Quantification analysis for saccadic eye movements

Traditional parameters, such as latency and peak velocity, have been used for quantitative analysis of saccade dynamics. However, saccades of equal magnitude which differ substantially in dynamics may still have the same peak velocity. This lack of selectivity might degrade the efficacy of the peak velocity measurement in discriminating patients from normal subjects. In this study, an alternative parameter, the damping ratio of a second-order response, is proposed to concisely and accurately describe saccadic dynamics. A measurement of damping ratio is made by considering the response curve of saccade as the step response of a second-order transfer function. Using the least-mean-square algorithm, the second-order function has been optimized to fit the response of the saccade dynamics with the latency removed. Nineteen normal subjects (ten young and nine older) and 16 patients of Parkinson's disease (eight mildly affected and eight advanced affected) were tested for eye movements in response to pseudorandom saccadic stimuli. While the traditional peak velocity had difficulties in differentiating between the saccadic dynamics of normal and Parkinson subjects, the damping ratio is sensitive enough not only to highlight the difference between a group of patients mildly affected with Parkinson's disease and an age-matched normal group (p<0.01; abnormal versus normal), but also to distinguish between groups at different ages (p<0.01; younger versus older). It is proposed that the damping ratio could be a useful parameter in analyzing saccadic dynamics.