Unequal saccades produced by aniseikonic patterns: a model approach

This study addresses a possible mechanism for fast disconjugate adaptation of binocular horizontal saccades. Disconjugacy of binocular saccades was elicited by two dichoptically presented, identical but aniseikonic, random checkerboard patterns. Adaptation was achieved with the patterns at far distance (144 cm). In this condition, which requires a relatively small (8%) size difference of the saccades, a short learning period was mandatory for the binocular saccades to become disconjugate. The saccadic modifications were superimposed on an idiosyncratic pattern of intra-saccadic yoking. A model of saccadic signal generation is described, that has been used to separate the contributions on saccadic disconjugacy provided by modification of visual inputs processing, which alters the motor-system inputs, and by modification of the control system: the adaptation. We identified three major components of the saccadic command (two phasic and one tonic) that contribute and in a specific way to the saccadic yoking and disconjugacy. The model analysis proposes that separate control mechanisms exist operating on these phasic and tonic signals. We show that the saccadic system can generate the vergence component shown by our aniseikonic saccades. We discuss a distributed-parallel implementation of the saccadic system able to provide both the conjugate and disconjugate components of control.

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