What is the coordinate frame utilized for the generation of express saccades in monkeys?

The latencies of saccades to suddenly appearing eccentric targets can have a bimodal distribution, with an early, express peak, and a late, regular peak (Fischer and Boch 1983, Brain Res 260: 21-26). Express saccades usually are a product of learning. The purpose of this study was to determine whether this learning is specific to the relative position of the target in space, the orbital position of the eye, or the vector of the saccade to be produced. Further, it was asked whether and how the frequency with which express saccades are generated is influenced by the immediately preceding saccadic vector and the familiarity of the targets. To this end, rhesus monkeys were trained to make saccadic eye movements to single targets and to two sequential targets that appeared at various positions relative to the head, relative to the initial fixation spot and relative to each other. The results show that the frequency with which express saccades are generated is determined by the saccadic vector that has to be generated and not by the relative position of a target in space, the orbital position of the eye, the immediately preceding saccadic vector, or the familiarity of the targets.