New mechanism that accounts for position sensitivity of saccades evoked inresponse to stimulation of superior colliculus

Electrical stimulation of the feline superior colliculus (SC) is known to e voke saccades whose size depends on the site stimulated (the "characteristi c vector" of evoked saccades) and the initial position of the eyes. Similar stimuli were recently shown to produce slow drifts that are presumably cau sed by relatively direct projections of the SC onto extraocular motoneurons . Both slow and fast evoked eye movements are similarly affected by the ini tial position of the eyes, despite their dissimilar metrics, kinematics, an d anatomic substrates. We tested the hypothesis that the position sensitivi ty of evoked saccades is due to the superposition of largely position-invar iant saccades and position-dependent slow drifts. We show that such a mecha nism can account for the fact that the position sensitivity of evoked sacca des increases together with the size of their characteristic vector. Consis tent with it, the position sensitivity of saccades drops considerably when the contribution of slow drifts is minimal as, for example, when there is n o overlap between evoked saccades and short-duration trains of high-frequen cy stimuli.