Eye-Head Coordination

Abstract

To direct head and eyes toward a target and ultimately fixate it with the fovea, an animal must solve three problems. First, he must compute the angular distance between his foveal lines of sight and the target to be acquired. This angular distance is referred to as retinal error; its absolute magnitude will determine the amplitude of the saccadic eye movement that will be produced. Second, the animal must initiate a head movement that will be compatible in amplitude with the saccadic eye movement. Third, since the eyes usually move first and with higher velocity than the head, their lines of sight will reach and fixate the target while the head is still moving. To stabilize his eyes with respect to a stationary target during head movement, the animal must perform a rotational eye movement that, by being opposite in direction from the movement of the head, but equal to it in amplitude and velocity, allows the fovea to remain constantly on the target. This movement is termed compensatory.