Figure 1.

Trial structure and study design. (A) Subjects performed saccades from square to square along the horizontal meridian. Instructional color cues (300 ms duration) at the currently foveated square signaled the direction of the next saccade (e.g., “blue = right” vs. “green = left”; here represented by horizontal vs. vertical lines) or indicated to maintain fixation (e.g., “red = stay”). After each saccade, the subjects’ gaze remained at the new square until the next saccade was triggered by a color cue. The stimulus onset asynchrony (SOA) was jittered between 1.5 and 6 s for all saccade and fixation cues. (B) Saccade vectors were oriented either toward (centripetal) or away from (centrifugal) the primary position (defined as head-centered straight gaze) and also differed in their proximity to the primary position (proximal vs. distal). The design thus resulted in four saccade trial types represented by vectors with specific start and end positions (start-to-end), i.e., proximal centrifugal (0-to-1), distal centrifugal (1-to-2), proximal centripetal (1-to-0), and distal centripetal (2-to-1), each of which could be performed to the left or right side. (C) Depending on the initial deviation of the eyes relative to primary position, rightward saccades of identical amplitude either re-center the eyes or move them further away from primary position.