The saccadic system is a prime example of motor control without continuous visual feedback. These systems suffer from a strong vulnerability against disturbances. The mechanism of saccadic adaptation allows adjustment of saccades to alterations arising not only from anatomical changes but also from external changes. The weighting of errors according to their reliability provides a strong benefit for an optimized control system. Thus the consistency of visual error should influence the characteristics of adaptation. In the typical adaptation paradigm a visual error is introduced by stepping the target during the saccade by a given amount. In this paradigm, the retinal error varies with the accuracy of the saccade and the step size. To study the influence of error consistency we use a variant of the adaptation paradigm which allows to specify a constant error size. Intrasaccadic target step sizes were calculated with respect to the predicted landing position of each individual saccade. The consistency of the visual error was varied by introducing different levels of noise to the intrasaccadic target step. Different mean intrasaccadic target step sizes were examined: positive target step, negative target step, and a condition in which the mean of the error distribution was clamped to the fovea. In all three conditions saccadic adaptation was strongest when the error was consistent and became weaker as the error became more variable. These results show that saccadic adaptation takes not only the average error but also the consistency of the error into account.

When asked to maintain their gaze steady on a given location, humans continually perform microscopic eye movements, including fast gaze shifts known as microsaccades. It has long been speculated that these movements may contribute to the maintenance of fixation, but evidence has remained contradictory. We used a miniaturized version of saccadic adaptation, an experimental procedure by which motor control of saccades is modified through intrasaccadic displacements of the target. We found that the statistical distribution of microsaccade amplitudes changes after brief exposure to systematic shifts of the fixation point during microsaccade occurrence. Shifts in the same directions as microsaccades produce movements with larger amplitudes, whereas shifts against microsaccade directions result in smaller movements. Our findings show that microsaccades are precisely monitored during fixation and that their motor program is modified if the postsaccadic target position is not at the expected retinal location. These results demonstrate that saccadic adaptation occurs even when the stimulus is already close to the foveal center and precise execution of the movement may not be critical. They support the proposal that adaptation is necessary to maintain a consistent relationship between motor control and its visual consequences and that the representation of space is intrinsically multimodal, even during fixation.

Havermann K, Zimmermann E, Lappe M. Eye position effects in saccadic adaptation.

The saccadic amplitude of humans and monkeys can be adapted using intrasaccadic target steps in the McLaughlin paradigm. It is generally believed that, as a result of a purely retinal reference frame, after adaptation of a saccade of a certain amplitude and direction, saccades of the same amplitude and direction are all adapted to the same extent, independently from the initial eye position. However, recent studies in humans have put the pure retinal coding in doubt by revealing that the initial eye position has an effect on the transfer of adaptation to saccades of different starting points. Since humans and monkeys show some species differences in adaptation, we tested the eye position dependence in monkeys. Two trained Macaca fascicularis performed reactive rightward saccades from five equally horizontally distributed starting positions. All saccades were made to targets with the same retinotopic motor vector. In each session, the saccades that started at one particular initial eye position, the adaptation position, were adapted to shorter amplitude, and the adaptation of the saccades starting at the other four positions was measured. The results show that saccades that started at the other positions were less adapted than saccades that started at the adaptation position. With increasing distance between the starting position of the test saccade and the adaptation position, the amplitude change of the test saccades decreased with a Gaussian profile. We conclude that gain-decreasing saccadic adaptation in macaques is specific to the initial eye position at which the adaptation has been induced.

Saccades are so called ballistic movements which are executed without online visual feedback. After each saccade the saccadic motor plan is modified in response to post-saccadic feedback with the mechanism of saccadic adaptation. The post-saccadic feedback is provided by the retinal position of the target after the saccade. If the target moves after the saccade, gaze may follow the moving target. In that case, the eyes are controlled by the pursuit system, a system that controls smooth eye movements. Although these two systems have in the past been considered as mostly independent, recent lines of research point towards many interactions between them. We were interested in the question if saccade amplitude adaptation is induced when the target moves smoothly after the saccade. Prior studies of saccadic adaptation have considered intra-saccadic target steps as learning signals. In the present study, the intra-saccadic target step of the McLaughlin paradigm of saccadic adaptation was replaced by target movement, and a post-saccadic pursuit of the target. We found that saccadic adaptation occurred in this situation, a further indication of an interaction of the saccadic system and the pursuit system with the aim of optimized eye movements.

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Synchronisation of spatiotemporal continuous disorder is realised in a Liquid Crystal Light Valve single feedback system with an incoherent, unidirectional master-slave-coupling scheme as excellent model system for synchronisation. Thus, complex states disordered in space and time were completely synchronised by using identical systems as master and slave. Thereby the impeding role of system differences is demonstrated in comparison to former experiments. A novel imaging method is introduced, in which the synchronisation process and effects like a time lag can be more easily characterised.

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