In measurements of the probability of observation of peripherally observable test flashes it has been found disadvantageous to have special attention for the direction in which the flashes can be seen instead of general attention for the whole visual field.
J. J. MERTENS, "Influence of Knowledge of Target Location upon the Probability of Observation of Peripherally Observable Test Flashes," J. Opt. Soc. Am. 46, 1069-1070 (1956)