Use of exocentric and egocentric representations in the concurrent planning of sequential saccades

摘要

The concurrentplanningofsequential saccadesoffersasimple modeltostudythenatureofvisuomotor transformationssincethesecond saccade vector needs to be remapped to foveate the second target following the first saccade. Remapping is thought to occur through egocentric mechanisms involving an efference copy of the first saccade that is available around the time of its onset. In contrast, an exocentric representation of the second target relative to the first target, if available, can be used to directly code the second saccade vector. While human volunteers performed a modified double-step task, we examined the role of exocentric encoding in concurrent saccade planning by shifting the first target location well before the efference copy could be used by the oculomotor system. The impact of the firsttarget shiftonconcurrent processing wastestedbyexamining the end-pointsofsecondsaccades followingashiftofthesecond target during the first saccade. The frequency of second saccades to the old versus new location of the second target, as well as the propagation of first saccade localization errors, both indices of concurrent processing, were found to be significantly reduced in trials with the first target shift comparedtothose without it.Asimilar decreaseinconcurrent processing was obtainedwhenweshifted the first target but kept constant the second saccade vector. Overall, these results suggest that the brain can use relatively stable visual landmarks, independent of efference copy-based egocentric mechanisms, for concurrent planning of sequential saccades.