The optokinetic response was used to assess the relationship between the optic tectum and primitive cortex. The effects of monocular vision as well as transection of the optic tectum commissure on movement detection in the turtle was examined. Ablation of the cortex or tectum indicated that the cortex had little functional significance and that the optic tectum was the primary visual center. The monocular study indicated that movement detection is relatively independent in the two eyes, each involved only with temporal to nasal movement. Transection of the tectal commissure abolished the supernormal response rate following sudden reversal of stimulus movement. This effect supported a proposed tectal integrative mechanism involved in movement detection in the turtle.