Adaptation of saccadic eye movements: behavioural evidence for different mechanisms controlling saccade amplitude lengthening and shortening

Abstract

Ocular saccades are fast and accurate movements of both eyes. Saccade accuracy can be maintained throughout life by adaptation mechanisms despite physiological or pathological alterations of the sensorimotor system. This study was aimed at testing whether these saccadic adaptation mechanisms involve changes in the sensory and/or motor stages of sensorimotor transformation. We used the double-step target paradigm to adapt reactive saccades directed to the right. Leftward (backward condition) and rightward (forward condition) intrasaccadic target steps were used to, respectively, decrease and increase saccade amplitude in different subjects groups. In each condition, we measured the effects of these adaptive changes of pro-saccades amplitude on anti-saccades (saccades directed to the opposite direction of a target). In the backward condition, all 8 tested subjects showed a statistically significant decrease of the amplitude of their rightward pro-saccades (−17.3 ± 1.9% on average). A group analysis showed a significant transfer of adaptation to rightward (“motor”) anti-saccades (average gain change = −18.4±2.8%), but not to leftward (“sensory”) anti-saccades (+3.6±4.8%). In the forward condition, only 8 of the 12 tested subjects showed a significant adaptive increase of the amplitude of their rightward pro-saccades (+11.5±2.1% on average). Moreover, the group analysis performed in these 8 adapted subjects did not reveal any transfer of adaptation to “motor” and “sensory” anti-saccades (+4.7±4% and +2.3±4.8%). Control experiments achieved in two further groups of 15 and 10 subjects rejected the contribution of unspecific factors (e.g. fatigue …) in this pattern of results, and duplicated these results for the other direction (leftward) of trained saccades. These findings suggest that, whereas the level of involvement of forward adaptation cannot be resolved, the mechanisms involved in the backward adaptation of reactive saccades 1) take place at a motor level, and 2) clearly differ from those involved in the forward adaptation.

Footnotes

 This work was supported by ANR grant to DP.