Sensorimotor integration compensates for visual localization errors duringsmooth pursuit eye movements

To localize a seen object, the CNS has to integrate the object's retinal lo cation with the direction of gaze. Here we investigate this process by exam ining the localization of static objects during smooth pursuit eye movement s. The normally experienced stability of the visual world during smooth pur suit suggests that the CNS essentially compensates for the eye movement whe n judging target locations. However, certain systematic localization errors are made, and we use these to study the process of sensorimotor integratio n. During an eye movement, a static object's image moves across the retina. Objects that produce retinal slip are known to be mislocalized: objects mo ving toward the fovea are seen too far on in their trajectory, whereas erro rs are much smaller for objects moving away from the fovea. These effects a re usually studied by localizing the moving object relative to a briefly fl ashed one during fixation: moving objects are then mislocalized, but flashe s are not. In our first experiment, we found that a similar differential mi slocalization occurs for static objects relative to flashes during pursuit. This effect is not specific for horizontal pursuit but was also found in o ther directions. In a second experiment, we examined how this effect genera lizes to positions outside the line of eye movement. We found that large lo calization errors were found in the entire hemifield ahead of the pursuit t arget and were predominantly aligned with the direction of eye movement. In a third experiment, we determined whether it is the flash or the static ob ject that is mislocalized ahead of the pursuit target. In contrast to fixat ion conditions, we found that during pursuit it is the flash, not the stati c object, which is mislocalized. In a fourth experiment, we used egocentric localization to confirm this result. Our results suggest that the CNS comp ensates for the retinal localization errors to maintain position constancy for static objects during pursuit. This compensation is achieved in the pro cess of sensorimotor integration of retinal and gaze signals: different ret inal areas are integrated with different gaze signals to guarantee the stab ility of the visual world.