Retinal application of γ-aminobutyric acid (GABA) antagonists block direction-sensitive (DS) responses in turtle in two ways: (1) the selectivity of DS retinal ganglion cells in vitro, and (2) the eye's ability to track the direction of full field image motion. The experiments described below demonstrate that an important locus for retinal slip computation by the accessory optic system (AOS) occurs in the retina. Visual responses were measured physiologically and behaviorally from turtles which had their telencephalon removed. Physiological responses to visual field movement were recorded in the AOS using an in vitro brain preparation. DS responses of single cells were blocked by intravitreal application of bicuculline. The behavioral approach was to measure optokinetic nystagmus (OKN) in lesioned animals. OKN occured in the absence of the telencephalon, yet was disrupted following an intravitreal injection of bicuculline. Thus, both experimental approachesm showed that DS processing exists without the telencephalon, yet is disrupted by GABA antagonists applied to the retina. © 1990.