Fixation errors and timing in sequences of memory-guided saccades

We analyzed the relation between position and amplitude errors during the performance of sequences of saccades to previously memorized target positions in complete darkness. Although a complete compensation (on the average) for fixation errors was observed, groups of successive saccades could be identified which showed propagation of position errors. These groups are characterized by a long fixation time prior to the first saccade and short fixations prior to the remaining saccades. These findings indicate that sequences of eye movements can be performed in two different modes: (1) extraretinal information about the actual eye position is used to correct fixation errors; (2) pre-programmed groups of saccades with limited length can be performed with fixed amplitudes neglecting the actual eye position. These groups tended to occur predominantly at the end of a sequence. (C) 1998 Elsevier Science B.V. All rights reserved.