The parietal cortex and the rostral part of the superior colliculus are involved in fixation.
Deactivation of the collicular fixation system leaves only express saccades.
To probe the quality of fixation one may try to evaluate the slow movements and the microsaccades observed during a task where the subjects has nothing to do but to fixate a small target stimulus.
Another way is to ask the subject to make saccades and measure the reaction times. If there are more express saccades in an overlap condition the fixation system is "weaker" and if the reaction times are "longer" the fixation system is "stronger".
Express saccade makers are subjects, who produce more than 30% express saccades in an overlap prosaccade task. From the deactivation studies we concluded that it is the fixation system, which is to weak in these subjects.