FBS Colloquia No.318Dynamic Brain Network Laboratory

Neural origins of the sense of proper direction of time

It is a long-lasting knowledge, or belief, of humans that time flows in one direction and not in reverse. To elucidate where and how the knowledge has been installed in the brain, we introduced a direction judgement task, in which participants judged whether short video clips in our daily scenes (3 s in duration, n = 360) were played in normal direction or in reverse. Interestingly, the reaction was more synchronous and faster for the detection of reversal than for the detection of normalcy, in a few critical situations, such as forward motions of animals, humans, and vehicles, free fall of objects, diffusion of particles, and division (or addition) of some materials by the hand. It seems that our brain is equipped with a "reversal" detector rather than a "normalcy" detector, but it does not make sense to assume that the brain is prepared for the reversal that never happens in the actual world.
An additional functional imaging study gave us a clue to solve the riddle. During the direction judgement task, there appeared a strong bidirectional functional connectivity between the left cerebellum, a putative site where the forward models are acquired, and the right middle temporal gyrus, where motions of living and non-living organisms are represented. Activations of these regions were enhanced when the video clips with fast detection of reversal were played in reverse. From the results we suggest that the sense of the proper direction of time is complementarily supported by a "reversal" detector that actually serves to detect prediction errors of forward models, which automatically and unconsciously predict how the external world unfolds itself in the next moment.