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Behavioral Evidence of Separate Adaptation Mechanisms Controlling Saccade Amplitude Lengthening and Shortening

¹Institut National de la Santé et de la Recherche Médicale U864, Espace et Action, Bron; ²Université de Lyon and Biologie Humaine, Université Lyon 1; ³Hospices Civils de Lyon, Mouvement et Handicap, Hôpital Neurologique, Lyon; ⁴Institut Fédératif de Recherche (IFR) 19, Institut Fédératif des Neurosciences de Lyon, Lyon and IFR23, Institut Fédératif de Recherche sur le Handicap, Bron, France; and ⁵Departments of Physiology, Psychology, and Medicine, Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada

Submitted 2 September 2008; accepted in final form 9 December 2008

Abstract

The accuracy of saccadic eye movements is maintained over the long term by adaptation mechanisms that decrease or increase saccade amplitude. It is still unknown whether these opposite adaptive changes rely on common mechanisms. Here, a double-step target paradigm was used to adaptively decrease (backward second target step) or increase (forward step) the amplitude of reactive saccades in one direction only. To test which sensorimotor transformation stages are subjected to these adaptive changes, we measured their transfer to antisaccades in which sensory and motor vectors are spatially dissociated. In the backward adaptation condition, all subjects showed a significant amplitude decrease for adapted prosaccades and a significant transfer of adaptation to antisaccades performed in the adapted direction, but not to oppositely directed antisaccades elicited by a target jump in the adapted direction. In the forward adaptation condition, only 14 of 19 subjects showed a significant amplitude increase for prosaccades and no significant adaptation transfer to antisaccades was detected in either the adapted or nonadapted direction. These findings suggest that, whereas the level(s) of forward adaptation cannot be resolved, the mechanisms involved in backward adaptation of reactive saccades take place at a sensorimotor level downstream from the vector inversion process of antisaccades and differ markedly from those involved in forward adaptation.