A characteristic of human conscious experience is the ability to mentally project oneself to a new time location, either in the past or in the future, i.e. the so-called Mental Time Travel (MTT). The aim of the present project is to study the neural basis of MTT.

It has been proposed that the self-projection in time is similar to the self-projection in space, emphasizing the role of spatial navigation in MTT. Indeed, the posterior parietal cortex, a key region for visual imagery in space-related tasks, and the temporo-parietal junction, a key region for mental projection, are both recruited by self-projection in time and space. However, the specific neural computation of the two processes remains unclear. One way to address this is by investigating the brain oscillations that may modulate MTT. One plausible candidate for MTT processing are beta oscillations because of their role in the production and perception of short temporal intervals, as opposed to alpha oscillations which correlate with illusory changes in self location in space. Thus, we hypothesize that the MTT may rely on posterior parietal cortex's beta oscillations to self-project in time. Moreover, because previous studies showed a different performance in time processing between young and elderly health population, we want to investigate also if there is a differences between these two populations in the MTT.

To assess the MTT we developed a new task in which participants see stimulus faces of different ages, one at a time. Each face is presented with a short phrase describing a particular life event, commonly happening in middle age. Participants perform a 2-alternative forced choice: in the 'Past Projection', they indicate if it is likely or unlikely that the stimulus person has lived the life event 10 years ago; in the 'Future projection' that the stimulus person will live the event 10 years in the future. We use transcranial alternating current stimulation (tACS) to test within subjects the role of target (Beta) and control oscillations (Alpha), in addition to a control non-stimulation condition (Sham). To rule out that tACS may simply affect age perception, participants are also asked to report the age of each stimulus face (Age Estimation task). Moreover, to investigate if there is any correlation between the MTT and the perception of short temporal interval, participants perform a time reproduction task before the brain stimulation and the MTT task. In the Time Reproduction task, a blue square is presented with a variable duration (encoding phase). Immediately after the encoding phase, a red square of the same size and position is presented. Subjects press a response button when they considered that the same of the previously studied time interval has elapsed.

A pilot study from 11 participants shows that beta-tACS corresponds to a shift in the psychometric curve towards a younger age in the 'Past condition', and towards an older age in the 'Future condition'. Thus, participants indicated a younger face as more likely to have lived an event 10 years ago and an older face as more likely to live an event 10 years in the future. This suggests that beta-tACS leads to an under-estimation of time relative to sham and alpha-tACS. Crucially, there were no significant differences in estimating the veridical ages of the stimulus faces. Thus, beta oscillation may modulate self-projection in time, since these data are consistent with an under-estimation of time.

If confirmed, results will allow us to specify that self-projection in time relies on the posterior parietal regions and beta oscillations. Moreover, if supported by our future experiments involving space processing, this result may also shed a new light on the distinction between self-projection in space, which is known to rely on alpha, and self-projection in time that relies on beta. Overall, these data may point to the fact that beta oscillations are critical for processing long durations, in addition to their well-known role in supporting short durations. This may be because beta oscillation may serve as a memory standard to which different durations are compared.