Effect of allocentric landmarks on primate gaze behavior in a cue conflict task.

The relative contributions of egocentric versus allocentric cues on goal-directed behavior have been examined for reaches, but not saccades. Here, we used a cue conflict task to assess the effect of allocentric landmarks on gaze behavior. Two head-unrestrained macaques maintained central fixation while a target flashed in one of eight radial directions, set against a continuously present visual landmark (two horizontal/vertical lines spanning the visual field, intersecting at one of four oblique locations 11° from the target). After a 100-ms delay followed by a 100-ms mask, the landmark was displaced by 8° in one of eight radial directions. After a second delay (300-700 ms), the fixation point extinguished, signaling for a saccade toward the remembered target. When the landmark was stable, saccades showed a significant but small (mean 15%) pull toward the landmark intersection, and endpoint variability was significantly reduced. When the landmark was displaced, gaze endpoints shifted significantly, not toward the landmark, but partially (mean 25%) toward a virtual target displaced like the landmark. The landmark had a larger influence when it was closer to initial fixation, and when it shifted away from the target, especially in saccade direction. These findings suggest that internal representations of gaze targets are weighted between egocentric and allocentric cues, and this weighting is further modulated by specific spatial parameters.