error latencies), the close similarity between the average distribu- tions for the prosaccade task (eg Figure 1D) and the antisaccade prosacccade error distributions (Figure 5D), and the correlation between the percentage of express saccades in prosaccade and antisaccade tasks. The dissimilarity in the ESM distributions (higher express peak, and lower subsequent peaks in prosaccade compared to prosaccade error distributions) could be explained by our using a synchronous rather than overlap antisaccade task. One appealing aspect of accumulator models in general, and race models in particular, is that various model elements appear to map to specific saccade-related neurophysiological structures and processes in a relatively straightforward manner [23,38]. Indeed the underlying neurophysiology, particularly with respect to the superior colliculus is a key component of some models [39,40]. The rate of rise of activity in saccade related neurons in both frontal eye fields (FEF) [41] and the superior colliculus (SC) [13,42] is related to saccade latency. There is some evidence that the trigger threshold is relatively fixed [41,43], while pretarget activity levels vary systematically in both FEF [44]and SC [13]. Compared to pretarget activity levels in both the FEF and SC when a normal latency saccade is executed, pretarget activity is increased prior to an express saccade, leading to the visual response in the SC being sufficient to trigger a saccade [12,13]. Pretarget activity levels in both the SC and FEF are also relatively higher in prosaccade errors relative to correct antisaccades [44,45]. In both cases, the behavioural result (an express saccade or prosaccade error) is explained in terms of increased pretarget activity in the SC; the decision signal rises from a higher baseline, and assuming a constant rate of rise and threshold, crosses threshold triggering a saccade earlier than would otherwise be the case. However, note that in the antisaccade task, a general increase in pretarget activity in the SC in ESMs, would imply higher antisaccade directional error rates and decreased prosaccade error latencies (which we observed) and decreased antisaccade latencies (which we did not observe). A change in trigger threshold at the level of the SC would also imply linked alteration in both error prosaccades and correct antisaccades. Rather, our results suggest that in ESMs there is a specific alteration in the timing and variability of reflexive prosaccades, perhaps related to descending inhibitory control, which compromises their performance in the antisaccade task. Descending inhibitory control in the oculomotor system involves a number of cortical areas (FEF; dlPFC; supplementary eye fields; parietal eye fields). Frontal lesions in humans have been shown to increase antisaccade directional error rates [46], with most recent attention directed at dlPFC [47,48]. However, effects on saccade, prossacade error and antisaccade latency were either not reported in these studies, or the results were unclear. Increased directional error rates have been produced in healthy humans using TMS of the dlPFC [49], and deactivation of dlPFC in monkeys using cooling [50]. However, microstimulation of dlPFC has also been shown to increase error rates, and increase saccade latency [51]. Given this background, it would be unwise to claim that a specific alteration in dlPFC can explain the pattern of results observed in the ESMs [52]. However, a specific and circumscribed alteration of frontal function in the ESMs, affecting inhibition of reflexive responses, leading to poorer performance in the antisaccade task, might explain our observations. References 1. Wu S, Keysar B (2007) The effect of culture on perspective taking. Psychological Science 18: 600–606. ESM Antisaccade Performance Given that in ESMs a high proportion of express saccades persists even in a testing paradigm which requires high levels of inhibition, it seems likely that they may also persist in other task contexts [53]. Thus in comparing Chinese and other participant groups (in which ESMs remain rare) using a range of oculomotor based measures, a low level effect (high numbers of express saccades in a large proportion of Chinese participants) not related to task context might bias dependant measures and be mistakenly assumed to be driven by cultural differences between groups. For example, Chinese groups have been observed to make more fixations and have shorter fixation times in scene processing tasks, compared to non-Chinese participants [19,20]. But suppose that a large proportion of the Chinese participants executed the equivalent of express saccades while scanning scenes (presumably sequences of saccades with short fixation durations), this might increase the average number of saccades and reduce average fixation duration compared to a group that did not contain a high proportion of ESMs. The cultural neuroscience literature assumes that environment (broadly constructed to include everything from the physical environment to the general cultural milieu) acts on a basically similar neurophysiological substrate to produce observed differ- ences between population groups (ie distinct cultural groups). Our results demonstrate a difference in reflexive oculomotor control within a particular cultural group. While that there were undoubtedly many ways in which the habits and environments of our individual participants differed, it is difficult to see how these might produce the specific alteration in saccade function we have observed. Our results could imply a difference in neuro- physiological substrate, at least as far as eye movement control is concerned, not primarily related to culture. It has recently been demonstrated that for certain relevant functions such as attentional processing [8] and eye movement strategies during face processing [9] ethnicity (or at least continent of origin) rather than culture is a key determinant of performance. For the difference between Chinese and non-Chinese groups we have observed, it remains to be established whether the high proportion of ESMs persist in Chinese groups whose main cultural exposure is non-Chinese. Supporting Information Figure S1 Individual frequency distribution histograms of latency for the 16 ESMs. In each plot the median prosaccade latency, and the percentage of express saccades is shown. Plots are ordered by %ES, from highest (top left) to lowest (bottom right). (TIF) Figure S2 Individual frequency distribution histograms for 60 normal participants. Conventions as for Figure S1. (TIF) Author Contributions Conceived and designed the experiments: PCK QG. Performed the experiments: PCK XJ. Analyzed the data: PCK. Contributed reagents/materials/analysis tools: PCK QG. Wrote the paper: PCK. 2. Bohns VK, Handgraaf MJJ, Sun J, Aaldering H, Mao C, et al. (2011) Are social prediction errors universal? Predicting compliance with a direct request across cultures. Journal of Experimental Social Psychology 47: 676–680. PLOS ONE | www.plosone.org 7 October 2012 | Volume 7 | Issue 10 | e47688