Used the task of discriminating double-length jumps of a clock hand to explore the relation between Ss' temporal uncertainty regarding critical signal appearances and the stimulus event matrix. 32 male college students, characterized as field independent or dependent according to their performance on the short-form Embedded Figures Test and the portable Rod and Frame Test, participated in a 1-hr vigil. 2 levels of critical signal regularity (varying the interstimulus interval) were combined factorially with 2 frequencies of the stimulus event matrix. A significant decrement in signal detection as a function of task duration was evidenced, as was a significant Critical Signal Regularity * Stimulus Event Matrix * Task Duration interaction. Field-independent Ss demonstrated overall superior performance relative to field-dependent Ss, but the prediction that this difference would be accentuated under the high stimulus event matrix conditions was not supported. Possible alternative tests of the contextual constraints in the vigilance paradigm are discussed. (PsycINFO Database Record (c) 2006 APA, all rights reserved)