Different dynamics of human smooth-pursuit onset and offset: a comparison of young and elderly subjects

The investigation of smooth-pursuit eye movements (SPEM) with step-ramp/ste p-stop stimuli, rather than the conventional sinusoidal stimuli, permits th e evaluation of smooth-pursuit initiation and termination. The similarity o f these onset and offset mechanisms is the subject of some controversy. The effects of target predictability, target velocity, and age on smooth-pursu it onset and offset were investigated using the search-coil technique. We m easured 21 healthy subjects in two age groups (21-30 years and 53-72 years) . SPEM were elicited by horizontal step-ramp/step-stop stimuli presented on a homogeneous background in a predictive and a nonpredictive mode and targ et velocities of 10 or 20 deg/sec. Latency, duration, and velocity overshoo t of SPEM onset and offset were analyzed. A velocity overshoot at the onset of pursuit was seen in all subjects under all conditions (predictable vs. nonpredictable, age, stimulus velocity). The magnitude of the overshoot dep ended on stimulus conditions and age. It was clearly smaller under predicta ble conditions, at a stimulus velocity of 20 deg/sec, and for the elderly. In contrast, none of our subjects showed an overshoot at the end of smooth pursuit. Our results suggest different dynamics for SPEM onset and offset.