Reaction times of head movements occurring in association with express saccades during human gaze shifts. 
During lateral gaze shifts to a visual target of 10 degrees eccentricity, the gap paradigm (presence of a 200 ms gap between offset of fixation lamp and onset of target lamp) produced a significant decrease in the reaction times (RTs) of head movements (54 ms) and saccades (82 ms) when compared with those obtained under the overlap paradigm across five subjects.  Correlation coefficients between the RTs of saccades and head movements were significantly lower under the gap paradigm than those under the overlap paradigm.  Particularly, the RTs of head movements occurring in association with express saccades were independent of the saccade RTs.  These results indicate that the head and eye motor systems are controlled by the separate neural mechanisms in the conditions which produce express saccades.