjects to identify the brain regions involved in temporal memory, which is based on a linear progression of events. Since 'recollecting a past episode involves remembering not only what happened but also when it happened', PET scans were used to find the areas of the brain that were activated when trying to remember a certain word in a sequence. The results show that temporal memory of past events involves the frontal and posterior brain regions.

Research
There has been a lot of research conducted in this area to map brain regions and discover whether animals can exhibit this phenomenon as well. Relationships have been established between people who exhibit mental time travel and various neurological disorders, such as neuroticism. Quoidbach J. et al. found that people with high neuroticism generate more negative future events than emotionally stable people.

One study showed that people with schizotypy have a greater feeling of mental time travel, along with more olfactory detail than normal people.

Episodic-like memory and planning for the future in great apes
Osvath et al. conducted a study on apes to show that they have the ability of foresight. The study consisted of testing for self-control, associative learning, and envisioning in chimpanzees and orangutans through a series of experiments. Critics questioned whether these animals truly exhibited mental time travel, or whether it was associative learning that caused them to behave as they did. The Bischof-Kohler hypothesis says that animals cannot anticipate future needs, and this study by Osvath tried to disprove the hypothesis.

The scientists showed that when the apes were presented with a food item in conjunction with a utensil that could be used to actually eat that particular food, these animals chose the utensil instead of food. They anticipated a future need for the utensil that overcame the current want for just a food reward. This is an example of mental time travel in animals. It was not a result of associative learning that they actually chose the utensil instead of the food reward since the scientists ran another experiment to account for that. Other examples, such as food caching by birds, may be examples of mental time travel in non-humans. Even survival instinct by certain animals such as elephants, in response to imminent danger, could involve mental time travel mechanisms.

Another study to show that great apes have the ability of foresight was conducted by Martin-Ordas G. et al. These scientists were able to show that 'apes remember in an integrated fashion what, where and when' a particular event had happened. Two experiments were conducted in this study-the first being an investigation of the content of the memories of apes i.e. could these animals remember when and where two types of food they were shown before were now hidden. The second experiment explored the structure of the memories. It was found that the apes' memories were formed in an integrated what–where–when structure. All these findings once again show that it is not instinctive or learning predispositions that made the animals behave the way they did; rather, they have the ability to mental time travel, just like humans can.

Episodic-like memory in western scrub-jays
In their study to show that birds exhibit episodic-like memory, Clayton et al. used 3 behavioral criteria: content, structure, and flexibility, to decide whether the food caching habits of these birds were evidence of their ability to recall the past and plan for the future. Content involved remembering what happened based on a specific past experience. Structure required the formation of a 'what-where-when' representation of the event. Finally, flexibility was used to see how well the information could be organized and re-organized, based on facts and experiences. Mental time travel involves the use of both episodic future thinking and semantic knowledge. This study also contradicts the Bischof-Kohler hypothesis by showing that some animals can mentally time travel into the future or back to the past.

Episodic future thinking in 3-5-year olds
In this study, the scientists show that children as little as 4 years old can exhibit episodic future thinking. While their study involved looking at 3-year olds as well, it was shown that those children could not handle the task as well as the older children, possibly due to development in linguistic abilities. The brain regions that were activated during the four tasks (episodic recollection, prospection, third-person theory of mind, and navigation) were similar. These regions were found to be the hippocampus, parahippocampal gyrus, retrosplenial cortex, posterior parietal cortex and the ventro-medial pre-frontal cortex.

Issues and Future Research
The two biggest questions in the field right now are whether mental time travel is unique to humans or not, and what the mechanisms of this phenomenon are. Suddendorf and Corballis say that while mental time travel is uniquely human, it does not mean that animals cannot exhibit future-oriented behavior. In contrast, it is that there are several characteristics that distinguish one from the other. For example, future-orientation can be based on instincts, and imagination may be involved. However, mental time travel does not involve these things. Rather, it is a mental state that cannot be directly observed. It involves flexibility in situations to adapt to individual goals, and therefore must be unique to humans. But, as Toomela describes in his study, since mental time travel is a complex episodic psychological phenomenon that is personal and subjective, evidence of it in animals can only be indirect. It may also be beneficial to look at research in the field of psychology to further understand this state.

The future of this field lies in understanding what the specific behavioral markers for mental time travel are, and being able to exactly identify which regions in the brain cortex correspond to mental time travel specifically. As was mentioned in the Clayton N.S. et al. study, research needs to be done to figure out whether episodic memory and future planning are linked. Also, if birds exhibit signs of episodic memory and future planning, how is it possible to link that to humans, since the mammalian cortex is different from that of a bird? It has been suggested to use more mammalian models in order to understand chronesthesia better, as well as to map out more regions in the cortex that are specifically devoted to mental time travel.