Smooth pursuit eye movements in response to 2-dimensional motions: the role of predictability and the two-thirds power law
Creators: name: Wang, Jie
Publisher: name: No Publisher Supplied
Year: 2023

description: Objects moving in natural environments change their direction of motion frequently (animals chasing one another; limbs or fingers performing visuomotor tasks). Smooth pursuit eye movements, which maintain the line of sight on smoothly moving targets, need to generate accurate and timely responses to changes in the direction of target motion. Models of smooth pursuit eye movements, however, are dominated by studies of responses to one-dimensional constant velocity target motions. The effects of the geometry of two-dimensional motion paths have not received adequate attention. The experiments reported in this thesis examined smooth pursuit of predictable and unpredictable two-dimensional target motions, including investigation of the covariation of movement curvature and speed. A major goal will be to determine whether the two-thirds power law, a mathematical relationship between movement curvature and speed that has been found for biological movements and smooth pursuit of repetitive elliptical motions, applies to the present results. Two experiments were run. In both a target moved at a constant speed toward a central location and then changed direction abruptly at one of five angles ranging between 0° and180°). The path of target motion was either predictable (path marked by a line) or unpredictable (unmarked). The time of the turn was either fixed (Experiment 1) or chosen at random (Experiment 2). Results showed that after the target began to move, eye speed reached values close to target speed and then began to decrease over time, reaching a minimum shortly after the time of the turn. Minimum eye speed depended on the turn angle of the target. The relationship between log pursuit speed (S) and the log radius of curvature (R) of the eye trajectory showed that log⁡S increased as a function of log⁡R, in approximate agreement with the classical two-thirds power law. The relationship between log⁡R and log⁡S, however, was best described by an exponential function. This exponential relationship appeared to arise from the ceiling in eye speed for the largest values of R and a floor value of the radius of curvature. These results show that pursuit depends in important ways on the, the curvature, of the eye’s path. The fact that the relationship between the geometry and kinematics of pursuit is so much like that of other biological motions has two implications: (1) The curvature-speed relationship in pursuit may reflect the same constraints on the kinematics of neural activity that have been found to apply to arm movements. For arm movements, the neural population of directionally- tuned cells in cortical area M1 shows limits in how quickly the firing rates change in response to the planned change in the movement direction. For high curvature movements, requiring large changes in movement direction, a broad range of cells with different preferred directions would be active simultaneously resulting in a lower population firing rate and consequently a slower movement speed. For pursuit, similar constraints may occur in the neural population of directionally-tuned cells in the smooth pursuit region of frontal eye field. (2) The compliance of pursuit with the two-thirds power law means that pursuit may be particularly well-suited to track the motions of living things, arguably the most important targets in the real-world for pursuit.