ESM Antisaccade Performance Figure 3. Data from antisaccade tasks. Comparison of mean695%CI between 60 normal (non-ESM) subjects and 16 ESMs. A. Antisaccade directional error rate. B. Mean error pro-saccade latency. C. Mean correct antisaccade latency. Note different y-axis scales in B and C. doi:10.1371/journal.pone.0047688.g003 histograms for each saccade type in each of the groups (Figure 5). While the distribution of CorrAS latency was identical, the lower mean latency for ESM ErrPS latency was explained by a prominent early peak in the distribution, which (as in overlap tasks) occurred in the express saccade range. The mean proportion of ErrPS with latencies in the express range was 36626%. This contrasted with the normal group in whom there were fewer errors with latency in the express range (11611%). The difference between these percentages was statistically significant (t = 5.69, p,0.0001). Relationship between Overlap and Antisaccade Performance The two groups, defined on the basis of their performance on the prosaccade task, exhibited different patterns of performance in the antisaccade task. What then of the relationship between performance in the two different tasks? Performance in the prosaccade task was summarised using median saccade latency for each participant and the percentage of express saccades. We investigated the relationship between these parameters and antisaccade directional error rate (expressed as a percentage) and median ErrPS latency (Figure 6). Antisaccade directional error rate was correlated both with the median latency in the prosaccade task (Figure 6A; r = 20.33, p = 0.003) and with the percentage of express saccades (6B; r = 0.36, p = 0.002). Correla- tion coefficients were slightly higher between the median prosaccade latency and antisaccade error prosaccade latencies (6C; r = 20.55, p,0.0001) and percentage of express saccades in prosaccade tasks and antisaccade prosaccade error latency (6D; r = 0.43, p,0.0001). The highest correlation coefficient was observed for the relationship between the percentage of express saccades in prosaccade tasks, and the percentage of ErrPS that were express saccades (Figure 6E; r = 0.63, p,0.0001). We also examined whether a general difference in the speed of prosaccade and antisaccade systems might have a strong influence on directional error rate. For each participant we calculated the difference between the median correct antisaccade latency and the median prosaccade latency, and plotted this against the anti- saccade directional error rate (Figure 6f). While there was a positive correlation, it was modest (r = 0.32, p = 0.004). Discussion Our first objective was to establish the proportion of express saccade maker participants (ESMs) in a large group of healthy, adult, na¨ıve Chinese participants using a prosaccade overlap task, Figure 4. Influence of target direction (Left vs Right) on the antisaccade directional error rate (%) for A. Normal participants; B ESMs. Solid line is the line of equality (x = y). doi:10.1371/journal.pone.0047688.g004 identical to that used in our previous study [10]. In such tasks, the central fixation target remains illuminated when the eccentric saccade target appears. Compared to prosaccade gap tasks, the continued presence of the fixation target provides no visual warning signal of the imminent appearance of the saccade target caused by early fixation offset, and makes fixation disengagement more difficult. This usually results in an increase in saccade latency and a reduction in express saccades [24,25]. Previous estimates [15] and studies in which relatively large numbers of participants had been tested [17], suggest that ESMs comprise no more than 5% of healthy adults. In our earlier experiment [10] 29% of a na¨ıve Chinese group were ESMs compared to 3% in a UK group. In the current study 17/77 (22%) were ESMs. Thus ESMs were again encountered much more frequently than expected. We have used the criterion employed previously [10] as developed in the original reports on ESMs [15,16] to divide the Chinese participants into two groups (ESMs and ‘‘Norms’’). While consistent with the previous literature, it might be argued that we have dichotomised what is continuous, producing groups where there is in fact a continuum. However, as plotted in Figure 2, the data suggest that the 30% ES criterion does capture something that other parameters (eg median prosaccade latency) do not. We reported previously that the distribution of %ES in Caucasian and Chinese groups is different (Figure 5 in [10]), with no occurrence in the Caucasian data of the bulge that appears to the right of the criterion line in Figure 2B. The data do not appear to be consistent with the hypothesis that the distribution of this parameter is simply shifted to the right in the Chinese group as a whole. There continue to be many Chinese participants who in overlap conditions execute few ES, who are indistinguishable from their Caucasian counterparts. The difference between the two popula- tions is the occurrence of the high number of participants in the Chinese population who execute many ES in prosaccade overlap conditions, defined here as ESMs. While the 30% criterion is to some extent arbitrary, it is not equivalent to other procedures such as performing a median split on a continuous variable. In fact the median proportion of ES for our dataset of 76 participants is 15%. The higher 30% criterion was originally adopted because it identified a particular type of na¨ıve participant [15]. Our main aim in the current experiment was to investigate how participants meeting this criterion behave in a voluntary saccade task (the antisaccade task). The difference between the ESMs, defined as discussed above, and other participants was not a simple speeding of saccade reaction times. There was considerable overlap between the PLOS ONE | www.plosone.org 4 October 2012 | Volume 7 | Issue 10 | e47688