If the brain clock malfunctions, the mind uses the 'shortcut' of space

When we talk about time, we often 'draw' it without realising it: we move our hands from left to right, we put the past 'behind' and the future 'in front', somehow trying to visualise it. This strategy has both cultural and biological origins. The left-right representation is ingrained in our reading and inspection habits. The back-front representation is rooted in our locomotion habits (in other words, if it is true that shrimps walk backwards, for these animals the future is behind and the past in front). But does all this really indicate that the brain represents time, and in particular the passage of time intervals, in an inherently spatial manner?

In a recent study of ours, carried out in collaboration between Sapienza University of Rome and the Fondazione Santa Lucia Irccs and published in NeuroImage, we showed that for the brain, the 'spatialisation' of time represents a reserve strategy. This strategy comes into play when the internal mechanisms responsible for measuring temporal durations are inefficiently activated.

In our experiment, we asked young adults to distinguish the duration of short (1 second) or long (3 seconds) visual stimuli by pressing a button placed to their left or one placed to their right. In this task, the spatial representation of time is manifested in the fact that people respond more quickly by pressing the left button when they judge an interval to be short, and the right button when they judge it to be long. It is as if time, i.e. the passage from a short duration to a longer one, mentally flows from left to right. In the literature, this widely documented effect is known as STEARC (Spatial-Temporal Association of Response Codes) and has long been considered strong evidence in favour of the idea that time is represented by the brain in an inherently spatial manner.

The key result of our study is to have demonstrated that the STEARC effect does not appear when decisions are fast, but only emerges when decisions are slow. By leveraging current knowledge of brain electrophysiological (EEG) responses associated with the estimation of temporal durations, we were able to clarify the functional significance of this result. In trials in which decisions were delayed, the brain's time-measuring mechanisms were not optimally activated and, precisely because of this, the brain resorted to a 'compensatory' spatial representation of durations: the so-called Mental Time Line.

Over the past two decades, knowledge of the neural mechanisms underlying the perception and representation of time has grown rapidly. Mechanisms have been identified, distributed at different levels of the nervous system, that specialise in encoding durations belonging to different time ranges, e.g. below or above the second. Different neural modalities of time measurement have also been described: neurons that progressively increase their activity with the passage of time, or neuronal populations that show distinct discharge patterns at different times of the same interval. At a higher level of processing, it was finally shown that in the cerebral cortex, neurons sensitive to different durations are organised in an orderly manner, so that anatomically close neurons encode temporally close durations.