This study investigated the response characteristics of single cells in the nucleus of the basal optic root (nBOR) of the accessory optic system of pigeons. One hundred and twenty-one nBOR cells were studied and quantitative data from 87 units indicated that they preferred very large random patterned stimuli, and did not respond to stimuli less than 30 degrees in diameter. In many cases the receptive field encompassed the entire monocular visual field. nBOR cells were directionally sensitive with approximately 70% preferring motion with an upward vector. They responded to very slow velocities in the range of 0.5 degrees to 5.0 degrees s-1, and did not adapt to continuous stimulation. Maximal responses occurred to very large randomly patterned stimuli moving slowly in an upward direction. These response characteristics together with the anatomical connections of the nBOR suggest that this nucleus may be involved in processing self-induced visual motion information required for vertical optokinetic stabilization.