Subthreshold violations of trajectory predictions are sensitive to TMS of cerebellum Crus I/II.

We recently suggested that when visual objects move towards each other, it may involve a trajectory prediction and sensory checking at the level of milliseconds. We examined this possibility by exploring the consequences of a sub-threshold manipulation of squares trajectories on a newly found illusion, i.e., the perception of a gap at the time of the collision. We first examined the consequences of the manipulation on the conscious illusion. In a more exploratory way, we studied the EEG correlates of the manipulation, as well as the impact of transcranial magnetic stimulation (TMS) on the cerebellum (right Crus I/II). The TMS was a typical intermittent theta-burst stimulation, but only one sequence of around 3 min, compared with a placebo stimulation. All participants participated to two TMS sessions, in the sham-verum or in the verum-sham order. The trajectory manipulation occurred within less than 60 ms before the collision and reliably decreased the illusion rate. The results suggest that the prediction is temporarily stopped after the trajectory change. The illusion was accompanied by late positive ERP signals in CP4. The amplitude of these signals was modulated by TMS verum on the cerebellum, but only in the sham-verum order. As a whole the results are consistent with automatic verifications of the trajectory regularity at the millisecond level.