Orbital position dependency is different for the gain of externally and internally triggered saccades

THE gain of visually triggered saccades depends on orbital position. Centrifugal saccades have smaller gains and are slower than centripetal saccades elicited by the same target amplitude. We determined whether internally triggered saccades, e.g. scanning or memory saccades, exhibit the orbital position dependency evident in visually guided saccades. The search coil technique was used to record eye movements of healthy subjects while they performed horizontal 12.5 degrees saccades under three paradigms (gap, scanning and memory saccades). Orbital position influenced externally triggered gap saccades but not the gain or peak eye velocity of scanning or memory saccades. These findings do not support the idea that position dependency caused by orbital mechanics is compensated for at the level of the common brainstem burst generator. Instead our results are consistent with the view that cortical output reflects the differences evident in the gain of visually triggered centrifugal and centripetal saccades. (C) 1999 Lippincott Williams & Wilkins.