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Saccadic adaptation maintains saccade accuracy and has been studied with targeting saccades, i.e. saccades that bring the gaze to a target, with the classical intra-saccadic step procedure in which the target systematically jumps to a new position during saccade execution. Post-saccadic visual feedback about the error between target position and the saccade landing position is crucial to establish and maintain adaptation. However, recent research focusing on two-saccade sequences has shown that exploring saccades, i.e. saccades that explore an object, resists this classical intra-saccadic step procedure but can be adapted by systematically changing the main parameter used for their coding: stimulus size. Here, we adapted an exploring saccade and a targeting saccade in two separate experiments, using the appropriate adaptation procedure, and we tested whether the adaptation induced on one saccade type transferred to the other. We showed that whereas classical targeting saccade adaptation does not transfer to exploring saccades, the reciprocal transfer (i.e., from exploring to targeting saccades) occurred when targeting saccades aimed for a spatially extended stimulus, but not when they aimed for an isolated target. These results show that, in addition to position errors, size errors can drive adaptation, and confirm that exploring vs. targeting a stimulus leads to two different motor planning modes., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Saccadic adaptation maintains saccade accuracy and has been studied with the intrasaccadic target displacement procedure: displacing a target backwards (or forwards) during saccade execution gradually decreases (or increases) subsequent saccade amplitude. Adaptation has traditionally been studied with targeting saccades which bring the eyes onto a new object. Within-object saccades take the eye from one position in an object to another position in the same object and have been shown to resist the intrasaccadic target displacement procedure. The amplitude of within-object saccades depends on object size rather than position, and we therefore hypothesized that within-object saccades might adapt in response to an intrasaccadic change in object size. In separate sessions, we increased or decreased object size during within-object saccade execution. Results showed amplitude lengthening or shortening, respectively. Furthermore, within-object saccade adaptation seems to share several characteristics with targeting-saccade adaptation.

In the antisaccade task, subjects must execute an eye movement away from a visual target. Correctly executing an antisaccade requires inhibiting a prosaccade toward the visual target and programming a movement to the opposite side. This movement could be based on the inversion of the visual vector, corresponding to the distance between the fixation point and the visual target, or the motor vector of the unwanted prosaccade. We dissociated the two vectors by means of saccadic adaptation. Adaptation can be observed when systematic targeting errors are caused by the displacement of the visual target during the saccade. Adaptation progressively modifies saccade amplitude (defined by the motor vector) such that it becomes appropriate to the postsaccadic stimulus position and thus different from the visual vector of the target. If antisaccade preparation depended on visual vector inversion, rightward prosaccade adaptation should not transfer to leftward antisaccades (which are based on the same visual vector) but should transfer to rightward antisaccades (which are based on a visual vector inside the adaptation field). If antisaccade preparation depended on motor vector inversion, rightward prosaccade adaptation should transfer to leftward antisaccades (which are based on the same, adapted motor vector) but should not transfer to rightward antisaccades (which are based on a nonadapted motor vector). The results are in line with the first hypothesis, showing that vector inversion precedes saccadic adaptation and suggesting that antisaccade preparation depends on the inversion of the visual target vector.

Executing sequences of accurate saccadic eye movements supposes the use of signals carrying information about the first saccade for updating the predetermined motor plan of the subsequent saccades. The present study examines the signals used in planning a second saccade when subjects made two successive saccades towards one long or two short peripheral objects displayed before the first saccade execution. Different first eye movement signals could be used: desired eye movement signals, representing the movement necessary for attaining the intended target, or actual eye movement signals, representing the movement actually executed. Experimental dissociation of desired and actual eye movement signals is made possible by adaptive modifications of the first saccade, obtained by transfer of single saccade adaptation, during which the motor vector was progressively modified in response to the systematic intra-saccadic step of a single target. Whether the second saccade used the actual eye movement signal to compensate or not for the adaptive changes in the first saccade depended on which object properties were relevant for saccade planning. Compensation was observed for saccades that aimed for a new object (between-object saccades) because adaptation modifies relative object location. No compensation was observed for saccades that explored an extended object (within-object saccades). Implications for the on-line control of subsequent eye movements are discussed.