Things always seem to go more smoothly in fairy tales than in real life. Little Red Riding Hood receives a straight answer to her simple remark. But when scientists say, “Oh, Mother Nature, what big brains you have given us,” Mother Nature only smiles, “All the better for you, my dear, to figure out my ways.” Perhaps.
Despite the spectacular advances that science has made, we still do not know why the human brain is so much bigger than the chimp's. Surely it has evolved to serve some adaptive functions that are not shared by our smaller-brained relatives, but, apart from language, it is not obvious what these functions might be. Interestingly, the difficulty of the problem itself may provide an important clue to the solution of the puzzle: Whatever these uniquely human functions are, they must be so subtle that even clever scientists do not see them. In other words, the clue is to look for differences between humans and nonhumans that are not obvious.
Nothing can be more subtle than the ability to reflect on what one has experienced in the past or might experience in the future. Although this ability has existed since time immemorial, until recently no one has paid much attention to it as an object of scientific interest. Now, however, mental time travel, the topic of Suddendorf & Corballis's (S&C's) target article, is moving to the center stage of brain/mind sciences with a vengeance. During the time alone that the S&C article spent in the publishing pipeline, a whole slew of prominent papers on mental time travel have appeared in leading journals (Addis et al. Reference Addis, Wong and Schacter2007; Buckner & Carroll Reference Buckner and Carroll2007; D'Argembeau & Van der Linden Reference D'Argembeau and Van der Linden2006; Schacter & Addis Reference Schacter and Addis2007b; Spreng & Levine Reference Spreng and Levine2006; Szpunar et al. Reference Szpunar, Watson and McDermott2007). There is every reason to believe that it is going to be among the hot topics of the future. Who can resist the allure of the idea of a universal yet invisible human trait?
We like the general thrust of S&C's story, as well as their novel attempt to decompose mental time travel into more basic components. This is the way of healthy science. We agree with many details of their argument. Specifically, we concur with their judgment that nonhuman animals probably do not possess the same kind of ability for mental time travel that humans do. Apart from the fact that, as yet, no convincing evidence of this ability has been shown by any creature other than humans, there are at least three other reasons why we side with S&C on this issue: (1) It helps with the puzzle of the large human brain. (2) The view opposing it helps one understand why mental time travel, until recently, had not been singled out for study. (3) It helps provide explicit guidance in the search for the neural underpinnings of mental time travel.
If we were permitted to offer one suggestion to S&C – and others who are or will be studying mental time travel – it would be to pay attention to the relevant concepts and their attendant terminology. In the forthcoming debate about mental time travel, arguments should be about facts and their interpretation, and not the words used to exchange ideas about them.
What exactly is mental time travel? What do we mean when we use the term? In the S&C story, the term “mental time travel” is used to refer to an evolved faculty or capacity, that “allows us not only to go back in time, but also to foresee, plan, and shape virtually any specific future event” (target article, Abstract) and thereby enhances flexibility in future-related behavior. At the same time, however, it is also used to refer to the kind of mental activity that it allows or enables, for example “anticipation of future needs” (sect. 3.3). Now, a capacity that allows or enables a particular activity is not the same as the activity itself. The distinction may be subtle, but it is fundamental, and it should be reflected in the terms that are used to refer to them.
The distinction between general capacity and particular behavioral/cognitive expressions of it may become especially relevant in the investigation of neural correlates of mental time travel, as illustrated by recent functional neuroimaging studies. In these studies, the temporal aspects of the mental time travel capacity (past vs. future) have been confounded with the details of the nature of the imagined activity: As a consequence of the employed tasks, what people imagine for future events is necessarily different from what they remember about past events. Ideally, and eventually, such confounding will be eliminated, and the neural correlates of the neurocognitive capacity that enables mental time travel (the “medium”) will be separated from the neural correlates of the contents of what is imagined or remembered (the “message”).
Our own terminological preference is to reserve the term “mental time travel” for the kinds of mental activity in which people engage when they remember particular past events or think about possible personal future happenings, and to use some other term, such as “chronesthesia” (Tulving Reference Tulving, Stuss and Knight2002a), to refer to the general neurocognitive capacity that makes such remembering and thinking possible. The concept of chronesthesia was introduced as a specifically time-oriented facet of autonoetic consciousness. The concept of autonoetic consciousness was introduced as a part of the description of a brain-damaged amnesic individual who could neither remember any personal past happenings nor imagine any personal future scenarios (Tulving Reference Tulving1985). This patient was said to have lost his autonoetic consciousness, a kind of consciousness that endows a healthy person with the capability of mental time travel, to “roam … at will over what has happened as readily as over what might have happened, independently of the physical laws that govern the universe” (Tulving Reference Tulving1985, p. 5).
If the Bischof-Köhler hypothesis survives the challenges from clever tiny-brained birds and their even cleverer keepers (e.g., Raby et al. Reference Raby, Alexis, Dickinson and Clayton2007), it will make sense to look for some unique properties – cytoarchitectonic, hodological, neurochemical – in the human brain. S&C mention the possible usefulness of this physiological criterion, and we think the optimism in this regard is well placed.
So, is the large human brain relevant to the story? Nobody knows at this stage; time will tell. But the world is full of testimony that larger, more recently evolved systems can perform feats that smaller, more ancestral ones cannot. If Mother Nature wants to hide her ultimate secrets, what better place for it than a humongous brain, the most elaborate structure known in the universe.
