The Neural Control of Visually Guided Eye Movements
B. Subcortical Mechanisms of Visually Guided Saccadic Eye Movements

Neurons in the upper layers of the superior colliculus have well-defined receptive fields. Neurons in the deeper layers discharge prior to and during eye movements; some of these cells also have visual receptive fields. Electrical stimulation here elicits saccades at low currents. The basic operational principle of this structure is shown in Figure 5. Schematized are three microelectrode recording sites; shown are the receptive field locations of the neurons at the tip of each site. Electrical stimulation at these locations results in a saccadic eye movement that shift the center of gaze to the place where the receptive field had been located prior to the eye movement. The basic process therefore involves the computation of the retinal error signal between the present and desired gaze direction; the signal so generated results in a saccade that nulls the retinal error. The code utilized is called a retino-centric code.