Figure 1. Perturbations shapes. We used the same 16 perturbation shapes for the two reference stimuli. The first 12 perturbation shapes were combinations of two Fourier components, and the last four ones were random combinations of them:
and
, with T the duration of the perturbation and t = 0 the beginning of the perturbation. The first perturbations j = 1…7 were
. For j = 8,…,10, they were the opposite of the three first ones:
. For j = 11, 12 we used
. Perturbations 13 and 14 were random combinations of perturbations 1, 2, 3, 11, and 12, constrained to be orthogonal. Perturbations 15 and 16 were random combinations of fj for j ∈ [1,8] and gk for k ∈ [1,7], allowing higher frequencies than perturbation directions 13 and 14. Perturbation direction 15 and 16 were also constrained to be orthogonal. The largest amplitude for each perturbation we presented was 115 µm. An exception was made for perturbations 15 and 16 applied to the second reference trajectory, as for this amplitude they had a discrimination probability below 70%. They were thus increased by a factor 1.5. The largest amplitude for each perturbation was repeated at least 93 times, with the exception of perturbation 15 (32 times) and 16 (40 times) on the second reference trajectory.