A Coordinate System Based on Experimental Work that Describes Ocular Kinematics

Abstract

Abstract: : Purpose:To present a model and a coordinate system based on experimental work that describes eye kinematics. Methods: A simple plastic transparent spherical globe was made to rotate around a fixed horizontal axis. A series of crosses composed of thin black plastic strips and spaced 15^o apart were pasted on the prime meridian. The center of each cross represented different positions of the visual line. The model was recorded on videotape while rotating from 0^o to 90^o with a Sony TVR950 Camcorder ^TM.The angular projections of the horizontal arms of the crosses from the spherical model onto a flat surface (the charged couple device of the camcorder) were recorded and measured directly in different gaze positions using Adobe Photoshop ^TM. The angular projections were also determined mathematically using the derivative of the general equation for an ellipse to calculate the angular projections of the crosses at different elevations and azimuths. Individual frames from video clips of five subjects were analyzed for angular projections from the eye to a flat surface. A horizontal guide line was drawn across the center of the pupil of each frame. The position of the center of the pupil was determined in different gaze directions and connected to an iris marking. The angle between the horizontal line and the line connecting the center of the pupil and the iris marking was measure in different gaze positions. Pins of different colors were pushed into a flat black-felt screen at different intervals and in different configurations to serve as fixation targets. Subjects exerted maximum effort in upward and downward circum-gaze.The angular projections were then compared with the angular projections from the digital images of the eye in comparable positions. Results: Angular projections from the model and from the iris markings were similar confirming the validity of the model for describing eye movements. For example the angle projected from the spherical model of the horizontal limb of the cross at elevation 25^o and azimuth 30^o was about 12^o. The projection from iris markings was about the same. Conclusions: The validity of the model for describing eye movements was confirmed. From the model a coordinate system was constructed. There is no torsion or counterrolling in any direction of gaze. The polar axis for eye rotation is the horizontal axis (the X-axis of coordinate systems described in the literature). The equitorial plane is 90^o from the polar axis. Tertiary gaze is achieved by two degrees of freedom of movement consisting of alternate great circle and small circle rotations. There are other inferences to be derived from the model.

Keywords: eye movements • eye movements: recording techniques • eye movements: conjugate