What determines saccade timing in sequences of coordinated eye and hand movements?

In action sequences, the eyes are generally fixated ahead of the stimulus being responded to, overlapping the processing of adjacent stimuli. What determines when the saccade to the next item is initiated and what processing is overlapped remain controversial. Saccade initiation has been linked directly to the difficulty of the underlying processing on the fixated item or to a rhythmic timer whose period is adjusted to the difficulty of the context. We compared the effects of item and context difficulty when subjects made speeded choice responses to five visual stimuli arrayed linearly. Item difficulty was manipulated by varying stimulus-response compatibility, creating contexts (sequences) whose average item difficulty differed. Results showed that saccade initiation time reflected item difficulty, not context difficulty, with evidence that saccade initiation was delayed until central processing was completed. We discuss the implications of these results for the control of saccades and processing in action sequences.