We found that ( Figure 3) compared to the yes rates under no context, the mean yes rates averaged over six observers are higher under low contextual contrast C c 0.05 and lower under higher contextual contrast C c ¼ 0.4, for any target contrast C t . We define a contextual facilitation index (CFI) as the average increase in the yes rate in a particular context (relative to no context), specifically CFI [ Mean Ct ½PðyesjC t ; a given contextÞ À PðyesjC t ; without contextÞ; Figure 2A The **data** points are the mean over six observers, and the error bars indicate the standard errors of the means (SEMs). On average and relative to the no-context condition, the weaker colinear contexts C c ¼ 0.01 and C c ¼ 0.05 raised the yes rates by CFI ¼ 38% 6 8% and 15% 6 8%, respectively, whereas the stronger context C c ¼ 0.4 lowered it by ÀCFI ¼ 17% 6 8%. The colored curves are Bayesian fits to **data** of the corresponding color, no fit is done for **data** without context. The root mean square normalized fitting error RMSNFE ¼ 0.66 in the unit of SEM. The fitted parameters (and their 95% confidential intervals) are k ¼ 1.9 (0.6, 3.2), r n ¼ 0.0025 (0.0020, 0.0029), and P(yes) ¼ 0. where Mean Ct ðxÞ [½ P Ct x=½ P Ct 1 stands for the average of x over C t . The weakest context C c ¼ 0.01 raises the yes rate by CFI ¼ 0.38 6 0.08, and the intermediate context C c ¼ 0.05 by CFI ¼ 0.15 6 0.08. In contrast, the strongest context C c ¼ 0.4 lowers the yes rate by jCFIj ¼ 0.17 6 0.08. Averaged over C t , the observers were more than twice as likely to perceive a target in the weakest than in the strongest context.