Adaptation of scanning saccades co-occurs in different coordinate systems

Abstract

Plastic changes of saccades (i.e., following saccadic adaptation) do not transfer between oppositely directed saccades, except when multiple directions are trained simultaneously, suggesting a saccadic planning in retinotopic coordinates. Interestingly, a recent study in human healthy subjects revealed that after an adaptive increase of rightward-scanning saccades, both leftward and rightward double-step, memory-guided saccades, triggered toward the adapted endpoint, were modified, revealing that target location was coded in spatial coordinates (Zimmermann et al. 2011). However, as the computer screen provided a visual frame, one alternative hypothesis could be a coding in allocentric coordinates. Here, we questioned whether adaptive modifications of saccadic planning occur in multiple coordinate systems. We reproduced the paradigm of Zimmermann et al. (2011) using target light-emitting diodes in the dark, with and without a visual frame, and tested different saccades before and after adaptation. With double-step, memory-guided saccades, we reproduced the transfer of adaptation to leftward saccades with the visual frame but not without, suggesting that the coordinate system used for saccade planning, when the frame is visible, is allocentric rather than spatiotopic. With single-step, memory-guided saccades, adaptation transferred to leftward saccades, both with and without the visual frame, revealing a target localization in a coordinate system that is neither retinotopic nor allocentric. Finally, with single-step, visually guided saccades, the classical, unidirectional pattern of amplitude change was reproduced, revealing retinotopic coordinate coding. These experiments indicate that the same procedure of adaptation modifies saccadic planning in multiple coordinate systems in parallel-each of them revealed by the use of different saccade tasks in postadaptation.