, 1999; Everling and Munoz, 2000; Sato and Schall, 2003). We verified that these results were not confounded by simple variation of RT across conditions and that modulation in the Accurate condition was not simply a byproduct of response withholding. First, we examined activity

in visually responsive and movement neurons on trials in which monkeys missed response deadlines and produced premature Accurate or late Fast responses (see Experimental Procedures). This necessarily reversed the RT effect (mean RT was faster after premature Accurate [367 ms] than late Fast [499 ms] trials, though error rates were unaffected; Figure 4A). If our results were due to RT rather than cognitive state, neural activity levels should also reverse. This did not occur; activity levels remained higher in the Fast condition Compound Library than the

Accurate condition for both visually responsive (Figure 4B) and movement (Figure 4C) neurons. Interestingly, we also observed that target selection time was delayed for late Fast responses relative to premature Accurate trials (Figure 4B, arrows), suggesting that response deadlines were missed Adriamycin ic50 due to late or premature target localization (Ho et al., 2012). Second, we compared neural activity in the three SAT conditions holding RT constant. We matched trials from the Accurate and Fast conditions to a restricted range of RTs around the median RT in the Neutral condition (see legend to Figure 4). Once again, neural activity varied with SAT condition independent of RT (Figures 4D and 4E). Together, these results demonstrate that changes in cognitive state elicited by SAT cues persisted across the range of RT. In other words, fast responses in the Fast condition and equally fast responses in the Accurate condition were qualitatively different. Were monkeys simply guessing in the Fast condition?

The high accuracy rates in the Fast condition (∼70%) indicate that they were not. To investigate further, we reasoned that fast guesses should result in a nonuniform distribution of errors in the Thymidine kinase Fast condition. Specifically, guesses should be more prevalent for the fastest responses than for comparably slower responses. We divided the Fast condition into RT quintiles and found that error rates differed by less than 0.3%. Further evidence against a guessing strategy is provided by our previous work showing that guesses are associated with attenuated, rather than magnified, neural activity in FEF (Heitz et al., 2010), opposite of the pattern reported here. Some investigators have suggested that SAT is mediated not by the level of a response threshold but rather by the excursion of firing rate from baseline to threshold (Forstmann et al., 2008, 2010; van Maanen et al., 2011). We observed variation in both baseline and presaccadic activity, so it is possible that the total excursion was larger in the Accurate than Fast condition.