Ultra-fine knowledge of gaze position in saccade planning

Sensorimotor integration is an important component of spatial representations, as retinotopic information needs to be combined with extra-retinal knowledge about eye movements to properly locate objects in space. Recent research has shown that extra-retinal oculomotor knowledge extends to fixational drift, the persistent wandering of the eye in between saccades: human observers are capable of inferring geometrical configurations even in the absence of spatial information on the retina, purely based on motor knowledge of eye drift (Zhao et al, Nature Communications, 2023). Here we examine whether extraretinal information about fixational drift is also used to control eye movements. Specifically, we study whether saccade planning takes into account drift-induced displacements of the line of sight from the intended fixation location. Emmetropic observers (N=5) attempted to maintain fixation on the location P0 of a previously briefly displayed marker (a 10’ dot at maximum contrast) in complete darkness. After an interval of 2s, the eye had drifted to a new location PE, and a saccade cue (another 10’ dot displayed for 50 ms) instructed subjects to perform a 4 degree horizontal saccade. The task of the subject was to perform a return saccade to the remembered location of P0. Our data show that the return saccade compensated for the previous drift displacement. This compensation accounted for 55% of the angular correction needed to fixate back to the original point. Similar results were found when the task was replaced by a 2AFC where subjects performed a saccade toward one of two cues placed at positions P0 and PE. In both conditions, performance was higher than when subjects visually selected P0 or PE with a button press instead of a return saccade, suggesting that the uncertainty in extraretinal drift estimation is smaller in the motor modality than for visual judgments.