Mental Time Travel, Language, and Rats
When you are not focusing on something and your mind starts to wander, what do you think of? Many people find that they think about about imagined events, either past or future. This is a process that has been called “mental time travel,” and some researchers have suggested that it is a uniquely human process. Interestingly, that is a theory that has come under fire.
The hippocampus is important in mental time travel, which has two aspects. Firstly, the hippocampus contains “place cells” that encode where a person or animal is located in space. A notable example is the hippocampus of London taxi drivers, who have to memorize a huge, complex map of London; studies have shown that they have enlarged hippocampi. The hippocampus also appears to be involved in human mental time travel, as this part of the brain is activated when people are asked to remember past episodes or imagine possible future ones.
But how can you tell if an animal is thinking about a past or possible future event? It is a simple matter of asking a research participant what they were thinking about, or asking them to recall an event, but this is impossible in animal subjects. What other options are there?
Well, for one, neuroimaging. Micro-electrode recordings have shown that place cells in the hippocampi of rats encode specific locations in a maze, and a later study showed that those same cells fire when the rats are outside of the maze, suggesting that the rats were re-experiencing (at least to a degree) their trajectory through the maze. Mental time travel.
Interestingly, the paths through the mazes that were encoded by specific cells were not always the paths that were shown in the “re-imagining” of the maze, suggesting that the rats were imagining future paths through the maze.
While the jump from place cells firing outside of a maze to mental time travel may be a bit of a stretch, the researchers could be onto something. The relationships between space and time in cognition are very complex, and any attempt to say that we have accurately pinned them down in an animal that cannot directly communicate its thoughts is certainly premature. That said, the neurological evidence is quite compelling.
An interesting corollary of this research is its relationship the development of language. It has been suggested that human language and the ability to discuss topics that were not immediately present co-evolved. The importance of mental time travel in this idea is clear — both future and past events would be extremely difficult to communicate without something as complex as human language.
Of course, this begs the question of why, if mental time travel is possible in animals like rats, humans are the only animals with advanced language capacities. This question is especially interesting when you take into account recent advances in the understanding of ape cognition, which shows that great apes have a deeper understanding of number, space, and other cognitive concepts than we previous thought.
So why are humans the only animals with language? No one has a definitive answer yet. But in my next post, I’ll address an interesting theory on the topic.
Corballis MC (2013). Wandering tales: evolutionary origins of mental time travel and language. Frontiers in psychology, 4 PMID: 23908641
Corballis MC (2009). Mental time travel and the shaping of language. Experimental brain research. Experimentelle Hirnforschung. Experimentation cerebrale, 192 (3), 553-60 PMID: 18641975
Keysers, C. (2012). Primate cognition: copy that. Nature, 482. 158–159. doi: 10.1038/482158a