Yarbus
34 suggested that vergence starts first, followed by the saccade and then by the residual vergence. He also suggested that during the saccade the vergence simply adds to the saccade. Subsequent studies
12 13 18 19 35 dealing with saccade–vergence interactions have questioned the second observation of Yarbus. Vergence has been reported to be accelerated by the saccade and, reciprocally, saccade was decelerated by the vergence.
12 19 Nevertheless, this is an unresolved controversy, and it is not known whether such a vergence component is central “facilitation” (see Zee et al.
19 ) or peripheral “enhancement” (see Hung
36 ). Collewijn et al.
14 observed that frequently vergence starts first, perhaps more frequently for convergence components than for divergence components. Combined divergent movements, nevertheless, when studied with high resolution also showed presaccade divergence. The average percentage of trials for which vergence starts first was 40% in our adult population, which is less than that reported by Collewijn et al.
14 It may be related to the different spatiotemporal arrangement of the target presentation used in the two studies. The most novel result of our study is the difference between children and adults in this aspect of behavior. Children clearly show frequent asynchronous behavior, namely the preceding of the vergence component. ANOVA, with adults versus children as the fixed factor applied on the percentage of simultaneously starting combined eye movements, shows a significant difference between adults and children (F
1,28 = 5.25,
P < 0.05). Moreover, our data show that the ability for synchronization develops very slowly with age and continues beyond the age of 10 to 12 years (see
Fig. 9C ). Thus, the study of combined eye movements provides a useful tool for examining this delicate aspect of visuomotor control in children, which is presumably related to cognitive development and ability to handle multiple motor commands together.