The goal of BiT was the understanding of the neural mechanisms underlying the perception of time in a range spanning from hundreds of milliseconds to a few seconds. To meet this goal, the entire project was structured in three working packages (WPs).
In WP1 with a series of fMRI experiments at ultra-high field (7T) we tested a new biologically plausible hypothesis of temporal representation via duration tuning and topography. In these studies, we showed the existence in parietal, occipital and frontal cortices of duration preferences, in SMA these preferences were topographically organized. Chronomaps show a high degree of flexibility of representation. They can indeed change according to the duration range and the temporal context hand, the perception, and the association with a different magnitude dimension like numerosity. Duration preferences in SMA and parietal cortex are present for both visual and auditory stimuli.
The aim of WP2 was to better specify the functional role of sensory cortices in time perception and to explore the link between spatial and temporal information within these areas. In a series of TMS experiments we first showed that spatial and temporal information are processed within the same neural circuits in visual cortex and follows the spatial topography. In addition, we showed that the causal engagement of primary visual cortex and extrastriate area V5/MT in time encoding is strongest while the stimulus unfolds over time and depends on the nature i.e. filled versus empty, of the temporal interval at hand. With two EEG experiments we then used perceptual adaptation (to either duration or temporal frequency) to distort stimulus duration perception and we showed that time distortions can be predicted by an early event related component (ERP N200) and an increase in the Beta band frequency spectrum in posterior electrodes contralateral to the adapted stimulus. Overall, our findings suggest that local and low-level perceptual processes are involved in generating a subjective sense of time. In WP3 we assessed the functional relationship and the temporal hierarchies between putative “time regions”. With two TMS and an EEG/TMS experiment we showed the existence of a cortical hierarchy of visual (V1) parietal (IPL) and frontal regions (SMA) associated to a temporal discrimination task, where early ERP component in V1 and SMA are necessary to temporal judgments. Moreover, we showed that while occipital areas are active during the encoding of stimulus duration probably via accumulation of sensory input, SMA is the area that reads-out this signal and has a high-level representation of stimulus duration.
Finally using Dynamic Causal Modelling, we were able to identify the effective connectivity structure between the cerebellum and this cortical network. The results highlight the role of the cerebellum as the network hub and that of SMA as the final stage of duration recognition. Interestingly, when a specific duration is presented, only the communication strength between the units selective to that specific duration and to the neighboring durations is affected. These findings link for the first time, duration preferences within single brain region with connectivity dynamics between regions, suggesting a communication mode that is partially duration specific.
