experimental data for our simulations and did thus not need to compute the activation maps for other possible fixation durations (see Rothkegel et al., 2016; Sch¨ utt et al., 2017) and ii) negative values of the grid and all values smaller than a constant η were transformed by an exponential function such that each value on the grid was larger than zero. This was done because the likelihood of a scanpath containing any impossible fixation would be zero. Thus we could not differentiate between models in terms of likelihood otherwise. After transforming these small or negative values the combined target map was normalized again to obtain a sum of 1. 

SceneWalk StartMap . Since the original SceneWalk model was not intended to reproduce the early CFB (see results of model simulations), we developed a modified version of the original model that takes the sudden image onset during scene perception into account. We made two changes. First, contrary to the original SceneWalk model with a constant activation across the entire attention map at the beginning of a trial, we decided to use an attention map with higher activations near the center of an image than at the periphery (see Fig. 7b). This was motivated by the sudden image onset that may lead to an initial prioritization of central locations. Second, we realized that the decay of the attention map was too fast during the initial fixation. Therefore, we added a parameter that specified the rate of decay during the initial fixation. For all other fixations we used the same decay parameter as during the original simulations. 

The initial center map was an elliptical Gaussian (cf., Center Bias model). We estimated two parameters for the standard deviations of the center map. The horizontal standard deviation σ$_{x}$ was estimated at values of 3 . 5 $^{◦}$, 1 . 8 ◦ and 3 . 9 ◦ for Experiments 1–3. The vertical standard deviation σ$_{y}$ for Experiment 1–3 was estimated at 2 . 3 $^{◦}$, 2 . 3 ◦ and 2 . 4 $^{◦}$. The parameters estimated for Experiment 4 were very large with σ$_{x}$ = 136 . 0 ◦ and σ$_{y}$ = 

