Velocity matching during smooth pursuit of different targets on different backgrounds

Human smooth pursuit is not completely understood. For example, its maximun average gain (eye velocity/ target velocity) has been the subject of controversy. Studies of smooth pursuit of unpr~i~able ramps have given contlieting results. Puckett and Steinman (1969) and Steiman, Skavenski and Sansbury (1969) reported that maximum average gain was less than 1. Rashbass (1961) claimed that gain was equal to 1 under the same conditions. A resolution of this conflict is important because if the eye, on the average, lags behind the target, retinal slip velocity (the difference between eye and target velocity) could be used to keep the eye smoothly pursuing the target once smooth pursuit is underway. Recent, interest in this problem was renewed by Robinson (1976, pp. 29-31) who suggested that there may be two smooth pursuit subsystems--One that uses retina1 image slip to keep going and another that, on the average, matches eye velocity to target velocity.2 This proposed velocity-matching “fast foveal” smooth pursuit subsystem, whose average gain would equal 1, uses retinal slip only to allow the eye to attain a velocity equai to target velocity. Once this velocity is attained it is “remembered” and used to keep the eye travelling at the same velocity as the target without any need for new retinal slip signals. Robinson proposed that such a fast fovea1 velocity matching subsystem would be activated when motivation is provided to minimize velocity and position errors. Such motivation might arise when an acuity target is tracked because its critical detail would be hard to resolve if the target image slipped appreciably on the retina or if the target image moved away from the center of best vision in the fovea. The present experiments set out to determine maximum average smooth pursuit gain under conditions designed to encourage the smooth pursuit subsystem to perform at its best. Specifically, smooth pursuit