Figure 1. Percentage frequency distribution histograms of saccade latency in the prosaccade overlap task. A. Example from an individual ‘‘normal’’ subject; B. individual ESM. In A. and B. the median saccade latency, and the percentage of express saccades is shown. C. Mean695%CI distribution for 60 ‘‘normal’’ subjects. D. Mean695%CI distribution for 16 ESMs. In C. and D. the intersubject mean (6SD) of the individual median latencies and the intersubject mean (6SD) percentage of express saccades is shown. The vertical grey region shows the range of express saccade latency (80 ms to 130 ms) and the dotted vertical line is at 100 ms. The three horizontal grey bars in C. show the three latency ranges (1 to 3) over which latency was compared between ESMs and normal subjects. doi:10.1371/journal.pone.0047688.g001 80 ms to 130 ms (the express saccade range; Figure 1C, range 1), 150 ms to 180 ms (range 2), and 200 ms to 230 ms (range 3) between the two groups. For each range the data were analysed with a repeated measures ANOVA, treating ‘‘latency bin’’ as a within subjects factor and group (normal vs ESM) as a between subjects factor. For all three ranges the effect of group was ESM Antisaccade Performance statistically significant (F1,74 = 577, 153 and 233 respectively, all p,0.001). Antisaccade Tasks Of the 17 ESM participants, antisaccade data were obtained for 16. Subsequent analysis is based on this group of 16 ESMs (median age 24 y, 7 males) compared to a group of 60 normal participants (median age 23.5 y, 30 males). ESM antisaccade directional error rate (41624%; Figure 3A) was statistically significantly higher (t = 2.5, p = 0.01) compared to the normal participants (28616%). Error pro-saccade latency (ie saccades directed at the target in the antisaccade task, ErrPS) and correct antisaccade latency (CorAS) were analysed. For all bar one of the normal participants, and all of the ESMs, median ErrPS latency was less than that of CorAS latency. However, the intersubject mean difference between the medians (CorAS-ErrPS) was 97654 ms and 127645 ms for the normal and ESM groups respectively; this difference was statistically significant (t = 2.01, p,0.05). The reason for this greater difference in the EMSs was that while CorAS latency was identical between the groups (Figure 3C; Norm: 290661 ms vs ESM:290637 ms), the ErrPS latency was lower in the ESMs (Figure 3B; Norm:193631 ms vs ESM: 164635 ms). When investigated with a repeated measures ANOVA with saccade type (ErrPS vs CorAS) as a within subjects factor, and group (Norm vs ESM) as a between subjects factor, saccade type was significant (F1,74 = 233.9, p,0.001) while group was not (F1,74 = 1.9, p = 0.187). The interaction between type and group was statistically significant (F1,74 = 4.1, p = 0.045). We examined the influence of target direction on the generation of errors by calculating the directional error rate separately for each participant in each direction (Figure 4). Note that by target direction we are referring to the side on which the visual target was presented, not the required direction of a correct antisaccade. For normal participants there was very little difference in the rightward (29619%) and leftward (20620%) error rates, with 29/60 participants (48%) having a larger absolute error rate for rightward targets. For the ESMs there was more evidence of an asymmetry with the rightward mean error rate (42620%) slightly higher than the leftward (38626%); 11/16 (69%) exhibited higher error rates when targets were presented on the right. We examined the distribution of CorAS and ErrPS latencies in the two groups by plotting mean (695% CI) distribution Figure 2. Data from prosaccade task. A. Plot of the percentage of express saccades against median prosaccade latency. B. Distribution of percentage of express saccades in the prosaccade task. Vertical dashed line shows the criterion used to define an ESM (30% ES in the prosaccade task). Columns to the right of this line show counts of ESMs. doi:10.1371/journal.pone.0047688.g002 PLOS ONE | www.plosone.org 3 October 2012 | Volume 7 | Issue 10 | e47688