In a further test, we repeated the whole above analysis considering fixations within ROIs only, and fed their number to the generator of random fixations (random viewer). The previous results were confirmed, i.e., significantly smaller KLDact values for non-primate images, and significantly larger KLDact values for primate images than expected (not shown). In order to investigate the existence of regions-of-interests (ROIs), defined as areas with high GDC-0980 price density of fixation positions, we identified spatial clusters of fixations by use of the mean shift algorithm (Comaniciu and Meer, 2002 and Funkunaga and Hosteler, 1975) adapted for eye movement

data (Santella and DeCarlo, 2004). This is an automatic, entirely data-driven method that derives the number and arrangement of clusters deterministically. The algorithm starts from the set of N   fixation positions vi,j→=xi,jyi,j, with i   ∈ (1, …, N  ) being the index of the fixation positions, and j   = 1 the original fixation positions on the 2D screen. The clustering algorithm proceeds iteratively, while moving at each iteration each of the points to its new position v→i,j+1, in dependence on the weighted mean of proximity and density of points around the reference point, v→i,j+1=∑iK(Vij−Vk,j)Vk,j∑i(Vij−Vk,j) with j ≠ k. The kernel K was defined as a

2D-Gaussian with mean and Angiogenesis inhibitor variance of 0: K(v→)=e(x2+y2)σ2. σ   was the only parameter of the clustering algorithm and defined the attraction radius of the points. We varied its value and found 2.5 to yield satisfying results, i.e., the algorithm did not lead to over

fitting or to coarse clusters. We used Oxymatrine this value to perform all of our analyses. At each iteration the positions were moved into denser configurations, and the procedure was stopped after convergence. Thereby fixations were assigned to a cluster whose reference points lay within a diameter of 1° apart, referred to as experimental cluster. Robustness to extreme outliers was achieved by limiting the support of points at large distances as defined by the kernel K(v→). In order to discard outlier clusters, we additionally applied a significance test to disregard clusters containing only a very small fraction of the data that deviate from expectation of independence. As a significance test on the experimental clusters, we proceeded as follows: we assigned n random locations on the screen by drawing n pairs of uniformly distributed numbers, with n being the total number of fixations on a specific image. This random fixation map was fed into the mean shift clustering algorithm, leading to a set of simulated clusters. Repeating this procedure 100 times, we obtained two distributions: one of fixation numbers per cluster and one of cluster point density.